Wind calculation model (Heat Overlay)
The wind speed on streetlevel (1.2 meter) is calculated using a translation of the 10 meter wind speed measured at a weather station. A single reduction field per wind direction can be calculated, which in turn can be multiplied with the 10 meter wind speed to obtain the wind speed at streetlevel. The calculation model is based on the McDonald (2000) method for wind in cities. It uses the vertical surfaces of buildings perpendicular to the wind direction to determine in what way the wind speed is slowed down. The DPRA Heat stress report has extended this method with perpendicular vertical tree surfaces as well.
Implementation
Our implementation differs slightly from the implementation described by DPRA Heat stress report. It does not do cubic interpolation from coarse grids to more detailed grids. We do all operations on a detailed grid.
Our implementation is as followed:
Average window
First of all, the wind window has to be selected. If the wind speed is lower than 1.5 m/s, a window of 175 by 175 is used for averaging. Else a window of 140x280 is taken. See the image below:
Per grid cell
Per wind window grid cell:
- Determine the average obstacle height per square meter. Buildings and trees lower than 2 meters are not taken into account. It is divided by the area of the search window.
- Determine the average vertical building surface perpendicular to the wind direction.