Pakistan Journal of Meteorology Vol. 9, Issue 17: July 2012 71 Persistent Heavy Downpour in Desert Areas of Pakistan in South Asian Monsoon 2011 Cheema, S. B. 1, 2 , Q. Zaman 2 , G. Rasul 2 ABSTRACT This diagnostic study carried out to understand the phenomenon of heavy rainfall which gripped up the Sindh in August and September 2011. National Center for Environmental Prediction (NCEP) Reanalysis data alongwith satellite and high resolution model (HRM) outputs are used for the post analyses of this downpour. The model output and satellite imagery captured well spatial and temporal distribution of the rainfall event. The analysis showed that heavy rainfall occurred over this province due to orientation of monsoon currents. A low pressure system developed over Bay of Bengal (BoB) passed over India and entered in Pakistan from 07 August, afterwards this tropical depression merged into seasonal low on 10th Aug and accentuated the weather system. Moisture was supplied from (BoB) and Arabian Sea (AS) during the monsoon season. Orientation of Tibetan high also played a crucial role for spatial distribution of precipitation over Sindh. This high blocked the monsoon current from penetration in north. At same time, another high pressure system was lying over Iran and its ridge was extended up to Baluchistan and Karachi Region. It blocked westward movement of monsoon depressions. A mid tropospheric cyclone (MTC) generated along the coastal boundary of Mumbai (India) and caused another heavy rainfall event from 29thAug to 16th September. The structure and intensification of this MTC has been discussed in this study. This paper also brings out the relationship between precipitation and large scale circulation such as monsoon orientation, tilting of Tibetan High, generation of MTC which are main causes for this rainfall event in 2011. Key words: Tropical Depression, Low pressure system, Tibetan high, Mid Tropospheric Cyclone, Monsoon Orientation. Introduction South Asia is a region with variant land surface conditions, complex topography and vast coastlines. The region is generally dominated by the South Asian monsoon along with East Asian Monsoon. South Asian monsoon yields a heavy rainfall in this area (Rasul et al., 2005). Monsoon circulation, its variability and heavy weather systems have been studied extensively by different scientist. Dr.Q.Z.Chaudhry (1991) fond inter annul variability of Monsoon rainfall and its relationship with global /regional circulation features by using a large data sets (1901-1990). He suggested that normal rainfall for country is 132.6 mm and Sindh contribute seasonal monsoon 67% to the annual rainfall. Cheema (2011) concluded by using different statistical tests that if the monsoon rainfall increased in future then it will shift towards south eastern parts of Punjab. Rana (2011) mentioned that severe flood of 2010 was the direct result of interaction between westerly and easterly flows. In Pakistan, the main contribution in the annual rainfall is summer monsoon. Summer monsoon rainfall contributes almost 60% of the total annual rainfall (Muslehuddin et al., 2005). In Summer (JAS) low depressions originate over Bay of Bengal and on the Arabian Sea that is the fundamental reason for precipitation in Pakistan. These Low Pressure Systems (LPS) that originate over Bay of Bengal move northwest cause heavy rainfall in upper Punjab and Kashmir. That caused flood and disaster in the country. But in 2011, monsoon track deviate towards Southwest and generated worst floods in Sindh. The province has not a long history of flooding, however foulest floods recorded in 2003, 2006 and 2010. All these deadliest floods happened during the monsoon season (JAS) which is augmented by heavy precipitation and snowmelt flows of glaciers. Rainfall in 2011 breaks all previous records since 1961-2010. In general, monsoon depressions reached the eastern border of Pakistan around first week of July. But in July 2011 rainfall was recorded below normal. It was recorded that rainfall was 18 % below than normal in country and 72% below in Sindh. In Aug and September accumulate rainfall above normal with 72% departure in whole country and 248% above than normal in Sindh. Heavy monsoon rainfall 1 Sohailbabar.cheema@gmail.com 2 Pakistan Meteorological Department