Chapter 11 - Clouds and Precipitation in Extratropical Cyclones

Abstract

Extratropical cyclones draw energy from the horizontal variation of temperature in the atmosphere; i.e., from the baroclinic structure of the atmosphere. Clouds form in response to the vertical air motions within baroclinic waves and are usually concentrated along warm, cold, and occluded fronts. The air motions within baroclinic waves are frontogenetical, meaning that they tend to bring isotherms together into sharp zones of horizontal temperature contrast to define frontal zones. Frontogenetic zones have strong secondary circulations that keep the flow in force balance. This overturning produces nimbostratus clouds along most fronts. These clouds are unique in the atmosphere because they are produced by the dynamics of frontogenesis. However, the frontal clouds exhibit considerable variability. They break down into mesoscale precipitation features known as narrow cold frontal rainbands, wide cold frontal rainbands, occlusion bands, and surge rainbands. The narrow cold frontal rainbands are associated with gravity wave dynamics at the edge of sharp cold fronts. Wide cold and warm frontal rainbands are connected with moist conditional symmetric instability. The rainbands of extratropical cyclones often contain embedded convective elements. These observed structures and associated dynamics are all reviewed in this chapter.