Evaluation of QuikSCAT wind vector performance with respect to field measurements for the Bulgarian part of the Black Sea

Winds over the ocean play an important role in meteorology, oceanography and climatology. They affect air-sea variations in heat, humidity, gases and particles, regulating the crucial relation between the ocean and the atmosphere that establishes and supports the climate on regional and global scale. Therefore, the knowledge of wind fields over the oceans is essential for global weather forecast purposes. Furthermore, the accurate forecasting of ocean surface winds contribute to good wave forecasts, which provide informativeness and safety in marine industry, navigation and other human activities in open and off-shore seas. The sources of surface observations that are generally used to make analyses of winds over the oceans not always can provide data with temporal and spatial regularity. These gaps can be filled in with wind data registered by satellite sensors like NASA\´s QuikSCAT satellite radar scatterometer. It provides on operational basis wind vector measurements over global oceans with considerable reliability and sufficient time frequency. The standard operational QuikSCAT wind vector product has a spatial resolution of 25 km times 25 km. However, the recent measurement methodology of QuikSCAT allows for wind vector retrievals at better spatial resolution of 12.5 km. In the present paper the performance of the latest QuikSCAT Level 2B (L2B) 12.5 km wind product was evaluated with respect to wind data acquired from meteorological station mounted on a fixed earth gas exploration platform "Galata" in the western Black Sea. The QuikSCAT satellite occupies a sun-synchronous, near-polar orbit with altitude of 803 km above the Earth and with local equator crossing time at the ascending node of 6.00 a.m. +/-30 min and swath width of 1800 km. The mission requirements of QuikSCAT SeaWinds Scatterometer for wind measurements are an accuracy of plusmn2 m/s in speed for the range 3-20 m/s and plusmn20deg RMS in wind direction for wind speed ranging from 3-30 m/s. On th- e other hand, the Galata platform is located on the Bulgarian shelf, 26 km off cape Galata to the east of city of Varna. The satellite sensor measurements of wind speed and direction were validated against anemometer observations co- located in time and space to the satellite pass in 2007 for winter (January-February) and summer (June-July) periods. The QuikSCAT wind vector processing algorithm estimates the wind speed on 10 m level above the sea surface. To make the satellite wind speed data comparable to those derived from the anemometer, the latter were recalculated to 10 m level applying a correction to every measurement. The validation of scatterometer data of wind speed and direction for both abovementioned time periods was performed by statistical analysis including calculation of RMSE, bias and correlation. The results indicate a good agreement between satellite and in situ data for moderate winds (3-15 m/s). Estimated coefficients of correlation R are 0.88 and 0.96 for wind speed and wind direction, respectively. The highest RMS error was found for the light winds with speed less or equal to 3 m/s. The study results have confirmed the reliability of QuikSCAT L2B wind vector product for the western Black Sea conditions, which imply on its further use for meteorological, oceanographic, and climate researches and forecasts in regions of sparse in situ data such as the Black Sea.