Oceansat-2 Scatterometer Research Articles

Validation of ocean surface winds from the OCEANSAT-2 scatterometer using triple collocation

Ocean surface winds from the OCEANSAT-2 scatterometer (OSCAT) were validated with equivalent neutral wind observations from 87 global buoys and winds from the European Centre for Medium Range Weather Forecasting (ECMWF) Numerical Weather Prediction (NWP) model using triple collocation for a period of 9 months. Functional relationship analysis (FRA) employing the error-in-variables method is found to be more ‘exact’ in comparison with classical linear regression analysis for the validation of the OSCAT data. Moreover, using the wind component domain for validation and error assessment rather than the speed and direction domain is confirmed to be favourable. The FRA method applied on the triple-collocated wind components shows that the error standard deviations of the OSCAT and buoy winds are quite similar. The calibration trends and biases for OSCAT, buoys and ECMWF are found to be close to unity and zero, respectively.

Comparison of oceanic winds measured by space-borne scatterometers and altimeters

Global ocean surface wind speeds from Quick Scatterometer (QuikSCAT) for 5 years (2005–2009) and from Oceansat-2 Scatterometer (OSCAT) for 1 year (2010) were compared with wind speeds estimated from JASON Altimeters for representative months to investigate the consistency in wind speeds between these sensors. The comparison was carried out through statistical analyses. The spatial window used for comparison with JASON was 0.25° for QuikSCAT and 0.5° for OSCAT, while the temporal window for both QuikSCAT and OSCAT was ±30 min. The results of the inter-comparison indicate that OSCAT wind speeds are almost as consistent with JASON as QuikSCAT wind speeds.

Comparison of Oceansat-2 scatterometer winds with buoy observations over the Indian Ocean and the Pacific Ocean

Oceansat-2 scatterometer (OSCAT)-derived winds were compared with in situ observations made by Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) and Triangle Trans-Ocean Buoy Network (TRITON) buoys in the Indian Ocean and the Pacific Ocean, respectively. Root mean square deviation (RMSD) for wind speed is 1.71 m s−1 for the Indian Ocean and 1.92 m s−1 for the Pacific Ocean. The wind speed accuracies are within the mission requirement (<2 m s−1), but the wind direction errors are higher than the mission requirement (>20°). The RMSD values of wind direction are 51.38° and 44.61° for the Indian Ocean and the Pacific Ocean, respectively. OSCAT winds tend to underestimate the buoy value when the wind speeds were low (<4 m s−1) and overestimate the buoy value when the wind speeds were high (>4 m s−1). The wind speed residuals show an increasing trend with the increase of wind speeds. Interestingly, the wind direction residuals decrease with increasing wind speed and deviations of wind directions are conspicuously large for lower wind speeds than for higher wind speeds. OSCAT wind speeds were relatively more accurate for the Indian Ocean than those for the Pacific Ocean.

Generation and validation of analysed wind vectors over the global oceans

Daily and 12 hourly gridded analysed wind vectors (AWVs) over global ocean were generated using the simple spatial interpolation scheme of ‘box averaging’, with a horizontal resolution of 0.5° × 0.5°. For daily analysed winds, observations only from Oceansat-2 Scatterometer (OSCAT) were used. The 12 hourly AWVs were generated by combining the data from both OSCAT and Advanced Scatterometer (ASCAT). Apart from ocean wind vectors, an effort was made also to produce analysed wind stress, divergence and curl of wind stress. The daily and 12 hourly analysed winds were validated using in situ observations from 97 global moored buoys and data from European Centre for Medium Range Weather Forecasting analyses for a period of 9 months. The validation result shows a good agreement between AWV products and the buoy and model analysis data, yielding a standard deviation of around 2 m s−1 in wind speed and around 20° in wind direction.

Intra-seasonal variability in Oceansat-2 scatterometer sea-surface winds over the Indian summer monsoon region

In September 2009, the Indian Space Research Organisation launched a Ku-band microwave scatterometer (OSCAT) onboard the polar orbiting satellite ‘Oceansat-2’. In this article, the capabilities of the newly available OSCAT sea-surface winds are demonstrated by studying the monsoon intra-seasonal variabilities during the 2010 summer monsoon season. A preliminary validation of OSCAT surface winds with European Centre for Medium Range Weather Forecasting (ECMWF) analysis surface winds carried out during June to August 2010 suggests that the quality of the OSCAT winds are able to meet the mission specifications. The observed mean monthly features of the Indian summer monsoon in July and August 2010 from OSCAT match well with those of ECMWF reanalysis winds. The OSCAT winds capture the known characteristics of the Indian summer monsoon, such as the northward propagation of a low level jet, and its preferred locations during active and break monsoon conditions, reasonably well. The Morlet wavelet transform is used for time series analysis. The OSCAT measured sea-surface winds were found to possess two dominant modes of variability during the 2010 monsoon season: one with a periodicity between 32 and 64 days, and another with a periodicity between 8 and 16 days. Rainfall activity over the Indian summer monsoon region is closely associated with the phases of the two above-mentioned dominant intra-seasonal variabilities. This study demonstrates that the OSCAT winds can be used very well and with confidence for meteorological studies.

