Abstract
The Weather Research and Forecasting (WRF) model is used to investigate convection–aerosol interactions in the United Arab Emirates (UAE) for a summertime convective event. Both an idealized and climatological aerosol distributions are considered. The convection on 14 August 2013 was triggered by the low-level convergence of the cyclonic circulation associated with the Arabian Heat Low (AHL) and the daytime sea-breeze circulation. Numerical experiments reveal a high sensitivity to aerosol properties. In particular, replacing 20% of the rural aerosols by carbonaceous particles has a comparable impact on the surface radiative fluxes to increasing the aerosol loading by a factor of 10. In both cases, the UAE-averaged net shortwave flux is reduced by ~90 W m−2 while the net longwave flux increases by ~51 W m−2. However, when the aerosol composition is changed, WRF generates 20% more precipitation than when the aerosol loading is increased, 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. An increase in the amount of aerosols also leads to drier conditions and a delay in the onset of convection due to changes in the AHL.
Highlights
It has long been known that aerosols, defined as solid or liquid particles suspended in the atmosphere from both from natural and anthropogenic sources, play an important role in the climate system [1,2,3]
The former will be denoted as aerosol–radiation interactions (ARI) and the latter as aerosol–cloud interactions (ACI) throughout the text
As far as the ACI effects are concerned, an increase in aerosol loading leads to a larger number of smaller cloud droplets, which leads to more scattering and a higher cloud albedo and optical depth [8]
Summary
It has long been known that aerosols, defined as solid or liquid particles suspended in the atmosphere from both from natural and anthropogenic sources, play an important role in the climate system [1,2,3]. The ARI effects are found to have the largest influence on the development of convection in dusty areas, leading to a stronger, albeit delayed, MCS This is because the heating of the dust layer during the day reduces convective instability, but the increase in downward longwave radiation flux at the surface [19] will lead to higher instability and a roughly 14% increase in precipitation. The two main objectives of this study are as follows: (i) investigate the added value of incorporating aerosols and accounting for their direct and indirect effects on the model-predicted convective activity, and (ii) explore the sensitivity of the WRF response to different aerosol loadings and properties and assess how it compares against observations.
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