P. C. Pandey

Remote sensing of atmospheric water content from Bhaskara SAMIR data

The 19-35 GHz and 22-235 GHz passive microwave radiometers (SAMIR) on board the Indian satellite Bhaskara have provided very useful data. From these data has been demonstrated the feasibility of deriving atmospheric and ocean surface parameters such as water vapour content, liquid water content, rainfall rate and ocean surface winds. Different approaches have been tried for deriving the atmospheric water content. The statistical and empirical methods have been used by others for the analysis of the Nimbus data. A simulation technique has been attempted for the first time for 19 35 GHz and 22-235 GHz radiometer data. The results obtained from three different methods are compared with radiosonde data. A case study of a tropical depression has been undertaken to demonstrate the capability of Bhaskara SAMIR data to show the variation of total water vapour and liquid water contents.

Empirical orthogonal function analysis of humidity profiles over the Indian Ocean and an assessment of their retrievability using satellite microwave radiometry

The paper examines the variability of vertical humidity profiles over the Indian oceanic region using a set of 1200 radiosonde observations spanning 10 years (1982–1991). The examination is based upon the method of empirical orthogonal function (EOF) analysis. The first EOF explains 61% of the total variance and the first three EOFs together account for 85% of the total variability. The first principal component is almost perfectly correlated with the total precipitable water (TPW) and the second one is well correlated with the ratio of boundary layer moisture and TPW. This fact and an inequality derived from the analysis of the variances of individual terms of the EOF expansion of specific humidity are utilised to establish an algorithm for retrieving humidity profile from satellite microwave measurement of TPW over the region of study. Power of the retrieval technique is demonstrated using 127 independent radiosonde measurements and by plotting the profiles of rms error and bias. The method is found to be distinctly superior compared to a power-law retrieval. A few examples of profile retrieval from satellite measurements of TPW have been checked against colocated radiosonde measurements. Some examples of retrieval show that the method is uniquely able to capture the humidity variability in the boundary layer, particularly the high moisture loading in the monsoon season.

Remote sensing of seamounts in the Central Indian Ocean Basin using satellite altimetry

Abstract We describe the use of satellite altimetry for the remote sensing of seamounts in the Central Indian Ocean Basin (CIOB). This is based on the statistical theory of detecting a given signal in the presence of background noise. Detection is achieved by passing the sea surface height data obtained by a satellite altimeter through a matched filter designed from a typical seamount signature model and power spectral density (PSD) of the background noise. A realistic seamount signature was computed for this purpose using gravitational potential theory and observed seamount dimensions. The PSD of an individual Geosat track was estimated using autoregressive modelling. For the sake of brevity, the results of processing only a few Geosat tracks in the region of study using the above technique have been reported in the paper. It was found that four charted seamounts were faithfully detected by the filter. Six uncharted seamounts were also predicted.

An algorithm for sea surface temperature estimation from ERS-1 ATSR using moisture dependent coefficients: a simulation study

A new algorithm for estimating sea surface temperature (SST) from ERS-1 ATSR data using moisture dependent coefficients has been proposed. The clear-column water vapour (WV) information required for selecting the appropriate coefficients has also been derived from ERS-1 ATSR data. The method has been tested with the simulated ATSR data by comparing the estimated SST with the true SST used for simulating the data. The comparison of SST estimation errors by present and standard methods, under different noise conditions, has indicated improvements by the present method over the standard method. The small errors anticipated in SST estimation due to the errors in WV estimation causing erroneous selection of coefficients have also been taken care of by making use of overlapping ranges of WV for moisture dependent SST estimation coefficients. The method could be used for operational SST from ATSR data.

A Geosat-derived noise spectrum for detecting seamounts in the Arabian Sea

Noise power spectral densities (PSDs) have been derived from the analysis often seamount-free Geosat tracks in the Indian seas. The average noise PSD has been fitted by a model function. The model noise PSD has been used in conjunction with a model seamount signature to construct a matched filter for detecting seamounts in the Arabian Sea. Performance of the technique has been verified by successfully detecting the Somali ridge and the Error Seamount which are located in the Arabian Sea. Several uncharted seamounts have also been predicted.

An optimal estimation technique for increasing the accuracy of crop forecasts by combining remotely sensed and conventional forecast results.

Abstract An optimal estimation (OE) technique has been used to increase the accuracy of crop acreage and yield estimates by combining results from remotely sensed (RS) data and conventional models. For crop acreage estimation the OE increased the accuracy of wheat acreage estimation when the first forecasts of the Directorate of Economics and Statistics (DES) were combined with state level RS estimates over the states of Haryana and Punjab in India. To increase the accuracy of wheat yield forecasts an autoregressive (AR) model was developed. Results of AR model were optimally combined with RS-based estimates for Hisar and Karnal districts in Haryana, India. The OE results for a total of eight forecasts had a lower mean absolute per cent deviation than the forecasts using RS and AR approaches. The power of OE was further demonstrated by combining weather-based wheat yield model results for the state of Punjab (India) with first order AR model results, suggesting an increase in accuracy of forecasts by opti...

