This paper describes a high resolution air mass transformation (AMT) model. The model is intended for short-range weather forecasts of the temperature and humidity profiles in the lower atmosphere, the structure of the boundary layer, the boundary layer height, and the amount of boundary layer clouds. The AMT model consists of a one-dimensional, multilayer boundary layer model, which is advected along trajectories from a source region to a receptor point. The trajectories are calculated within a larger scale (limited area) model. The initial profiles for temperature and humidity are obtained from observed radiosondes. The paper describes the physical and dynamical background of the model. With the model we have made case studies of the development of stratocumulus over the North Sea, and have simulated the representation of clear skies over land. The output of the model is compared with the output of the ECMWF model and the current operational bulk AMT model. Sensitivity of the model to boundary and initial conditions is discussed. In addition to the case studies the model has been used as an operational forecast tool in 77 cases. These cases have been evaluated by independent forecasters. Since the model performed well and because no large computing facilities are needed, it is concluded that the model is a useful tool for the short-range weather forecaster.

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