Micrometeorology Research Papers - Academia.edu (original) (raw)

The SOLAS air-sea gas exchange experiment (SAGE) was a multiple-objective study investigating 2 gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in

- by and +2
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- Geochemistry, Oceanography, Microstructure, Turbulence

The Greater Accra region is the most urbanized region in Ghana. Over the last decade, the region has undergone rapid population shift and urban sprawl. This has modified the natural environment of the area and led to the formation and warming of Urban Heat Island (UHI). Previous studies have not established enough evidence on the existence of UHI phenomenon in the region. For this reason, this study examined the existence, magnitude, and spatial extent of UHI warming as well as its effect on temperature extremes in the region. The study used Landsat satellite imagery captured in 1991, 2002 and 2017, and in-situ daily minimum and maximum temperature data spanning the period 1980 to 2017. The satellite images were processed and analyzed using an integrated Geographic Information System (GIS) and remote sensing technique while extreme temperature indices were assessed with the ClimPACT2 software. The study found that UHI existed in the region with an increasing spatial coverage and a magnitude of about 4.07 • C, 5.79 • C, and 4.86 • C in 1991, 2002, and 2017 respectively. The built-up and bare land areas experienced the strongest effect of UHI warming. In addition, enhanced UHI warming effect caused an increase in the frequency and intensity of warm temperature extremes in the region. A faster increase in night time temperatures than day time resulted in a decrease in diurnal temperature range of the region. Considering the high rate of warming amidst rapid urban expansion, more warming is expected in the region. This is expected to exacerbate climate extreme events and weather-related health issues. It is also expected to increase energy consumption, air pollution and human discomfort.

- by Leonard Amekudzi
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- Environmental Science, Atmospheric Science, Climatology, Micrometeorology

Water stress in plant is associated with reduced availability of soil moisture under higher ambient temperature and wider vapour pressure deficit for a considerable period of time. Instruments like pressure chambers and porometers are being used to quantify crop water stress under field conditions, but their use is limited because of the numerous time-consuming measurements that must be made. The application of thermal indices involving canopy temperature for monitoring crop water stress and irrigation scheduling has been demonstrated by several researchers in the last five decades since the evolution of portable infrared thermometers in the 1960s. As the temperature of plant canopy is a manifestation of canopy energy balance, a water-stressed canopy is hotter than a well-watered one under the same environmental conditions. Infrared thermometer integrates the thermal radiation from all exposed surfaces in the field of view of the instrument that included the plant surface and exposed soil surfaces into a single measurement and converts it into temperature unit applying the principle of Stefan-Boltzmann law. However, different plant physiological as well as micro-climatic factors like solar radiation, turbulence, air temperature and humidity must influence the canopy temperature at the time of observation. Hence, stoma-tal conductance and transpiration rates cannot be estimated by canopy temperature alone. In other words, canopy temperature alone is not enough to make estimates of plant water status. For this reason many researchers have attempted to normalize the canopy temperature to account for the influence of other variable

The extent of African elephant (Loxodonta africana) induced damage on shrub and mature Baikiaea plurijuga trees was investigated around artificial and natural watering points in northern Hwange National Park, Zimbabwe. Damage was assessed in three zones of elephant occupancy during the dry season i.e. high elephant occupancy zone (≤ 1 km from water points), moderate elephant occupancy zone (> 1-2 km from water points) and low elephant occupancy zone (> 2 km from water points). A total of 48 plots along baseline transects were sampled among four artificial watering points and four natural watering points at increasing distance from the watering points. Damage to recruits, mature B. plurijuga and overall woody vegetation decreased with distance from artificial watering points. In addition, damage to mature B. plurijuga and overall woody vegetation decreased with distance from natural watering points, whereas damage to recruits did not change with distance from water points. Our results show that artificial watering points are associated with higher damage to B. plurijuga recruits and overall woody vegetation within ≤ 1 km radius from water points compared to natural watering points. Other changes associated with increasing distance from artificial watering points were increase in canopy cover and decrease in woody species diversity. In the natural watering points, we recorded an increase in canopy cover, mean basal area of B. plurijuga shrubs and height B. plurijuga shrubs, and a decrease in species diversity with distance from watering points. Overall, woody species diversity was higher around natural watering points than around artificial watering points. Our findings suggest that browsing by large herbivores near watering points leads to the degradation of vegetation.

- by Kanisios Mukwashi and +1
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- Animal Science, Wildlife Ecology And Management, Micrometeorology, Wildlife Conservation

The Greater Accra region is the most urbanized region in Ghana. Over the last decade, the region has undergone rapid population shift and urban sprawl. This has modified the natural environment of the area and led to the formation and warming of Urban Heat Island (UHI). Previous studies have not established enough evidence on the existence of UHI phenomenon in the region. For this reason, this study examined the existence, magnitude , and spatial extent of UHI warming as well as its effect on temperature extremes in the region. The study used Landsat satellite imagery captured in 1991, 2002 and 2017, and in-situ daily minimum and maximum temperature data spanning the period 1980 to 2017. The satellite images were processed and analyzed using an integrated Geographic Information System (GIS) and remote sensing technique while extreme temperature indices were assessed with the ClimPACT2 software. The study found that UHI existed in the region with an increasing spatial coverage and a magnitude of about 4.07 • C, 5.79 • C, and 4.86 • C in 1991, 2002, and 2017 respectively. The built-up and bare land areas experienced the strongest effect of UHI warming. In addition, enhanced UHI warming effect caused an increase in the frequency and intensity of warm temperature extremes in the region. A faster increase in night time temperatures than day time resulted in a decrease in diurnal temperature range of the region. Considering the high rate of warming amidst rapid urban expansion, more warming is expected in the region. This is expected to exacerbate climate extreme events and weather-related health issues. It is also expected to increase energy consumption, air pollution and human discomfort.

- by cosmos wemegah and +2
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- Atmospheric Science, Climatology, Micrometeorology, Remote sensing and GIS

L’obiettivo principale della tesi sperimentale è quello di valutare gli effetti, sullo stato idrico di piante di agrume, dell’adozione del regime irriguo deficitario denominato Partial Root-zone Drying (PRD). L’applicazione di tecniche geofisiche minimamente invasive (ERT), supportata da misure di umidità del suolo, ha permesso, inoltre, di caratterizzare le interazioni tra suolo e apparato radicale delle piante di agrume presso l’area di studio sottoposte a regime PRD.

