Mean fields of precipitable water over the Indian Ocean during the 1979 summer monsoon from TIROS-N soundings and FGGE data

Abstract

An assimilation of precipitable water in three layers (Surface-700 mb. 700-500 mb. 500-300 mb) as inferred by TIROS-N precipitable water data and Level 111-b humidity fields from ECMWF (European Center for Medium Range Weather Forecast) during summer 1979 is presented. The domain of the study is the summer MONEX area (30-120° E: 30° N-30° S) and the period covers May. June, July. August and the first half of September 1979. An objective analysis of the satellite data was carried out every 12 hours at 0000 and 1200 GMT. To overcome the lack of satellite data over cloudy areas, some subjective data were inserted taking mean TIROS-N temperature profiles in disturbed areas and assuming a certain profile of relative humidity. The fields obtained from the assimilation of TIROS-N data were merged with Level Ill-b fields. During the merging process higher weight was given over land to Level 111-b fields than to TIROS-N fields whereas the opposite prevailed over the ocean. The accuracy of the final fields has been estimated using the dropsonde soundings performed from research aircraft during MONEX-79. The relative accuracy is about 13 %).

The fields reveal the variations of precipitable water associated with the establishment of the SW monsoon. Preonset conditions were characterized by dry air over the Arabian Sea and humid air over the equator. Two weeks prior to the onset, maxima of PW began to move from equatorial regions northward to tht: Arabian Sea. Post-onset fields show the existence of dry air along the African coast and western Arabian Sea due to the inversion layer. and humid air over the eastern Arabian Sea where the inversion layer weakens and convection develops. The major break which occurred around mid-July affected the fields of precipitable water. The first week of the break in the activity of the rnonsoon was characterized by a drop of about 15% of the amount of precipitable water over the Arabian Sea compared to an active period. The fields of vertical wind reveal that convection was inhibited over the eastern Arabian Sea during the break. Two-latitude cross-sections of weekly mean fields were constructed at different longitudes. They show the time evolution of the fields associated with the onset of the Southwest monsoon and with its activeibreak cycle.