Assimilation of Oceansat-2-Scatterometer-Derived Surface Winds in the Weather Research and Forecasting Model

This paper describes, for the first time, the impact of Oceansat-2 scatterometer (OSCAT) surface winds in the Weather Research and Forecasting (WRF) 3-D variational (3-D-Var) assimilation system. Before using OSCAT winds into WRF assimilation system, we compared OSCAT surface wind retrievals against National Centers for Environmental Prediction analyzed winds, Advanced Scatterometer retrievals, and buoy-measured winds. After the initial assessment of the quality of the OSCAT winds, the control (CNT) (without OSCAT) as well as experimental (which assimilated OSCAT surface winds) runs were made for 48 h starting daily at 0600 Universal Time Coordinated (UTC) during July 2010. The assimilation experiments demonstrated positive impact of OSCAT winds on both the analysis state as well as subsequent short-term forecasts of surface winds. Compared to CNT run, the assimilation of the OSCAT winds improved the surface wind analysis as large as 25%, when compared with Advanced Microwave Scanning Radiometer (AMSR-E) measured winds. The assimilation of OSCAT winds also showed small, but positive, impact on the forecast (particularly later hours of forecast) of midtropospheric moisture, temperature, and upper tropospheric winds. Compared to the CNT run, the assimilation of OSCAT winds improved precipitation forecast for moderate to heavy rainfall thresholds when validated against Tropical Rainfall Measuring Mission precipitation.

Variability of scatterometer based surface vorticity over Arabian Sea and its use in monsoon onset forecasting

This paper presents the results of the study undertaken to investigate the changes in the surface wind and vorticity fields over south Arabian Sea and adjoining north Indian Ocean and their association with monsoon onset over Kerala. For this purpose QuikSCAT daily gridded data of recent 10 years (2000-2009) have been utilized. The results show that there is a rising trend in the surface wind speed and vorticity over south Arabian Sea about 4-7 days before the monsoon onset over Kerala. Thus, the rising trends in the surface wind speed and vorticity over south Arabian Sea seem to be a good precursor of monsoon onset over Kerala. Continuous monitoring of Oceansat-2 scatterometer surface wind and vorticity fields over south Arabian Sea and adjoining north Indian Ocean during 15th May to 15th June could provide predictive indications of monsoon onset over Kerala on medium range scale.

Relative Calibration Using Natural Terrestrial Targets: A Preparation Towards Oceansat-2 Scatterometer

Scatterometer instruments transmit a series of microwave pulses and measure the returned echo to determine the normalized radar cross section (σ0) over the target to derive the near-ocean-surface wind vector. Accuracy of the derived wind vector over the data sparse oceans therefore depends on the accuracy of σ0 measurement. For this purpose, accurate calibration of the scatterometer is required. As a preparation toward calibration of the Oceansat-2 mission, of the Indian Space Research Organisation, a relative calibration technique has been proposed in this study by selecting homogeneous areas over the globe with isotropic radar response and temporally stable signature of σ0. For this purpose, the daily averaged σ0 and Level-2A (L2A) σ0 measurements of the QuikSCAT scatterometer have been used. Analyzing the monthly mean and standard deviation in σ0 for the period of 2005-2006, several regions are chosen which have a quasi-isotropic radar response and minimal temporal variation in σ0. The analysis shows that the selected areas over Antarctica and Greenland with permanent ice covers have temporally stable signatures of σ0. The regions like the Amazon forests and parts of Australia also show high temporal stability of σ0 but greater standard deviation than the snow-covered areas. The QuikSCAT L2A data have also been used to study the day-night variation and azimuthal dependence of the σ0 over these targets. The present work demonstrated that quasi-uniform natural sites such as Sahara, Amazon forest, Kutch, Greenland region, and Antarctica region, covering wide dynamic range of σ0, can be used for the purpose of calibration.

Spatio-Temporal Coherence Based Technique for Near-Real Time Sea-Ice Identification from Scatterometer Data

The identification of sea-ice has frequently been cited as one of the most important tasks for deriving the sea-ice parameters and to avoid erroneous retrieval of wind vector over sea-ice infested oceans using space-borne scatterometer data. Discrimination between sea-ice and ocean is ambiguous under the high wind and/or thin/scattered ice conditions. The pre-launch technique developed for Oceansat-2, utilizes the dual-polarized QuikSCAT scatterometer data by using the spatio-temporal coherence properties of sea ice in addition to backscatter coefficient and the Active Polarization Ratio. Results were compared with the operational sea-ice products from National Snow and Ice Data Center. The threshold API value of −0.025 was found optimum for sea-ice and ocean discrimination. The overall sea-ice identification accuracy achieved was of the order of 95 per cent, ranging from 92.5% (during December in Southern Hemisphere) to 98% (during March in Northern Hemisphere). The applicability of the algorithm for both the Arctic as well as Antarctic makes it suitable for its operational use with the Oceansat-2 scatterometer data.