• Abhijit Sarkar

Articles written in Journal of Earth System Science

• Wind dependence of quasi-specular sea scatter

Earlier investigations have shown that a power law relationship of the typeσ=aWb exists between the scattering coefficientσ and sea surface wind speedW at microwave frequencies. The coefficientsa andb are usually quoted for different frequencies, look angles, polarizations etc. This paper attempts to define the angular dependence using such a power law relationship in the quasi-specular range.

• A study on the sensitivity of the radar scattering coefficient to oceanic winds

Sensitivity of the radar scattering coefficient to the oceanic wind vector in the midangular range for frequencies from L- to Ku- band is studied. This is based on computations of scattering coefficient via the two-scale scattering theory employing a semi-empirical model for the ocean spectrum suggested by Fung and Lee and the slope distribution by Cox and Munk. Higher frequency and incident angles of over 45 degrees seem to yield better wind sensitivity.

• Quasi-specular sea scatter with modified reflection coefficient

The paper deals with the relationship between the sea surface wind speed (U) and the backscattering coefficient (σ0) for microwave frequencies and quasi specular range of angles. Valenzuela’s effective reflection coefficient based on the theory of scattering of electromagnetic waves from rought dielectric surfaces has been introduced in the expression of backscattering coefficient and the final result compared with the σ0-U relationship derived fromSEASAT scatterometer and concurrent sea truth data.

• Selection of optimum frequency of a wind scatterometer

The problem of selecting the optimum operating frequency of a scatterometer, used for remote sensing of sea surface wind speed has been addressed by applying the criteria of maximum sensitivity of backscattering coefficient to wind speed as well as its correlation with wind speed. The backscattering coefficient values for sea surface were computed by the two-scale scattering theory. To compute the atmospheric transmittance, 753 clear sky atmospheres over Indian Ocean were used. While the correlation coefficient was uniform (0.94) throughout the frequency range of 1 to 30GHz, only frequencies above 5GHz were found sensitive enough to yield a wind speed accuracy of ±2msec−1 and better, the accuracy improving with frequency.

• Selection of optimum frequency of a wind scatterometer

• Numerical study of the effects of urban heat island on the characteristic features of the sea breeze circulation

A two-dimensional numerical model is employed to study the effect of the coastal urban heat island on the sea breeze front and the thermal internal boundary layer height. The temperature at the land surface is determined by solving an energy budget equation. The effect of the urban heat island is studied by varying the width of the region and its intensity. During the early afternoon, the presence of the urban heat island enhances the strength of convergence of the sea breeze front and also reduces its inland penetration. The presence of the urban heat island causes increased thermal internal boundary layer height. Larger urban width causes larger vertical velocity and higher thermal internal boundary layer. Stronger convergence and higher thermal internal boundary layer are also obtained in case of larger heat island intensity.

• An analytical source function for a coupled hybrid wave model

An analytical form for the source function is formulated by comparing the fetch-limited approximation of the Ocean Wave Transport equation and the empirical equation for the fetch-dependent wave forecast nomograms. The source function thus generated has been utilised in the numerical model based on Toba’s formulation of wave transport equation and tested for the seas around the Indian subcontinent (5°S to 25°N latitude; 45°E to 100°E longitude). The grid averaged hindcast wave heights are found to be moderately matching with the GEOSAT altimeter measured significant wave heights of the 1987–1989 period, particularly for waves higher than 1 meter.

