Annual Report 1999-2000

Autonomous Institutions

Indian Institute of Tropical Meteorology, Pune

The Indian Institute of Tropical Meteorology functions as a National Centre for basic and applied research in Tropical Meteorology. Its primary functions are to promote, guide and conduct research in the field of Meteorology and Atmospheric Sciences in all its aspects with special reference to the tropics and sub-tropics. The Institute organises manpower development programmes by arranging specialised training programmes in different areas of atmospheric sciences. The Institute also collaborates in the national and international collaborative research programmes, and also arranges field experiments of its own and through participation in the multi-agency missions.

The highlights of the progress made during the year are summarised below:

i) Technology Development Projects

Research models for the long range prediction of Indian monsoon rainfall for the year 1999 based on different statistical techniques and by using different Global Circulation Models have been developed and seasonal forecasts for the monsoon have been communicated to the India Meteorological Department. All the models had forecast the monsoon rainfall for 1999 to be on the positive side of the normal.

A new method named extrapography, which involves time series modeling and one step ahead extrapolation, has been developed for the prediction of winter, summer, June, July+August, September and Post-monsoon rainfalls in sequential manner over relatively plain and contiguous India.

Under the Indo-Japanese collaborative project on "Dendroclimatological reconstruction and estimation of global environmental changes in monsoon Asia", a study of climatic response of tree ring density variation in cedrus deodera over the western Himalayas was carried out, which revealed the importance of early wood density into tree-ring and climatic relationships, which is quite different from the results of similar studies in other region. Under this project field programmes were also organized in the tropical forest sites of western and eastern Maharashtra State and several samples of tectona gradis (teak) were collected.

Numerical simulation of tropical cyclone intensity and structure using limited area model (LAM) was carried out and its sensitivity to sea surface temperature, pressure radiation and wind field was studied. It was found that with lower SST, growth rate of cyclone was slow in terms of drop in surface pressure and the maximum intensity reached was also less. In case of no radiation, the time evolution of surface-pressure showed delayed intensification but maximum intensity reached was same as in the case with radiation.

The relationship between the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO) (1881-1998) was investigated using 108 years of data. The analysis showed that the April NAO is a useful parameter in the long-range prediction of rainfall for the 3 homogeneous regions (viz. South Peninsular India, NW India, and West Central India).

Response function analysis based on monthly and seasonal climate parameters (temperature and precipitation) of the region and various density parameters of tree-ring indicated the significant relationship between pre-monsoon (March-April-May) summer climate and earlywood density parameters as well as total ring width. Based on the relationship derived from the response function analysis, attempts were made to reconstruct the pre-monsoon climate of the region.

A generalized method for estimation of Arial Probable Maximum Precipitation (PMP) for plain areas of Maharashtra State has been developed by considering the maximum average raindepths for different size areas and durations from major rainstorms over plain areas of Maharashtra and its neighbourhood were considered using 105 years rainfall (1891-1995).

By developing a generalised physical approach of estimating the Arial probable maximum precipitation (PMP) the generalized PMP estimates for 1000, 5000 and 10,000 KM2 areas at different locations of the non-orographic regions of Godavari basin were obtained and accordingly generalized charts were prepared. These maps of PMP for different size areas and duration would be very useful for estimating design storm of PMP magnitude for sub catchments falling in the range of 100 to 10,000 km2 size.

The wind tunnel experiments conducted to study the evaporation of charged and uncharged water drops showed that the effect of ventilation is stronger on the evaporation of uncharged drops than that of charged drops. Further, the calculations showed that in order to reach the earth’s surface with the same size, the charged drop at the cloud base is required to be smaller in size as compared to the uncharged one.

ii) Basic Research Projects

Several global and regional predictors have been identified and regression equations have been developed for seasonal forecasting of all India monsoon rainfall. Studies have also been carried out to examine the relationship between the monsoon rainfall activity and meteorological factors such as El Nino, La Nina, ENSO, NAO, NPO, temperature and pressure anomalies, planetary waves etc.

