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N. Gautam
,
B. Simon
, and
P. C. Pandey

Abstract

The main objective of this work is to study the latitudinal and seasonal variation of latent beat fluxes (LHF) and associated atmospheric and oceanic parameters over the Arabian Sea (AS) and the Bay of Bengal (BB) for the year 1988.The atmospheric and oceanic parameters, like precipitable water (PW), ocean surface wind (WS), and cloud liquid water content, are derived from the Special Sensor Microwave/Imager (SSM/I), and SST is obtained from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer. The latent heat fluxes have been estimated from the bulk aerodynamic formula using the above satellite-derived parameters. The surface-level humidity, an essential parameter required for computing LHF, has been estimated using Liu's global relation between monthly mean surface-level humidity and precipitable water. A comparison of these variables and the relation among them has also been made over the AS and the BB.

A significant latitudinal variation is observed in LHF for most of the months over the AS and the BB, while other oceanic and atmospheric parameters are characterized by a strong latitudinal variation in nonmonsoon months. Seasonal variations in LHF are more significant at higher latitudes compared to lower latitudes over the AS and the BB. The effect of coastal upwelling near the Somali coast decreases LHF, while surface winds near the Indian coast during monsoon months increases LHF. A comparative study over the AS and the BB demonstrates higher PW and SST over the BB than over the AS. LHF is found to be greater over the AS than over the BB for nonmonsoon months. Correlation analysis indicates that LUF is found to be highly correlated with DQ (difference between the humidity at the surface and humidity near the surface) over the AS and weakly correlated over the BB during nonmonsoon months. Throughout the year, DQ is found to be a dominant factor for LHF over the AS. However, WS exercised better control over the BB in generating LHF. SST and PW are found to be highly correlated with each other over the AS (r = 0.87) and the BB (r = 0.75) for nonmonsoon months. The correlation becomes weakly negative over the AS (r = 0.15) and weak over the BB (r = 0.26) during monsoon months. Precipitable water is found to have a high correlation with WS over the AS (r = 0.72). This unique feature is revealed by SSM/I data and has not been reported earlier due to paucity of data over this region.

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P. C. Pandey
,
E. G. Njoku
, and
J. W. Waters

Abstract

The Scanning Multichannel Microwave Radiometer (SMMR) on the Seasat and Nimbus-7 satellite measured microwave radiation at 6.6, 10.69, 18.0, 21.0 and 37.0 GHz with both horizontal and vertical polarizations. Numerical simulations have been performed to explore the potential of using the 18.0, 21.0 and 37.0 GHZ SMMR channels with simultaneous infrared measurements of cloud top height for retrieving cloud temperature differential and thickness over the ocean. The results suggest it is possible to infer cloud vertical thickness to ∼0.4 km rms accuracy and cloud temperature differential to ∼3°C rms. These accuracies are approximately half the a prior variances.

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N. Gautam
,
Sujit Basu
,
C. M. Kishtawal
,
R. M. Gairola
, and
P. C. Pandey

Abstract

A new regional relationship is derived for wet tropospheric range correction h for radar altimeter and precipitable water (W) over the Indian Ocean using ship observations for the period 1982–91. The W varied over the range of 20–80 mm, thus providing total variability expected over tropical oceans. A fifth-order polynomial between h and W gave an rms error of 2.3 mm when compared with h computed using direct relation. Model results have also been compared with an earlier relation over the Indian Ocean and with the global relation, and it has been found that the present model yields the lowest rms error in h values over the Indian Ocean. Comparisons with earlier models show a factor of 2 improvement in the accuracy of the correction over this region.

The W values have also been derived using the NOAA High-Resolution Infrared Sounder data for the years 1980, 1981, and 1984. These monthly mean W (3-yr averaged) values have been used to study the variabilities in W. The Indian Ocean depicts large variabilities of W even on a monthly scale. The monthly mean map of h has also been given to get a rough idea about the values of corrections required over this region.

Here h obtained using W from a sample pass of the Seasat Scanning Multichannel Microwave Radiometer (SMMR) and h provided by the Fleet Numeric Oceanographic Center (FNOC) have been compared to point out deviations of FNOC model-derived h values from h obtained from SMMR-derived W. The Special Sensor Microwave/Imager-derived W values for a sample pass over the Arabian Sea and the Bay of Bengal have been used to estimate h values from our derived relation between h and W. The Bay of Bengal exhibits high h values compared to those over the Arabian Sea. This study demonstrates the usefulness of the proposed regional relation between h and W for application to satellite-borne altimeter data, such as the ERS-1 and Topex/Poseidon missions, where an onboard microwave radiometer provides instantaneous W measurements for studying various oceanographic phenomena.

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N. Jithendra Raju
,
Mihir K. Dash
,
Prasad Kumar Bhaskaran
, and
P. C. Pandey

Abstract

Strong bidirectional internal solitary waves (ISWs) generate from a shallow channel between Car Nicobar and Chowra Islands of Nicobar Islands, India, and propagate toward the Andaman Sea (eastward) and Bay of Bengal (westward). Batti Malv Island separates this shallow channel into two ridges, north of Batti Malv (NBM) and south of Batti Malv (SBM). First, this study identifies the prominent mode-1 and mode-2 ISWs emerging from NBM and SBM using synthetic aperture radar images and then explores their generation mechanism(s) using a nonlinear, unstructured, and nonhydrostatic model, SUNTANS. During spring tide, flow over NBM is supercritical with respect to mode-1 internal wave. Model simulations reveal that mode-1 ISWs are generated at NBM by a “lee wave mechanism” and propagate both in the east and west directions depending on the tidal phases. However, the flow over SBM is subcritical with respect to mode-1 internal wave. The bidirectional propagating mode-1 ISWs evolve from a long-wave disturbance induced by “upstream influence.” But, during spring tide, with an increased tidal flow over SBM, it is observed that the westward propagating ISWs are formed by a dispersed hydraulic jump observed over the ridge. Moreover, the bidirectional mode-2 waves from SBM are generated by a lee wave mechanism. An energy budget comparison reveals that the region surrounding NBM is efficient in radiating low-mode baroclinic energy (0.98 GW), while SBM is highly efficient in converting barotropic to baroclinic energy (4.1 GW).

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