Upper tropospheric drying and the transition to break'' in the Indian summer monsoon during 1999

METEOSAT pixel measurements reveal a striking new observation that rapid southward propagating deep clouds, on diurnal scale, are embedded in the northward migrating cloud structures from near equator upto 20N during active phases of 1999 summer monsoon. For the first time, we show that, while the deep convection is on'' during the active phase, an abrupt atmospheric drying occurs three days before the commencement of break in layers between 600 and 200 hPa over Continental Tropical Convergence Zone (CTCZ) with drastic reduction in Upper Tropospheric relative Humidity to similar to 30%. East-West circulation cells induce subsidence over CTCZ and hence upper tropospheric drying. Continued drying and simultaneous weakening of low level convergence by similar to 50% causes the transition to break. Subsidence associated with intense convection near foot hills of Himalayas weakens low level convergence. A rapid shift occurs in divergence field during transition with convergence replacing intense divergence at upper levels over CTCZ.