Using the objectively analyzed FGGE II-b upper-air data, the large-scale circulation, heat sources and moisture sinks over the Tibetan Plateau and surrounding areas are examined for a 9-month period from December 1978 to August 1979. In addition to the FGGE data, special soundings obtained during the Chinese Qinghai-Xizang (Tibet) Plateau Meteorological Experiment (QXPMEX) from May to August 1979 are also used in the objective analyses. The evolution of the large-scale flow patterns, temperature, outgoing longwave radiation (OLR) and vertical circulation is described in order to identify the distinct seasonal changes from winter to summer that lead to the onset of the Asian summer monsoon. The Tibetan Plateau maintains a large-scale thermally driven vertical circulation which is originally separated from the planetary-scale monsoon system. The rising motion exists only on the western Plateau in winter and then spreads to the whole Plateau as the season progresses. The monsoon onset over Asia is an interaction process between the Plateau-induced circulation and the circulation associated with the principal rainbelt migrating northward. During winter the Plateau is a heat sink, but it is surrounded by regions of more intense cooling. In spring the Plateau becomes a heat source, but the cooling in the surrounding areas continues. The sensible heat flux from the surface provides the major source of heating on the Plateau. However, additional contribution from condensation heating is observed in the western Plateau during all seasons and, more significantly, in the eastern Plateau during summer. The sensible heating of the elevated Plateau surface and the radiative cooling in the environment maintain the horizontal temperature contrast that drives the thermally direct vertical circulation. The detailed examination of the warming process of the upper troposphere during two transition periods, i, e., the onset of the Southeast Asian monsoon in May and that of the Indian monsoon in June, reveals that the temperature increase over the eastern Plateau during the first onset was mainly the result of diabatic heating, whereas that over the Iran-Afghanistan-western Plateau region leading to the second onset was caused by intense subsidence. There are large diurnal variations in the boundary layer and vertical circulation over the Plateau. As a result of diurnal heating of the surface, a deep mixed layer of nearly uniform potential temperature exists over the Plateau in the evening (1200 UTC), suggesting the role of thermal convection in the upward transport of heat. However, moisture is not well mixed vertically and there is a large horizontal temperature gradient in the boundary layer. From late spring to summer the boundary layer becomes more stable for dry convection. On the other hand, the vertical distributions of equivalent potential temperature in late spring and afterwards show a conditionally unstable stratification for moist convection with the increase of moisture of surface air.

https://www.jstage.jst.go.jp/article/jmsj1965/70/1B/70_1B_319/_pdf

Seasonal Heating of the Tibetan Plateau and Its Effects on the Evolution of the Asian Summer Monsoon

The long-term mean date of the monsoon onset over Kerala (MOK) varies between 30 May and 2 June according to different estimates, with a standard deviation of 8–9 days. The earliest date of MOK, and the most delayed one, during the last 100 years differ by 46 days (7 May and 22 June, respectively). MOK switches on a spatially large and intense convective heat source over south Asia, lasting from June to September, whose moisture supply is made available through the cross-equatorial low-level jet stream. Superposed epoch analysis of 10 years of outgoing longwave radiation (OLR) data shows that MOK is a significant stage in the evolution of the OLR field in the tropics of the eastern hemisphere. At the time of MOK there is increased convection in a band about 5–10 degrees wide meridionally, extending from the south Arabian Sea to south China, and convection is suppressed all around, particularly in the western Pacific Ocean. In 1983 when MOK was delayed by about 3 pentads, OLR data showed that the ...

Interannual Variability of the Onset of the Indian Summer Monsoon and Its Association with Atmospheric Features, El Niño, and Sea Surface Temperature Anomalies

Using the National Centers for Environmental Predictions (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis, distributions of the heat source Q1 and moisture sink Q2 between 50°N and 50°S are determined for a 15-yr period from 1980 to 1994. Heating mechanisms operating in various parts of the world are examined by comparing the horizontal distributions of the vertically integrated heat source 〈Q1〉 with those of the vertically integrated moisture sink 〈Q2〉 and outgoing longwave radiation (OLR) flux and by comparing the vertical distributions of Q1 with those of Q2. In northern winter, the major heat sources are located (i) in a broad zone connecting the tropical Indian Ocean, Indonesia, and the South Pacific convergence zone (SPCZ); (ii) over the Congo and Amazon Basins; and (iii) off the east coasts of Asia and North America. In northern summer, the major heat sources are over (i) the Bay of Bengal coast, (ii) the western tropical Pacific, and (iii) Central America. Throughout the ...

