Satellite observations of interaction between tropics and mid-latitudes

In certain seasons and over certain locations, the mid-latitude westerlies invade subtropical and tropical areas. Short wave perturbations moving in the broad mid-latitude westerlies amplify the. long wave troughs creating new baroclinic zones in relatively southern latitudes. These. baroclinic zones Interact .with the low-latitude circulations thus leading to development of new circulation pattern .In which low level easterlies extend northward over the Peninsula, central and northwest .India. The paper describes the role of short waves in the interaction between tropics and mid-latitudes and presents satellite data of a few sequences In which such Interactions have actually taken place.

A diagnostic study of flood producing rainstorm of September 1988 over northwest India with the aid of a fine mesh numerical analysis system

A numerical analysis of the synoptic situation leading to devastating floods in Punjab and adjoining states during September 1988 has been carried out. The analysis is done by three dimensional multivariate optimum interpolation (OI) scheme cast on 1° x 1° Lat./Long. Grid. Software has been developed for computation of several derived parameters and linked with the basic flow variable analysis. A diagnostic study of day-to-day rainfall versus the objectively analysed grid point fields of integrated horizontal flux divergence of water vapour is carried out, The study brings out a close spatial correspondence between the area of net moisture flux convergence on the analysis day and the area of heavy rainfall on the following day. The study suggests that the numerical analysis products can be of a good predictive value to a synoptic forecaster In heavy rainfall predictions under difficult and uncertain synoptic situations.

Potential for long-range regional precipitation prediction over India

The potential for long-range precipitation prediction over the Indian monsoon region is generally good where climate noise (i.e., variability due to daily weather fluctuations) is small as compared to the climate signal (i.e., variability due to year to year fluctuations in monthly/seasonal means) being in the tropical belt. In order to understand the potential on smaller spatial scales, the ratios of inter-annual variability to that associated with climate noise have been computed for precipitation of four seasons as well as SW monsoon sub-seasons/months over 1656 stations in the Indian subcontinent. Precipitation in SW monsoon has been found potentially predictable on seasonal as well as intra-seasonal scale. The west coast and contiguous northwest India, part of the 'northeast India are more predictable. Potential for long-range prediction over northwest India is highest during the active monsoon period from July to September. Over eastern peninsula potential for prediction is generally found low whereas over north-central India it is always moderate. Over northern latitudes precipitation due to western disturbances during January to May is potentially predictable. Precipitation over southeast India and Sri Lanka during October to February due to northeast (NE) monsoon shows good potential for long-range prediction. It is manifested that long-range precipitation forecasting schemes for SW monsoon season, sub-seasons and months and for the other seasons over India on point to regional scale have good scope by taking into account the potential predictability at the individual stations as well as at contiguous resemblance areas over the country.

The role of Himalayan Massif-Tibetan Plateau and the mid-tropospheric sub-tropical ridge over north India during the advance phase of the southwest monsoon

The advance of monsoon over the Indian sub-continent is not a continuous process. It advances in a phased manner. It has been observed from large sample of the data that the monsoon current often stagnates outside northwest India. Gujarat, northwest Madhya Pradesh and west Uttar Pradesh for a long period resulting in delaying its advance considerably over these areas. The cause of such prolonged stagnation can be identified to the mechanic Himalayas - Tibetan plateau on the mid-tropospheric westerly flow in absence of any synoptic scale forcing over north India. During 1976, 1982 and 1991 there was prolonged stagnation of southwest monsoon over northwest India and neighbouring areas and, in 1985, it was over north Konkan during June-July. In those years, the sub-tropical ridge (STR) in the middle troposphere over India was weak or absent during the initial phase of advance of monsoon. In absence of the STR, the westerly trough activity in the mid-tropospheric levels extended to southern latitude disrupting the monsoon flow and bringing prolonged stagnation. The observation was confirmed on a test study conducted during the prolonged stagnation of the monsoon of 1995. On the other hand, in the year 1990, the mid- tropospheric STR became prominent from middle of June over north India and it helped in restricting southward extension of westerly troughs. Consequently the equatorial trough remained organize gradually over north India and caused the gradual advance of SW monsoon over the entire country without considerable.

Association between mid-latitude circulation and Indian monsoon rainfall

The statistical relationship between the summer monsoon rainfall over India and mid-latitude general calculation at the 500 hPa level was investigated for the period 1971-1989. The index used to characterise general circulation feature is the perturbation of the zonal flow (ratio of meridional to zonal index) for the latitudinal band 35°N - 70°N over different geographical area and the hemisphere. It was found that the perturbation of the zonal flow during preceding January over the geographical sector 1 (45°W - 90°E) shows significant relationship with the subsequent Indian summer monsoon rainfall in an inverse manner. Thus, the perturbation of the mid-latitude circulation during preceding January over the geographical sector seems to be a useful predictor of the subsequent Indian monsoon rainfall. Significant simultaneous inverse relationship also exists between perturbation of mid-latitude zonal flow during July to September over Sector 2 (90°E- 160°W) and summer monsoon rainfall over northwest India.

Effect of eastward moving cloud clusters over northwest India and neighbourhood on the Indian summer monsoon

A number of studies have been made relating to the prediction of date of onset and also the activity of southwest monsoon over India. In this study an attempt has been made to link the behaviour of rainfall in northern and peninsular India to the satellite cloud clusters over the region of Afghanistan, Iran and northwest India. The monsoon rainfall over India commences in June. Hence variation of monsoon rainfall pattern with the movement of cloud clusters from northwest in pre-monsoon and monsoon months have been studied to elucidate the pattern of changes. The variability of monsoon performance largely depends on the number and movement of cloud clusters over northwestern region during pre-monsoon and monsoon months.

Re-determination of normal dates of onset of southwest monsoon over India

The onset of southwest monsoon over various parts of the country can be early, timely or late as compared to a set of normal dates. Advance of monsoon could be either rapid or slow or sometimes there could be prolonged stagnation in the advance of monsoon. The timing of the monsoon onset is of vital importance to the agricultural sector and water replenishment. The climatological normal dates of onset which are being used at present are based on a much older data set (1901-1940) obtained from 149 stations. In this study, the climatology of the summer monsoon onset over different parts of India is derived using the mean pentad precipitation data of 569 stations spread all over the country, from 1971-2000. It has been observed that the revised dates of onset of monsoon over the Andaman Sea is 20 May, over Kerala is 1 June, over the northeastern parts is 5 June and the date of monsoon covering the entire country is 15 July (same as the existing dates). Considerable differences between the existing and the re-determined dates of onset are noticed over parts of south peninsula and western parts of central and adjoining northwest India where the re-determined dates are advanced by 1 to 3 days and delayed by 10 to 15 days respectively with respect to the existing normal dates of arrival of monsoon. The Standard Deviation of the re-determined normal dates ranges between 7 to 14 days with larger values over the northwest and west central parts and interior peninsular India.