Study of wind speed attenuation at Kavaratti Island using land-based, offshore, and satellite measurements

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

Monsoon and cyclone induced wave climate over the near shore waters off Puduchery, south western Bay of Bengal

Seasonal and annual variations in wave characteristics are studied based on measurements during 2009-2011 using wave rider buoy. Presence of swells generated by the south Indian Ocean cyclones are found over the south western Bay of Bengal during pre-monsoon season. Maximum significant wave height is measured 6 m during Thane cyclone in December, 2011. Long period waves are observed mostly during the summer (SW) monsoon and are negligible in the winter (NE) monsoon period. Short period waves dominate (63) the wave climate during the NE monsoon. Wave spectra during the SW monsoon are multi peaked whereas during the post monsoon season single peaked spectra are found. Single peaked spectra observed during SW monsoon of 2011 coincides with the presence of positive Indian Ocean Dipole. Waves during the pre monsoon and SW monsoon season are influenced by sea breeze. Analysis indicates that directional width is minimum for waves from the NE since they are wind waves and maximum for waves from SE since they are swells. Study indicates that wave climate of the south western Bay of Bengal is in contrast to that in eastern Arabian Sea during the SW monsoon

Wave forecasting and monitoring during very severe cyclone Phailin in the Bay of Bengal

Wave fields, both measured and forecast during the very severe cyclone Phailin, are discussed in this communication. Waves having maximum height of 13.54 m were recorded at Gopalpur, the landfall point of the cyclone. The forecast and observed significant wave heights matched well at Gopalpur with correlation coefficient of 0.98, RMS error of 0.35 m and scatter index of 14%. Forecasts were also validated in the open ocean and found to be reliable (scatter index < 15%). The study also revealed the presence of Southern Ocean swells with a peak period of 20-22 sec hitting Gopalpur coast along with the cyclone-generated waves

Hindcasting and Validation of Mumbai Oil Spills using GNOME

Oil spill trajectory forecasting became mandatory for providing advisory services to the regulatory authorities during the event of oil spill, for planning their remediation and clean up measures. The present study describes a method to simulate the trajectory of the spilled oil using GNOME and validating it using available Radar data. The trajectory forecasting of two oil spill events, happened in mumbai high region, during 2010- 2011 has been executed in hindcast mode using General NOAA Operational Modeling Environment. The forcing parameters such as, forecasted European Center of Medium Range Weather Forecast winds and Regional Ocean Modeling system currents were used for the execution. The likely areas which are to be affected are found from the prediction. The trajectory obtained from GNOME is compared with oil spill signatures obtained from the radar data of a particular time step. The observed oil slicks were found within the average distance of 3.73 km and 4.16 km from the prediction for MSC chitra spill and Mumbai uran trunk pipeline spill respectively. This trajectory model can be used for making the contingency plans, conducting the mock drills and during oil spill response and preparedness operation

Influence of winds on temporally varying short and long period gravity waves in the near shore regions of the eastern Arabian Sea

Wave data collected off Ratnagiri, west coast of India, during 1 May 2010 to 30 April 2012 are used in this study. Seasonal and annual variations in wave data controlled by the local wind system such as sea breeze and land breeze, and remote wind generated long period waves are also studied. The role of sea breeze on the sea state during pre-and postmonsoon seasons is studied and it is found that the maximum wave height is observed at 15:00 UTC during the premonsoon season, with an estimated difference in time lag of 1-2 h in maximum wave height between premonsoon and postmonsoon seasons. Observed waves are classified in to (i) short waves (Tp &lt;8 s), (ii) intermediate waves (8 &lt; Tp&lt; 13 s), and (iii) long waves (&lt;T&lt; p 13 s) based on peak period (Tp) and the percentages of occurrence of each category are estimated. Long period waves are observed mainly during the pre-and the postmonsoon seasons. During the southwest monsoon period, the waves with period &gt; 13 s are a minimum. An event during 2011 is identified as swells propagated from the Southern Ocean with an estimated travelling time of 5-6 days. The swells reaching the Arabian Sea from the south Indian Ocean and Southern Ocean, due to storms during the pre-and postmonsoon periods, modify the near surface winds due to higher phase wave celerity than the wind speed. Estimation of inverse wave age using large-scale winds such as NCEP (National Centers for Environmental Prediction) reflects the presence of cyclonic activity during pre-and postmonsoon seasons but not the effect of the local sea breeze/land breeze wind system

