Sensitivity of Summertime Convection to Aerosol Loading and Properties in the United Arab Emirates

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.

Assessment of the WRF Model as a Guidance Tool Into Cloud Seeding Operations in the United Arab Emirates

“…In fact, the air temperature biases in Figure 7a resemble those obtained by Fonseca, Francis, Weston, et al. (2021) which employed a similar WRF configuration in their simulations. Aerosols are known to scatter and absorb the Sun's shortwave radiation and trap the longwave radiation emitted by the Earth's surface (Francis, Chaboureau, et al., 2021).…”

“…The reason for this discrepancy is likely to be the use of a climatological aerosol distribution in the cloud microphysics scheme and representation of the aerosol direct and indirect effects in the model in the runs discussed in this paper (Table 2). In fact, the air temperature biases in Figure 7a resemble those obtained by Fonseca, Francis, Weston, et al (2021) which employed a similar WRF configuration in their simulations. Aerosols are known to scatter and absorb the Sun's shortwave radiation and trap the longwave radiation emitted by the Earth's surface (Francis, Chaboureau, et al, 2021).…”

“…The stronger daytime winds may also contribute to the cold bias as they promote enhanced vertical mixing, as noted by Fonseca, Francis, Weston, et al. (2021). The overestimation of the wind speed in WRF may arise from the deeper AHL and associated enhanced circulation in the model (cf.…”

“…One reason for this is a shift in the wind direction for some of the stations, as highlighted by the larger error bars for this run which represent one standard deviation from the mean, and resulting cooler air advection. The stronger daytime winds may also contribute to the cold bias as they promote enhanced vertical mixing, as noted by Fonseca, Francis, Weston, et al (2021). The overestimation of the wind speed in WRF may arise from the deeper AHL and associated enhanced circulation in the model (cf.…”

“…(2016) and Fonseca, Francis, Weston, et al. (2021), interior grid (analysis) nudging is employed in the two outermost nests, a set‐up which is found to improve the model's innermost grid forecasts. Following the latter study, the potential temperature perturbation, horizontal wind components and water vapor mixing ratio are relaxed toward the forcing data above ∼800 hPa and on a time‐scale of 1 hr.…”