Identification of a surface layer structure and analysis of humidity data in two weather situations at Jodhpur (26180N, 73040E), India, during MONTBLEX 1990
N Das1, M Bose2 and U K De1;
1Environmental Science Programme, Jadavpur University, Kolkata 700 032, India
2Atmospheric Science Research Group, Physics Department, Jadavpur University, Kolkata 700 032, India
e-mail: deutpal@hotmail.com

The Monsoon Trough Boundary Layer Experiment held in 1990 was a multi-institutional effort to probe the atmospheric boundary layer over the monsoon trough over northern India. For this experiment, four micrometeorological towers were set up at four dierent locations along the normal position of the trough. One such tower of 30m height was located at Jodhpur (26180N; 73040E), Rajasthan. The fast and slow response data available during the experiment have been used in the present study to determine a suitable layer-structure of the surface layer for evaluation of sensible heat flux using the multilayer hypothesis of Kramm (1989).


Isotopic evidence of Middle Proterozoic magmatism from Bombay High Field: Implications to crustal evolution of western offshore of India
S S Rathore1;, A R Vijan2, M P Singh2, B N Prabhu3 and Anand Sahu1
1Forward Base Oce, Assam & Assam Arakan Basin-Tripura, ONGC, Agartala 799 014, India.
email: ssrathore62@hotmail.com
2KDM Institute of Petroleum Exploration, ONGC, Dehradun 248 195, India.
3Regional Chemical Laboratory, ONGC, Sibsagar, Assam 785 640, India.
Now at: Geochronology Laboratory, KDM Institute of Petroleum Exploration, ONGC,
Dehradun 248 195, India.

Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446  67Ma with an initial 87Sr=86Sr ratio of 0:7062  0:0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of dierent grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385  21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458  43Ma and 1465  43 Ma, respectively.

Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-
Ar ages of 1452  42Ma and 1425  40Ma with an overall mean age of 1438  19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age
within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400{1450Ma on the  western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.


Study of momentum transfers within a vegetation canopy
Mahendra Mohan and M K Tiwari
Radio and Atmospheric Science Division, National Physical Laboratory, New Delhi 110 012, India.
e-mail: mmohan@mail.nplindia.ernet.in

Two models have been developed by applying conditions of continuity between the roughness sub- layer and the top of vegetation canopy having constant foliage distribution. Massman's cosh-type of wind profile and Albini's exponential wind profile have been used to derive expressions for shear stress, displacement height and roughness length in analytical forms. The computed results compared with those of Massman models (1987, 1997) show similarity with the present models.


Remote sensing of spectral signatures of tropospheric aerosols
M B Potdar1;, S A Sharma1, V Y Parikh1, P C S Devara2, P E Raj2, Y K Tiwari2,
R S Maheskumar2, K K Dani2, S K Saha2, S M Sonbawne2, Y Jaya Rao2 and
G Pandithurai2
1Space Applications Centre, Ahmedabad 380 015, India.
2Indian Institute of Tropical Meteorology, Pune 411 008, India.
e-mail: potdar51@hotmail.com

With the launch of the German Aerospace Agency's (DLR) Modular Opto-electronic Scanner (MOS) sensor on board the Indian Remote Sensing satellite (IRS-P3) launched by the Indian Space Research Organization (ISRO) in March 1996, 13 channel multi-spectral data in the range of 408 to 1010nm at high radiometric resolution, precision, and with narrow spectral bands have been available for a variety of land, atmospheric and oceanic studies. We found that these data are best for validation of radiative transfer model and the corresponding code developed by one of  the authors at Space Applications Centre, and called ATMRAD (abbreviated for ATMospheric RADi-ation). Once this model/code is validated, it can be used for retrieving information on tropospheric aerosols over ocean or land. This paper deals with two clear objectives, viz.,
(1) Validation  of ATMRAD model/code using MOS data and synchronously measured atmospheric data, and if found performing well, then to
(2) derive relationship between MOS radiances and Aerosol Optical Thickness (AOT). The data validation procedure essentially involves  near-synchronous measurements of columnar aerosol optical thickness and altitude profiles of aerosol concentration using ground-based multi-lter solar radiometers and Argon-ion Lidar, respectively and  computation of the top-of-the-atmosphere (TOA) radiances from a low reflecting target (near clear water reservoir in the present study) using the ATMRAD model. The results show that the model performance is satisfactory and a relationship between the spectral parameters of MOS radiances and aerosol optical thickness can be established. In this communication, we present the details of the experiments conducted, database, validation of the ATMRAD model and development of the relationship between AOT and MOS radiance.

Estimation of spectral reflectance of snow from IRS-1D LISS-III sensor over the Himalayan terrain
J Srinivasulu and A V Kulkarni

Marine and Water Resources Group, Space Applications Centre, Ahmedabad, 380 015, India. The sensor onboard the satellite views the earth as a plain surface and consequently the satellite obtained spectral radiances cannot represent true values over a mountainous terrain. The relative magnitudes of terrain slope and its aspect with respect to the sun's position will determine the amount of direct solar radiation incident on an undulating surface. Estimation of spectral reflectance from satellite data forms an important component in many of the snow and glacier studies. The spectral reflectance of snow is influenced by its various parameters. The changes in snowpack characteristics as a result of various metamorphic processes, with age, can cause variations in its spectral reflectances. Since, the terrain geometry also modifies the amount of reflected radiation from a rugged surface, one has to correct the estimated spectral reflectances for terrain topography so as to use them in deriving the snowpack characteristics accurately. Also, the amounts of melt runoff originating from glaciers having different slopes and orientations will not be the same. Considering these aspects, a model has been developed to estimate the terrain corrected spectral reflectances over the Himalayan terrain using the Linear Imaging Self Scanner-III data of the Indian Remote Sensing Satellite. The model computes spectral reflectances from satellite based radiance measurements and includes the effect of the terrain topography on the incident solar radiation. The terrain slope and its aspect are generated from the digital elevation model of the region.

