• Mannil Mohan

      Articles written in Journal of Earth System Science

    • Selection of optimum frequency of a wind scatterometer

      Abhijit Sarkar Raj Kumar Mannil Mohan

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      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

      Abhijit Sarkar Raj Kumar Mannil Mohan

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    • Influence of circulation parameters on the AOD variations over the Bay of Bengal during ICARB

      Marina Aloysius Mannil Mohan S Suresh Babu Vijayakumar S Nair K Parameswaran K Krishna Moorthy

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      MODIS (Moderate Resolution Imaging Spectroradiometer) level-3 aerosol data, NCEP (National Centers for Environmental Prediction) reanalysis winds and QuikSCAT ocean surface winds were made use of to examine the role of atmospheric circulation in governing aerosol variations over the Bay of Bengal (BoB) during the first phase of the ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) campaign (March 18–April 12, 2006). An inter-comparison between MODIS level-3 aerosol optical depth (AOD) data and ship-borne MICROTOPS measurements showed good agreement with correlation 0.92 (𝑝 > 0.0001) and a mean MODIS underestimation by 0.01. During the study period, the AOD over BoB showed high values in the northern/north western regions, which reduced towards the central and southern BoB. The wind patterns in lower atmospheric layers (< 850 hPa) indicated that direct transport of aerosols from central India was inhibited by the presence of a high pressure and a divergence over BoB in the lower altitudes. On the other hand, in the upper atmospheric levels, winds from central and northern India stretched south eastwards and converged over BoB with a negative vorticity indicative of a downdraft. These wind patterns pointed to the possibility of aerosol transport from central India to BoB by upper level winds. This mechanism was further confirmed by the significant correlations that AOD variations over BoB showed with aerosol flux convergence and flux vorticity at upper atmospheric levels (600–500 hPa). AOD in central and southern BoB away from continental influences displayed an exponential dependence on the QuikSCAT measured ocean surface wind speed. This study shows that particles transported from central and northern India by upper atmospheric circulations as well as the marine aerosols generated by ocean surface winds contributed to the AOD over the BoB during the first phase of ICARB.

    • Role of dynamics in the advection of aerosols over the Arabian Sea along the west coast of peninsular India during pre-monsoon season: A case study based on satellite data and regional climate model

      Marina Aloysius S Sijikumar S S Prijith Mannil Mohan K Parameswaran

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      The circulation dynamics of an event marked by the formation of an aerosol cluster off the coast of Maharashtra on April 22, 2006, its southward migration along the Indian west coast with a mean speed of ∼200 km/day and its final dissipation after reaching the end of the peninsula by April 28, 2006 as revealed by MODIS (Moderate Resolution Imaging Spectroradiometer) against the pre-monsoon conditions of April 2006 are examined in this study. The maximum aerosol concentration in the cluster was found getting confined to lower and lower altitudes during its southward movement. The NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis wind field indicates that the atmospheric circulation, especially the horizontal wind convergence is the major factor that guides the formation and the dynamics of the cluster. Fine mode fraction from MODIS suggests that the cluster mainly consists of coarse dust particles. The regional climate model, RegCM3 with an efficient dust generation module simulates the formation and movement of the cluster appreciably well. The simulations which also exhibit the altitudinally descending nature of the cluster during its southward movement confirm the mechanism which governs the cluster dynamics suggested based on MODIS and NCEP/NCAR reanalysis data.

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