A predictive numerical model of the Atmospheric Boundary Layer based on the Turbulent Energy Equation.

Type: Report
Series: RMK 13
Author: Bodin, S.


The interest in numerical modeling of the atmospheric boundary layer has grown considerably over the last decade. At SMHI and elsewhere boundary layer models find applicatoions in local forecasting, especially at airports, in air pollution diffusion and dispersion studies and in wind energy programmes .

In this report an one - dimensional numerical boundarylayer model is derived and numerical simulations of boundary layer data from Australia and Finland are presented and discussed. The model, which is the first step towards a three-dimensional model, is based on the so-called Gutman approach and incorporates the turbulent energy equation for turbulence closure. A scale analysis is performed, that shows that unless a grid distance of 20 km or less can be used in a threedimensional model it is more profitable to use an onedimensional one with more sophisticated physical parameterizations.

The model also includes condensation, i e fog and clouds, and complete radiation computations. A predictive equation for surface temperature is used in conjunction with a simple soil moisture model.

The numerical solution employs a variety of the Crank-Nicolson scheme called Laasonen's scheme. The vertical coordinate is transformed log-linearly into a new height coordinate to allow better resolution close to the ground. 35 grid points are used to describe the boundary layer up to 2000 m. A time step of 4 minutes has been used in the simulations.

Two versions of the model, the Gutman version and a usual "Ekman" version, have been tested on day 33 and 34 of the Wangara data. The two versions have been compared and the Ekman version has also been compared with the simulations of Yamada & Mellor (1975).

The results show that the Ekman version is superior when simulating the wind of the Wangara data. The thermal boundary layer development is very well predicted by both versions. Comparisons with Yamada & Mellor speak in favour of the present model.

Conclusions are drawn and some future work is outlined. The model is intended to undergo operational tests at Arlanda airport in the near future.