Ocean Society of India Proceedings of National Conference OSICON 2013 "Role of Oceans in Earth System", IITM, Pune, 26-28 November 2013 Global Analysis of OSCAT winds for Sea-State prediction using Third Generation Wave Models J. Swain and P. A. Umesh Naval Physical & Oceanographic Laboratory (NPOL), Kochi – 682 021, India E-mail: drjswain2010@gmail.com Abstract Indian Space Research Organization (ISRO) launched Oceansat-2 (OSCAT) on 23 September 2009 and the scatterometer onboard is a space borne sensor capable of providing ocean surface winds (both speed and direction) over the globe. Observations of ocean surface winds from such space borne sensors are the potential source of data covering the global oceans suitable for driving the state-of-the-art numerical wave models for sea-state prediction. In this study, the OSCAT winds have been processed and analysed for a selected period. In the absence of gridded analysed winds, i.e the blend of OSCAT and outputs of a global weather prediction model, the OSCAT measured winds are subjected to spatio-temporal interpolation and the 6- hourly gridded wind fields are generated over 1x1 degree resolution to drive the wave models WAM and WAVEWATCH-III. However, the accuracy of input ocean surface winds is expected to have a perceptible influence on the predicted sea-state parameters. Attempts have been made to compare OSCAT interpolated winds with buoy measurements and found that they are reasonably in good agreement. It is also proposed further to blend the interpolated OSCAT wind field with the analysed winds of global weather prediction models for improved sea-state prediction. The work presented here was a part of the collaborative project between NPOL and SAC, Ahmedabad (Project: NPOL/SAC-II). 1 Introduction Knowledge of surface wind fields over the sea is essential for a range of oceanographic, meteorological, and climate investigations, as well as for improving regional and global operational weather predictions. The surface wind field is a key variable in estimating the exchanges of momentum (kinetic energy) between the atmosphere and sea. Surface wind is fundamental to estimate fluxes of heat longwave radiation, moisture across the interface, and for development, operationalisation and validation of coupled ocean and atmosphere models. The global ocean surface wind field is observable from space at near-mesoscale resolution. Measurement techniques using satellite-borne active microwave Scatterometers have been developed tested and being refined over the past three decades. Scatterometers are real aperture radar capable to perform a set of simultaneous Normalized Radar Cross Section (NRCS) measurements under different azimuth angles for each resolution cell at high radiometric accuracy. Oceansat- 2 is India’s second satellite built for the study of oceans as well as the interaction of oceans and the atmosphere to facilitate global and regional climatic studies [1]. Oceansat-2 is the second Indian satellite dedicated to ocean research, a continuity of the operational services of Oceansat-1 with high application potentials. It was launched from Satish Dhawan Space Centre, Sriharikota on 23rd September, 2009, using Polar Satellite Launch Vehicle (PSLV-C14). It has three payloads, (i) Ocean Colour Monitor (OCM), (ii) Ku-band Pencil Beam Scatterometer (OSCAT) developed by Indian Space Research Organization (ISRO), and (iii) Radio Occultation Sounder for Atmosphere (ROSA) developed by the Italian Space Agency. The mission goal of Oceansat-2 is to provide wind speed measurements of 4-24 m/s, with an accuracy of 2 m/s, and direction, with an accuracy of 20°. The scatterometer, a Ku-band pencil beam sensor similar to that onboard QuikSCAT satellite, provides surface vector winds over global oceans with a two days repeativity. Pencil beam scatterometer has the advantage of estimating wind direction at low wind speeds because of its viewing from four azimuth angles [2]. As stated before, accurate observations of surface wind stress and wind velocity over global oceans are required for a wide range of meteorological and oceanographic studies. Surface winds are essential for nowcasting of weather and sea state, as well as for improving weather forecasts via data assimilation in numerical weather prediction (NWP) models. Measurements of surface winds over global oceans still continue to be sparse in space and time, thus causing hurdles for improvements in numerical predictions. Development of space-based technologies and geophysical retrieval techniques over the past decades has resulted in significant improvement in the observation frequency of surface winds over global oceans. The most prominent among these is the space-borne scatterometer [3]. Space- based radiometers and altimeters are additional sources of ocean surface wind speed. More recently, synthetic aperture radar data have also been used to retrieve wind fields [4]. Limitations and strengths of wind-measuring capability of each type of sensor and technique are briefly described by Sarkar [5]. For weather forecasting with NWP models to be reliable, it is necessary that the input data (such as ocean surface winds) be drawn from sources that are mutually consistent. This study examines the consistency of oceanic winds measured by the new space-borne Oceansat-2 Scatterometer (OSCAT) with that measured by JASON Altimeter [6] and buoy measurements. The wind component standard deviations of OSCAT with respect to the European Centre for Medium-range Weather Forecasts (ECMWF) model winds and in situ buoy winds are better than 2.5 m/s [7]. Singh et al. [8] investigated the quality of the OSCAT winds by comparing them with the global model surface