The Effects of Dust Emissions on the Summertime Climate of Southwest Asia: Incorporating Sub-grid Variability in a Regional Climate Model’s Dust Scheme

Improvements are made to the dust scheme in a regional climate model by representing sub-grid variability of wind speeds and surface roughness lengths. The new module quantifies wind variability by using model meteorology, assuming wind speeds follow a Gaussian distribution. More specifically, wind variability is approximated by dry convective eddies within the boundary layer, forced by surface sensible heat fluxes. Incorporating sub-grid variability of wind increases dust emissions over Southwest Asia by nearly 40% while reducing incoming shortwave radiation by 5-15 W/m^2. Likewise, the dust scheme is modified to include variability of surface roughness length over Southwest Asia. An empirical distribution of roughness length for each grid-cell is calculated based on the USGS's 4km resolution land cover data. However, incorporating roughness length variability does not significantly alter dust emissions. Nevertheless, aerosol optical depth values in simulations including the spatial variability of wind lie closer to observations. Using the improved dust module in RegCM3, this work also presents the effects of dust on the surface summertime climate of Southwest Asia. It is shown that dust emissions reduce average summertime surface temperatures by approximately 0.5-1.0C while attenuating shortwave incident radiation by 35 W/m^2. Likewise, analysis of the diurnal cycle of energy and surface temperatures yields significant changes caused by the radiative effects of dust. That is, afternoon temperatures are reduced by over 1C while shortwave radiation is reduced by nearly 100 W/m^2. Lastly, results indicate that dust alters not only mean temperature, but also extreme daily temperatures experienced over Southwest Asia.