On the climatology and dynamics of heavy rainfall events and the winter low level jet in and near Saudi Arabia

Abstract

<p>This study supports the weather organizations in Saudi Arabia (SA) and possibly the surrounding countries by providing new understanding of the weather systems that produce heavy rainfall and the dynamics that are involved. The study focuses generally on classifying the synoptic patterns inducing heavy rainfall in SA and specifically on understanding the climatology of the Red Sea Low Level Jet (RSLLJ) and its role in moisture advection toward the central Red Sea as well as understanding the dynamics behind the jet formation. The study uses 15 years of observed daily rainfall and the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP/UCAR) reanalysis data set to classify the synoptic patterns using a correlation-based map pattern classification. The ERA-Interim dataset from the European Center for Medium-Range Weather Forecasts (ECMWF) was used to create a detailed climatology of the RSLLJ for a 10-year period. The structure and the causes of the RSLLJ were investigated using the Weather Research and Forecasting (WRF) model and the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) analyses data set. The results summarize six major synoptic features and classify 28 detailed synoptic patterns emerging from 186 cases of heavy rainfall in SA. The results also reveal that the RSLLJ is a relatively frequent phenomenon and the frequency of its occurrence varies according to location. The jet was found to have a significant role in moisture advection toward the central Red Sea area, which can contribute toward precipitation there. A linear relationship between the RSLLJ speed and the mean moisture concentration was observed in the central Red Sea region, where more moisture content was associated with greater jet speed. As for the jet causes, WRF simulations confirm that the RSLLJ is a terrain induced phenomenon that initiates as a gap flow as the jet forms north of the strait of Bab el-Mandab as a result of a hydraulic effect when a stably stratified cool layer is channeled through the mountainous gap at low levels over the strait into the southern Red Sea. In addition, because the WRF successfully simulated the structure of the RSLLJ, the results reveal the usefulness of numerical simulations as an important analytical tool to investigate and examine weather phenomena in locations with scarce observations, as in the case of the RSLLJ.</p>