R. Vijayakumar

Study of surface ozone behaviour at urban and forested sites in India

Abstract Surface ozone concentrations were measured continuously at Pune for a period of one year during 1991–1992 and for a period of 10 days in January 1992 at Upper Kargudi and in April 1992 at Bandipur, core zones of the Nilgiri Biosphere Reserve forests located in Tamil Nadu and Karnataka States, respectively, in south India. There is a marked diurnal variation in the concentration of surface ozone which clearly follows the diurnal variation of surface temperature. The monthly maximum concentration was observed during the summer season (March–May) and minimum during the monsoon season (June–September). The annual average concentration of ozone was 27 ppb and hourly values varied between 2 and 69 ppb at Pune. However, in the forest environment the 10-day average concentration was 15 ppb and hourly values varied between 5 and 31 ppb. The values of surface ozone at Pune and forest environment are below the U.S. EPA standard (113 ppb, maximum 1 h permissible concentration).

Some thermodynamical and microphysical aspects of monsoon clouds

The thermodynamical and microphysical characteristics of monsoon clouds in the Poona, Bombay and Rihand regions were investigated using extensive aircraft in-cloud observations. The number of clouds sampled at Poona, Bombay and Rihand is 2199, 169 and 104 respectively.The temperatures inside the cloud are colder than its environment at Poona and Rihand. The maximum difference is about 3°C at the cloud base level and the difference decreased with height. At Bombay the difference is less than 1°C and at some levels the temperatures inside the cloud are warmer than its environment.The lapse rates of temperatures inside the cloud are slightly less than those in the immediate environment of the cloud. The environmental lapse rates are nearly equal to the saturated adiabatic value.The positive increments in liquid water content (LWC) are associated with the increments in temperature inside the cloud. Similarly positive increments in temperatures inside the cloud are associated with the increments in temperature of its immediate environment at the same level or the layer immediately above.The maximum cloud lengths observed at Poona and Bombay respectively are 14 and 3 km. The horizontal cross-section of LWC showed a maximum number of 13 peaks in clouds at Poona while only 7 peaks were observed at Bombay. The location of maximum LWC in the horizontal cross-section is more or less at the centre of the cloud. The LWC profile showed an increase with height from the base of the cloud at Poona and Bombay. There is no marked variation of LWC with height at Rihand.The total droplet concentration at different altitudes at Poona and Bombay is in the range 28–82 cm−3. The size distribution of cloud droplets experienced a broadening effect with increase in height from the cloud base at Poona. The broadening effect at Bombay is not as marked as that at Poona.

Electrical and microphysical measurements in warm cumulus clouds before and after seeding

Abstract Electrical and microphysical measurements were made in warm maritime and inland cumulus clouds, before and after seeding, by repeated aircraft penetrations at a single level, a few hundred meters above cloud base. Such measurements were also made in non-seeded clouds by single penetrations during transient flights. The electric field initially was negative in the maritime clouds which developed rain. In the cloud case which dissipated without rain it was initially positive. The field showed sign reversal with time, occasionally preceded by intensification, in all maritime clouds. The field initially was positive in inland clouds. It showed no time variation except in one cloud case where both positive and negative fields were recorded during the period of heavy rain. The droplet charge, droplet median volume diameter and liquid water content showed no marked time variation in either maritime or inland clouds. However, in the cloud case which developed heavy rain marked increases in droplet median...

The Variability of the Interannual Oscillations of the Indian Summer Monsoon Rainfall

A new method of analysis namely, Singular Spectrum Analysis (SSA) is applied to the Indian Summer Monsoon (June-September) Rainfall (ISMR) series. The method is efficient in extracting the statistically significant oscillations with periods 2.8 and 2.3 year from the white noise of the ISMR series. The study shows that 2.8 / 2.3 year cycle captures the variability of the ISMR related to Southern Oscillation / Quasi Biennial Oscillation. The temporal structure of these oscillations show that these are in phase in extreme (excess and drought) monsoon conditions as well as in El Nino Southern Oscillation (ENSO) years. Both these oscillations show minimum variability during the period 1920–1940 and there is an increasing trend in the variability of these oscillations in the recent decades. The study enables to obtain pure signal consisting of reconstructed time series using these two oscillations, from the original white noise series.

Some physical aspects of summer monsoon clouds-comparison of cloud model results with observations

The physical characteristics of the summer monsoon clouds were investigated. The results of a simple cloud model were compared with the aircraft cloud physical observations collected during the summer monsoon seasons of 1973, 1974, 1976 and 1981 in the Deccan Plateau region.The model predicted profiles of cloud liquid water content (LWC) are in agreement with the observed profiles. There is reasonable agreement between the model predicted cloud vertical thickness and observed rainfall.The observed cloud-drop spectra were found to be narrow and the concentration of drops with diameter > 20μn is either low or absent on many occasions. In such clouds the rain-formation cannot take place under natural atmospheric conditions due to the absence of collision-coalescence process. A comparison of the model predicted and observed rainfall suggested that the precipitation efficiency in cumulus clouds of small vertical thickness could be as low as 20 per cent.The clouds forming in the Deccan Plateau region during the summer monsoon are, by and large, cumulus and strato-cumulus type. The vertical thickness of the cumulus clouds is in the range of 1.0-2.0 km. The LWC is found to be more in the region between 1.6-1.9 km A. S. L., which corresponds to the level at almost 3 / 4 th of the total vertical thickness of the cloud and thereafter the LWC sharply decreased. Nearly 98 percent of the tops of the low clouds in the region are below freezing level and the most frequent range of occurrence of these cloud—tops is in the range of 2.0-3.0 km A. S. L. The dominant physical mechanism of rain-formation in these summer monsoon clouds is the collision-coalescence process.

