Investigating the Relationship Between Water Vapor Convergence and Severe Convection Using the WRF Model at 1KM Resolution (original) (raw)
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Quarterly Journal of the Royal Meteorological Society, 2005
This study presents a comprehensive analysis of the variability of water vapour in a growing convective boundary-layer (CBL) over land, highlighting the complex links between advection, convective activity and moisture heterogeneity in the boundary layer. A Large-eddy Simulation (LES) is designed, based on observations, and validated, using an independent data-set collected during the International H 2 O Project (IHOP 2002) fieldexperiment. Ample information about the moisture distribution in space and time, as well as other important CBL parameters are acquired by mesonet stations, balloon soundings, instruments on-board two aircraft and the DLR airborne water-vapour differential-absorption lidar. Because it can deliver two-dimensional cross-sections at high spatial resolution (140 m horizontal, 200 m vertical), the airborne lidar offers valuable insights of small-scale moisture-variability throughout the CBL. The LES is able to reproduce the development of the CBL in the morning and early afternoon, as assessed by comparisons of simulated mean profiles of key meteorological variables with sounding data. Simulated profiles of the variance of water-vapour mixing-ratio were found to be in good agreement with the lidar-derived counterparts. Finally, probability-density functions of potential temperature, vertical velocity and water-vapour mixing-ratio calculated from the LES show great consistency with those derived from aircraft in situ measurements in the middle of the CBL. Downdraughts entrained from above the CBL are governing the scale of moisture variability. Characteristic length-scales are found to be larger for water-vapour mixing-ratio than for temperature
Measure of Divergence at the Top of Tropical Convective Systems from Water Vapor Winds
The aim of this work is to study the feasibility of estimating the high level wind divergence associated with tropical convective cloud systems. This approach could be useful to monitor the activity of the deep convection. We explore here the use of full resolution water vapor channel images to estimate the wind vectors in the high troposphere. Numerous and consistent wind vectors can be derived from water vapor image sequencies, mainly in the vicinity of high level cloud regions. The water vapor wind vectors are here derived using an algorithm similar to the algorithms used in the operational centers. The main difference consists in adjusting the position of the computation window to center on the brightness gradient maximum, leading to a better tracer selection and a reduction of the number of erroneous vectors passing the automatic quality control. From the drift wind vectors, an interpolation scheme is applied to obtain the wind field on a regular grid. The divergence field is t...
SOLA, 2015
Water vapor variations associated with a meso-γ scale convection were investigated using GNSS (Global Navigation Satellite System) derived PWV (precipitable water vapor) and high resolution numerical model data with a 250 m horizontal grid interval. A rapid increase of GNSS-derived PWV that occurred prior to the initiation of surface rainfall was well simulated by the numerical model. In the model, PWV values began to increase 16 min before the rainfall occurred at the surface. A local maximum of PWV was formed because of the generation of shallow free convection and surface water vapor flux convergence due to a lifting of an air parcel at approximately 1 km elevation by a preceding surface wind convergence. Due to the existence of a stable inversion layer between 2.2 and 3.5 km elevation, the shallow free convection took 11 min to rise above the inversion layer to form a deep convection. These results suggest that observation of local distributions of GNSS-derived PWV is useful for monitoring the generation of deep moist convection.
Coupling of water vapor convergence, clouds, precipitation, and land-surface processes
Journal of Geophysical Research, 2007
1] On daily timescales, the climate over land is a complex balance of many coupled processes. ERA40 reanalysis data for subbasins of the Mississippi in summer are used to explore the links between these processes in a fully coupled model system, and observed surface precipitation and surface short-wave fluxes derived by the International Satellite Cloud Climatology Project are used for evaluation. This paper proposes that the effective cloud albedo viewed from the surface is a useful link which connects the cloud fields to both surface and large-scale processes. The reanalysis has a low bias in cloud albedo in all seasons except summer. In the coupled system in the warm season, on daily timescales, the lifting condensation level falls as soil moisture and precipitation increase. The ratio of the cloud short-wave radiative forcing at the surface to the diabatic precipitation heating of the atmosphere is less in the reanalysis than in the observations. The surface energy budget is split into the surface net radiation and the evaporative fraction. The surface cloud radiative forcing largely determines the surface net radiation, while evaporative fraction, with fixed vegetation, is largely determined by temperature and near-surface soil moisture.
Journal of Geophysical Research, 2006
In this study, the precipitable water (PW) and ice water path (IWP) simulated by the Global Data Assimilation System (GDAS) are compared to those observed by NOAA's Microwave Surface and Precipitation Products System. Results show small root-meansquare (RMS) differences in PW but large RMS differences in IWP between the two data sets, indicating the existence of model errors in reproducing clouds. To examine the possible linkage between the small PW and large IWP differences, three experiments are conducted with a two-dimensional cloud-resolving model in which the observed zonal wind and the GDAS-derived large-scale vertical velocity are imposed. The model initial conditions of PW are perturbed by ±10% in the first two experiments, respectively, while treating the third one without any perturbation as a control simulation. Thermodynamic, cloud microphysics, and precipitation budgets are then calculated from the zonally averaged and vertically integrated data at hourly intervals from these experiments. Results show the generation of larger differences in the cloud hydrometeors and surface rain rates, with the given PW perturbations. This indicates that the model-simulated clouds and precipitation are extremely sensitive to the initial errors in PW, primarily through the biased condensation process.
