B. K. Mukherjee

Quasi-biennial oscillation in stratospheric zonal wind and Indian summer monsoon

Abstract In an earlier study a relationship was pointed out between phases of the quasi-biennial oscillation (QBO) in the lower stratospheric (30 mb) zonal wind and percentage departures of summer monsoon rainfall of India. That study was based on analysis of wind data for Thumba (8°32′N, 76°52′E) and the rainfall data for India for a short-period (1971–76). Wind data for Balboa (9°N, 80°W), which is also an equatorial station, and rainfall activity over India are now examined for a longer period (1951–82). About 15% of the variability in rainfall over India during the summer monsoon is associated with the pattern of the QBO.

High-Level Warmings, Winds and Indian Summer Monsoon

Abstract Temperature and wind data for the troposphere, stratosphere and mesosphere obtained from rocket-sonde/radiosonde/rawin observations made at a tropical station (Thumba, 8°32′15″N, 76°51′48″E) during five summer monsoons (1971–73, 1975–76) with differential monsoon activity were examined. There is agreement between the occurrence of high-level warmings and monsoon activity in four out of five monsoons studied. There were no warmings in the year with very weak monsoon activity. The temperatures of the stratopause and the tropopause were significantly warmer in 1972 when the monsoon was very weak than in other years when the monsoon was active or very active. There is a high positive correlation between the monsoonal activity (precipitation departure from normal over Indian subcontinent) and the 25 km mean zonal wind, and a strong negative correlation with the winds near 16 and 50 km. The change in the sign of correlation coefficient was due to the observed phase change with altitude of the quasi-bie...

High-Level Warmings Over a Tropical Station

Abstract Examination of the available rocketsonde data for the tropical station, Thumba, India, has shown that warmings, less pronounced than in the case of middle and high-latitude stations but of noticeable magnitude, and coolings of similar magnitude occurred in the mesopheric and upper stratospheric levels during the period, December 1970–March 1971. This was the only winter period when observations were made at the station during all of the 4 mo. No prominent change in wind has been observed in association with the warnings, however. The maximum warming observed over a period of 1 week in the upper stratosphere was 26°C at 45 km. The upper mesosphere had been subjected to a continuous process of warming for over 3 weeks in December–January 1971 during which period the temperature rose by 48°C at 70 km. There is no definitive indication that these warmings were of the propagating type. However, they appear to have moved in the vertical at a rate of 3–5 km/day. The temperature behavior of the lower str...

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.

Perturbations in tropical middle atmosphere during winter, 1984–1985

SummaryUsing high altitude rocketsonde data for Thumba (8.5 N, 76.9E) and Balasore (21.5 N, 86.9E) and the stratalert messages for high latitudes for the winter (December–March) 1984–1985, an examination has been made to study the perturbations in the temperature and winds in the tropical middle atmosphere and their linkage with the dynamical events occurring over the high-latitude middle atmosphere during that winter.The results of analysis indicated occurrence of strong cooling in the mesosphere over a period of seven days (5–12 December 1984) and the depth of the cooling layer was 15 km. This incident was followed by a strong warming over a period of seven days (12–19 December 1984) and the depth of the warming layer was 13 km. The major warming event, which occurred over high latitudes during the later part of December and the first week of January, was followed by the cooling in the mesosphere and warming in the stratosphere at Thumba. Also the zonal winds were strong easterly and the meridional winds were northerly in the upper stratosphere and the lower mesosphere over tropics during the same period. Weaker zonal winds/stronger easterly winds were generally noticed to be associated with coolings/warmings over tropics.

Vertical motions within the Indian tropical middle atmosphere.

Abstract Computations of vertical motions in the middle atmosphere over the Indian tropical region have been made based on the thermodynamic equation with the geostrophic approximation. The authors have used the once weekly rocketsonde temperature and wind data for the tropical station Thumba India, (8°32′N, 76°52′E) for the four summers (1972, 1973, 1975 and 1976) and two winters (1971 and 1972) which are also years of varying monsoon activity. In the tropical middle atmosphere, downward motion (subsidence) is the dominant feature when the motion field is considered in a longer time scale. The trend of fluctuations in vertical motion suggests wave structures in the tropical middle atmosphere. The magnitude of the extreme values of the vertical motion in the stratosphere over the high latitudes is larger, by a factor of 2 or more, than those obtained over the low latitudes (tropics). Whereas the magnitude of the vertical motion in high latitudes is associated with stratospheric warmings during winter, the...

High-level warmings and total ozone over a tropical region

The rocketsonde data obtained from the launchings made at Thumba (8°32′15″N, 76°51′48″E) during the winter period 1970–71, as already reported, have indicated that warmings of noticeable magnitude occurred at high levels (upper stratosphere and mesosphere) over this tropical station during the period mentioned. The mean monthly radiosonde temperatures of 50, 100 and 300 mb levels at Thumba (Trivandrum) and Delhi (28°35′N, 77°12′E) during the same period have also pointed out certain anomalies consistent with the warmings referred to above at Thumba. The radiosonde temperatures of the two stations, Thumba (Trivandrum) and Delhi, have now been examined, along with the values of total ozone, for the ten winter periods commencing from 1961–1962. The analysis has pointed out the possibility of high-level warmings also having occurred in the past over the Indian region during the winters of 1963–1964 and 1967–1968, which are also the periods when prominent warmings are definitely known to have occurred at higher latitudes. The behaviour of total ozone has been found to be different in the different years of the warmings. The features noticed have been presented and discussed.

