The Wind Field Generator

One 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 domain-scale 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:

STEP 0:	selection of the appropriate parameterization for the given meteorological conditions
STEP 1:	construction of an inert vertical wind profile, depending on atmospheric stability and determination of a set of stability parameters (Ermak 1991)
STEP 2:	parameterization of kinematic terrain effects (Liu 1980)
STEP 3:	intermediate divergence minimization to adjust the horizontal wind components in each vertical level (Goodin 1980)
STEP 4:	computation of thermodynamically generated slope flows, modifies the horizontal surface wind components (Allwine 1985)
STEP 5:	Froude number adjustment for the horizontal wind (Allwine 1985)
STEP 6:	smoothing of the horizontal wind field
STEP 7:	divergence computation of the horizontal field, new vertical wind components
STEP 8:	vertical adjustment of the vertical wind (zero at the top or at the mixing height) (O'Brien 1970)
STEP 9:	final divergence minimization to adjust the horizontal wind --> final wind field.

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

NX is the number of grid elements in East direction, NY the number of grids in the North direction, DX, DY the grid space [m] in x,y-direction (horizontal resolution),
GAMMA is the vertical lapse rate [K/m] below the mixing height, STABILITY CLASS refers to one of six stability classes, MIXING HEIGHT defines the top of mixed layer,
WIND SPEED [m/s] and WIND DIRECTION [DEG] are wind observations at
MEASUREMENT HEIGHT [m] above the surface.

Data availability regarding the lapse rate at the moment of a hypothetical accident seems to be very questionable. For this reason, an experienced user has the option of selecting an appropriate stability class, or he can enter a lapse rate.

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.

Stability
class description gamma
[K/m] mixing
height [m]
1 very unstable -0.025 2500
2 unstable -0.020 1500
3 slightly unstable -0.015 800
4 neutral -0.010 500
5 slightly stable -0.001 300
6 stable -0.010 200

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.

ground type recommended
roughness length [m]
water bodies, ice 0.001
snow 0.015
barren land, dessert 0.030
grassland 0.100
crops, agricultural land 0.200
forest 1.100
suburbs 1.000
central urban areas 1.500

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 stability-adjusted 3D wind field with its appropriate stability parameters.