• S G Narkhedkar

      Articles written in Journal of Earth System Science

    • An efficient optimum interpolation scheme for objective analysis over Indian region

      S K Sinha S G Narkhedkar S Rajamani

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      In the optimum interpolation scheme, the weights for the observations are computed by solving a set of linear equations for every grid point. As the number of observations increases particularly over data-rich regions, the matrix dimension increases and the computer time required to solve these equations to determine weights increases considerably. In order to reduce the computer time for computing the weights, Tanguay and Robert suggested schemes in which the gaussian function representing the autocorrelation function has been approximated by a second-order and also by a fourth-order Taylor series expansion. This resulted in the solution of matrices of order 4 or 9 respectively to obtain weighting functions irrespective of the number of observations used in the analysis. In the present study, the analyses of mean sea level pressure and geopotential height at 700 mbar level have been carried out for five days using the above two schemes and the regular OI scheme. The analyses are found to be similar in all the three cases suggesting that a lot of computer time could be saved without sacrificing the analysis accuracy by using the modified scheme in which the second-order approximation is utilized.

    • Proper depiction of monsoon depression through IRS-P4 MSMR

      P N Mahajan R M Khaladkar S G Narkhedkar Sathy Nair P C Joshi P K Pal

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      In this paper, daily variations of satellite-derived geophysical parameters such as integrated water vapour (IWV), cloud liquid water content (CLW), sea surface temperature (SST) and sea surface wind speed (SSW) have been studied for a case of monsoon depression that formed over the Bay of Bengal during 19th-24th August 2000. For this purpose, IRS P4 MSMR satellite data have been utilized over the domain equator — 25‡N and 40‡-100‡E. An integrated approach of satellite data obtained from IRS-P4, METEOSAT-5 and INSAT was made for getting a signal for the development of monsoon depression over the Indian region. Variations in deep convective activity obtained through visible, infrared and OLR data at 06 UTC was thoroughly analyzed for the complete life cycle of monsoon depression. Geophysical parameters obtained through IRS-P4 satellite data were compared with vorticity, convergence and divergence at 850 and 200 hPa levels generated through cloud motion vectors (CMVs) and water vapour wind vectors (WVWVs) obtained from METEOSAT-5 satellite. This comparison was made for finding proper consistency of geophysical parameters with dynamical aspects of major convective activity of the depression.

      From the results of this study it is revealed that there was strengthening of sea surface winds to the south of low-pressure area prior to the formation of depression. This indicated the possibility of increase in cyclonic vorticity in the lower troposphere. Hence, wind field at 850 hPa with satellite input of CMVs in objective analysis of wind field using optimum interpolation (OI) scheme was computed. Maximum cyclonic vorticity field at 850 hPa was obtained in the region of depression just one day before its formation. Similarly, with the same procedure maximum anticyclonic vorticity was observed at 200 hPa with WVWVs input. Consistent convergence and divergence at 850 and 200 hPa was noticed with respect to these vorticities. In association with these developments, we could get lowest values of OLR (120 W/m2 ) associated with major convective activity that was consistent with the maximum values of integrated water vapour (6-8gm/cm2) and cloud liquid water content (50-60 mg/cm2 ) persisting particularly in the southwest sector of the monsoon depression.

    • Variational method for objective analysis of scalar variable and its derivative

      S G Narkhedkar S K Sinha

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      In this study real time data have been used to compare the standard and triangle method by performing the objective analysis of mean sea level pressure.In the standard method,derivative fields are obtained from the grid point data using finite difference scheme whereas in the triangle method,a set of non-overlapping triangles are formed from the observations and the scalar and the spatial derivatives are computed directly at the centroid of each of the non-overlapping triangles.These scalars and their derivatives are then mapped to uniform grids by using the standard method.It has been found that objectively analysed scalar field obtained using standard method is superior to the scalar field derived by the triangle method,whereas the derivative fields produced by triangle method are superior to the derivative fields produced using standard method. A variational objective analysis scheme has been developed and an experiment has been carried out with depression case of June (11 –15)2004.It is found that the new scheme (variational)is able to extract the better parts of both triangle and standard methods.The results of this study will be useful in carrying out diagnostic calculations that involve derivative estimates.

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