Convection-Aerosol Interactions in the United Arab Emirates: A Sensitivity Study

Abstract

Abstract. The Weather Research and Forecasting (WRF) model is used to investigate convection-aerosol interactions in the United Arab Emirates for a summertime convective event. Both an idealised and scaled versions of a 7-year climatological aerosol distribution are considered. The convection on 14 August 2013 was triggered by the low-level convergence of the circulation associated with the Arabian Heat Low (AHL) and the daytime sea-breeze circulation. The cold pools associated with the convective events, as well as the low-level wind convergence along the Intertropical Discontinuity (ITD) earlier in the day, explain the dustier environment, with Aerosol Optical Depths (AODs) in excess of two. Due to a colder surface and air temperature, the AHL is incorrectly represented in WRF, which leads to a mismatch between the observed and modelled clouds and precipitation. Employing interior nudging in the outermost grids of the three-nested simulation has a small but positive impact on the model predictions of the innermost nest. This is because the higher temperatures from more accurate boundary conditions are offset by colder temperatures from locally enhanced precipitation, the latter arising from a shift in the position of the AHL. Numerical experiments revealed a high sensitivity to the aerosol properties. In particular, replacing 20 % of the rural aerosols by carbonaceous particles has an impact on the surface radiative fluxes comparable to increasing the aerosol loading by a factor of 10, with a daily-averaged reduction in the UAE-averaged net shortwave radiation flux of ~90 W m−2 and an increase in the net longwave radiation flux of ~51 W m−2. However, in the former, WRF generates 20 % more precipitation than in the latter, due to a broader and weaker AHL. The surface downward and upward shortwave and upward longwave radiation fluxes are found to scale linearly with the aerosol loading, while the downward longwave radiation flux varies by less than ±12 W m−2 when the aerosol amount and/or properties are changed. An increase in the aerosol loading also leads to drier conditions due to a shift in the position of the AHL and rainfall occurring in a drier region, with a domain-wise decrease in the daily accumulated rainfall of 16 % when the aerosol loading is increased by a factor of 10. In addition, the onset of convection is also delayed.

Evaluation of Nine Operational Models in Forecasting Different Types of Synoptic Dust Events in the Middle East

“…In these cases, a dust storm may occur in front of or behind the frontal system, characterized as pre-frontal or post-frontal dust, respectively [47,67]. Sometimes a dynamic low-pressure center that is not necessarily associated with a front may also cause dust emissions [43,68]. In this case, the presence of high relative vorticity, convergence and upward motions at the surface initiate the dust outbreak [69].…”

“…In this study, four types of synoptic dust events in the Middle East (cyclonic, prefrontal, post-frontal and Shamal dust) are investigated. The first mechanism for the dust emission is due to the presence of a surface low-pressure associated with intense nearsurface winds [43,68,69]. In these cases, the rotation of dust particles is observed around the low-pressure closed center, and due to the upward movements in the center of the cyclone, the dust particles are transported to higher altitudes.…”