Numerical Simulation of the Structure and Evolution of a Polar Mesocyclone over the Kara Sea.Part 1. Model Validation and Estimation of Instability Mechanisms

P. S. Verezemskaya and V. M. Stepanenko

Numerical experiments based on the WRF model were conducted to analyze the structure and evolution of the polar mesoscale cyclone developed over the Kara Sea on September 29–30, 2008. It was found that baroclinic instability in the lower troposphere and convective instability (including that due to the wind-induced surface heat exchange) did not play a significant role. Significant contribution was made by the downward advection of potential vorticity from the upper troposphere and by the conditional instability of second kind. It is demonstrated that if water phase transitions are not taken into account, the mesocyclone intensity is reduced by 7–20% and the time of its deve- lopment increases by 4 hours. The advection of potential vorticity was not the only process causing the intensification of the lower potential vorticity anomaly associated with cyclonic circulation.