Assessment of the ASCAT wind error characteristics by global dropwindsonde observations and examination its impact on tropical cyclone simulations

This research focus on the examination of the accuracy the ASCAT wind vector around the tropical cyclone by the dropwindsondes deployed by the DOTSTAR (Dropwindsonde Observations for Typhoon Surveillance near the TAiwan Region), NOAA/HRD (National Oceanic and Atmospheric Administration, Hurricane Research Division) during 2007-2010, T-PARC (THORPEX Pacific Asian Regional Campaign) in 2008, and IOP (Impacts of Typhoons on the Ocean in the Pacific) in 2010. For the result of the accuracy of ASCAT wind as comparing with the dropwindsondes data observed from three projects, the bias and root-mean-squared differences of wind speed between ASCAT and dropwindsonde data were -2.0 m s^-1 and 4.7 m s^-1 based on the calculation from 754 matching samples. The matching samples are defined when the time and space difference between ASCAT wind vector and dropsondes observation are within 1 hour and 12.5 km. Further analyses also indicate that large wind direction differences occur for low wind regime (below 12 m s^-1), and large wind speed differences occur for higher wind regime (above 18 m s^-1). The negative wind speed bias becomes larger when wind speed increase. Moreover, the accuracies of wind vector in weak and high wind regimes are significantly degraded in wet condition, while, those in middle wind regime (between 12 and 18 m s^-1) has much better quality, in good agreement with the satellite's design specification. Because the ASCAT-dropwindsonde matched samples are located around tropical cyclones with high wind conditions, the overall error characteristics are larger than previous studies on the validation of the scatterometer wind. Nevertheless, results from this study imply that ASCAT winds around the 12-18 m s^-1 have high enough accuracy for the application to determine the critical wind radius of 34-knot wind. Furthermore, a new error bound of the ASCAT wind estimate in stronger wind regime near tropical cyclones is suggested to be set at about 8 m s^-1. Such error characteristic of ASCAT wind has been applied to the WRF-VAR data assimilation system to evaluate the impact of the ASCAT data on tropical cyclone simulations.