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Study Investigates Impacts of Physical Parameterizations and Resolution on WRF Gray‐zone Precipitation Simulations over Middle East and UAE

The climate of the Middle East region, which consists of various landscapes, is highly affected by the Indian monsoon in the south and the Mediterranean synoptic‐scale systems in the north. Due to the complex interactions between synoptic systems, topography, dust, and convective‐scale motions, capturing rainfall variability over the Middle East can be quite challenging for atmospheric models. "Gray zone resolution" refers to model grid spacing in the range of 200 m to 10 km. This resolution allows to numerically resolve complex phenomena such as turbulence, convective transport and cloud evolution which are subject to high uncertainties - hence the term "gray" indicating not well understood.

In an attempt to develop an efficient regional model to address this issue, Dr. Lulin Xue, a third cycle awardee of the UAE Research Program for Rain Enhancement Science (UAEREP) and the Chief Scientist at Hua Xin Chuang Zhi Science and Technology LLC, worked on a study investigating the impacts of physical parameterizations and resolution on the Weather Research and Forecasting (WRF) gray‐zone precipitation simulations over the Middle East and the UAE.

The study configured and tested a WRF model for the realistic representation of precipitation over the UAE and the Middle East. 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 study compared different physical configurations in WRF in simulating rainfall over the Middle East region and explored the rainfall sensitivity to different physical processes and resolutions. It also examined the annual evolution of precipitation to avoid any conclusions depending on the specific synoptic conditions. This approach assessed how the rainfall varies with different treatment of physical processes (namely cloud microphysics and PBL) and resolution.

The study 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%).

Furthermore, the results also revealed that reduction in horizontal grid spacing (5 km) does not improve the winter precipitation but marginally increases the summertime precipitation over the Middle East.

The study indicates that gray‐zone simulations can perform as good as convection‐permitting simulations by carefully choosing the specific model physics packages for synoptic and meso‐scale precipitation.

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