The problem of phase locking related to monsoon circulation by examining the triad energy exchanges among the intraseasonal oscillation was investigated over global tropics (20° S-30° N) at 850 hPa and 200 hPa for June, July and August 1988. The results reveal that when the phases of the three interacting low frequency oscillations (LFO) are close to each other and the amplitude of two adjacent ones are much larger as compared to the third, then a rapid exchange of energy transfer can occur. The role of barotropic process was found to be important through nonlinear triad interactions related to phase locking for maintenance of LFOs during summer monsoon.

Dependence of the amount of seasonal rainfall on that of the monthly rainfall was studied. For this study, the long homogeneous series of seasonal and monthly scale rainfall data for 124 years (1871-1994) was analysed. The analysis indicated a rare possibility of occurrence of the seasonal rainfall to be excess/deficient when the monthly rainfall of any month of summer monsoon period (June to September) is deficient/excess.

The intraseasonal variations of southwest monsoon rainfall activity over India for 1999 were studied using weekly mean wind patterns for the troposphere. The observed wind data at radiosonde stations over India for five standard levels - 850, 700, 500, 300 and 200 hPa for the period June - September were used in this analysis. The relative vorticity ( z ) and divergence (d) were computed for 19 small triangular areas, each formed by three adjacent stations over the country. The weekly mean circulation patterns showed that cyclonic circulation extended upto mid-tropospheric levels during the active phases when large positive weekly rainfall departures were observed over the sub-divisions of the monsoon trough area. The variations in the magnitude of the z - and d-values during such epochs were found to be related with the weekly rainfall variability over India on sub-divisional scale.

Oceanic response to a symmetric idealised moving cyclone was studied for three observed tracks in the Arabian Sea. The right bias in the model currents is seen in all the cases, whereas the right bias in the upper layer thickness deviation field is noticed only for the cases having northward component in the track of the cyclone.

iii) Technologies Transferred/ Commercialized

Scientists of the Institute participated in the cruise of Sagar Kanya under the INDOEX IFP-99 programme during 20 January to 12 March 1999 and collected observational data of concentration and size distribution of atmospheric aerosols, Aitken nuclei, atmospheric electric field and conductivity over the marine environment of Indian Ocean and Arabian Sea regions. One scientist of the team also participated in the cruise of the American ship "Ron Brown" from Mauritius to Maldives.

As a part of the BOBMEX-99 (Bay of Bengal Monsoon Experiment) Programme, the Institute participated in the ORV Sagar Kanya Cruise No.147B August 11-31,1999. Extensive observations of atmospheric aerosols, Aitken Nuclei, water vapour, ozone and precipitation chemistry over the marine environment (Bay of Bengal) using hand held multi-band solar radiometer and Aitken Nuclei counter were collected during the above period. A field phase programme to study the air-sea interface coupling for understanding variations in north-south oscillations ITCZ over Bay of Bengal during different phases of monsoon was also conducted during this cruise.

The Institute also participated in the ORV Sagar Kanya Cruise No.148 during 4 September to 10 October 1999 for collection of meteorological data over the sea surface. A 3-meter horizontal boom was fabricated and installed on board ORV Sagar Kanya. Radiation, wind speed (slow and fast sampling), temperature and humidity sensors were also mounted on the boom. Observations were taken in the post-southwest monsoon 1999.

An automatic visible spectrometer developed at the Institute has been installed at the University of Mauritius, Reduit, Mauritius on 16 September 1998 for the monitoring of the distributions of NO2 and O3 in the atmosphere as a part of the Indian Ocean Experiment (INDOEX)-India Programme. The spectrometer has been developed indigenously based on the Czerny-Turner type spectrometer.

iv) Technology Demonstration Projects undertaken in the area of Socio-economic Development

A Field Observational Programme in collaboration with G.B.Pant Institute of Himalayan Environment and Development , Shamshi, H.P. was organised during 17 May-4 June 1999. Special observations of atmospheric aerosols and trace gases were carried out at Kullu and Kothi.

v) Research Highlights

The Institute has made significant contributions in the challenging areas of Atmospheric Sciences like Weather Forecasting, Climatology, Hydrometeorology, Monsoon Studies, Climate Modeling, Weather Modification, Cloud Physics, Atmospheric Chemistry, Atmospheric Electricity and studies relating to Land-Surface processes. The important highlights are summarised below:

NWP Modeling and Model Diagnostics

Numerical experiments were performed to see the sensitivity of the cyclone tracks to the merging of only mass fields (pressure and temperature) and wind field of the artificial vortex. Results showed that merging of wind improved the cyclone track.