Trends and variability in column-integrated atmospheric water vapor
Climate Dynamics, 2005
An analysis and evaluation has been performed of global datasets on column-integrated water vapor (precipitable water). For years before 1996, the Ross and Elliott radiosonde dataset is used for validation of European Centre for Medium-range Weather Forecasts (ECMWF) reanalyses ERA-40. Only the special sensor microwave imager (SSM/I) dataset from remote sensing systems (RSS) has credible means, variability and trends for the oceans, but it is available only for the post-1988 period. Major problems are found in the means, variability and trends from 1988 to 2001 for both reanalyses from National Centers for Environmental Prediction (NCEP) and the ERA-40 reanalysis over the oceans, and for the NASA water vapor project (NVAP) dataset more generally. NCEP and ERA-40 values are reasonable over land where constrained by radiosondes. Accordingly, users of these data should take great care in accepting results as real. The problems highlight the need for reprocessing of data, as has been done by RSS, and reanalyses that adequately take account of the changing observing system. Precipitable water variability for 1988-2001 is dominated by the evolution of ENSO and especially the structures that occurred during and following the 1997-98 El Nin˜o event. The evidence from SSM/I for the global ocean suggests that recent trends in precipitable water are generally positive and, for 1988 through 2003, average 0.40±0.09 mm per decade or 1.3±0.3% per decade for the ocean as a whole, where the error bars are 95% confidence intervals. Over the oceans, the precipitable water variability relates very strongly to changes in SSTs, both in terms of spatial structure of trends and temporal variability (with a regression coefficient for 30°N-30°S of 7.8% K À1 ) and is consistent with the assumption of fairly constant relative humidity. In the tropics, the trends are also influenced by changes in rainfall which, in turn, are closely associated with the mean flow and convergence of moisture by the trade winds. The main region where positive trends are not very evident is over Europe, in spite of large and positive trends over the North Atlantic since 1988. A much longer time series is probably required to obtain stable patterns of trends over the oceans, although the main variability could probably be deduced from past SST and associated precipitation variations.
2004
An idealized case-study has been designed to investigate the modelling of the diurnal cycle of deep precipitating convection over land. A simulation of this case was performed by seven single-column models (SCMs) and three cloud-resolving models (CRMs). Within this framework, a quick onset of convective rainfall is found in most SCMs, consistent with the results from general-circulation models. In contrast, CRMs do not predict rainfall before noon. A joint analysis of the results provided by both types of model indicates that convection occurs too early in most SCMs, due to crude triggering criteria and quick onsets of convective precipitation. In the CRMs, the first clouds appear before noon, but surface rainfall is delayed by a few hours to several hours. This intermediate stage, missing in all SCMs except for one, is characterized by a gradual moistening of the free troposphere and an increase of cloud-top height. Later on, convective downdraughts efficiently cool and dry the boundary layer (BL) in the CRMs. This feature is also absent in most SCMs, which tend to adjust towards more unstable states, with moister (and often more cloudy) low levels and a drier free atmosphere. This common behaviour of most SCMs with respect to deep moist convective processes occurs even though each SCM simulates a different diurnal cycle of the BL and atmospheric stability. The scatter among the SCMs results from the wide variety of representations of BL turbulence and moist convection in these models. Greater consistency is found among the CRMs, despite some differences in their representation of the daytime BL growth, which are linked to their parametrizations of BL turbulence and/or resolution.
Journal of Geophysical Research, 2012
North American Model (NAM) analysis data and the Weather Research and Forecasting (WRF) Advanced Research WRF (ARW) model version 2.2 are used to investigate the effect of a mesoscale convective system (MCS) in extratropical regions on the transport of water vapor in the upper troposphere and lower stratosphere (UTLS). In addition, physical mechanisms contributing most to the water vapor distribution in the UTLS and the amount of water vapor transported during the most active period of the convective system are examined. In an MCS occurring over the Midwest, the primary focus of the present study, simulated by WRF on 13-14 July 2006, hourly water vapor amount averaged near the system in the UTLS increased substantially during the time that convective system activity developed, and reached maximum values at the same time that the strongest convection and heaviest precipitation occurred at the surface. In the upper troposphere, large positive hourly water vapor tendencies were mainly due to vertical advection with highest rates at the time of highest rain rates. Water vapor tendencies due to microphysical processes tended to oppose the moistening due to advection. Near the tropopause and in the lower stratosphere, however, positive hourly water vapor tendencies were primarily due to microphysics and mixing within the MCS. Horizontal advection also transported some moisture in regions downstream from the MCS at most times, with the largest impacts later in the MCS lifetime. Around the tropopause, microphysical processes related to the presence of convectively injected ice appeared to be the largest contributor to moistening for this case. The results were not found to be sensitive to model microphysical schemes.