Low Level Jet [LLJ] is a meteorological phenomenon that occurs principly close to mountain terrain area during all time, but especially during summer heating period. Mostly LLJ occurs between sunset and sunrise only and more particularly between 2300 LT to 0600 LT. Consider the recent rainfalls in Tamilnadu and even in Coastal Andhra Predesh during Feb 7, 2008 to Feb 13, 2008. There is a diffused highpressure ridge sliding in the Bay of Bengal{BOB}. Cross equtorial flow of wind from Indian Ocean enters south penninsula from Southerly /SEly direction. During date time land is heated fast and the atmospheric layer close to earth, which is commonly termed as Planetary Boundary Layer[PBL] and which extends from earth surface to 2000 meter is afftected by surface heating. At night, when synoptic conditions are calm a stable temperature "inversion" (a decrease in temperature with height) forms as cooler more dense air from aloft subsides to the ground and the earth radiates energy stored during the day. The rapid decrease (with height) in surface frictional effects which decouples the layer of air near the earth’s surface from the air just above it (in the residual layer).
As noted above, an inertial acceleration, driven by the loss of friction in the residual layer after sunset, is present on nearly every occasion that a nocturnal boundary layer develops – that is, in all but high wind and precipitation cases. At what point this increase in wind speed aloft becomes a "jet" is not patently obvious and no clear LLJ threshold exists. This is what happened in western foothills and hill areas where there is heavy to very heavy rainfall. Coonor, Kodaikanal, Sivagiri, Srivilliputhur, Karur,RAjapalayam, Courtlam experienced heavy rainfall. It has been suggested that it is produced by either thermally driven flow, a local circulation, generated by the large scale topographical structure or a large/synoptic scale circulation enhanced by the topographical barrier and with a significant influence of the intrusion of the BOB High and enhanced atmospheric low in south Indian Ocean