The response of surface sensible heat flux to total cloud cover and daily rainfall was studied using MONTBLEX (Monsoon Trough Boundary Layer Experiment) observations of 1989 and 1990 at two stations, Jodhpur in the western end sector of monsoon trough and Kharagpur in the eastern sector. The transition hours just after sunrise and around sunset have shown quick response in altering or modifying the surface fluxes. A general inverse relationship between total cloud cover and surface-sensible heat-flux at these hours is well established. The surface sensible heat fluxes respond to soil wetness at these locations in association with rainfall due to low-pressure systems.

Objective Analysis including satellite input for NWP Models

Intercomparison of the algorithms of geophysical parameters developed by various researchers in the recent years was made using satellite microwave radiometry data obtained from DMSP-SSM/I and radiosonde data obtained from Minicoy and Port Blair island stations, over the Indian seas. After comparing rms errors, correlation coefficients and biases, the algorithms of Petty, 1993a (for surface wind speed), Petty, 1993b (for integrated water vapour), Weng and Grody, 1994 (for integrated cloud liquid water) and Smith et al. 1996 (for precipitation rates) were found to be most accurate for the retrieval of geophysical parameters. These algorithms were further assessed for different weather phenomena during monsoon periods. It is inferred from the results of the study that the estimated geophysical parameters obtained from DMSP-SSM/I data are of potential use for better depiction of monsoon systems over the Indian seas.

Extended Range Prediction

Multiple regression equations have been developed for the seasonal prediction of monsoon rainfall (% departure) over three smaller homogeneous regions of India (viz. south Peninsular India, northwest India and west central India) and India as a whole. For this purpose, 30 years data (1961-1990) were used. The parameters used are North Atlantic Oscillation index in the month of April; De-Bilt temperature anomaly in the month of January and Northern Hemisphere surface air temperature anomaly during winter (January and February).

The analysis of the NE monsoon rainfall (October -December) over Tamil Nadu and North Atlantic Oscillation (NAO) and El Nino-Southern-Oscillation (ENSO) is carried out using 100 years of data (1881-1980) on monthly to climatological scale. The analysis revealed that the relationship between NAO in April and SOI in May, with NE monsoon rainfall is statistically significant at 1- percent level. The study is useful for the long range forecasting of the north east monsoon rainfall.

The comparative study of the reconstructed winter NAO indices from the tree ring analysis and the actual indices was carried out using statistical and spectral analysis techniques. The actual indices were prepared by taking the pressure gradient between the mean sea level central pressure of the Icelandic Low and Azores High. Both the series were found to be highly correlated and the relationship is significant at 1 per cent level. The spectral analysis revealed that Quasi-Biennial Oscillations (QBO) signal is present in both the series of indices.

Monsoon Studies and Forecasting

Daily pre-monsoon thunderstorm activities for selected stations of south peninsular India were analysed for the months March-June. A characteristic peak in the cumulative thunderstorm frequencies for the stations was seen to occur about 6 weeks prior to the onset of monsoon almost every year. Such a peak may be followed by subsequent peak(peaks) in the activities. Analysis of satellite derived OLR data over the region also showed a characteristic fall below a threshold value, which coincides with such peaks. A regression equation was fitted to predict onset of summer monsoon over Kerala (using the information of the characteristic peak) well in advance (about 6 weeks). The relationship is significant at 99% level.

Climate Variability Studies

Densitometric analysis was carried out for several tree-ring cores of Cedrus deodara from two different sites of Western Himalayas namely Kufri and Kanasar. Chronolies of various parameters viz. earlywood, latewood, minimum, maximum and mean densities and total ring width were prepared. Most of the parameters except latewood and maximum density show moderately high values of common variance. Signal to noise ratios of the series were also large compared to those of maximum and latewood density chronologies, suggesting usefulness of density parameters of the species in dendroclimatic studies. Response function analysis based on monthly and seasonal climate parameters (temperature and precipitation) of the region and various density parameters of tree-ring indicated the significant relationship between pre-monsoon (March-April-May) summer climate and earlywood density parameters as well as total ring width. Based on the relationship derived from response function analysis, attempts were made to reconstruct the pre-monsoon climate of the region.

