Diabatic effects on the evolution of storm tracks

Abstract. Despite the crucial role of moist diabatic processes in midlatitude storm tracks and related model biases, we still lack a more complete theoretical understanding of how diabatic processes affect the evolution of storm tracks. To alleviate this shortcoming, we investigate the role of diabatic processes in the evolution of the Northern Hemispheric storm tracks using a framework based on the tendency of the slope of isentropic surfaces as a measure of baroclinic development. We identify opposing behaviours in the near-surface and free troposphere for the relationship between the flattening of the slope of isentropic surfaces and its restoration by diabatic processes. Near the surface (900–800hPa), cold air advection associated with cold air outbreaks initially acts to flatten isentropic surfaces, with air–sea interactions ensuing to restore surface baroclinicity. In the free troposphere (750–350hPa), on the other hand, diabatic generation of slope of isentropic surfaces precedes its depletion due to tilting by eddies, suggesting the primary importance of moist diabatic processes in triggering subsequent baroclinic development. The observed phasing of the diabatic and tilting tendencies of the slope is observed both in upstream and downstream sectors of the North Atlantic and North Pacific storm tracks, rendering the phasing a general feature of midlatitude storm tracks. In addition, we find a correspondence between the diabatic generation of slope of isentropic surfaces and enhanced precipitation as well as moisture availability, further underlining the crucial role of moisture and moist processes in the self-maintenance of storm tracks.