Temperatures and winds over tropical middle atmosphere during two contrasting summer monsoons, 1975 and 1979

Using the monthly geopotential heights and winds for 700 and 200 hPa for India during July and August, and the weekly M-100 Soviet rocketsonde temperature and wind data for Thumba (8.5°N, 76.9°E) during the last week of June and the first week of September for the two contrasting summer monsoon years 1975 (a very strong monsoon year) and 1979 (a very weak monsoon year), a study has been made to examine the mean circulation features of the trophosphere over India, and the structures of the temperatures and the winds of the middle atmosphere over Thumba. The study suggested that the axis of the monsoon trough (AMT) at 700 hPa shifted southward in 1975 and northward towards the foothills of the Himalayas in 1979, from its normal position. Superimposed on the low-pressure area (AMT) at 700 hPa, a well-defined divergence was noticed at 200 hPa over the northern India in 1975.The mean temperatures at 25, 50 and 60 km (middle atmosphere) over Thumba were cooler in 1975 than in 1979. While a cooling trend in 1975 and warming trend in 1979 were observed at 25 and 50 km, a reversed picture was noticed at 60 km. There was a weak easterly/westerly (weak westerly phase) zonal wind in 1975 and a strong easterly zonal wind in 1979. A phase reversal of the zonal wind was observed at 50 km. A tentative physical mechanism was offered, in terms of upward propagation of the two equatorially trapped planetary waves i.e. the Kelvin and the mixed Rossby-gravity waves, to explain the occurrence of the two spells of strong warmings in the mesosphere in 1975.

An Investigation of Heaviest Rainfalls over Coastal Andhra Pradesh of India during October

Abstract An investigation of the dates of occurrence of heaviest rainfall is very important for flood forecasting. We have considered this aspect in the present study by examining the daily rainfall data for four coastal stations Kakinada, Masulipatnam, Nellore and Visakhapatnam for the month of October for a 10-year period 1973–82. The data have been analyzed by considering the heaviest rainfalls for each station separately. We have defined the heaviest rainfall as that which exceeds two or more times the mean rainfall of the month. The study shows that heaviest rainfalls occur during the period 16–23 October. This feature is repeated year after year, constituting a rainfall singularity for the region.

Interannual variability of middle atmosphere and Indian summer monsoon

SummaryThe interannual variability of the mean zonal wind and the mean temperature, obtained from the rocketsonde data for January (symbolizing winter), of the middle atmosphere over the Northern Hemisphere for the 6-year period, 1970–1972 and 1974–1976 has been investigated. The stations considered for the study are Thule, Poker Flat, Fort Churchill, Wallops Island, Cape Kennedy, Fort Sherman, Kwajalein and Ascension Island. The study has shown marked interannual variation in the zonal wind northward of 35–40°N. In the stratosphere (25–45 km), the westerly zonal wind reached its maximum during 1970, 1971 and 1975 near 35–40°N whereas it reached its maximum during 1972, 1974 and 1976 near 60°N, where the polar night jet is located. At stratopause level (50 km), the temperature gradient between equator-middle latitude and the polar region showed interannual variability. The gradient was stronger during 1970, 1971 and 1975 than during 1972, 1974 and 1976.The rainfall activity of the following summer monsoon (June–September) over India during the 6-year period of study has also been examined. The monsoon was weak to normal during 1972, 1974 and 1976 when the maximum westerly flow was strong, stable and located at its normal position (near 60°N), and the equator-middle latitude to pole temperature gradient at stratopause level was weak. The monsoon was normal to strong during 1970, 1971 and 1975 when the maximum westerly flow was weak, diffuse and shifted south-ward (35–40°N) and the equator-middle latitude to pole temperature gradient at stratopause level was strong.ZusammenfassungDie Schwankungen des mittleren zonalen Windes und der mittleren Temperatur wurden von Jahr zu Jahr aus Raketensondendaten für Januar für die Sechsjahres periode 1970–1972 und 1974–1976 untersucht. Dabei wurden deutliche Veränderungen des zonalen Windes nördlich von 35–40° nördl. Breite nachgewiesen. In der Stratosphäre (25–45 km) erreichte der zonale Westwind sein Maximum 1970–1971 und 1975 zwischen 35 und 40° nördl. Breite, dagegen 1972, 1974 und 1976 nahe 60°N, wo der Polarjet auftritt. Im Niveau der Stratopause (50 km) wurde eine Veränderung des Temperaturgradienten zwischen niedrigen und polaren Breiten festgestellt. Der Gradient war 1970, 1971 und 1975 größer als 1972, 1974 und 1976.Parallel dazu wurde der Niederschlag des folgenden Sommermonsuns (Juni bis September) untersucht. 1972, 1974 und 1976 war der Monsun schwach bis normal, 1970, 1971 und 1975 normal bis stark.