Dependence of the amount of seasonal rainfall on that of the monthly rainfall was studied. For this study, the long homogeneous series of seasonal and monthly scale rainfall data for 124 years (1871-1994) was analysed. The analysis indicated a rare possibility of occurrence of the seasonal rainfall to be excess/deficient when the monthly rainfall of any month of summer monsoon period (June to September) is deficient/excess.

Long-term analysis of the Total Ozone Mapping Satellite (TOMS) data was carried out for the study of ozone variations. The analysis revealed that there is no depletion in the amount of ozone over equator, though the depletion is continued in the polar region. The monthly mean distribution of the ozone along the latitude circle over equator showed that one maximum located near zero longitude and one minimum near 180° longitude are found common to all the seasons.

Analysis of the 140-year historical record suggested that the inverse relationship between the El-Niño - Southern Oscillation (ENSO) and the Indian summer monsoon (weak monsoon arising from warm ENSO event) has broken down in recent decades. Two possible reasons emerge from the analysis. A southeastward shift in the Walker circulation anomalies associated with ENSO events may lead to a reduced subsidence over the Indian region, thus favouring normal monsoon conditions. Additionally, increased surface temperature over Eurasia in winter and spring, which is a part of the mid latitude continental warming trend, may favour the enhanced land-ocean thermal gradient conducive to a strong monsoon. These observations raise the possibility that the Eurasian warming in recent decades helps sustain the monsoon rainfall at a normal level despite strong ENSO events.

Hydrometeorology

Based on the analysis of 231 severe rainstorms affected Indian region, it has been observed that under favourable synoptic conditions more than one rainstorm can be obtained especially when the movement of a low latitude west-moving disturbance is in phase with east-moving westerly troughs or western disturbances in mid latitudes. The second rainstorm normally occurs in northwest India and if it happens towards the fag end of the monsoon season, severe floods occur in the rivers of the northwest India.

Atmospheric Modeling

The various dynamical processes of low frequency monsoonal transients that result from nonlinear y -c energy conversion due to Coriolis force, vorticity and divergence were examined using the computational model developed earlier. The result revealed that the conversion from the transient rotational motions to divergent motions by wave-wave interaction due to vorticity on the 30-45 day time scale is associated with planetary scale wavenumbers 1 and 2 over tropics at 200 hPa.

Ocean Modeling

Mixed layer response of ocean to Arabian Sea cyclone, using IRG (IITM Reduced Gravity) model was studied considering three observed tracks in the Arabian Sea. The right bias in the model current was present in all the three cases but the right bias in the upper level thickness deviation (ULTD) field was present only for the case when the track was having northward component. Reduction in the time step from 30 min. to 15 min., in the model integration, does not show any change in the currents and ULTD fields.

The upper ocean response to moving idealised cyclone in all the possible directions from a fixed initial position was also examined. The right bias in the model currents and the lag between the storm centre and the maximum current position for northeast and northwest tracks were 1.5 times greater than that for the due north, east and west tracks. Further, it was noticed that reduction in the upper layer thickness enhances by about 3-4 meter for the tracks along east/west direction. It was also seen from the sensitivity experiments that as the initial position of the westward moving cyclone is away from the equator, the storm centre is found to be closer to the upwelled region.

Climate and Global Modeling Studies

The Artificial Neural Network (ANN) technique with error-back-propagation algorithm was used to provide prediction (hindcast) of ISMR on monthly and seasonal scales. The ANN technique was employed on the 5 time-series of June, July, August, September and seasonal (JJAS) rainfall from 1871 to 1994. Previous 5 years values from all the 5 time-series were used to predict for the next year. The performance of the models was examined and it was found that they could be used as a forecasting tool not only at seasonal scale but also on monthly scales. With the passage of time the relationship between various predictors and Indian monsoon are changing, leading to change in unpredictability of monsoon. This issue was examined using neural networks and rainfall time series from 1871 to 1994 and it was found that the monsoon system inherently has a decadal scale predictability variation.

