Wind Retrieval Simulation in Tropical Cyclone for FY-3 Dual-Frequency WFR

Tropical cyclone is one of the primary disastrous synoptic systems in China. With the continuous observation, global coverage and the ability to penetrate through precipitation layer, microwave sensors on polar orbit satellites can provide more precise observations of the tropical cyclone location and intensity for marine extreme weather forecasting, which will compensate for the shortage of conventional observations. The FY-3 satellite microwave scatterometer, named Wind Field Radar (WFR), is the only way to measure ocean vector winds (OVW).Compared with single-frequency scatterometer, dual-frequency scatterometer has advantages in higher spatial resolution and better response to winds in extreme weather conditions, where high winds are usually associated with high rain rates. The Jet Propulsion Laboratory (JPL) has developed a conceptual design for a Dual Frequency Scatterometer (DFS) in the Extended Ocean Vector Winds Mission put forward in 2007. The concept of Rotating Fan Beam Scatterometer which will be extended to dual-frequency mode has been studied under ESA. The WFR installed on FY-3 satellite which will be launched in 2016 is designed to use the dual-frequency and dual-polarization time-sharing observation pattern.Rain perturbations result from volume scattering and attenuation by precipitation in the atmosphere, as well as changes of sea surface roughness by impinging rain drops. Few studies which investigate effects of rain on Ku-band and C-band scatterometer data indicate that the impact of rain to sea surface is a complex phenomenon not yet fully understood, and it's not dominating in high wind and strong rain fall areas. Therefore, changes in surface roughness are not considered here.The purpose of this study is to investigate the potential of the WFR proposed to fly aboard FY-3 satellite to measure OVW in rain fall areas of Typhoon Ivan. A theoretical model based on radiation transfer equation including rain attenuation and scattering, has been developed to quantify the modification by rain of the measured backscatter. Based on the simulated normalize radar cross section (NRCS) dataset generated by forward model, the impact on the retrieved winds has been analyzed, and the dual-frequency retrieval algorithm has been firstly studied. Analysis shows that changes of contaminated NRCS briefly depend on relative size of volume scattering item and attenuation item. Wind vectors measured at Ku-band can be more severely altered by rain than those at C-band. Precipitation in tropical cyclone can significantly degrade the OVW accuracy. Retrieval results show that new methods combined Ku-band and C-band have higher spatial resolution than C-band retrieval, and better performance in rain fall region than Ku-band retrieval. Especially, partitioned wind retrieval technique can significantly reduce the rainfall error, is an effective way to improve the wind retrieval accuracy in tropical cyclone with the synchronous observation by microwave humidity sounder (MWHS) aboard FY-3 satellite.