- by Giovanni Zocco
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- Agricultural Engineering, Agrarian Studies, Agriculture, Environmental Monitoring

The mitigation of the heat island effect can be achieved by the use of cool materials that are characterized by high solar reflectance and infrared emittance values. Several types of cool materials have been tested and their optical and thermal properties reveal that these materials can be classified as "cool" with the ability to maintain lower surface temperatures. Cool materials can be used on buildings and other surfaces of the urban environment. Based on these results, a modeling study was undertaken to assess the urban heat island effect over Athens, Greece, a densely populated city, by trying to analyze the impacts of large-scale increases in surface albedo on ambient temperature. Numerical simulations were performed by the "urbanized" version of the nonhydrostatic fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5, version 3-6-1). Two scenarios of modified albedo were studied: a moderate and an extreme increase in albedo scenario. It was found that large-scale increases in albedo could lower ambient air temperatures by 2°C. Furthermore, the impact of high albedo measures on heat island magnitude was estimated by creating a spatial representation of the urban heat island effect over the modeled area. The results of this study can help to promote the adoption of high albedo measures in building energy codes and urban planning regulations.

- by Afroditi Synnefa
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- Environmental Engineering, Meteorology, Case Studies, Micrometeorology

In this study, simple parameterizations, used to determine the net radiation, have been evaluated with dedicated field data from a tropical location at Ile-Ife, Nigeria. Separately determined were the net radiation for a clear-sky condition and a modification of this, incorporating the relative cloud amount as a multiplicative factor. Cloud amount has been computed as a ratio between the measured global radiation and that calculated for the clear-sky condition. In the absence of direct measurement of the global radiation, a simple empirical expression is given. From these estimations, differences between the clear-sky and the cloud-adjusted net radiation values were found to be substantial, thus indicating that cloudiness is an important factor influencing radiative balance of the tropical atmosphere. The scheme was compared with the actual measurements and remarkably good agreements have been obtained with correlations ranging between 0.87 and 0.94.

- by Gbenga Jegede
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- Micrometeorology, Cloudiness, Parameterization, Net Radiation

Knowledge of soil-vegetation-atmosphere energy exchange processes is essential for examining the response of agriculture to changes in climate in both the short and long term. However, there are relatively few sites where all the flux measurements necessary for evaluating these responses are available; where they exist, data are often incomplete and/or of limited duration. At the same time, there is often an extensive observation network available that has gathered key meteorological data (sunshine, wind, rainfall, etc.) over decades. Simulating the terms of the surface energy balance (SEB) using available meteorological, soil and vegetation data can improve our understanding of how agricultural systems respond to climate and how this response will vary spatially. Here, we employ a physically-based scheme to simulate the SEB fluxes over a mid-latitude, maritime temperate environment using routine weather observations. The latent heat flux is a critical SEB term as it incorporates the response of the plant to environmental conditions including available energy and soil water. This response is represented in modeling schemes through surface resistance (r s), which is usually expressed as a function of near-surface water vapor alone. In this study, we simulate the SEB over two grassland sites, where eddy flux observations are available, representing imperfectly-and poorly-drained soils. We employ three different formulations of r s , representing varying degrees of sophistication, to estimate the surface fluxes. Due to differences in soil moisture characteristics between the sites, we ultimately focused our attention on an r s formulation that accounted for soil water retention capacity, based on the Jarvis conductance model; the results at both hourly and daily intervals are in good agreement, with RMSE values of ≈ 40 W m −2 for sensible and latent heat fluxes at both sites. The findings show the potential value of using routine weather observations to generate the SEB where flux observations are not available and the importance of soil properties in estimating surface fluxes. These findings could contribute to the assessment of past and future climate change on grassland ecosystems.

- by Kazeem A B I O D U N Ishola and +1
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- Micrometeorology, Surface energy balance, Land-Atmosphere Interaction, Land Surface Modeling

As the acreages of agricultural lands increase, changes in surface energetics and evapo-transpiration (ET) rates may arise consequently affecting regional climate regimes. The objective of this study was to evaluate summertime ET dynamics and surface energy processes in a subarctic agricultural farm in Interior Alaska. The study includes micrometeoro-logical and hydrological data. Results covering the period from June to September 2012 and 2013 indicated consistent energy fractions: LE/R net (67%), G/R net (6%), H/R net (27%) where LE is latent heat flux, R net is the surface net radiation, G is ground heat flux and H is the sensible heat flux. Additionally actual surface evapotranspiration from potential evaporation was found to be in the range of 59 to 66%. After comparing these rates with those of most prominent high latitude ecosystems it is argued here that if agroecosystem in high latitudes become an emerging feature in the land-use, the regional surface energy balance will significantly shift in comparison to existing Arctic natural ecosystems.

- by Aon Watcharee and +1
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- Environmental Science, Agroecology, Micrometeorology, Environmental Sustainability
- by Vladimir Janković
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- Political Ecology, Cold War and Culture, History of Science, Computer Networks
- by Xiaopeng Gao
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- Earth Sciences, Modeling and Simulation, Micrometeorology, Biological Sciences

Daily time-step normalized difference vegetation index (NDVI) time series from satellite-derived (NOAA/AVHRR, SPOT/VEGETATION, TERRA/MODIS) and ground-based micrometeorological sensors were evaluated for a coniferous pine forest (Pinus sylvestris L.) located in Hyytiälä, Finland. Micrometeorology-based broadband NDVI was calculated from observed upward and downward photosynthetically activity radiation (PAR) and global radiation measurements. The composite satellite-derived NDVI time series were smoothed with a best index slope extraction method (BISE) and adjusted Fourier transform (AFT) in order to downscale from the compositing period to daily scale.The broadband and satellite-derived NDVIs were highly correlated during the main growth period (Julian days 90–270), but poorly correlated when the entire year was considered, i.e., large differences occurred during winter. High correlations were also found between the seasonal courses for broadband NDVI and daily air temperature. The analysis revealed that the onset of greenness in spring was consistently determined from broadband NDVI time series in different years, but that fluctuations in NDVI during the late season transition to winter dormancy prevented reliable prediction of the termination in physiological activity. Efforts to retrieve the same relationships during spring from satellite-derived NDVI failed.After comparing the smoothed time series from different NDVI determinations, we examined the relationship between NDVI, gross primary production (GPP) and FAPAR. An obvious exponential relationship is found between broadband NDVI and GPP (R2=0.72 for clear weather conditions; also detectable from the satellite sensors), while a linear relationship occurs between broadband NDVI and FAPAR (R2=0.79). FAPAR in relation to satellite-derived NDVI is best described with a logistic curve under clear weather conditions, but the level of correspondence is low (R2=0.53). Overall, broadband NDVI is a good index to describe physiological activity of the pine forest during certain periods, i.e. provides a means for obtaining other physiological parameters that are required by ecosystem models. However, during the late season, broadband NDVI estimated over the pine stand is influenced by more than vegetation physiological activity. Though satellite-derived NDVI is more difficult to link to GPP, it may still provide useful information under clear weather conditions. Satellite-derived NDVI remains our only choice for generalization in large-scale investigations. Thus, intensified examination of the influences of smoothing and downscaling of satellite-derived NDVI is inevitable.