Atmospheric Electricity

Using monthly data on number of thunderstorm days and mean sea surface temperature (SST) for a period of 11 years (1970-1980) over one island station, each in the Bay of Bengal and Arabian Sea, and for several stations along the east and west coasts of Indian Peninsula, the monthly mean variation of thunderstorm days was examined and compared with the SST. The comparison revealed that both the parameters show clear signals of their semi-annual variations, which are in phase with each other. Sensitivity of occurrence of thunderstorms to SST variation was examined which gives an important result towards the response of tropical convection to modest changes in the surface thermodynamics over the oceans. The analysis showed that the occurrence of the thunderstorms over the Bay of Bengal for 1° C change in SST was twice more than that over the Arabian Sea. SST analysis showed that the higher frequency of thunderstorms over the Bay of Bengal may be attributed to the warmer (0.4 to 1.2° C) temperature conditions and partly due to higher concentrations of Aitken Nuclei than over the Arabian Sea. The analysis also showed that the peak warming period of the SST in the month of April usually precedes the occurrence of the ensuing monsoon by about 30 days.

Boundary Layer Studies

A study was carried out to simulate the boundary layer parameters and their interaction with soil hydrology using a numerical one-dimensional model coupled with a two-layer model of soil hydrology and thermodynamics. This model approximates the growth of the daytime mixed layer and considers the surface heat flux during the transition between the stable and unstable atmospheric conditions.

Atmospheric Chemistry

Special field observations of aerosols, Aitken Nuclei and trace gases have been carried out at Kullu and Manali regions of the Himalayan Ecosystem. The results of the analysis of observations indicated that (i) the average concentration of TSP (165 m g/m3) at Kullu was found to be higher than that observed at the same site in 1996, (ii) the mass size distribution of aerosols showed bimodal with dominant coarse mode, (iii) chemical composition of aerosols showed higher SO4 concentrations at Manali as compared to that at Kullu and (iv) the average concentration of Aitken Nuclei at Kullu was found to be 5818 cm-3.

Twenty-one rainwater samples were collected at the Institute’s Rain and Cloud Physics Research (RCPR) Centre, New Delhi during the monsoon period of 1998. pH value of these samples were measured and found in the range of 5.68 to 8.94. A total of 64 rainwater samples were collected at Kanpur from three different locations during monsoon period of 1998. pH values of these samples were found in the range of 6.11 to 7.68.

To examine the effect of global change induced perturbations in several alkaline neutral compounds and consequently on the distribution of ionization for the troposphere, a first steady state ion composition model was developed indigenously and tested at the Power Challenge-SG-Computer at the Institute. It is envisaged to use this model for studying various problems related to tropospheric phenomena and global electric circuit.

Lidar Probing of the Atmosphere

The Argon-Ion Lidar data archived at the Institute for the period October 1986 - September 1998 were analysed for studying the cloud macro-physical characteristics such as cloud-base and-ceiling heights, as well as multi-layer structures over Pune. The polarization lidar data collected during 1997 - 1998 were also analysed to investigate the scattering / composition properties of aerosol characteristics with regard to the laser light polarization. The results indicated that the lidar has the capability to capture the multi-layer cloud structures interface between cloud condensation nuclei in the sub-cloud layer and in the vicinity of cloud-base and anisotropic of aerosol scattering. The results further suggested, (i) lower cloud-base heights (varying between 500 m and 1500 m) during south-west monsoon season as compared to rest of the year, (ii) fine-scale structures in the depolarization ratio indicating space-time variations in polarization properties of aerosol scatter, and (iii) more anisotropy in aerosol scatter in the lower atmosphere and its decay with increasing altitude.

Spectroscopic Measurements of Atmospheric Minor Constituents

Spectrometer data collected at Pune during July-September 1999 were analysed and total column densities of NO2 and O3 are derived. Similarly spectrometer data collected at Iceland station Reykjavik (64° N, 22.6° W) for the period December 1993 - March 1994 and December 1994 - March 1995 were analysed. Slant column densities of NO2 and O3 derived for SZA 87° - 93° were utilised to retrieve the vertical profile over Iceland. The spectrometer data collected during the period January-July 1999 at Mauritius was also analysed for total column densities of NO2. The annual variation of NO2 over Mauritius when compared with annual variation over Pune showed an anti-phase relationship between these two stations.