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Seasonal and Interannual Variability of Atmospheric Heat Sources and Moisture Sinks as Determined from NCEP–NCAR Reanalysis

On the 40-50 day Oscillations During the 1979 Northern Hemisphere Summer

The concentration of air pollutants in ambient air is governed by the meteorological parameters such as atmospheric wind speed, wind direction, relative humidity, and temperature. This study analyses the influence of temperature and relative humidity on ambient SO2, NOx, RSPM, and SPM concentrations at North Chennai, a coastal city in India, during monsoon, post-monsoon, summer, and pre-monsoon seasons for 2010-11 using regression analysis. The results of the study show that both SO2 and NOx were negatively correlated in summer ( for SO2 and for NOx) and moderately and positively correlated ( for SO2 and for NOx) during post-monsoon season with temperature. RSPM and SPM had positive correlation with temperature in all the seasons except post-monsoon one. These findings indicate that the influence of temperature on gaseous pollutant (SO2 & NOx) is much more effective in summer than other seasons, due to higher temperature range, but in case of particulate, the correlation was found contradictory. The very weak to moderate correlations existing between the temperature and ambient pollutant concentration during all seasons indicate the influence of inconstant thermal variation in the coastal region. Statistically significant negative correlations were found between humidity and particulates (RSPM and SPM) in all the four seasons, but level of correlation was found moderate only during monsoon ( and ) in comparison with other three seasons and no significant correlation was found between humidity and SO2, NOx in all the seasons. It is suggested from this study that the influence of humidity is effective on subsiding particulates in the coastal region.

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Influence of Temperature, Relative Humidity and Seasonal Variability on Ambient Air Quality in a Coastal Urban Area

The influence of heat, moisture and moist static energy (MSE) budget, over the Arabian Sea and adjoining area (0°-30°N and 30°E-75°E), on the onset and activities of Asian summer monsoon has been studied in detail. The data base for this study consists of twice daily FGGE Level IIIb analysis for the period 16 May to 15 July 1979.
The pentad mean variation, the vertical distribution and period averages of the various terms in energy budget equations are closely examined to find out their influence on the activities of monsoon. The study indicates the significant increase in the net enthalpy, latent heat energy (LHE), MSE and a number of budget parameters, well in advance of the onset of monsoon over Kerala coast. Further, a decreasing trend is observed in most of the above parameters about 5 days before the break monsoon condition which started over India on 16 July, 1979.
The vertical distributions of the budget parameters reveal that during active monsoon period secondary maxima of horizontal heat and MSE flux divergences are observed in the upper troposphere which are replaced by minima during weak monsoon circulation.
The broad features of the budget studies over the Arabian Sea are in good agreement with the large scale energetics (Mohanty et al. 1982a, 1982b). Some of the significant departures in the results of the two studies have been discussed.

A study of heat and moisture budget over the Arabian Sea and their role in the onset and maintenance of summer monsoon

Annual variations of mixed-layer characteristics at New Delhi, India have been studied for a weak monsoon (1987) and a strong monsoon (1988) year. In the weak monsoon year (1987), the maximum mixing depthhmax was found to have a value of around 3000 m during the pre-monsoon, less than 2000 m during the summer monsoon, around 2000 m during the post-monsoon, and less than 1000 m in the winter season. For the strong monsoon year (1988),hmax values were less than 1987 values for comparable periods throughout the year. The seasonal and yearly differences ofhmax were explained by the surface energy balance and potential temperature gradient γ at a time close to sunrise. According to the spatial patterns of γ obtained by an objective analysis of the 850 to 700 hPa layers. mixed-layer characteristics obtained at New Delhi are representative of the north and central regions of India.

M. P. Singh

Papers

A study of heat and moisture budget over the Arabian Sea and their role in the onset and maintenance of summer monsoon

Mixed-layer characteristics as related to the monsoon climate of New Delhi, India

Comparative study of line source models for estimating lead levels due to vehicular traffic in Delhi