- by Nguyen Quoc
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- Remote Sensing, Meteorology, Time Series, Micrometeorology

- by Shakkie Kativu and +1
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- Animal Science, Wildlife Ecology And Management, Micrometeorology, Wildlife Conservation

Biogeochemical models are useful tools for integrating the effects of agricultural management on GHG emissions; however, their development is often hampered by the incomplete temporal and spatial representation of measurements. Adding to the problem is that a full complement of ancillary measurements necessary to understand and validate the soil processes responsible for GHG emissions is often not available. This study presents a rare case where continuous N 2 O emissions, measured over seven years using a flux gradient technique, along with a robust set of ancillary measurements were used to assess the ability of the DNDC model for estimating N 2 O emissions under varying crop-management regimes. The analysis revealed that the model estimated soil water content more precisely in the normal and wet years (ARE 3.4%) than during the dry years (ARE 11.5%). This was attributed to the model's inability to characterize episodic preferential flow through clay cracks. Soil mineral N across differing management regimes (ARE 2%) proved to be well estimated by DNDC. The model captured the relative differences in N 2 O emissions between the annual (measured: 35.5 kg N 2 O-N ha À1 , modeled: 30.1 kg N 2 O-N ha À1 ) and annual-perennial (measured: 26.6 kg N 2 O-N ha À1 , modeled: 21.2 kg N 2 O-N ha À1 ) cropping systems over the 7 year period but overestimated emissions from alfalfa production and underestimated emissions after spring applied anhydrous ammonia. Model predictions compared well with the measured total N 2 O emissions (ARE À11%) while Tier II comparison to measurements (ARE À75%) helped to illustrate the strengths of a mechanistic approach in characterizing the site specific drivers responsible for N 2 O emissions. Overall this study demonstrated the benefits of having near continuous GHG flux measurements coupled with detailed ancillary measurements towards identifying soil process interactions responsible for regulating GHG emissions.

- by Pietro Goglio and +2
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- Modeling and Simulation, Micrometeorology, Cropping Systems, Environmental Sciences

Virtually all reviews dealing with aerosol-sized particle deposition onto forested ecosystems stress the significance of topographic variations, yet only a handful of studies considered the effects of these variations on the deposition velocity (V d). Here, the interplay between the foliage collection mechanisms within a dense canopy for different particle sizes and the flow dynamics for a neutrally stratified boundary layer on a gentle and repeating cosine hill are considered. In particular, how topography alters the spatial structure of V d and its two constitutive components, particle fluxes and particle mean concentration within and immediately above the canopy, is examined in reference to a uniform flat-terrain case. A two-dimensional and particle-size resolving model based on first-order closure principles that explicitly accounts for (i) the flow dynamics, including the two advective terms, (ii) the spatial variation in turbulent viscosity, and (iii) the three foliage collection mechanisms that include Brownian diffusion, turbo-phoresis, and inertial impaction is developed and used. in V d across the hill near the canopy top. Just after the hill summit, the model results suggest that V d fell to 30% of its flat terrain value for particle sizes in the range of 1-10 µm. This reduction appears consistent with maximum reductions reported in wind-tunnel experiments for similar sized particle deposition on ridges with no canopies.

- by davide poggi
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- Boundary Layer Meteorology, Micrometeorology, Atmospheric sciences, Topography

Micrometeorological studies in urban zones are more complex than in rural zones. Inside urban canopy, 3-D flow structures are created by the urban geometry (streets with different configurations of buildings). In this paper, the application of microscale models (CFD models) to urban micrometeorology is studied and a CFD model is applied to an array of cubes representing a portion of a city. The model results are validated against wind tunnel measurements made with the same geometry and are analysed to understand the flow properties in this case. Strong horizontal inhomogeneities of the mean and turbulence variables are found inside the canyons showing that the representativeness of measurement in one point is limited spatially. In addition, the CFD results are spatially averaged over thin slices encompassing a cube-canyon unit in the central zone of the array showing the special importance of average properties as the dispersive stress inside the urban canopy created by the cubes.

- by Alberto Martilli
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- Heat Transfer, Micrometeorology, Validation, RANS

We evaluate a new approach to estimate regional evapotranspiration (ET) across a montane, Mediterranean climate gradient in the San Jacinto and Santa Rosa Mountains of Southern California. Spatially distributed evaporative fraction (EF) measurements were made monthly from October 2008 to September 2009 at 54 locations across an elevational gradient using a mobile measurement platform, called the Regional Evaporative Fraction Energy Balance (REFEB) method. We used these measurements and the Enhanced Vegetation Index (EVI) from MODerate resolution Imaging Spectroradiometer (MODIS) observations to derive EF at a regional scale. We converted EF to monthly ET using remote-sensing based observations of available energy. We compared the REFEB ET estimates, along with modified Priestly-Taylor (PT) ET estimates driven by MODIS data against four eddy covariance (EC) towers and eight gauged catchments. Both of the satellite-based ET estimates were highly correlated with tower ET observations (r 2 = 0.66 for REFEB and 0.95 for PT). The PT MODIS approach overestimated ET compared to precipitation estimates and stream gauge measurements, while REFEB ET was moderately lower than PT ET. The annual regional REFEB ET (193 mm) was 87 mm less than precipitation (280 mm). REFEB ET underestimated EC tower ET (regression slope = 0.78, p < 0.001). Regional PT ET (288 mm) exceeded precipitation by 8 mm and significantly overestimated EC tower ET (regression slope = 1.43, p < 0.001). The relationship between precipitation and ET is not linear, with a break around 290 mm/year, at which point ET becomes nearly constant at 200-300 mm/year with increasing precipitation. This causes a break in the precipitation-runoff relationship, with a disproportionate increase in runoff when precipitation exceeds 290 mm/year. REFEB provides a viable method to estimate regional ET, which is applicable to areas that are poorly constrained by other remote sensing approaches.

