Boundary Layer Stability Regime at DACCIWA Site Using Gradient Richardson Number (original) (raw)
Related papers
2018
This study surveyed the levels of atmospheric stability across the atmospheric boundary layer in Jos, northcentral Nigeria. Five years (2011-2015) meteorological data for temperature and wind speed at 1000mbar pressure level was retrieved and processed from ECMWF Era-Interim Re-analysis platform. The data were for 6-hourly synoptic hours: 0000H, 0600H, 1200H and 1800H at 0.125° grid resolution. The gradient Richardson (Rig) number technique was used to assess stability conditions across distinct layers: 10-50m (surface layer); 50-100m (mid layer) and 100-1300m (upper layer). Results indicated that the surface layer is always in unstable state as over 90% and 100% of Rig values were below Richardson Critical (Ric) value of 0.25 and Richardson Termination level (RT) of 1, respectively. Stable conditions exist at the mid layer across the hours and all Rig values were greater than RT level of 1. Rig values for the upper layer were largely negative and ranged between -52 to -360. This in...
An Assessment of Atmospheric Boundary Layer Turbulence in Maiduguri, Nigeria
Open Journal of Air Pollution
This study examined the level of turbulence in the atmospheric boundary layer in Maiduguri, northeastern Nigeria. Five years (2011-2015) temperature and wind speed data at 1000 mbar pressure level retrieved from Era-Interim Reanalysis Platform was used. These data were gotten at 6-hourly synoptic hours: 0000H, 0600H, 1200H and 1800H at 0.125˚ grid resolution. The gradient Richardson (R ig) number method was utilised in analysing turbulence across three layers: 10-50 m (surface layer); 50-100 m (mid layer) and 100-1300 m (upper layer). Findings shows that the surface layer is always in a turbulent state as over 95% of R ig values were below Richardson Critical (R ic) value of 0.25 with range 0.02-0.94. However, all values across the hours were below the Richardson Termination (R T) value of 1. Laminar conditions exist at the mid layer across the hours as 99.9% of R ig values ranging 0.88-8.02 were greater than R T of 1. R ig values for the upper layer were largely negative and ranged between −78.71 to −724.14. This indicates robust turbulent conditions. Turbulence generated through forced and free ascents prevailed at the surface layer and upper layer respectively. This shows that wind shear is dominant at the surface while thermal buoyancy prevails at the upper level. The months of February and September at 1200 and 1800 hours respectively are the periods of maximum (about 134 m) and minimum (below 15 m) heights were free convection destabilises forced convection in the study area. Relating findings to emission dispersion suggests that air pollutants will be transported across far and near distances at the upper layer and surface layers respectively. This is due to the stable nature of the mid layer that will limits vertical emission dispersion. Policy makers should ensure that potential emission sources stacks are above 50 m to ensure pollutants are dispersed aloft in the area.
Advances in Meteorology, 2019
The characteristics of the wind vertical profile over the coast of Cotonou during wind convective diurnal cycle were explored in this study. Wind data at 10 m above the ground and the radiosonde data in the lower 60 m of the surface boundary layer were used over the period from January 2013 to December 2016. Based on Monin–Obukhov theory, the logarithmic and power laws have allowed characterizing the wind profile. The error estimators of the Root Mean Square Error (RMSE) and the Mean Absolute Error (MAE) were, respectively, evaluated at 0.025; 0.016 (RMSE; MAE) and 0.018; 0.015. At the site of Cotonou, the atmosphere is generally unstable from 09:00 to 18:00 MST and stable for the remainder of the time. The annual mean value of the wind shear coefficient is estimated at 0.20 and that of the ground surface roughness length and friction velocity are, respectively, of 0.007 m, 0.38 m·s−1. A comparative study between the wind extrapolation models and the data was carried out in order to...
