Parmjit Singh Sehra’s research while affiliated with City University and other places

Using the upper tropospheric and stratospheric temperatures and total ozone amount data obtained from the TIROS-N satellite over the Indian region during the period May to July 1979 subdivided into six different phases of the monsoon, possible interactions between the 400-1mb atmospheric region and the summer monsoon have been examined in this paper. This investigation shows a noticeable decrease of the stratospheric temperatures and total ozone amount at the onset phase of the southwest monsoon with an increasing trend during its active phase. Variations in the stratospheric temperatures and total ozone amount are found to be consistent with each other parcularly in the 100-70mb layer.

Using M-100 meteorological rocketsonde data collected over Thumba Equatorial Rocket Launching Station (8°32′N, 76°52′E), Trivandrum, India for the period January to May 1972, a sudden mesospheric warming followed by an almost equal cooling has been detected over this region on 1st March 1972. The Thumba equatorial mesospheric temperature showed an increase of about 50 to 60 °C in an altitude region between 60 and 70 km from 23 February to 1 March 1972 which was followed by a cooling of about 50 to 70 °C from 1 to 15 March in that altitude region. Sudden warmings in the stratosphere at high latitudes during winter time are well known. However, sudden mesospheric warnings over equatorial regions have not been observed too frequently and are thus relatively less studied compared to high latitude warnings. The sudden warming observed in the equatorial mesosphere on 1st March 1972 is quite rare and unique event which followed minor midwinter stratospheric warmings at high latitudes in the Northern Hemisphere, although the equatorial stratosphere remained practically undisturbed in its thermal structure during this period.

An investigation of the atmospheric wind structure over Antarctica from surface upto about 80km is made using data from about 52 successful M-100 meteorological rocket soundings at Molodezhnaya, Antarctica where the author worked during 1971-73. The Antarctic summer was found to be characterised by light easterly winds which increased in strength with altitude. The South Polar winter was marked by strong westerly winds which had jet speeds of about 90m/s. The winter westerly jet increased in strength as the season progressed. The autumn reversal of winds from easterly to westerly occurred in the first week of February in the stratosphere and in February end in the troposphere. The vernal wind reversal from westerlies to easterlies occurred in the 3rd week of November in the stratosphere. It was found that the reversal first occurred in the upper layers and subsequently in the lower layers showing a downward propagation of the disturbance. The summer to winter shift was a relatively rapid change, while the winter to summer shift was slow and gradual. The winter and early spring period was marked by large wind perturbations. The easterly flow in the upper mesosphere was found to be predominant for about 8-9 months.

Seasonal wind variations over Antarctica in an altitude region from surface up to about 80 km are studied using data from 52 successful M-100 meteorological rocket soundings at Molodezhnaya, Antarctica where the author worked during 1971-73. It is found that the South Polar summer was characterised by light easterly winds increasing in strength with altitude and the winter exhibited strong westerly winds halving jet speeds of about 90 m/s increasing in strength as the season progressed, while the autumn and the spring were the wind reversal periods. The Antarctic winter regime was the most active period with a wind persistence of about 95% marked by strong westerly flow. Departures of the actual zonal winds from the Groves Model were found to be quite significant showing hemispheric asymmerties in the Northern and the Southern Hemispheres. The physical properties of the immense southern oceans play an important role in these differences.

Upper-atmospheric wind and thermal structures in Antarctica have been discussed earlier. The present study is an extension of that work and gives a complete picture of the atmospheric temperature exploration from the surface up to an altitude of about 80 km, with particular emphasis on seasonal temperature variations, by means of meteorological rocket sonde data from 52 successful M-100 rocket launchings carried out at the Soviet antarctic station Molodezhnaya in 1972.

Average vertical profiles of zonal and meridional components of winds, and of temperatures as obtained from M-100 meteorological rocket soundings conducted at Molodezhnaya, Antarctica in January-February 1972 (southern summer) are compared with those of the corresponding ascents from Thumba, South India. The actual results are also compared with the Groves atmospheric model and the corresponding departures of the actuals from the Model are worked out. It is found that in the southern summer the polar tropopause and stratopause were about 27°C and 13°C warmer than the corresponding equatorial tropopause and stratopause, while the mesopause was about 25°C colder. At both the stations the zonal winds in the stratosphere were predominantly easterly in January with speed less than 50ms-1, and westerly in February with speed less than 35ms-1, while the meridional winds were variable. Zonal wind departures of the actuals from the Groves model were found to be in a range of about ±35ms-1, while the temperature departures were mostly negative by about 25°C.

THE mesospheric wind results derived for atmospheric altitudes between 50 and 90 km from meteorological rocket flights carried out at Molodezhnaya station (67° 40' S, 45° 51' E) in Antarctica have been discussed in ref, 1. The rapid shifts in both zonal and meridional components of the winds during May to July indicated a sudden `explosive' change in temperature distribution in the upper mesosphere over Antarctica in the winter regime. I have investigated this phenomenon and extended the study down to the stratosphere; the results are discussed here. The meridional and the zonal temperature gradients at altitudes of 65, 70 and 75 km, derived from the thermal wind equations using the corresponding upper wind and temperature results, have also been studied. Atmospheric temperatures up to an altitude of about 80 km were also measured and the accidental root mean square errors in the determinations were as follows: in the altitude region 60 to 80 km it does not exceed 7 to 10 °C at 50km it is 5 °C, and below 40km the error is less than 3 °C.

UNDER a joint Indo-Soviet agreement I was the first Indian scientist to winter in Antarctica during the 17th Soviet Antarctic Expedition, 1971-73. In particular, I participated in the meteorological rocket soundings of the upper atmosphere carried out at Molodezhnaya, a station located at 67° 40'S, 45° 51'E at a height of 42 m above mean sealevel in Enderby Land, East Antarctica (Fig. 1).