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Title: Study of wind speed attenuation at Kavaratti Island using land-based, offshore, and satellite measurements
Authors: Joseph A 
Rivankar P 
Balakrishnan Nair TM 
Keywords: Satellites;Signal attenuation;Wind measurement;Photo activated localization microscopy;Turbulence;Wind energy;Sensors;Fluid dynamics;Imaging systems
Issue Date: 2012
Publisher: Proceedings of SPIE - The International Society for Optical Engineering
Project: Ocean Sensing and Monitoring IV 
Maryland, USA 
April 23, 2012 
Abstract: The role of dense coconut palms in attenuating the wind speed at Kavaratti Island, which is located in the southeastern Arabian Sea, is examined based on land-based and offshore wind measurements (U10) using anchored-buoy-mounted and satellite-borne sensors (QuikSCAT scatterometer and TMI microwave imager) during an 8-year period (2000-2007). It is found that round the year monthly-mean wind speed measurements from the Port Control Tower (PCT) located within the coconut palm farm at the Kavaratti Island are weaker by 15-61 relative to those made from the nearby offshore region. Whereas wind speed attenuation at the island is ~15-40 in the mid-June to mid-October south-west monsoon period, it is ~41-61 during the rest of the year. Wind direction measurements from all the devices overlapped, except in March-April during which the buoy measurements deviated from the other measurements by ~20o. U10 wind speed measurements from PCT during the November 2009 tropical cyclone "Phyan" indicated approximately 50-80 attenuation relative to those from the seaward boundary of the island's lagoon (and therefore least influenced by the coconut palms). The observed wind speed attenuation can be understood through the theory of free turbulent flow jets embodied in the boundary-layer fluid dynamics, according to which both the axial and transverse components of the efflux of flows discharged through the inter-leaves porosity (orifice) undergo increasing attenuation in the downstream direction with increasing distance from the orifice. Thus, the observed wind speed attenuation at Kavaratti Island is attributable to the decline in wind energy transmission from the seaward boundary of the coconut palm farm with distance into the farm. Just like mangrove forests function as bio-shields against forces from oceanic waves and stormsurges through their large above-ground aerial root systems and standing crop, and thereby playing a distinctive role in ameliorating the effects of catastrophies such as hurricanes, tidal bores, cyclones, and tsunamis, the present study provides an indication that densely populated coconut palms and other tall tree vegetation would function as bio-shields against the damaging effects of storms through attenuation of wind speed.
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