The boundary layer wind regime of a representative tropical African region, Central Sudan
1988
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Climatology of West Africa boundary layer
Terrestrial, Atmospheric and Oceanic Sciences
Monthly means, seasonal variances, and trends of a global climatology boundary layer height (BLH) over West Africa are presented based on 36 years (1979-2014) of six-hourly ERA-Interim reanalysis. In this region, we found that there is a link between the West Africa Monsoon (WAM) and the monthly means of BLH where largest values of BLH variances are developed in the tropics close to the Inter Tropical Convergence Zone (ITCZ). High temperatures and sufficient moisture are also available close this area. Seasonal trend magnitudes vary from-20 to 20 m per decade during the period 1979-2014 and characterize by negative trend over east of the Sahara region. Trends are only included in the analysis, if their probability exceeds the 95% significance level. Case study of diurnal variation was done during the African monsoon multidisciplinary analyses Special Observing Period 3 (AMMA SOP3) experiment (August 2006). We found that the lower boundary-layer appears around 875 hPa in the monsoon layer where the wind decreases at midday in interaction through exchange processes with air originating from above the boundary layer. In the same time, the dust in Saharan Air Layer (SAL) seems to modify the atmospheric boundary layer (ABL) thermodynamic attributes by altering the shortwave and longwave radiative budget.
Study of Boundary Layer Height over West Africa
Journal of Geoscience and Environment Protection, 2019
Monthly means of boundary layer height (BLH) over West Africa are presented based on 36 years (1979-2014) of six-hourly ERA-Interim reanalysis. In this region, we found that there is a link between the West Africa Monsoon (WAM) and the monthly means of BLH in the summer. The trend and empirical orthogonal function (EOF) of BLH are presented, including the mid July variability of BLH with the precipitation. The dominant EOF of BLH accounts for around 42% of the variance with slightly large amplitude in the north while relatively small in the equatorial band. The second EOF which accounts for 16.4%, describes a longitudinal contrast with a zonal gradient. The relationship between BLH and precipitation is found using the canonical correlation analysis (CCA). Significant trends of the first and second pairs of BLH with precipitation are shown. The first and second CCA pair has a correlation of 68% and 60% with 12.2 and 10.8 degrees of freedom respectively. The critical correlation coefficients at the 95% level are 0.21 and 0.65 for the first and second CCA pairs respectively. This first CCA pair mostly determines the arid and semi-arid areas where the rate of explained regional variance is about 78% in the arid area and 73% in the semi-arid area. For the second pair of CCA, the rate of explained regional variance is more than 60% in the Guinea coast and wet equatorial area.
In this paper, we present the micro-spatial scale and temporal variability of Obukhov length (L), Kinematic momentum flux or turbulent friction velocity ( * ) and heat flux ( * ) in the Stable Boundary Layer (SBL) using observational data collected by a dense network of five Automatic Weather Stations (AWSs). The stations were in operation in the Highveld Priority Area (HPA) of the Republic of South Africa during 2008-2010. The L, * and * are solved using Monin Obukhov similarity theory. Analysis of spatial variability of stability regimes as presented by L suggests that HPA is dominated by strong stability regime. The analysis also revealed inter-station variability of * and L while * indicates no significant spatial variability between stations. Temporal variability showed that most of the low values for Obukhov length that fall within strong stable regimes (0<L<16) occurs at night. Almost 30-50% of * in the range of *
Correlation of meteorological parameters to characterize wind sites: A case study of N'guigmi, Niger
Global NEST Journal, 2022
To assess the impact of climate change on wind regimes, we performed a statistical analysis of some meteorological parameters. Indeed, variables such as temperature, humidity, pressure, and the insolation are rarely or not considered in the estimation of wind regimes and therefore in the characterization of wind sites. In setting wind farm projects, one is limited to assessing the wind potential with data that fluctuate greatly. In this work, we studied the influence of meteorological parameters on the wind regimes in the eastern part of Niger. We have firstly, based on data collected at the station of N'Guigmi, over 10 years, characterized each of the parameters by their monthly and interannual average. In a second step, we studied the interannual variability of these variables. At this level, the curves represented show large fluctuations. We then represented trend curves which showed us that during these ten years, humidity, insolation, and wind have undergone slight increases while temperature and pressure have significantly decreased. Finally, we used principal component analysis with the statistical tool XLSTAT to analyze the correlation of the different meteorological parameters on wind speed. The preponderance of the influence of each variable is evaluated from a factorial design provided by XLSTAT. Thus, after the analysis of the factorial design, we concluded that temperature is a preponderant factor in wind flow. The second variable that conditions the wind flow is humidity. These results meet our expectations. Indeed, the wind being air in circulation, under a high temperature, its density decreases, and its circulation is facilitated. However, with humidity, it becomes heavier, and the wind circulates less quickly.