Grey-Zone Simulations of Rainfall over UAE and Arabian Peninsula

A multiple year’s cloud permitting simulations are tested using Weather Research and Forecasting (WRF) model over United Arab Emirates (UAE) and Arabian Peninsula at the gray-zone resolution, about at 9-km. Such resolution is both too coarse to accurately simulate convective updrafts, and too fine to rely on the underlying averaging approximation that are inherent to most cumulus parameterization for deep convection. Yet, many studies have demonstrated that cloud permitting model at the gray-zone resolution can capture many regional precipitation patterns, including the Madden Julian Oscillation and the Indian Summer Monsoon.

In this study, a regional configuration of the WRF model is shown to capture the spatial distribution of precipitation over the Arabian Peninsula as well as the timing of occurrences of precipitation with respect to TRMM3B42 and GPM observations. Circulation features are also realistic in WRF. Wintertime precipitation events are credibly captured by the WRF, but systematic dry biases occur during summer precipitation. Wintertime precipitation events are for the most part initiated by extratropical intrusions. Mid tropospheric potential vorticity anomalies induce southerly wind over the Arabian Peninsula, which trigger a moisture transport from the Arabian and Red Seas. This inflow of moisture feeds precipitation over the frontal region. In contrast, Summer time precipitation over the UAE is strongly tied to the local land-sea contrast and its interaction with regional topography. While the 9-km configuration is unable to reproduce the Summer time rainfall, it does capture the variability in precipitable water and other cloud properties. This suggests that the coarse resolution is insufficient to capture the convective initiation of summertime precipitation events. Multiple yearlong simulations with the WRF model is used to investigate the variability of the precipitation over the Arabian Peninsula and also its sensitivity to the change in large scale atmospheric patterns.