WRF Gray‐Zone Simulations of Precipitation Over the Middle‐East and the UAE: Impacts of Physical Parameterizations and Resolution

Abstract

Several single-year-long (2017) simulations with different configurations of the Weather Research and Forecasting (WRF) model over the United Arab Emirates (UAE) and the Middle-East at the convective gray-zone resolution (9 km) are evaluated in their ability to capture the temporal and spatial distributions of precipitation. Annual rainfall over the Middle-East is dominated by wintertime precipitation and is mostly initiated by the frontal systems intrusion. WRF at 9 km resolution shows good skill in capturing the synoptic and meso-scale precipitation distributions. The aerosol-aware Thompson microphysics scheme outperforms the WRF double moment 6 class microphysics by about 20% in annual mean precipitation over the Middle-East. Different Planetary Boundary Layer (PBL) physics leads to large differences in the annual rainfall over both the Middle-East (about 30%) and the UAE (about 45%). The Quasi-Normal Scale Elimination (QNSE) PBL scheme produces stronger precipitation than the Asymmetrical Convective Model and is in better agreement with observations. This difference is attributed to the former scheme producing a warmer and moister lower-level atmosphere that builds more substantial instability. Regions of stronger instability also depict decreases in lifting condensation level heights and increases in boundary layer height, suggesting that the boundary layer reaching the cloud base helps to trigger the convection and increase precipitation. Refinement in grid spacing (5 km) marginally improves the summertime precipitation over the Middle-East but significantly increases the computational cost. Current study also indicates that gray-zone simulations can perform as good as convection-permitting simulations by carefully choosing the right model physics packages for the synoptic and meso-scale precipitation.Plain Language Summary Capturing rainfall variability over the Middle-East can be quite challenging for atmospheric models due to the complex interactions between synoptic systems, topography, dust, and convective-scale motions. The Weather Research and Forecasting (WRF) model is configured and tested over the United Arab Emirates (UAE) and the Middle-East for the realistic representation of precipitation. Various combinations of cloud microphysics and boundary layer physics schemes were tested to unearth the most suitable configuration at the convective gray-zone resolution (9 km). The model simulates precipitation comparable to observations in terms of its synoptic and meso-scale variability. It is found that altering the cloud microphysical schemes inflicts ∼20% change in annual mean precipitation over the Middle-East region. However, changing the boundary layer physics dramatically impacts the annual mean rainfall over the Middle-East (∼30%) and specifically over the UAE (∼45%). Reduction in horizontal grid spacing (5 km) does not improve the winter precipitation but marginally increases the summertime precipitation over the Middle-East region. Note that the reduction in horizontal grid spacing significa...

The First Characterization of Fog Microphysics in the United Arab Emirates, an Arid Region on the Arabian Peninsula

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