- by Ray Anderson
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- Micrometeorology, Watershed Hydrology, Evapotranspiration

It is known that when an isolated porous windbreak is represented by a sink in the mean momentum equations, even the simplest turbulence closures lead to reasonably good simulations of the mean wind close to the barrier, irrespective of whether or not sources are introduced in the turbulent kinetic energy (TKE) and dissipation rate ( ) equations. However, unless the barrier is considered to dissipate TKE in addition to opposing the mean flow, the pattern of turbulence is poorly simulated.

- by Eugene Yee
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- Earth Sciences, Turbulence, Micrometeorology, Biological Sciences
- by George Odhiambo
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- Micrometeorology

- by Alberto Martilli
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- Heat Transfer, Micrometeorology, Validation, RANS

The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100 m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (σ u , σ w ) and temperature (σ T ) are more planar homogeneous than their vertical flux of momentum (u 2 * ) and sensible heat (H ) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO 2 and water vapour. While the spatial variability in fluxes was significant (>15%), this unique data set confirmed that single tower measurements represent the 'canonical' structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the 'moving-equilibrium' hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u * , especially when u * was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (I w ) and the arrival frequency of organized structures (ū/ h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed.

- by Narasinha Shurpali and +1
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- Micrometeorology

The aim of this work is the statistical analysis of temperature measurements carried out in three vineyards and in several neighbouring meteorological stations. Measurements have been carried out in northern Italy during the 2008, 2009, 2010 vegetative seasons. The collected data series have been compared with those measured in neighboring stations belonging to Agrometeorological and Regional Meteorological Networks, allowing to study the vineyard micrometeorology. The aim of this analysis is to evaluate the accuracy of the measurements and to reconstruct vineyard microclimate by means of data series from meteorological stations.

- by Claudio Cassardo
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- Climate Change, Micrometeorology, Wine, Italy

Constant in the mixed-layer growth parameterization, 0.2 Constants in the force-restore model Gravitational acceleration, 9.81 m s-* Height of the mixed layer, m Von Karman's constant, 0.4 Soil thermal conductivity, W m-t "C-' Surface hydraulic resistance, s m-' Aerodynamic resistance of the surface layer, s m-' Time variable, s Windspeed at height zl,, m s-' Friction velocity, m s-' Height of the surface layer, m Screen height, m Height of known windspeed, m Surface roughness length, m Constant in the mixed-layer growth parameterization, 5.0 Volumetric heat capacity of the air at constant pressure, J m-a 'C-r Volumetric heat capacity of the soil, J m-s 7-r Surface soil heat flux density, W m-* Surface sensible heat flux density, W m-* Latent heat of vaporization of water, J kg-' Latent heat flux density, W me2 Monin-Obukhov length, m Time period of interest, s Net radiation flux density to the surface. W m-* Solar radiation flux density, W mm2 Underlying soil temperature, "C Mean absolute air temperature, K Soil surface albedo de; Idh in the free atmosphere, "C m-' Boundary-Layer

- by Michael Novak
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- Boundary Layer Meteorology, Modeling, Micrometeorology, Sensitivity Analysis

This study quantitatively investigated the scientific progress of walkability research landscape and its future prospects using bibliometric indicators to highlight the research hotspots. The results accentuated multifaceted nature of walkability research landscape with a strong association towards public health disciplines. Keyword co-occurrence analysis emphasized that majority of the walkability studies centred on the interactions between walking and other three main factors such as built environment attributes, transportation and obesity. Based on the identified research hotspots, a brief state-of-the-art review of walkability studies was presented. Future prospects based on the unexplored research gaps within the hotspots were also discussed. High correlation (r = 0.99, p < 0.05) between annual publications and citation counts demonstrated the significance of walkability studies to the contemporary scientific community. Being one of the comprehensive studies to evaluate the historic trajectory of walkability research landscape, the findings were expected to accelerate a comprehensive understanding of the walkability research domain that will assist future research direction.

We analyzed three data sets to examine methods to estimate the height of the stable boundary layer (SBL) in the complex setting of an urban area. We focused on a data set from a field study that we conducted in Riverside, California, in 2011 in which surface micrometeorological variables were measured using a sonic anemometer. In addition, profiles of temperature and relative humidity were measured with a tethersonde up to a height of 500 m. These measurements, supplemented with data from other field studies, were used to evaluate representative diagnostic and prognostic models for the SBL height. As expected, these models performed poorly in the spatially inhomogeneous setting of the urban area. The time integrated friction velocity (u*), with time starting at sunset, provided the best estimate of the height. This formulation is useful because we show that the surface friction velocity can be estimated with a measurement of wind speed at one height, and an estimate of the surface roughness length.► Structure of the urban nocturnal boundary layer was investigated through a field study. ► Different models for the stable boundary layer heights (SBL) were evaluated. ► These models performed poorly in the spatially inhomogeneous setting of the urban area. ► Observed SBL heights, correlated well with the time integral of friction velocity. ► Surface friction velocity can be estimated using the measured wind speed at one level.

- by Sam Pournazeri
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- Micrometeorology

A first-order closure model that has the general utility for predicting the wind flow within and above vegetation canopies is presented. Parameterization schemes that took into account the influence of large turbulent eddies were developed for the Reynolds stress and the mixing length in the model. The results predicted by the model were compared with measured data for wind speeds within and above six types of vegetation canopy, including agricultural crops, deciduous and coniferous forests, and a rubber tree plantation during fully leafed, partially leafed and leafless periods. The predicted results agreed well with the measured data; the root-mean-square errors in the predicted wind speeds (non-dimensionalized by the friction velocity above the canopy) were about 0.2 or less for all of the canopies. The secondary wind maxima that occurred in the lower canopies were also correctly predicted. The influence of foliage density on the wind profiles within and above a vegetation canopy was successfully simulated by the model for a rubber tree plantation during fully leafed, partially leafed and leafless periods. The bulk momentum transfer coefficients (C M) and the values of λ (which are defined by z 0 =λ(h−d), where z 0 is the roughness and d is the zero-displacement height of the canopy) for the vegetation canopies were also studied, and the relationships C Mh =0.0618 exp(0.792C F) and λ=0.209 exp(0.414C F) were determined; here, C Mh is the bulk momentum transfer coefficient at the canopy top; C F =C d PAI z max /h, where C d is the effective drag coefficient of the canopy, PAI is the plant area index and z max is the height at which the plant area density is maximum. The values of λ ranged from 0.22 to 0.32 for the canopies studied.