Airborne electrical and microphysical measurements in clouds in maritime and urban environments

Abstract A study by Khemani and Ramana Murty (1973) has indicated significant increases of rainfall downwind of the urban industrial complex at Bombay during the period of increased industrialisation. In order to understand the physical processes responsible for the observed increases in rainfall, aircraft measurements of cloud electrical and microphysical parameters and of free-air temperatures were made in the maritime (upwind) and urban (downwind) regions at Bombay. The study has pointed out differences in the electrical and microphysical state of clouds in maritime and urban environments. In the maritime clouds the electric field was both positive and negative and the cloud droplet charges were positive. In urban clouds the electric field was negative and the droplet charge was also negative. The cloud condensation nuclei and the total measured droplet concentrations were higher in the urban environment. In urban clouds the droplet spectra showed a multimodal distribution and in maritime clouds it was unimodal. Also, the concentration of large droplets of diameter 50 μm and above, and the integrated cloud droplet liquid water content were more in the urban environment. The tail of the droplet spectra extended to higher sizes in urban clouds. The free air temperature in the urban environment was higher by about 1°C.

Measurement of Atmospheric Total Ozone by the Filter Photometric Method

The total ozone content in the atmosphere was determined from the multichannel photometer observations of direct solar radiation made in the urban environment at Pune (18° 32′ N, 73° 51′E, 559 m ASL) and Sinhagad hill station (18° 22′N, 73° 45′E, 1305 m ASL) during March 1980-February 1982. The total ozone content of the atmosphere was computed making use of the differential absorption of solar radiation due to ozone at 0.4 and 0.6 μm wavelengths in the Chappuis band. The values of the ozone data obtained from the photometer observations at Pune and Sinhagad were compared with the corresponding ozone data obtained from the Dobson spectrophotometer located at Pune. Values of ozone obtained by the photometric method were found to be smaller by 8–18% than the Dobson values when Vigrouxs absorption coefficients were used. Similarly, when the absorption coefficients of Inn and Tanaka (1953) were used, the ozone values obtained by the photometric method were smaller by 4–14% than the Dobson values. The ozone values at the hill station obtained from the photometric method were in better agreement (5%) with the Dobson values.

Variations in the atmospheric electric field and Meteorological parameters at a Tropical coastal urban station during 1936–40 and 1962–66

SummaryThe seasonal and diurnal variations in the vertical component of the atmospheric electric field, air temperature, relative humidity and horizontal wind speed were studied using the surface data for the two periods (1936–40) and (1962–66) recorded at the tropical urban station, Colaba, Bombay (18°51′N, 72°49′E, 11 m ASL), located on the west coast of India.The atmospheric electric field during the latter period (1962–66) is significantly higher (up to 42.3%) than the earlier period (1936–40). This has been attributed to the enhanced particulate concentrations in the atmosphere. The increase noticed in the atmospheric electric field is a maximum during winter and minimum during the monsoon. The atmospheric electric field exhibited a marked semi-diurnal oscillation with peaks at 0900 LST and 2200 LST during winter, premonsoon and post-monsoon seasons of both the periods. During the monsoon season the double oscillation is not marked.The variations noticed in the surface air temperature and the relative humidity are in agreement with those observed in the atmospheric electric field. The horizontal wind speed showed a decrease which has been attributed to the surface roughness resulting from urbanization.

Electrical, microphysical and dynamical observations in summer monsoon clouds

Abstract Electrical, microphysical and dynamical conditions in ground-based clouds were studied using the observations carried out at Mahabaleshwar (17°56′N, 73°40′E; 1382 m ASL), a hill station, during the summer monsoon season of 1977. There is a significant correlation between the rain intensity and the corresponding atmospheric electric potential gradient. The diurnal curves of rainfall and negative electric potential gradient exhibited two peaks. The peak in rainfall during the morning hours and the afternoon peak in electric potential gradient were statistically significant. The early morning peak in rainfall was attributed to the enhanced convergence caused by the radiational imbalance in the cloud and cloud-free regions during active monsoon conditions. The afternoon peak in the electric potential gradient was attributed to the cloud formation due to convection. The atmospheric electric potential gradient showed sign reversal from its normal fair weather positive to negative at the time of the onset of rain. The reversal of the electrical potential gradient and the incidence of positively charged raindrops at the surface were almost simultaneous. Positive raindrop charges were recorded 1–2 minutes prior to the occurrence of negative electric potential gradient showing a steep increase during heavy rain spells. The raindrop charges were predominantly positive. The possible physical mechanisms for electrification of monsoon clouds have been discussed. The cloud condensation nuclei and surface temperature also showed peaks during the morning hours. The microphysical observations suggest that the rain formation takes place in monsoon clouds both by the collision-coalescence and ice crystal processes. A simple 1-D model was used to compute different cloud physical parameters. For a cloud with its top at 9 km the precipitation efficiency is in the range 60–70% of the adiabatic liquid water content. It has an average rainfall rate of 15 mm hr −1 , vertical velocity of 3–5 m s −1 and average life time of 30 minutes.

Retrieval of aerosol size distribution from extinction measurements and verification with observations at a tropical station

Abstract Aerosol size distributions were retrieved by computing aerosol extinction parameters using extensive measurements of direct solar radiation made in the 0.4 and 0.6 μm wavelengths at Pune with Volz type sunphotometer during winter (November–February), pre-monsoon (March–May), monsoon (June–August) and post-monsoon (September–October) of 1980–1981. The computer aerosol size distributions are compared with the direct measurements made using Anderson eight-stage cascade impactor. There is agreement between the retrieved and measured size distributions. The retrieval method is simple and useful for intensive aerosol measurement programmes.