Anomalous Atmospheric Events Leading to the Summer 2010 Floods in Pakistan

orogrAphiC rAinstorms seen by trmm. Major rainstorms can occur in mountainous regions when air is lifted over the terrain. When the environment is especially unstable, storms may form that contain locally intense buoyant updrafts, heavy downpours of rain, and sometimes hail. High winds may occur where downdrafts, forming in raining regions, reach the ground and spread out. When a sequence of convective upand downdrafts occurs, the precipitating cloud systems take the form of a “mesoscale convective system” (MCS; for a thorough description see Houze’s 2004 article in Reviews in Geophysics), in which some “convective cells” die out while other new ones form, and the older dying storms morph into regions of wider-spread “stratiform rain.” This process is described by Houze in a 1997 Bulletin article. The stratiform region features gentler rain but covers a much greater area and can last for long periods of time if the environment is favorable. Even if the instability is modest but the environment is humid enough to support the persistence of the stratiform regions of MCSs, or if the MCS stratiform region is enhanced by orographic or synoptic-scale upward motion, a great amount of rain can be produced by the MCS. As a result, if MCSs are the principal rain-producing cloud systems over a region such as the upslope side of the Himalayas, the mountains can affect the rainstorms in two important ways: 1) Airflow over rising terrain may trigger new intense convective cells, thus revitalizing and maintaining the vigorous convective region of the MCS for a longer period of time; and 2) the orographic upward air motions may sustain and broaden the stratiform region of the MCS so that the gentler (but nonetheAnomalous Atmospheric Events Leading to the Summer 2010 Floods in Pakistan