Numerical experiment with modelled return echo of a satellite altimeter from a rough ocean surface and a simple iterative algorithm for the estimation of significant wave height

We have simulated the return echo of a satellite altimeter from a rough ocean surface using an analytical formula and have studied its sensitivity with respect to various oceanic and altimeter parameters. Our numerical experiment shows that for normally observed significant wave heights (SWH) the effect of off-nadir angle (ONA) up to 0.5° on the leading edge is not severe. Also, small surface roughness skewness seems to have little effect on the overall shape of the echo.Newton’s iterative scheme has been used to retrieve SWH from the mean return waveform without noise and with additive Gaussian noise typical of Seasat and Geosat altimeters. It has been observed that SWH can be retrieved in the presence of noise with an accuracy of ±0.6 m for ONA less than 0.5°. For higher ONA, accurate retrieval requires the use of precomputed look-up table along with our scheme.

A simulation technique for the determination of atmospheric water content with Bhaskara satellite microwave radiometer (SAMIR)

A simulation technique has been developed to estimate the integrated atmospheric water content over oceans using the 19·35 and 22·235 GHz brightness temperature data from satellite microwave radiometer (SAMIR) on board the Indian satellite Bhaskara. The results obtained have been compared with those from linear statistical regression and empirical methods as well as from the nearest radiosonde observations. Based on the simulation method, a map of total precipitable water for some of the Bhaskara passes in July 1980 over Bay of Bengal is given. The possible applications of such maps in the study of Indian summar monsoon and boundary layer characteristics have been pointed out.

Eddy Detection Over Southern Indian Ocean Using TOPEX/POSEIDON Altimeter Data

Mesoscale eddies constitute the most energetic component of the variability of ocean currents. An attempt has been made for the detection of oceanic mesoscale eddy signatures over the Southern Indian Oceanic (SIO) regions using the dynamic topography derived from TOPEX/POSEIDON (T/P) altimeter data, by the signal processing technique, called matched filtering. After applying all the ocean and atmospheric corrections, data of a complete cycle of T/P over SIO has been used for detection of eddy signatures. The geoid undulations are removed from the data of corrected sea surface height from T/P and the resulting dynamic topographic data are passed through a matched filter designed to detect a generic eddy signature of Gaussian signal embedded in noise. The filter is optimized to detect eddies with amplitude 20 to 30 cm and diameters roughly 100?250 km. Out of all the analyzed data of T/P orbits over SIO a few examples are presented for brevity. Qualitative verification of eddies is done with some independent T/P sea level anomaly data over the region. The analysis shows that the matched filtering technique is most suitable for monitoring eddy signatures along the subsatellite track instantly over the remote and most hostile regions of the southern global oceans.Mesoscale eddies constitute the most energetic component of the variability of ocean currents. An attempt has been made for the detection of oceanic mesoscale eddy signatures over the Southern Indian Oceanic (SIO) regions using the dynamic topography derived from TOPEX/POSEIDON (T/P) altimeter data, by the signal processing technique, called matched filtering. After applying all the ocean and atmospheric corrections, data of a complete cycle of T/P over SIO has been used for detection of eddy signatures. The geoid undulations are removed from the data of corrected sea surface height from T/P and the resulting dynamic topographic data are passed through a matched filter designed to detect a generic eddy signature of Gaussian signal embedded in noise. The filter is optimized to detect eddies with amplitude 20 to 30 cm and diameters roughly 100?250 km. Out of all the analyzed data of T/P orbits over SIO a few examples are presented for brevity. Qualitative verification of eddies is done with some independent T/P sea level anomaly data over the region. The analysis shows that the matched filtering technique is most suitable for monitoring eddy signatures along the subsatellite track instantly over the remote and most hostile regions of the southern global oceans.

A comparative study of near concurrent DMSP-SSM / I and geosat-altimeter measurements of surface winds over the Indian Oceanic region

Abstract In this paper, colocated, near simultaneous measurements of ocean surface wind speed near the Indian subcontinent from the Special Sensor Microwave/Imager (SSM/I) and Radar Altimeter onboard, respectively, DMSP and Geosat satellites are analysed. It is found that for about 84 per cent of cases, difference of two winds is within 3 m s 1. However, when the colocated winds are averaged for entire period and over the grid boxes of 2.5 2.5, the correlation between the two winds appears to be very high. It is found that the absolute difference of the two winds has no apparent relation with the amount of water vapour and the amount of cloud liquid water present in the atmosphere. Further, the variations of absolute difference of two winds with SSM/I winds, altimeter winds and significant wave height have indicated that altimeter measurements of high wind speed are likely to be more inaccurate compared to SSM/I measurements. The variation of bias and standard deviation of the difference of the two winds ...