Convection in the linear model framework

The linear model of orographic precipitation (Smith and Barstad, 2004) is used by a range of scientists. The balance between the model's simplicity and its relatively good performance have attracted users. In the model, air lifted by terrain produces cloud water which is transported downstream while rain water is formed. Eventually, the rain water falls out as precipitation at the ground. The model uses vertically-integrated properties with the time-scale of convection and advection controlling the solution. Given a near-saturated flow without too much structure such as wind shear and melting layers, the model predicts upslope precipitation intensities relatively well. However, over the mountain top and particularly in the lee, the model performance is more doubtful. The model was originally design for stable stratiform air masses. For warmer temperatures, convection is more likely to occur. Since the convective precipitation has a very different behaviour, it is questionable to what extent the linear model can be used. In this paper, we propose to include precipitation from convective vigour.

A relation between the static stability represented by the Brunt-Vaisala frequency, cloud cover and convection vigour is found by Kirshbaum and Smith (2009). This relation is coupled to a vertical velocity profile and a moisture profile, giving the vertically-integrated condensation rate. The convective condensation rate is advected downstream in a similar manner as in the stratiform version of the model.

Results from tests and comparison with observations will be presented and discussed.

Kirshbaum, DJ; Smith, RB, 2009: Orographic Precipitation in the Tropics: Large-Eddy Simulations and Theory. J. Atmos. Sci., 66, 2559-2578.

Smith, RB; Barstad, I. 2004. A linear theory of orographic precipitation. Journal of the Atmospheric Sciences 61 (12): 1377-1391.