- by Hidenori Takahashi
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- Earth Sciences, Environmental Science, China, Micrometeorology

We report initial measurements of concentrations and net air-water exchange fluxes of target persistent bioaccumulative toxicants (PBTs) in Lake Superior utilizing techniques not previously applied for this purpose. Gaseous PBTs are collected in diffusion denuders containing sections of commercial chromatography columns and subsequently thermally extracted into the cooled injection inlet of a high-resolution gas chromatograph. The PBT sampling/analytical methods enable accurate determination of gas-phase PBT concentration and micrometeorological measurement of fluxes to be carried out. PBT fluxes are measured by the modified Bowen ratio technique in which sensible heat flux is related to PBT flux, with the assumption of identical transfer velocities of heat and PBTs between two heights in the atmospheric surface layer. Micrometeorological measurement of flux accounts for all sources of resistance to mass transfer, including atmospheric stability effects, surface films, waves, sea spray, and bubbles. The sensible heat flux, PBT concentration, and PBT flux measurements carried out in 14 2-or 3-h periods during seven sampling events in Lake Superior in summer and fall 2002 and spring 2003 demonstrate advantages under the constraints of the techniques. The uncertainty of the flux measurements was typically in the range from 1% to 160%. Gaseous concentrations of R-hexachlorocyclohexane (R-HCH) and hexachlorobenzene (HCB) over Lake Superior were in the range from 6 to 170 and 12-95 pg/m 3 , respectively. Fluxes out of Lake Superior were measurable in 75% of the cases in which a concentration gradient was measured, and were in the range from-0.17 to +0.064 ng/m 2 ‚h for R-HCH and from-0.60 to-0.093 ng/m 2 ‚h for HCB.

- by David F. Tobias
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- Environmental Monitoring, Heat Treatment, Micrometeorology, Multidisciplinary

Applied previously to momentum and heat fluxes, the present study extends the flux-variance method to latent heat and CO 2 fluxes in unstable conditions. Scalar similarity is also examined among temperature (θ), water vapour (q), and CO 2 (c). Temperature is adopted as the reference scalar, leading to two feasible strategies to estimate latent heat and CO 2 fluxes: the first one relies on flux-variance similarity relations for scalars, while the second is based on the parameterization of relative transport efficiency in terms of scalar correlation coefficient and a non-dimensional quantity. The relationship between the θ-to-q transport efficiency (λ θq) and θ-q correlation coefficient (R θq) is used to describe the intermediate hydrological conditions. We also parameterize the θ-to-c transport efficiency (λ θ c) as a function of the θ-c correlation coefficient (R θ c) by introducing a new non-dimensional ratio (α). The flux-variance method is a viable technique for flux gap-filling, when turbulence measurements of wind velocity are not available. It is worth noting that the extended method is not exempt from a correction for density effects when used for estimating water or carbon exchange.

- by M. Leclerc
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- Physics, Boundary Layer Meteorology, Micrometeorology, Atmospheric sciences

While the breakdown in similarity between turbulent transport of heat and momentum (or Reynolds analogy) is not disputed in the atmospheric surface layer (ASL) under unstably stratified conditions, the causes of this breakdown are still debated. One reason for the breakdown is differences between how coherent structures transport heat and momentum, and their differing responses to increasing instability. Monin – Obukhov Similarity Theory (MOST), which hypothesizes that only local length-scales play a role in ASL turbulent transport, implicitly assumes that large-scale structures are inactive, despite their large energy content. Widely adopted mixing-length models also rest on this assumption in the ASL. The difficulty of characterizing low-wavenumber turbulent motions with field measurements motivates the use of high-resolution Direct Numerical Simulation (DNS), which is free from subgrid-scale parametrizations and ad hoc assumptions near the boundary. Despite the low Reynolds number and idealized geometry of the DNS, DNS-estimated MOST functions are consistent with ASL field experiments, as are low-frequency features of the spectra. Parsimonious spectral models for MO stability correction functions for momentum (φ m) and heat (φ h) are derived, based on idealized vertical velocity variance and buoyancy variance spectra fit to the corresponding DNS spectra. For φ m , a spectral model, based only on local length-scales, matches DNS and field measurements well. In contrast, for φ h , the model is substantially biased unless contributions from larger length-scales are also included. These results are supported by sensitivity analyses based on field measurements that are independent of the DNS. They show that ASL heat transport is not MO-similar, even under mild stratification, and in the absence of entrainment, non-stationarity and canopy effects. It further suggests that the breakdown of the Reynolds analogy is at least partially caused by the influence of large eddies on turbulent heat transport.

- by Kaighin McColl
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- Boundary Layer Meteorology, Micrometeorology, Atmospheric Turbulence

The effects of disturbance history, climate, and changes in atmospheric carbon dioxide (CO 2 ) concentration and nitrogen deposition (N dep ) on carbon and water fluxes in seven North American evergreen forests are assessed using a coupled water-carbon-nitrogen model, canopy-scale flux observations, and descriptions of the vegetation type, management practices, and disturbance histories at each site. The effects of interannual climate variability, disturbance history, and vegetation ecophysiology on carbon and water fluxes and storage are integrated by the ecosystem process model Biome-BGC, with results compared to site biometric analyses and eddy covariance observations aggregated by month and year. Model results suggest that variation between sites in net ecosystem carbon exchange (NEE) is largely a function of disturbance history, with important secondary effects from site climate, vegetation ecophysiology, and changing atmospheric CO 2 and N dep . The timing and magnitude of fluxes following disturbance depend on disturbance type and intensity, and on post-harvest management treatments such as burning, fertilization and replanting. The modeled effects of increasing atmospheric CO 2 on NEE are generally limited by N availability, but are greatly increased following disturbance due to increased N mineralization and reduced plant N demand. Modeled rates of carbon sequestration over the past 200 years are driven by the rate of change in CO 2 concentration for old sites experiencing low rates of N dep . The model produced good estimates of between-site variation in leaf area index, with mixed performance for between-and within-site variation in evapotranspiration. There is a model bias : S 0 1 6 8 -1 9 2 3 ( 0 2 ) 0 0 1 0 8 -9 186 P.E. Thornton et al. / Agricultural and Forest Meteorology 113 (2002) toward smaller annual carbon sinks at five sites, with a seasonal model bias toward smaller warm-season sink strength at all sites. Various lines of reasoning are explored to help to explain these differences.

