Model description

Nickovic et al. 2001; Pérez et al. 2006a, Pérez et al. 2006b) and conducts modelling research and developments for short-term prediction.

The model predicts the atmospheric life cycle of the eroded desert dust and was developed as a pluggable component of the Eta/NCEP (National Centers for Environmental Prediction) model. It solves the Euler-type partial differential non-linear equation for dust mass continuity and it is fully inserted as one of the governing prognosis equations in the atmospheric Eta/NCEP atmospheric model equations.

The model simulates all the major processes of the atmospheric dust cycle. During model integration, calculation of the surface dust injection fluxes is made over the model grid points declared as deserts. Once injected into the air, dust aerosol is driven by the atmospheric model variables: by turbulent parameters in the early stage of the process when dust is lifted from the ground to the upper levels; by model winds in the later phases of the process when dust travels away from the sources; finally, by thermodynamic processes and rainfall of the atmospheric model and land cover features which provide wet and dry deposition of dust over the Earth surface. One of the key components of the dust model is the treatment of the sourcing terms in the concentration continuity equation. Failure to adequately simulate/predict the production phase of the dust cycle leads to wrong representation of all other dust processes in the model. Therefore, special attention is made to properly parameterize dust production phase. Wind erosion in the emission scheme scheme is controlled mainly by the following factors: type of soil, type of vegetation cover, soil moisture content, and surface atmospheric turbulence.

 

 

The main general features of the model (Nickovic et al. 2001) are listed below:

• Dust production scheme (Shao et al. 1993) with introduced viscous sub-layer (Janjic, 1994).

• Soil wetness effects on dust production (Fecan et al. 1999).

• Dry deposition (Giorgi, 1986) and below cloud scavenging.

• Horizontal and vertical advection, turbulent and lateral diffusion (Janjic, 1994) represented as for other scalars in the Eta/NCEP model.

 

In the latest model version (BSC-DREAM8b) (Pérez et al. 2006a, Pérez et al. 2006b), grid points acting as desert dust sources are specified using arid and semiarid categories of the global USGS 1-km vegetation data set. Another data participating in dust production calculations is the FAO 4-km global soil texture data set from which particle size parameters are evaluated. Other schemes in the model have been updated:

• Eight size transport bins between 0.1 and 10 um are considered. In this interval, the aerosol effects on solar radiation are the most significant. Within each transport bin, dust is assumed to have time-invariant, sub-bin log-normal distribution employing the transport mode with mass median diameter of 2.524 um and geometric standard deviation 2.0.

• Dust affects the radiative fluxes at the surface and the top of the atmosphere and the temperature profiles at every model time step when the radiation module is processed. These changes influence the atmospheric dynamics, moisture physics, and near-surface conditions. Furthermore, dust emission is modified by changes in friction velocity and turbulent exchange coefficients; dust turbulent mixing, transport, and deposition are altered by changes in atmospheric stability, precipitation conditions, and free-atmosphere winds.

Since there are not yet satisfactory three-dimensional dust concentration observations, the initial state of dust concentration in the model is defined by the 24-hour forecast from the previous-day model run. The model at the starting day is run using “cold start” conditions, i.e., the zero-concentration initial state. The meteorological fields are initialized every 24h and boundary conditions are updated every 6h with the NCEP/NCAR I global analysis (0.5ºx0.5º).

 

References