• Auto-correlation analysis of ocean surface wind vectors

The nature of the inherent temporal variability of surface winds is analyzed by comparison of winds obtained through different measurement methods. In this work, an auto-correlation analysis of a time series data of surface winds measuredin situ by a deep water buoy in the Indian Ocean has been carried out. Hourly time series data available for 240 hours in the month of May, 1999 were subjected to an auto-correlation analysis. The analysis indicates an exponential fall of the autocorrelation in the first few hours with a decorrelation time scale of about 6 hours. For a meaningful comparison between satellite derived products andin situ data, satellite data acquired at different time intervals should be used with appropriate ‘weights’, rather than treating the data as concurrent in time. This paper presents a scheme for temporal weighting using the auto-correlation analysis. These temporal ‘weights’ can potentially improve the root mean square (rms) deviation between satellite andin situ measurements. A case study using the TRMM Microwave Imager (TMI) and Indian Ocean buoy wind speed data resulted in an improvement of about 10%.

• Auto-correlation analysis of wave heights in the Bay of Bengal

Time series observations of significant wave heights in the Bay of Bengal were subjected to auto-correlation analysis to determine temporal variability scale. The analysis indicates an exponential fall of auto-correlation in the first few hours with a decorrelation time scale of about six hours. A similar figure was found earlier for ocean surface winds. The nature of variation of auto-correlation with time lags was also found to be similar for winds and wave heights

• Study of inter-annual variations in surface melting over Amery Ice Shelf, East Antarctica, using space-borne scatterometer data

The widespread retreat of glaciers can be considered as a response to the climate change. Being the largest retreating glacier–ice shelf system in East Antarctica, the Amery Ice Shelf–Lambert Glacier system plays an important role in contributing to sea level rise as well as the surrounding environment and climate. The present study is focused on the investigation of surface melting over the ice shelf using QuikSCAT Ku-band scatterometer data for more than 100 months covering the period from January 2000 to July 2009. The corresponding weather data of Davis Station was obtained from the website of Australian Antarctic Division. Very prominent dips in the backscatter observed in the month of January form a distinct signature caused by physical process of significant melting of the ice/snow surface. The steep increase again in February is attributed to the initiation of the freezing phenomenon. The derived melting index compared well with the passive microwave-based melting index derived by other researchers. A strong relationship was found between the scatterometer-derived melting index and the cumulative monthly mean air temperature. The highest melting was observed in the summer (January) of 2004, and thereafter gradual cooling appeared to take place in the subsequent years. The snow pack thickness, inferred from the backscatter variations, was found to be higher during winters (June) of 2004 and 2005, compared with other years.

• Atmospheric correction for sea surface temperature retrieval from single thermal channel radiometer data onboard Kalpana satellite

An atmospheric correction method has been applied on sea surface temperature (SST) retrieval algorithm using Very High Resolution Radiometer (VHRR) single window channel radiance data onboard Kalpana satellite (K-SAT). The technique makes use of concurrent water vapour fields available from Microwave Imager onboard Tropical Rainfall Measuring Mission (TRMM/TMI) satellite. Total water vapour content and satellite zenith angle dependent SST retrieval algorithm has been developed using Radiative Transfer Model [MODTRAN ver3.0] simulations for Kalpana 10.5–12.5 𝜇m thermal window channel. Retrieval of Kalpana SST (K-SST) has been carried out for every half-hourly acquisition of Kalpana data for the year 2008 to cover whole annual cycle of SST over Indian Ocean (IO). Validation of the retrieved corrected SST has been carried out using near-simultaneous observations of ship and buoys datasets covering Arabian Sea, Bay of Bengal and IO regions. A significant improvement in Root Mean Square Deviation (RMSD) of K-SST with respect to buoy (1.50–1.02 K) and to ship datasets (1.41–1.19 K) is seen with the use of near real-time water vapour fields of TMI. Furthermore, comparison of the retrieved SST has also been carried out using near simultaneous observations of TRMM/TMI SST over IO regions. The analysis shows that K-SST has overall cold bias of 1.17 K and an RMSD of 1.09 K after bias correction.

