Effects of mineral aerosols on the summertime climate of southwest Asia: Incorporating subgrid variability in a dust emission scheme

Abstract

[1] Improvements in modeling mineral aerosols over southwest Asia are made to the dust scheme in a regional climate model by representing subgrid variability of both wind speed and surface roughness length. The new module quantifies wind variability by using model meteorology while assuming that wind speed follows a Gaussian distribution. More specifically, wind variability is approximated by dry convective eddies within the planetary boundary layer, forced by sensible heat fluxes at the surface. Incorporating subgrid variability of wind increases aerosol optical depth (AOD) over the region by nearly 35% while reducing incoming shortwave radiation by an additional 5–10 W/m[superscript 2]. Likewise, the dust scheme is modified to include the variability of surface roughness length over southwest Asia. Here an empirical distribution of roughness length for each grid cell is calculated based on the USGS's 4 km resolution land cover data set. However, incorporating roughness length variability does not significantly alter dust emissions over the region due to the relatively homogeneous land cover conditions. Nevertheless, including spatial variability for wind results in aerosol optical depth values closer to observational data sets, particularly MISR, which performs better than MODIS over this region. However, RegCM3's dust model still underestimates AOD over southwest Asia. In addition to improvements made in RegCM3's dust model, this work examines the effects of mineral aerosols on the mean monthly, surface summertime climate of southwest Asia. It is shown that dust emissions reduce average summertime surface temperatures by approximately 0.5°C while attenuating shortwave incident radiation by nearly 25 W/m2. Thus, the emission of dust is an important surface process in shaping the summertime climate over southwest Asia. However, both a warm bias in surface temperatures and overestimation of incoming shortwave radiation still exist in RegCM3 and need to be further addressed.