- by Peter Thornton
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- Earth Sciences, Carbon Dioxide, Climate variability, Carbon Sequestration
- by cosmos wemegah
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- Atmospheric Science, Climatology, Micrometeorology, Remote sensing and GIS

E Prinsloo (CSIR) for the technical support and assistance during the setting of the experiments. My thanks also go to Mr SJ Hilcove the owner of the Bellevue farm (Pietermaritzburg), the farm manager Mr H Ovenstone, and the workers of the farm for allowing the use of the facilities and for providing their grassland field available for this work; the Komatiland Plantations for allowing the use of their facilities and for providing the Outeniqua Yellow wood plot; the research staff of the Hluhluwe Game reserve for providing the Chromolaena odorata site and their facilities available for this work. Last but not least, I would like to express my special thanks to my parents and friends for their continuous encouragement and moral support during my study period. Praise, honour, and glory be to the Lord of heaven and earth! iii

- by Michael Mengistu
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- Micrometeorology

The demand for the world’s increasingly scarce water supply is rising rapidly, challenging its availability for agriculture and other environmental uses, especially in water scarce countries, such as South Africa, with mean annual rainfall is well below the world’s average. The implementation of effective and sustainable water resources management strategies is then imperative, to meet these increasingly growing demands for water. Accurate assessment of evaporation is therefore crucial in agriculture and water resources management. Evaporation may be estimated using different micrometeorological methods, such as eddy covariance (EC), Bowen ratio energy balance (BR), surface renewal (SR), flux variance (FV), and surface layer scintillometry (SLS) methods. Despite the availability of different methods for estimating evaporation, each method has advantages and disadvantages, in terms of accuracy, simplicity, spatial representation, robustness, fetch, and cost. Invoking the shortened su...

- by Michael Mengistu
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- Micrometeorology

The turbulent exchanges of CO2 and water vapour between an aggrading deciduous forest in the northeastern United States (Harvard Forest) and the atmosphere were measured from 1990 to 1994 using the eddy covariance technique. We present a detailed description of the methods used and a rigorous evaluation of the precision and accuracy of these measurements. We partition the sources of error into three categories: (1) uniform systematic errors are constant and independent of measurement conditions (2) selective systematic errors result when the accuracy of the exchange measurement varies as a function of the physical environment, and (3) sampling uncertainty results when summing an incomplete data set to calculate long-term exchange. Analysis of the surface energy budget indicates a uniform systematic error in the turbulent exchange measurements of-20 to 0%. A comparison of nocturnal eddy flux with chamber measurements indicates a selective systematic underestimation during calm (friction velocity < 0.17 m S-l) nocturnal periods. We describe an approach to correct for this error. The integrated carbon sequestration in 1994 was 2.1 t C ha-l y-l with a 90% confidence interval due to sampling uncertainty of :!:0.3 t C ha-l y-l determined by Monte Carlo simulation. Sampling uncertainty may be reduced by estimating the flux as a function of the physical environment during periods when direct observations are unavailable, and by minimizing the length of intervals without flux data. These analyses lead us to place an overall uncertainty on the annual carbon sequestration in 1994 of-0.3 to +0.8 t C ha-l y-l.

- by Steven Wofsy
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- Monte Carlo Simulation, Photosynthesis, Carbon Sequestration, Micrometeorology

Analyses of soil carbon loss following a change in land use from natural forest or grassland to cultivated arable land have shown that between 20-30% of soil organic carbon (SOC) can be lost from the soil within 20 years of cultivation (Mann, 1986; Davidson and Ackerman, 1993; Bellamy et al., 2005). Reducing disturbance to soils by changing arable management practices from conventional tillage to no-tillage or non-inversion tillage can increase SOC storage at rates of between 0.10 and 0.57 t [C] ha À1 y À1 (Lal et al., 1999; Smith et al., 2002; West and Post, 2002). This change in management is a potential method for increasing rates of soil carbon sequestration. In Ireland a growing number of larger farms are moving from conventional tillage (CON; ploughing to a depth of 20-25 cm) to non-inversion tillage practices (NIT; harrowing the soil to a depth of 10-15 cm), mainly due to the reduced costs and time associated with this method of cultivation. In order to provide a more accurate assessment of the long-term impact of this management practice on crop productivity and carbon sequestration, detailed assessments of net biome productivity (NBP) that account for both spatial and temporal variations are required. Clearly, any changes in SOC content associated with altered tillage practice would have important consequences for Ireland's national carbon inventory.

- by Kingsley Uzoma
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- Earth Sciences, Modeling and Simulation, Micrometeorology, Biological Sciences

In trials of a mass balance method for measuring methane (CH 4) emissions, sonic anemometers and an open-path laser were used to measure the transport of CH 4 released from a ground-level source across a downwind face 50 m long and 6 m high. Release rates matched emissions expected from dairy herds of 2 to 40 cows. The long laser path permitted inferences from measurements in only two planes, one upwind and one downwind, while the fast-response instruments allowed calculation of instantaneous horizontal fluxes rather than fluxes calculated from mean wind speeds and mean concentrations. The detection limit of the lasers was 0.02 ppmv, with the separation between the transmitters and reflectors being about 50 m. The main conclusions from the 23 trials were: (1) Emissions calculated from mean wind speeds and concentrations overestimated the true emissions calculated from instantaneous measurements by 5%. (2) Because of small changes in methane concentration, the minimum sample size in animal trials would be 10 dairy cows, producing about 40 mg CH 4 s À1. (3) For release rates greater than 40 mg CH 4 s À1 and with sufficient replication, the technique could detect a change in production rate of 9% (Po ¼ 0:05). (4) Attention to perceived weaknesses in the present technique should help towards detecting changes of 5%.

