Forecasting Dust Storms using HYSPLIT

 Roland R. Draxler

Prepared for the Sino-US Workshop on Dust Storms and Their Effects on Human Health, November 25-26, 2002, Raleigh, North Carolina

 

Introduction -  A model for the emission of PM10 dust had been constructed (Draxler et al., 2001) using the concept of a surface roughness dependent threshold friction velocity. Surface roughness is correlated with soil properties.  A dust emission rate is computed from each model grid cell when the local wind velocity exceeds the threshold velocity for the soil characteristics of that emission cell.  The dominant mechanism for the PM10 emission is "sand-blasting". The emitted material is dispersed and transported using HYSPLIT, a Lagrangian transport and dispersion model (Draxler and Hess, 1998). 

 

            The model was initially tested over Kuwait, Iraq and Saudi Arabia, where it predicted about the right number of dust events (18%).  The model results also agreed quantitatively with measurements at four locations in Saudi Arabia and one in Kuwait for one major dust event (>1000 g/m3).  However, for several smaller scale dust events (200-1000 g/m3) the model substantially over-predicted the air concentrations.  Part of the over-prediction was attributed to the model's sensitivity to the threshold friction velocity and the surface soil texture coefficient (the soil emission factor), and the difficulty in accurately representing these parameters in the model. 

 

            To apply the model over other domains where detailed digital soil characteristics are not available, the emission module was modified to use HYSPLIT's one-degree land-use file by assuming that a "desert" land-use grid cell corresponds to the Kuwait "active sand sheet" soil type category. To compensate for the greater number of potential dust emission cells, the original PM10 flux equation was replaced by a relationship not dependent upon soil type and which had a substantially lower emission flux.

 

            The modified model was tested for two recent Chinese dust storms and one Australian event, the results of which are discussed in more detail in this presentation. The meteorological data for the examples and model executables are provided on a limited number of CD-ROMs for distribution to workshop participants.

 

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