Harilal B. Menon

Temporal variation of aerosol optical depth and associated shortwave radiative forcing over a coastal site along the west coast of India

Optical characterization of aerosol was performed by assessing the columnar aerosol optical depth (AOD) and angstrom wavelength exponent (α) using data from the Microtops II Sunphotometer. The data were collected on cloud free days over Goa, a coastal site along the west coast of India, from January to December 2008. Along with the composite aerosol, the black carbon (BC) mass concentration from the Aethalometer was also analyzed. The AOD0.500 μm and angstrom wavelength exponent (α) were in the range of 0.26 to 0.7 and 0.52 to 1.33, respectively, indicative of a significant seasonal shift in aerosol characteristics during the study period. The monthly mean AOD0.500 μm exhibited a bi-modal distribution, with a primary peak in April (0.7) and a secondary peak in October (0.54), whereas the minimum of 0.26 was observed in May. The monthly mean BC mass concentration varied between 0.31 μg/m(3) and 4.5 μg/m(3), and the single scattering albedo (SSA), estimated using the OPAC model, ranged from 0.87 to 0.97. Modeled aerosol optical properties were used to estimate the direct aerosol shortwave radiative forcing (DASRF) in the wavelength range 0.25 μm4.0 μm. The monthly mean forcing at the surface, at the top of the atmosphere (TOA) and in the atmosphere varied between -14.1 Wm(-2) and -35.6 Wm(-2), -6.7 Wm(-2) and -13.4 Wm(-2) and 5.5 Wm(-2) to 22.5 Wm(-2), respectively. These results indicate that the annual SSA cycle in the atmosphere is regulated by BC (absorbing aerosol), resulting in a positive forcing; however, the surface forcing was governed by the natural aerosol scattering, which yielded a negative forcing. These two conditions neutralized, resulting in a negative forcing at the TOA that remains nearly constant throughout the year.

Variability of remote sensing reflectance and implications for optical remote sensing—A study along the eastern and northeastern waters of Arabian Sea

[1] Situated in the eastern and northeastern regions and subjected to similar oceanic processes, the inshore waters north of 20°N and east of 65°E are more optically complex than the offshore waters and waters south of 20°N and east of 65°E. This has been observed through the analysis of variations of the remote sensing reflectance (R rs (λ)) in the optical spectrum of Electromagnetic radiation. The optical complexity has further been studied through the examination of optically active substances (OAS) such as chlorophyll_a, suspended sediment and coloured dissolved organic matter (CDOM). It is found that CDOM is the significant component in making the area optically nonlinear. For the first time multi-spatial/temporal band-ratio algorithms are developed to map OAS from these waters through Ocean Colour Monitor flown on IRS - P4 satellite.

Directional spectrum of ocean waves from array measurements using phase/time/path difference methods

Abstract Wave direction has for the first time been consistently, accurately and unambiguously evaluated from array measurements using the phase/time/path difference (PTPD) methods of Esteva in case of polygonal arrays and Borgman in case of linear arrays. We have used time series measurements of water surface elevation at a 15-gauge polygonal array, in ≅8 m water depth, operational at the CERCs Field Research Facility at Duck, North Carolina, USA. Two modifications have been made in the methodology. One modification is that we use the true phase instead of the apparent phase, the other modification being that estimates of wave direction are registered only if the relevant gauges in the array are coherent at 0.01 significance level. PTPD methods assume that in a spectral frequency band the waves approach from a single direction, and are simple, expedient and provide redundant estimates of wave direction. Using Estevas method with the above modifications, we found that at Duck: (i) the directions of swell and surf beat, when energetic swell is present, conform to the schematic diagram of surf beat generation given by Herbers et al., (ii) surf beat of remote origin occurs when the significant wave height, H mo , falls below 0.41 m, (iii) the surf beat of remote origin is not normally incident at the shore contrary to Herbers et al. In fact we found that the surf beat of remote origin is incident at angles in excess of 45° with respect to the shore normal, and (iv) the surf beat of remote origin is largely trans-oceanic in origin.

Calibration of an optical equation to analyse the atmospheric turbidity and water quality of an estuarine environment

Mapping of oceanic productive zones through an optical sensor on board a satellite requires a detailed knowledge about the interaction of solar flux with the atmosphere and water column. Hence the radiative transfer on February 14, 2002 over estuaries of Goa has been examined through a couple radiative transfer model (CRTM). This has been carried out in the context of an optical sensor when it looks down at nadir. The forcing of extraterrestrial solar irradiance with atmosphere results in its absorption and scattering due to aerosol particles and gaseous constituents and rayleigh particles. The optical spectrum has been subjected to atmospheric forcing and radiation has been calculated for every 1 nm. Special reference is given to two wavelengths (443 and 510 nm), which are important for ocean optics. The atmospheric components pertaining to these wavelengths are 0.04193 w/m2/nm/sr and 0.0225 w/m2/nm/sr (rayleigh) and 0.0121 w/m2/nm/sr and 0.015 w/m2/nm/sr (aerosol). Within the optical frame work (400 –700 nm), the effect of aerosol overrides that of rayleigh from 545 nm onwards. The specular reflectance at the sea surface contributes 1.725 x l0-12 w/m2/nm/sr and 1.947x 10-12 w/m2/nm/sr while the respective signature from water column is 0.418x 10-3w/m2/nm/sr and 0.536x 10-3 w/m2/nm/sr. The simulated water signature has been compared with radiances measured by a radiometer (Satlantic radiometer). The correlation (R2) is found to be around 0.97. The sensitivity analysis further reveals the respective wavelengths of the optical spectrum affected by atmospheric and oceanic constituents. Moreover, the effect of change in solar zenith angle on water signature has also been carried out. This could be applied to invert satellite data so as to retrieve coastal water constituents and productivity.

Wave directional spectrum from SAR imagery

Gaussian smoothed SAR image spectra have been evaluated from 512 x 512 pixel subscenes of image mode ERS-1 SAR scenes off Goa, Visakhapatnam, Paradeep and Portugal. The two recently acquired scenes off Portugal showed the signature of swell of wavelength ≈ 200m and internal waves of wavelength > 400m. Only internal waves of wavelength > 400m were seen in the scene off Goa observed on 11 March 1992. The scenes off Visakhapatnam and Paradeep did not show any wave like features, the latter appearing to be of “white noise” nature.There exists a 180° ambiguity in wave direction observed from radar imageries. Based on the method of Atanassov et al (1985), a computer program has been developed for the removal of the 180° ambiguity by using two images of the same area separated by a time interval which is small compared to the period of the dominant waves. The computer program has been successfully tested with computer simulated images.