Incorporating the effect of small-scale circulations upon dust emission in an atmospheric general circulation model

Abstract

Realistic simulation of dust emission in an atmospheric general circulation model (AGCM) is inhibited by the model's coarse resolution compared to the scale of the circulations observed to mobilize dust. We construct a probability distribution of wind speed within each grid box that depends upon the speed explicitly calculated by the AGCM and the magnitude of fluctuations about this speed. This magnitude is calculated by incorporating information from the AGCM's parameterizations of the planetary boundary layer along with dry and moist convection. Emission depends on the fraction of the wind speed distribution above the threshold value. As a consequence, emission can occur even if the explicitly resolved wind speed is less than the threshold, as long as the subgrid scale variability is large enough. In the AGCM, subgrid wind fluctuations are dominated by dry convection. This favors dust emission over deserts, where there is continuous mixing within the boundary layer due to intense solar heating of the surface. Particles emitted over arid regions are farther from precipitation and mixed higher above the surface by dry convection. This increases their wet and dry deposition lifetimes, respectively, increasing the aerosol load for a given emission. The AGCM's identification of "preferred meteorology" for emission by subgrid circulations complements the preferred sources of erodible particles included in other models. Given the introduction of subgrid variability, the AGCM's dust aerosol burden improves significantly, compared to the Total Ozone Mapping Spectrometer (TOMS) and advanced very high resolution radiometer (AVHRR) aerosol optical thickness (AOT) retrievals, over the Sahara, Sahel, and the Taklimakan as well as downwind of these regions, considered to be major sources of dust emission. This mechanistic representation of subgrid variability allows us to calculate the atmospheric burden of dust under different climates, where emission can change due to altered boundary layer variability in addition to changes in the mean wind speed. Copyright 2004 by the American Geophysical Union.