Radar‐duct and boundary‐layer characteristics over the area of The Gulf

Abstract

AbstractRadar‐duct characteristics and the associated atmospheric boundary layer over the The Gulf (50 ° N, 28 ° E) on 28 April 1996 were simulated with the NCAR–Penn State MM5V3 numerical model. The model was validated against meteorological observations and duct data gathered in a special observational programme. A benchmark study with horizontal grid‐length of 5 km revealed complete coverage of The Gulf by ducts throughout the whole twenty‐four hours. Over land, ducts existed only at night. Over The Gulf, meso‐β scale variation occurred: shallow (250 m) surface ducts existed within about 200 km of the north coast; elevated ducts, over 500 m deep, near the southern coast, and S‐shaped ducts (about 400 m deep) over the majority of the area. Duct strength also increased from north to south. These features were related to the marine internal boundary layer (MIBL) that formed over The Gulf in the ambient north‐west wind. Meso‐γ scale variations in duct‐top height occurred within 200 km of the north coast, immediately downwind of Qatar and off the central east and west coasts. Duct characteristics were influenced at the meso‐γ scale by coastal configuration and at the meso‐β scale by sea surface temperature (SST), land–sea temperature difference, orography and ambient wind. An increase in grid length to 25 km lost much of the meso‐γ scale structure but retained the meso‐β scale features. At the meso‐β scale, duct‐top height and strength were consistently lower in the sensitivity runs than those in the control run because of changes in the depth of the MIBL. Elimination of the north–south SST gradient resulted in a shallower and more stable MIBL. Maximizing the land–sea temperature difference increased the subsidence associated with the stronger sea‐breeze circulation in the sensitivity case and therefore decreased the depth of the MIBL. When the orography was much reduced the lower wind‐speeds in the sensitivity run were again associated with a shallower MIBL. The lower wind‐speeds resulted from the lack of the thermal‐wind effect induced by the high orography around much of The Gulf. The explicit reduction of the ambient wind also led to a reduction in depth of the MIBL. Duct type also changed in the sensitivity studies, surface ducts replacing S‐shaped ducts in those areas where the depth of the MIBL decreased. Of the meso‐β scale factors influencing the ducts, ambient wind speed was most important, closely followed by orography. The distribution of SST had the smallest effect. Copyright © 2005 Royal Meteorological Society.