- by Richard Snyder
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- Earth Sciences, Micrometeorology, Viticulture, Biological Sciences

The application of micrometeorological models of heavy particle dispersal to winged diaspores (samaras) requires that the diaspores maintain stable flight and that descent velocity is constant with respect to vertical air motion. Others have argued that these assumptions may be invalid for autorotating samaras and that bilaterally symmetric samaras will disperse farther than asymmetric samaras in conditions of large horizontal wind velocities (and (or) high turbulence) because the asymmetric morphology is inherently less stable in flight. Therefore, application of these models may not be appropriate for asymmetric samaras. Experimental and natural releases of asymmetric and symmetric samaras reported here indicate that the two samara morphologies respond similarly to a given flow regime. Thus, in modelling dispersal, the autorotation of the samaras can be ignored, and the samaras can be regarded as heavy particles descending at a constant rate from source to deposition site. Key wor...

- by David Greene
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- Plant Biology, Morphology, Micrometeorology, Dispersion

Measurements of sap flow, vapour fluxes, throughfall and soil water content were conducted for 19 months in a young beech stand growing at low elevation, in the Hesse forest. This experiment is part of the Euroflux network, covering 15 representative European forests. Study of the radial variation of sap flow within tree trunks, showed a general pattern of sap flux density in relation to the depth below cambium. Among-tree variation of sap flow was also assessed, in order to determine the contribution of the different crown classes to the total stand transpiration. Stand sap flow and vapour flux, measured with eddy covariance technique, were well correlated, for half hourly as well for daily values, the ratios of the fluxes for both averaging periods being 0.77. A strong canopy coupling to the atmosphere was found, omega factor ranging between 0.05 and 0.20 relative to the windspeed. Canopy conductance variation was related to a range of environmental variables: global radiation, vapour pressure deficit, air temperature and soil water deficit. In addition to the effect of radiation and of vapour pressure deficit often found in various other tree species, here beech exhibited a strong reduction in canopy conductance when air temperature decreased below 17 • C. The model of transpiration was calibrated using data measured in the Hesse forest and applied to another beech stand under mountainous conditions in the Vosges mountains (east France). Measured and modelled stand transpiration were in good agreement.

- by Damien Lemoine
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- Earth Sciences, Hydrology, Micrometeorology, Canopy

Continuous measurements of carbon exchange using the eddy covariance (EC) technique were made at three boreal forest mature sites including Southern Old Aspen (SOA), Southern Old Black Spruce (SOBS) and Southern Old Jack Pine (SOJP) in 2000. Climatic conditions were slightly warmer than normal with precipitation exceeding evapotranspiration at each site. Annual ecosystem respiration (R) derived from daytime analyses of EC data was 1141, 815 and 521 g C m −2 per year and was consistently lower than nighttime EC estimates of 1193, 897 and 578 g C m −2 per year for SOA, SOBS and SOJP, respectively. The differences, however, were not statistically significant given the large uncertainty associated with each analytical technique. The uncertainty in annual net ecosystem productivity (NEP) was assessed by randomly simulating missing data and gap filling using simple biophysical algorithms. The uncertainty analysis supports the finding that each site was a net sink, and that differences in NEP were only significant between SOA and SOBS. The annual NEP and uncertainty for SOA, SOBS and SOJP was 122 (64-142), 35 (18-53) and 78 (61-91) g C m −2 per year, respectively. These relatively old growth forests represent a weak to moderate carbon sink. Despite having the shortest growing period, carbon sequestration was greatest at SOA because of its relatively large photosynthetic capacity (A max ). At the evergreen sites, A max was marginally larger at SOBS; however, annual carbon sequestration was smaller as a result of greater R. The evergreen sites exhibited a pronounced mid-season reduction in NEP, which was attributed to a large increase in R while A max had not reached its full capacity. Non-growing season R resulted in a carbon loss of 285, 120 and 64 g C m −2 and accounted for 70, 80 and 46% of the summertime NEP at SOA, SOBS and SOJP, respectively. Six years of EC data at SOA indicate that carbon sequestration at boreal aspen sites may benefit from warmer climatic conditions because R is relatively conservative and photosynthesis increases in response to a longer growing period.

- by Zoran Nesic
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- Earth Sciences, Climate Change, Photosynthesis, Carbon Sequestration

The lack of energy closure has been a longstanding issue with Eddy Covariance (EC). Multiple mechanisms have been proposed to explain the discrepancies in energy balance including diurnal energy storage changes, advection of energy, and larger scale turbulent processes that cannot be resolved by field EC. To investigate the energy balance issue, we used a year of data from paired EC towers in irrigated sugarcane in Maui, Hawai'i, USA. The towers were in identical crops and cultivation practices and had similar climate with the notable exception of wind. One tower was in a location where nearby orographic features funneled Trade Winds, resulting in sustained, continuous turbulence. The other was in a leeward location with less turbulence, particularly at night (u*). We found significantly improved closure (8.5-10%) at both sites using daily sums of Available Energy in closure regressions as opposed to 30 min data, illustrating the importance of storage terms. The energy budget closed for both fields when only days with continuous turbulence (all 30 min u* > critical u*) were considered, with significantly larger uncertainty in the leeward field (±13%) due to the small number of days (n = 13) with this condition. Significant energy imbalance appeared in both fields with even 30 min of subcritical turbulence in a day, and each field had different turbulence-closure patterns. Closure with continuous turbulence was sensitive to choice of critical u*; an arbitrary u* of 0.1 m s −1 resulted in non-closure. The results show the value of paired EC towers in contrasting turbulence conditions to assess energy budget closure.

- by Ray Anderson
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- Micrometeorology, Eddy Covariance
- by Amal Kar
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- Arid environments, Aeolian Geomorphology, Micrometeorology

- by Edmund I . Yamba
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- Environmental Science, Atmospheric Science, Climatology, Micrometeorology

Fluxes of aerosol particles with sizes larger than 10 nm together with fluxes of momentum, sensible and latent heat and CO 2 were measured 10 m above a Scots pine forest with the eddy covariance method. During days when nucleation events were observed particle size distribution measurements showed particle growth from 3 nm sizes to the Aitken mode. Analysis of the experimental data showed systematic differences in fluxes during the days when new particle production was observed compared to other days. During the nucleation events the particle flux measurements showed downward aerosol particle transport, i.e., indicating an elevated source, with respect to the measurement level, of particles larger than 10 nm. Furthermore the turbulence intensity and the heat fluxes were observed to be significantly higher. Evidences of mesoscale circulation were observed in wind speed records as well as in turbulent fluxes on nucleation days. The measurement results show that micrometeorology, the synoptic scale conditions and the particle formation are closely related.

