The Persian Gulf summertime low-level jet over sloping terrain



There are few observational and numerical studies of the summertime low-level jet (LLJ) over the Persian Gulf, known as the shamal. In this study, the Weather Research and Forecasting (WRF) model accurately simulates the LLJ's vertical structure, nocturnal features and strong diurnal variation of the wind. The case-study period is divided into a period during which the shamal is spatially extended (strong winds over the Gulf and Iraq) and a period during which it is less spatially extended (strong flow only over the Gulf). Orography, mountain slope and land/sea breeze are found to be key factors for the shamal diurnal variation of wind speed and wind direction; the Blackadar mechanism appears to be secondary to the heating in forcing the shamal. The Zagros Mountains not only channel the northwesterly winds but also provide a barrier for the easterly monsoon airflow, which maximizes the wind speed. Only during the less extensive period does the Zagros Mountains slope significantly affect the shamal wind intensity and direction. Both shallow and steep mountain slope sensitivity studies feature a jet-like flow over the Persian Gulf. The steep slopes cause larger wind speeds; however, the shallow slopes reveal a stronger diurnally varying wind direction due to larger heating and cooling of the sloping terrain. It is also shown that, during the less extensive period, the land breeze and the lower friction over the sea increase the intensity of the nocturnal jet over the Gulf. Copyright © 2011 Royal Meteorological Society