Mannil Mohan

Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba (.∘N, .∘E) during Different Seasons

Vertical profiles of meteorological parameters obtained from balloon-borne GPS Radiosonde for a period of more than two years are analyzed for characterization of the coastal atmospheric boundary layer (CABL) over Thumba (8.5∘N, 76.9∘E, India). The study reports seasonal variability in the thickness of three different sublayers of the CABL, namely, mixed layer, turbulent flow, and sea breeze flow. Among the three, the vertical thickness of sea breeze flow showed considerable dominance on the other two throughout the year. Mixed layer heights derived through gradients in virtual potential temperature (𝜃𝑣) showed large seasonal variability with a peak in the Summer and Post-Monsoon. On the other hand, the vertical thickness of turbulent flow remained steady all through the year. Results from the present study indicate that the magnitudes of mixed layer heights are often larger than the turbulent flow thickness.

Detection of marine aerosols with IRS P4-Ocean Colour Monitor

The atmospheric correction bands 7 and 8 (765nm and 865nm respectively) of the Indian Remote Sensing Satellite IRS P4-0CM (Ocean Colour Monitor) can be used for deriving aerosol optical depth (AOD) over the oceans. A retrieval algorithm has been developed which computes the AOD using band 7 data by treating the ocean surface as a dark background after removing the Rayleigh path radiance in the sensor-detected radiances. This algorithm has been used to detect marine aerosol distributions at different coastal and offshore locations around India.A comparison between OCM derived AOD and the NOAA operational AOD shows a correlation ∼0.92 while that between OCM derived AOD and the ground-based sun photometer measurements near the coast of Trivandrum shows a correlation of ∼0.90.

Comparison of ocean color chlorophyll algorithms for IRS-P4 OCM sensor usingin-situ data

In-situ chlorophyll concentration data and remote sensing reflectance (Rrs) measurements collected in six different ship campaigns in the Arabian Sea were used to evaluate the accuracy, precision, and suitability of different ocean color chlorophyll algorithms for the Arabian Sea. The bio-optical data sets represent the typical range of biooptical conditions expected in this region and are composed of 47 stations encompassing chlorophyll concentration, between 0.072 and 5.90 mg m-3, with 43 observations in case I water and 4 observations in case II water. Six empirical chlorophyll algorithms [i.e. Aiken-C, POLDER-C, OCTS-C, Morel-3, Ocean Chlorophyll-2 (OC2) and Ocean Chlorophyll-4 (OC4)] were selected for analysis on the Arabian Sea data set. Numerous statistical and graphical criterions were used to evaluate the performance of these algorithms. Among these six chlorophyll algorithms two chlorophyll algorithms (i.e. OC2 and OC4) performed well in the case I waters of the Arabian Sea. The OC2 algorithm, a modified cubic polynomial function which uses ratio of Rrs490 nm and Rrs555 nm (where, Rrs is remote sensing reflectance), performed well with r2=0.85; rms =0.15. The OC4 algorithm, a four-band (443, 490, 510, 555 nm), maximum band ratio formulation was found best on the basis of statistical analysis results with r2=0.85 and rms=0.14. Both OC2 and OC4 algorithms failed to estimate chlorophyll inTrichodesmium dominated waters. The OC2 algorithm was preferred over OC4 algorithm for routine processing of the OCM data to generate chlorophyll-a images, as it uses a band ratio of 490/555 nm and atmospheric correction is more accurate in 490 nm compared to 443 nm band, which is used by OC4 algorithm.

Simulations for optimal payload tilt to avoid sunglint in IRS-P4 Ocean Colour Monitor (OCM ) data around the Indian subcontinent

Ocean Colour Monitor (OCM) payload, onboard Indian Remote Sensing Satellite (IRS)-P4, detects the water constituents from the spectrum of solar radiation backscattered from the ocean waters. The radiation received by the sensor is contaminated by the specularly reflected solar radiation from the water surface. This specularly reflected radiation, called sunglint, contains no information on the water constituents, as it has not entered into the seawater and interacted with it. The intensity and spread of sunglint is determined by the solar illumination and sensor viewing directions and the sea surface roughness caused by the wind. For the accurate estimation of oceanic constituents, it is essential to minimize the sunglint in the detected radiances (preferably below ∼2-3%). In this work, sunglint simulations were computed for the instrument specifications of OCM and the optimal sensor viewing tilt angle identified for each month for the oceans around India.

Radiant heating rates and surface biology during the Arabian Sea Monsoon Experiment

Radiometric observations are carried out to estimate the net shortwave solar radiation in the upper ocean during the Arabian Sea Monsoon Experiment Phase II (ARMEX-II). Radiant heating rates in the Arabian Sea Warm Pool (ASWP) region from in situ radiometric measurements are presented for the first time. The estimated shortwave fluxes (300–700 nm) from the observed data are 101, 62, 40 and 29 W m −2 at 10, 20, 30 and 40 m depths respectively. About 5% of the surface light reaches below 50 m. Heating of the water column by penetrating solar radiation is substantial and estimated to be 0.18, 0.11, 0.08 and 0.07 ◦ Cd ay −1 in the upper 10, 20, 30 and 40 m respectively. Simultaneous observations from space-based sensors (SeaWiFS and IRS-P4 Ocean Colour Monitor) show an increase in solar absorption in regions with enhanced concentration of biologically active constituents.

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.

Aerosol optical depth modulations by lower tropospheric meteorological fields

The role of local production and transport in governing the spatial distribution of aerosol loading over the Indian subcontinent and adjoining oceanic regions are examined using satellite derived aerosol optical depth (AOD) and wind speed obtained from National Centre for Environmental Prediction (NCEP) reanalysis for the year 2004 (study domain: 20S°-40°N, 40°-120°N). The association of AOD with wind speed, wind convergence, vorticity and vertical velocity are examined for the Arabian Sea, Bay of Bengal, equatorial Indian Ocean as well as over the land surface in central India for different seasons. This analysis shows that the AOD dependence on these dynamical variables are significantly different for different seasons. Introducing MODIS derived AOD and the NCEP winds into an aerosol flux continuity equation, the locations of aerosol generation are identified and their strengths estimated for different months.

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.