Instrumentation for Cloud Physics and Weather Modification Studies

The data obtained on atmospheric electric field and conductivity obtained during the XVI Indian Scientific Expedition to Antarctica were analysed to study the universal diurnal variation of atmospheric electric field. Diurnal variation of electric field does not follow the universal diurnal variation and shows a maximum at 1300 UT and a secondary maximum at 1900 UT. The results were compared with the observations made earlier in the Indian Ocean region and in the Antarctic region.

Russian-Indian Working Group

Under the Russian-Indian Sub-Working Group on Meteorology a delegation of two Russian Scientists viz., (I) Dr.Valerij N.Dyaduchenko, State Secretary and Deputy Head, Russian Federal Service for Hydrometeorology and Environmental Monitoring, Moscow and (ii) Prof.Albert A.Chernikov, Director, Central Aerological Observatory, Dolgoprudny, Moscow visited the Institute on 2 December 1999. The scientists had discussions with the Director and scientists of the Institute.

Fourth Meeting of this Working Group was held at New Delhi during December 3-7, 1999. The Working Group agreed to implement the co-operation in the five major areas viz., (i) Studies of Monsoon, (ii) Study of Global and Regional Climate Changes and their Impact on Environment and Economy, (iii) Study of the Atmospheric Ozone, (iv) Automated Computation of Vertical Pressure Profiles using Radar Data and Studies in Weather Modification and Cloud Physics. The Institute has been identified as one of the Indian collaborators in two of these areas

IITM-CSU Collaborative Project

Indo-US (DST-NSF) collaborative project entitled, "Studies of Mesoscale (Beta and Gamma) Weather Systems over Indian Region using Regional Atmospheric Modeling Systems (RAMS) of Colorado State University (CSU)" has been sanctioned for a period of three years.

Indo-US Collaboration

As a part of the Institute’s on-going INDO-US NSF Research Project ‘Investigations of Atmospheric Chemistry - Aerosols - Climate Interactions’, Dr. V. Ramaswamy of GFDL / NOAA, USA visited the Institute during 11-18 February 1999 and had discussions with the Institute’s scientists involved in the project. The immediate work plan and preparation of data products from the experimental facilities (lidar, radiometers and spectrometer) available with the Institute, to suit to the radiative transfer models of GFDL, were finalised during the discussion.

IITM-SAC Collaborative Programme

Special observations of atmospheric aerosols using the lidar and radiometric techniques were carried out at the IITM, the Mulshi dam sites, and at the Khadakwasla dam site in synchronization with overpasses of ISRO’s IRS-P3 polar orbiting satellite over Pune.

Special observations of atmospheric aerosols, water vapour and ozone were carried out on 25 November 1999 as a part of inter-sensor calibration of IRS satellite series at Chharodi experimental site of Space Applications Centre, Ahmedabad.

IITM-IIA Collaboration Experiments

As a part of IITM-IIA (Indian Institute of Astrophysics, Bangalore) collaborative field experiments at Hanle-Leh, Aitken Nuclei observations were carried out at the High Altitude station during 26 September- 7 October 1999. Feasibility studies for future experiments related to aerosols and trace gases at Henle were also carried out.

Collaboration with SHAR, ISRO

Intercomparison of atmospheric electrical parameters from the cylindrical field mill developed at IITM and that available with SHAR was carried out. The results showed good agreement. A proposal was submitted to ISRO for the development of a network of field mill stations at SHAR for thunderstorm warning.

Collaboration with BARC, Mumbai

An intercomparison experiment, in a campaign mode, involving aerosol and radiation instrumental facilities available with the Institute and BARC was conducted at the Institute, during 28 December 1998 - 01 January 1999 for validation of radiative transfer model available with the BARC.

Collaboration with Universities

The Institute has been recognised as Ph.D. Research Centre (Ph.D. Degree of the University) for research on the topics in Environmental Science by the Bharati Vidyapeeth (Deemed University), Pune.

A memorandum of understanding between the University of Pune and IITM was signed on 26 May 1999 for collaboration in teaching and research in Atmospheric Physics.