
RiskWare Online Reference Manual

The Wind Field GeneratorOne of the most important features of an atmospheric emergency simulation system is the computation of a realistic wind field in a complex terrain.The core part of the wind field model in RiskWare is based on the Diagnostic Wind Model (DWM, Douglas 1990). The model generates gridded wind fields for a specific poiny in time. It adjusts the domainscale mean wind (input) for terrain effects (kinematic effects, such as lifting and acceleration of the airflow over terrain obstacles, as well as thermodynamically generated slope flows). It performs a divergence minimization to ensure mass conservation. The following steps are performed:
The diagnostic wind model bases on a digital elevation map of the model domain. A grid based mean elevation for every grid for the desired resolution must be provided by the customer. A typical application range of the model reaches from medium scale to regional scale (1 km x 1 km up to 200 km x 200 km) and covers vertical levels up to the mixing height. The horizontal resolution depends on the given data (recommended 1D horizontal grid size is 1/100 of the domain length, e.g. for a 10 km x 10 km model domain an appropriate grid size is 100 m x 100 m). The vertical resolution is computed depending on the stability (mixing height) and rises usually from a few meters at the bottom to some hundred meters at the top of the model domain. The number of vertical layers used also depends on the mixing height, e.g. for stable conditions and low mixing heights, the model uses 14 layers whereas it takes 20 layers for atmospheric unstable conditions. The model is not able to resolve very local wind structures (e.g. lee waves at a building) but it provides a mean wind field for the area of interest. This is an example of the input data set for the wind field model:
BERLIN DATASET NX 49 NY 53 DX [M] 100. DY [M] 100. GAMMA [K/M] 0.038 STABILITY CLASS 0 MIXING HEIGHT [M] 2000. WIND SPEED [M/S] 5. WIND DIRECTION [DEG] 30. MEASUREMENT HEIGHT [M] 100. ROUGHNESS LENGTH [M] 0.5 SURFACE TEMPERATURE [K] 297. where
The procedure for selecting a mixing height is similar. If it is known from measurements, it can be specified. Otherwise a mixing height compatible with the chosen stability class is assigned. Table 1 lists default lapse rates and mixing heights for the six stability classes.
The ROUGHNESS LENGTH [m] is a parameter determining aerodynamic effects. The influence of small obstacles in a scale smaller than the numerical grid size is parametrized by the roughness length. Usually, the value for the roughness length is directly coupled with the land use of the grid cell. If the land use of the model is not given in the input grid, a value for ROUGHNESS LENGTH can be selected from Table 1 which is representative for most of the model domain.
In addition to the parameter set which has to be specified for every new run, a second set of parameters defines internal model parameters such as accuracy, the parametrization type or strength. This option is only valid for an experienced user of the system and for training purposes. The model output is a terrain and atmospheric stabilityadjusted 3D wind field with its appropriate stability parameters. 
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