• Wave hindcast experiments in the Indian Ocean using MIKE 21 SW model

Wave prediction and hindcast studies are important in ocean engineering, coastal infrastructure development and management. In view of sparse and infrequent in-situ observations, model derived hindcast wave data can be used for the assessment of wave climate in offshore and coastal areas. In the present study, MIKE 21 SW Model has been used to carry out wave hindcast experiments in the Indian Ocean. Model runs have been made for the year 2005 using QuickSCAT scatterometer winds blended with ECMWF model winds. In order to study the impact of southern ocean swells, the model has been run in two different domains, with the southern boundary being shifted far south for the Domain 60S model. The model simulated wave parameters have been validated by comparing with buoy and altimeter data and various statistical yardsticks have been employed to quantify the validation. Possible reason for the poorer performance of the model in the Arabian Sea has also been pointed out.

• Evaluation of OSCAR ocean surface current product in the tropical Indian Ocean using in situ data

The OSCAR (ocean surface current analysis real-time),which is a product derived from various satellite observations,has been evaluated in the tropical Indian Ocean (TIO)in two di ﬀerent ways.First,the OSCAR-derived monthly climatology has been compared with available drifter-derived climatology in the TIO.From the comparison of the two climatologies,one can infer that OSCAR product is able to capture the variabilities of the well-known surface current systems in the TIO reasonably well.Fourier analysis of the major current systems,as reproduced by OSCAR,shows that the dominant annual and semiannual periodicities,known to exist in these systems,have been faithfully picked up by OSCAR. Next,the evaluation has been carried out by comparing the OSCAR currents with currents measured by moored buoys.The zonal component of OSCAR-current is in good agreement with corresponding component of buoy-observed current with a correlation exceeding 0.7,while the match between the meridional components is poorer.The locations of the peaks of the mean and eddy kinetic energies are matching in both the climatologies,although the peak in the drifter climatology is stronger than the same in the OSCAR product.Finally,an important feature of Indian Ocean circulation,namely the reverse Wyrtki jet,occurring during anomalous dipole years,has been well-reproduced by OSCAR currents.

• Probabilistic forecasting of Super Cyclone ‘Amphan’ using NCMRWF global and regional ensemble prediction systems

Probabilistic forecasting of tropical cyclone (TC) from ensemble prediction systems provides flowdependent uncertainty associated with the model forecast and helps in better decision making. NCMRWF global and regional ensemble prediction systems (NEPS-G and NEPS-R) have been used in forecasting the intensity and track of the Super Cyclone ‘Amphan’, which hit eastern India and Bangladesh in May 2020. Observation shows very heavy rainfall (${\ge}$11.5 cm/day) over West Bengal and Bangladesh on 20th May 2020. NEPS-R predicted very heavy rainfall in day-2 forecasts with a probability in the range of 50–70%. NEPS-G also could predict very heavy rainfall in day-2 forecast with probability lying in the same range but over a small area. In its day-5 forecast also, NEPS-G was able to predict very heavy rainfall with a probability lying in the range of 30–50%. The prediction of time and magnitude of the maximum intensity by NEPS-R was predicted better from the initial condition of 00 UTC 16th May 2020. Prediction of intensi-fication is better in NEPS-R forecast. Both NEPS-G and NEPS-R are under-dispersive in 10 m maximum wind forecasting. The RMSE-spread relationship of maximum wind speed is better in the NEPS-R forecast till 72 hrs forecast lead time. The reliability of both NEPS-Gand NEPS-R strike probability forecasts is good and NEPS-G shows better reliability at lower probability values. The mean direct position error (DPE) from NEPS-G does not exceed 350 kmin day-5 and 270km in day-3 forecast lead time. The meanDPEof NEPS-R forecast is about 175 km in day-3 forecast lead time. Both NEPS-G and NEPS-R predicted early landfall, but the error in the landfall time does not exceed 3 hrs within the last 72 hrs before landfall. NEPS-R at short range and NEPS-G at longer range show reasonable skill in prediction of tropical cyclone ‘Amphan’.

• Journal of Earth System Science

Volume 132, 2023
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Continuous Article Publishing mode

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019