- by E. Douglas Nilsson
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- Data Analysis, Heat Transfer, Micrometeorology, Growth

We present a description of the process of estimating surface fluxes of CO 2 , latent heat and sensible heat from estimates of fractions of satellite-based land cover types in the flux footprint. The study is conducted at two heterogeneous sites in the boreal forest of Central Canada. Using a Twin Otter aircraft, fluxes were measured in a grid pattern during three Intensive Field Campaigns (IFCs) and Landsat thematic mapper data were used for land cover classification. Using a footprint function developed from tracer gas release experiments in the boreal forest, the fractions of cover types within the footprint were determined, and used in a regression analysis against observed fluxes. The results showed that the surface cover types within the flux footprint accounted for about 90% of the variations in the measured airborne fluxes of CO 2 , sensible heat and latent heat, at two different study sites. The attempted validation of the regression models, by comparing flux estimates over regional transects outside the grid area for which the regression model had been developed or over site-specific runs within the grid area against observed fluxes, based on fractional distributions of surface cover types, were encouraging. They indicate the potential for extrapolating models developed for a given location to another location, based simply on the fractions of cover types, at least for similar land cover types.

- by Samuel Kaharabata
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- Earth Sciences, Micrometeorology, Carbon Footprint, Biological Sciences

Alternating dry-wet paddy field management such as System of Rice Intensification (SRI) had become an interesting subject in research and development in paddy cultivation which also been subject for trial for its implementation. The field's environment's variation of biophysical parameters related to production had also become important to be studied. This study aims to simulate the variation of evaporation and thermal condition over a wet and dry regime of paddy field. The simulation model used in this study was a combination of numerical surface energy balance and soil water flow model consisting two layered resistance energy balance model for non-ponded field, one-dimensional atmospheric boundary layer model of wind, temperature and vapor changes, and soil heat transfer and soil water flow models. Meteorological parameters at the site were measured and utilized as input for the simulation. The simulation shows the fluctuating latent, sensible and ground heat flux and also the variation of temperature, and soil condition for wet and dry regime of paddy field.

- by IAEME Publication
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- Hydrology, Micrometeorology, Evapotranspiration, Numerical Model

The lack of comprehensive solar radiation monitoring during the longest total solar eclipse of the 21st century at Tianhuangping (Zhejiang), China, on 22 July 2009, has led to this investigation in order to evaluate the cloudiness contribution in estimating the impact on global solar radiation throughout this phenomenon. In doing so, we applied a cloud cover empirical model to obtain the global solar radiation and, at the same time, we deduced a theoretical model to get the direct solar radiation in which both the occultation and obscuration functions of this eclipse are included. We took limb darkening and atmospheric transmission into account. Though the weather during our eclipse observations agreed with the forecasts for that day, clouds and some rain, we were nonetheless able to observe all phases of the eclipse from our observation site at Tianhuangping. This experience suggests that for coming eclipses a record of the in situ observation protocol of cloudiness is mandatory. Our results for comparing global solar radiation models indicate that our total solar eclipse radiation model is quite acceptable and representative of that which could have happened at that time. Plain Language Summary A total solar eclipse is a situation where the Sun is obscured by the Moon viewed from Earth. In this particular arrangement in space a shadow is cast over a particular region. Thus, the ideal circumstances to observe a total solar eclipse are those in which the sky is cloudless in that region or zone. Yet from time to time this phenomenon occurs in the presence of interfering clouds preventing a direct observation of the event. However, under these adverse circumstances effects over the environment can still be felt and measured as, for example, the rapid reduction of solar energy reaching the surface. In that case such measurements would be lacking, but with some information about cloudiness one can have an idea of how the variation of this energy during the eclipse took place. In the procedure a quantitative knowledge of how the Moon disk is going to progressively cut the Sun's brightness during the eclipse is necessary. From an environmental point of view it is shown that a scientific observation of a total solar eclipse matters even though it was made under the influence of blocking clouds.

- by Marcos A Peñaloza-Murillo and +1
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- History of Science, Climate variability, Atmospheric Aerosols, Micrometeorology

The problem of areally averaging descriptions of land-atmosphere energy and mass exchange is common to both the leaf-canopy and canopy-region scale translations. This paper attempts a unified treatment. The starting point is a review of both the basic and equilibrium-departure forms of the Penman-Monteith or combination equation (CE) for the latent and sensible heat fluxes at an evaporating surface; the equilibrium-departure form expresses the evaporation as a linear combination of two other scalar fluxes, those of available energy and saturation deficit. Next, tools for scale translation are established, including (1) the basic conservation requirement that net scalar fluxes average linearly over the land surface and (2) the matching of model form between scales. These tools are applied to the CE for latent and sensible heat fluxes FE and FH, leading to a ‘flux matching’ averaging scheme based on term-by-term matching of the linearly additive scalar flux terms in the CE for FE. Several variations of this scheme are discussed. For FE, all variations satisfy the scalar conservation requirement that fluxes average linearly, but for FH, none satisfies this requirement exactly, except when long-wave radiative coupling is ignored. By considering a range of canopy-region scale translations, errors in the averaging of FH are shown to be small in practice. Finally, three averaging schemes (the flux-matching scheme and two schemes based on parallel sums of elemental aerodynamic and surface conductances, respectively) are compared for leaf-canopy scale translations, by constructing a sequence of analytic model canopies at increasing levels of detail. For every canopy model considered, all three averaging schemes give nearly identical definitions of the bulk canopy conductance for typical dry canopies. However, schemes other than the flux matching scheme can become inconsistent over partially wet surfaces, yielding undefined or negative bulk conductances.

- by Halifax Highlander
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- Applied Mathematics, Earth Sciences, Water, Climatology

Modelling of pollen dispersal and cross-pollination is of great importance for the ongoing discussion on thresholds for the adventitious presence of genetically modified material in food and feed. Two different modelling approaches for pollen dispersal are used to simulate the cross-pollination rate of pollen emerged from an adjacent transgenic crop field. The models are applied to crosspollination data from field experiments with transgenic maize (Zea mays). The data were generated by an experimental setup specifically designed to suit the demands of mathematical modelling. First a Gaussian plume model is used for the simulation of pollen transport in and from plant canopies. This is a semiempirical approach combining the atmospheric diffusion equation and Lagrangian methodology. The second model is derived from the localised near field (LNF) theory and based on the physical processes in the canopy.