Evaluation of oil spill trajectory model with the observed SVP drifter track

Indian National Centre for Ocean Information Services (INCOIS) collaborated with Indian Coast Guard(ICG) and conducted Surface Velocity Program (SVP) drifter experiment at Mumbai High region for evaluating the operational oil spill trajectory model. INCOIS adopted General National Oceanic and Atmospheric Administration(NOAA) Operational Modelling Environment(GNOME) from NOAA and customised it in diagnostic mode for Indian ocean. GNOME was operationalised during May 2014. The objective of this experiment is to compare the drift pattern obtained from an oil spill trajectory model with the observed drifter track. SVP drifter was procured from M/s. Pacific Gyre, USA. It gives the Lagrangian trajectory path over the ocean. It was deployed by officials of Indian Coast Guard at Mumbai High region on 20 November 2016,12.45 hrs at 72.2295º E, 18.91035º N off Mumbai. It gave its drifted path along the west coast of India for ten days, before it beached near Diu on 3 December 2016.This observed track was considered for comparing the simulated positions obtained from GNOME when forced with currents of different ocean general circulation models. The results show that the positions of the drifter obtained, while forced with analysed currents of GODAS -MOM4p1 (GM4p1) and Hybrid Co-ordinate Ocean Model (HYCOM) are found to be in better agreement with the actual position of the drifter

Observational evidence of summer shamal swells along the west coast of India

Wave data collected off Ratnagiri, which is on the west coast of India, in 2010 and 2011 are used to examinethe presence of the summer shamal swells. This study also aims to understand variations in wave characteristics and associated modifications in wind sea propagation at Ratnagiri. Wind data collected using an autonomous weather station (AWS), along with Advanced Scatterometer (ASCAT) and NCEP data, areused to identify the presence of summer shamal winds along the west coast f the Indian subcontinent and on the Arabian Peninsula. NCEP and ASCAT data indicate the presence of summer shamal winds over the Arabian Peninsula and northwesterly winds at Ratnagiri. This study identifies the presence of swells from the northwest that originate from the summer shamal winds in the Persian Gulf and that reach Ratnagiri during 30 of the summer shamal period. AWS data show the presence of northwest winds during May and southwest winds during the strong southwest monsoon period (June-August). Another important factor identified at Ratnagiri that is associated with the summer shamal events is the direction of wind sea waves.During the onset of the southwest monsoon (May), the sea direction is in the direction of swell propagation (northwest); however, during the southwest monsoon (June-August), a major part of the wind sea direction is from the southwest. The average occurrence of summer shamal swells is approximately 22 during the southwest monsoon period. An increase in wave height is observed during June and July at Ratnagiri due to the strong summer shamal event

Simulation of coastal winds along the central west coast of India using the MM5 mesoscale model

A high-resolution mesoscale numerical model (MM5) has been used to study the coastal atmospheric circulation of the central west coast of India, and Goa in particular. The model is employed with three nested domains. The innermost domain of 3 km mesh covers Goa and the surrounding region. Simulations have been carried out for three different seasons-northeast (NE) monsoon, transition period and southwest (SW) monsoon with appropriate physics options to understand the coastal wind system. The simulated wind speed and direction match well with the observations. The model winds show the presence of a sea breeze during the NE monsoon season and transition period, and its absence during the SW monsoon season. In the winter period, the synoptic flow is northeasterly (offshore) and it weakens the sea breeze (onshore flow) resulting in less diurnal variation, while during the transition period, the synoptic flow is onshore and it intensifies the sea breeze. During the northeast monsoon at an altitude of above 750 m, the wind direction reverses, and this is the upper return current, indicating the vertical extent of the sea breeze. A well-developed land sea breeze circulation occurs during the transition period, with vertical extension of 300 and 1,100 m, respectively

Coastal sea level response to the tropical cyclonic forcing in the northern Indian Ocean

The study examines the observed storm-generated sea level variation due to deep depression (event 1: E1) in the Arabian Sea from 26 November to 1 December 2011 and a cyclonic storm "THANE" (event 2: E2) over the Bay of Bengal during 25–31 December 2011. The sea level and surface meteorological measurements collected during these extreme events exhibit strong synoptic disturbances leading to storm surges of up to 43 cm on the west coast and 29 cm on the east coast of India due to E1 and E2. E1 generated sea level oscillations at the measuring stations on the west coast (Ratnagiri, Verem and Karwar) and east coast (Mandapam and Tuticorin) of India with significant energy bands centred at periods of 92, 43 and 23 min. The storm surge is a well-defined peak with a half-amplitude width of 20, 28 and 26 h at Ratnagiri, Verem and Karwar, respectively. However, on the east coast, the sea level oscillations during Thane were similar to those during calm period except for more energy in bands centred at periods of ~ 100, 42 and 24 min at Gopalpur, Gangavaram and Kakinada, respectively. The residual sea levels from tide gauge stations in Arabian Sea have been identified as Kelvin-type surges propagating northwards at a speed of ~ 6.5 m s−1 with a surge peak of almost constant amplitude. Multi-linear regression analysis shows that the local surface meteorological data (daily mean wind and atmospheric pressure) is able to account for ~ 57 and ~ 69% of daily mean sea level variability along the east and west coasts of India. The remaining part of the variability observed in the sea level may be attributed to local coastal currents and remote forcin