Katabatic wind over gentle slope on the Majorca Island

An analysis of a katabatic flow dynamics is made through a two-layer model following the framework set out in Mahrt (1982). Data is provided by a very-high resolution mesoscale simulation of the isle of Majorca in western Mediterranean sea. The downslope momentum budget shows that buoyancy force is generally balanced by the advection term and the stress divergence at the beginning of the night. Over time, the latter term becomes smaller. Thermal wind terms and Coriolis effects are smaller probably due to the small spatial scale of the problem. According to these results, the katabatic wind can be considered a shooting flow, in agreement with the Froude number analysis from hydraulic theory.

Katabatic-layer average velocity and surface cooling rate have also been studied from simulation. Stationary oscillations of both variables appear along the slope direction, where maximum surface cooling rate matches well with minimum average velocity from gravity flow. The pattern of surface radiative temperature from two NOAA images confirms the oscillatory behavior detected with the cooling rate from simulation.