The effect of vertical and slantwise convection on the simulation of polar lows

Abstract

Two polar low cases during February 1984 are simulated with a mesoscale limited area grid point model. The role of released latent heat on different scales as a driving mechanism for polar lows is investigated by studying a series of prognoses with condensation schemes for the large-, meso-, and cloud-scales. For the mesoscales, a parameterization method for slantwise convection is developed and applied. The simulations show that release of latent heat from condensation is crucial in order to obtain sufficiently strong developments. When working alone, large-scale condensation leads to unstable lapse rates and to fast developments. Convection shifts the area of maximum heating upwards and stabilizes the atmosphere, while slantwise convection in addition spreads the heating horizontally away from the surface disturbance. The large-scale response to parameterized slantwise convection is to stabilize the model atmosphere with respect to symmetric instability, to shift areas of ascending motion towards sloping absolute momentum surfaces and to enhance low-level winds.