The analysis carried out over the Beas Basin, Himachal Pradesh, India, indicated a variation of 22% in the amount of incident solar radiation for an increase of 10 in terrain slope. Further, the terrain with south-east aspect received maximum amount of solar radiation. The large differences observed between the uncorrected and terrain corrected reflectances in the shortwave infrared band (B5), which is not saturated over the snow covered region, suggest that the terrain slope and its aspect cannot be neglected in estimating the accurate spectral reflectances over the Himalayan terrain.

Alkaline intrusion in a granulite ensemble in the Eastern Ghats belt, India: Shear zone pathway and a pull-apart structure
S Bhattacharya1 and Rajib Kar2
1Geological Studies Unit, Indian Statistical Institute, 203 B. T. Road, Kolkata 700 108, India.
e-mail: samar@isical.ac.in
2Department of Geology, J.K. College, Purulia 723 101, India.

The alkaline complex of Koraput, Orissa, India, is one of several bodies in the high-grade Eastern Ghats belt, but this one is an integral part of the high-grade belt and remote from the western boundary against the Bastar craton. The Koraput complex forms a lozenge-shaped intrusion into the metapelitic granulites and is bounded by shear zones. The combined eect of movement along these shear zones, is a northeasterly elongated sygmoidal cavity with maximum width along the northwesterly trending Reidel shear. Thus the Koraput alkaline complex can be considered to have been emplaced in a pull-apart structure, developed in the granulitic country rocks. Moreover, in view of the fact that the western margin of the high-grade Eastern Ghats belt bears clear evidence of collisional features, rather than that of rifting or break-up, the rift-valley model for the alkaline magmatism in this high-grade belt appears untenable.

Artificial neural network approach for estimation of surface specific humidity and air temperature using Multifrequency Scanning Microwave Radiometer
Randhir Singh, B G Vasudevan, P K Pal and P C Joshi
Meteorology and Oceanography Group, Space Applications Centre (ISRO), Ahmedabad 380 015, India.

Microwave sensor MSMR (Multifrequency Scanning Microwave Radiometer) data onboard Oceansat-1 was used for retrieval of monthly averages of near surface specic humidity (Qa) and air temperature (Ta) by means of Articial Neural Network (ANN). The MSMR measures the microwave radiances in 8 channels at frequencies of 6.6, 10.7, 18 and 21 GHz for both vertical and horizontal polarizations.

The artificial neural networks (ANN) technique is employed to nd the transfer function relating the input MSMR observed brightness temperatures and output (Qa and Ta) parameters. Input data consist of nearly 28 months (June 1999 { September 2001) of monthly averages of MSMR observed brightness temperature and surface marine observations of Qa and Ta from Comprehensive Ocean- Atmosphere Data Set (COADS).The performance of the algorithm is assessed with independent surface marine observations. The results indicate that the combination of MSMR observed brightness temperatures as input parameters provides reasonable estimates of monthly averaged surface parameters. The global root mean square (rms) differences are 1:0C and 1:1 g kg−1 for air temperature and surface specific humidity respectively.

Physical properties of aerosols at Maitri, Antarctica
C G Deshpande and A K Kamra
Indian Institute of Tropical Meteorology, Pune, India.

Measurements of the submicron aerosol size distribution made at the Indian Antarctic station, Maitri (70450 S, 11440 E) from January 10th to February 24th, 1997, are reported. Total aerosol concentrations normally range from 800 to 1200 particles cm−3 which are typical values for the coastal stations at Antarctica in summer. Aerosol size distributions are generally trimodal and open- ended with a peak between 75 and 133nm and two minima at 42 and 420 nm. Size distributions remain almost similar for several hours or even days in absence of any meteorological disturbance. Total aerosol concentration increases by approximately an order of magnitude whenever a low pressure system passes over the station. Based on the evolution of aerosol size-distributions during such aerosol enhancement periods, three types of cases have been identified. The nucleation mode in all three cases has been suggested to result from the photochemical conversion of the DMS emissions transported either by the marine air or by the air from the ice-melt regions around Maitri. Subsidence of midtropospheric air during the weakening of radiative inversion is suggested as a possible source of the nucleation mode particles in the third case. Growth of the nucleation mode particles by condensation, coagulation and/or by cloud processes has been suggested to be responsible for other modes in size distributions.

A direct inversion scheme for deep resistivity sounding data using artificial neural networks
Jimmy Stephen, C Manoj and S B Singh
National Geophysical Research Institute, Hyderabad 500 007, India
e-mail: jimmy@ngri.res.in

Initialization of model parameters is crucial in the conventional 1D inversion of DC electrical data, since a poor guess may result in undesired parameter estimations. In the present work, we investigate the performance of neural networks in the direct inversion of DC sounding data, without the need of a priori information. We introduce a two-step network approach where the first network identifies the curve type, followed by the model parameter estimation using the second network. This approach provides the flexibility to accommodate all the characteristic sounding curve types with a wide range of resistivity and thickness. Here we realize a three layer feed-forward neural network with fast back propagation learning algorithms performing well. The basic data sets for training and testing were simulated on the basis of available deep resistivity sounding (DRS) data from the crystalline terrains of south India. The optimum network parameters and performance were decided as a function of the testing error convergence with respect to the network training error. On adequate training, the nal weights simulate faithfully to recover resistivity and thickness on new data. The small discrepancies noticed, however, are well within the resolvability of resistivity sounding curve interpretations.