Sea Surface Meteorological Parameters Retrieval Under Moderate And High Sea States Using Spaceborne Microwave Radiometer Measurements

At present, under moderate sea states, the spaceborne passive microwave radiometer observations can measure the sea surface meteorological parameters such as sea surface wind speed (SWS) over wide areas. However, in hurricane, typhoon, or other serve marine conditions, the radiometer measurements obtained at frequencies higher than 10.7 GHz are more affected by raindrops and atmospheric water vapor than those obtained at lower frequencies, and the microwave radiometer operational wind speed products are retrieved from the radiometer observations, which acquired at moderate and high frequencies. Therefore, the radiometer operational algorithm cannot obtain the sea surface wind speed information under high sea states.We present a method to retrieve wind speeds in hurricane and typhoon conditions from spaceborne passive microwave radiometer data. Brightness temperature (TB) observations acquired at the 6.9 GHz horizontal polarization channel by the AMSR-E and AMSR2 on board the Earth Observing System AQUA and Global Change Observation Mission-Water 1 satellites are selected for wind retrieval due to the fact that the signal at this frequency is sensitive to high wind speeds but less sensitive to raindrops and atmospheric water vapor than those acquired at other higher frequency channels, and the TB at a horizontal polarization are much more sensitive to SWS than at a vertical polarization. The AMSR-E and AMSR2 observations of 53 hurricanes between 2002 and 2014 are collected and collocated with Stepped Frequency Microwave Radiometer (SFMR) measurements. Based on the small slope approximation/small perturbation method (SSA/SPM) model and an high sea state surface roughness spectrum (H13 spectrum), the wind speeds are retrieved from the TB data and validated against the SFMR measurements.The statistical comparison of the entire data set shows that the bias and root-mean-square error (RMSE) of the retrieved wind speeds are 1.11 and 4.34 m/s, respectively, which suggests that the proposed method can obtain high wind speeds under hurricane and typhoon conditions. Two case studies show that the wind speed retrieval bias and RMSE are 1.08 and 3.93 m/s for Hurricane Earl, and 0.09 and 3.23 m/s for Hurricane Edouard, respectively. The retrieved wind speeds from the AMSR-E and AMSR2 continuous three-day observations clearly show the process of hurricane intensification and weakening. Moreover, the retrieved wind speeds for typhoon Muifa (2011) and Chan-hom (2015) also illustrated that our algorithm can applied to monitor the process of typhoon.under extreme marine conditions, we also use the AMSR-E and AMSR2 TB measurements acquired at the 6.9 GHz horizontal polarization and vertical polarization channels also to retrieve the sea surface temperature (SST), the SST statistical comparison of the entire dataset shows that the bias and RMSE are 0 and 2.52℃. In our study, the retrieval algorithm also be applied to retrieve SWS and SST under moderate sea states, the results also illustrated that our algorithm can obtain the accurate SWS and SST information.There are some factors can affect the SWS and SST retrieval errors. Examples of these factors included the SST, Fresnel reflection coefficient, ocean surface roughness spectrum, and rain effects. Further studies are certainly needed to correct these factors, and separate rain-induced TB from the radiometer TB observations under hurricane conditions, which has the potential to improve high wind speed retrieval accuracy particular in the hurricane eyewall regions.