Organization and structure of clouds and precipitation on the mesoscale and microscale in cyclonic storms

Our current understanding of the organization and structure of clouds and precipitation in extra‐tropical cyclonic storms is reviewed. The regions of relatively heavy precipitation are organized into large mesoscale (∼ 10³–10 4 km²) rainbands which are classified into five types: warm‐frontal, warm‐sector, cold‐frontal (wide and narrow), prefrontal cold‐surge, and postfrontal bands. The rainbands themselves are composed of smaller mesoscale areas (∼ 10–10² km²) or ‘cores’ of precipitation. In some of the rainbands the precipitation cores often originate in higher‐level generating cells, probably produced by the lifting of potentially unstable air. The generating cells provide ‘seed’ ice crystals, which grow by aggregation and riming as they fall through lower cloud layers produced by large mesoscale or synoptic scale lifting of the air. In these cases the rainbands move with the velocity of the upper‐level winds at the levels of the generating cells and therefore tend to move through the cyclonic storm. Other rainbands have their origins in lower‐level convection. For example, narrow cold‐frontal bands are fed by moisture from a low‐level, southerly jet of air which the cold front progressively overtakes. The narrow cold‐frontal band is therefore anchored to the cold front. Appreciable precipitation on the mesoscale in cyclonic storms is invariably associated with high concentrations (∼ 1–100 1 −1 ) of ice particles. Small hills may play an important role in triggering or enhancing mesoscale rainbands in cyclonic storms in regions where the air is unstable. The precipitation over and downwind of large mountain barriers, on the other hand, may be disrupted because the orography interferes with the low‐level, southerly flow of air which provides the front with its principal source of moisture.