A Study Of The Effect Of Geophysical Noise On CFOSAT Scatterometer Measurements

The Ku-band scatterometer onboard China France Oceanography Satellite(CFOSAT)adopts a new observing mechanism,i.e.,viewing the sea surface with two conically scanning fan beams.Compared to the prior Ku-band pencil beam scatterometers,CFOSAT scatterometer(CSCAT)provides more independent backscatter measurements at varying incidence and azimuth angles,which opens up opportunities to retrieve high-quality winds,but also brings challenges for the sea surface wind inversion at the presence of geophysical noises.For the remote sensing of sea surface winds,rain,wind variability and sea surface temperature(SST)are the common geophysical noises affecting scatterometer measurements.Rain impacts on scatterometer backscatters by attenuating or scattering the microwave signal directly,hence it is the major factor in degrading scatterometer wind quality.While SST,though a second-order effect for scatterometer measurements,has more extensive effect than rain.This paper first investigates the effects of rain on CSCAT measurements.A simplified radar backscatter(?~0)model is proposed,in which the rain effects are categorized into two types,namely attenuation and intensification,respectively.The results show that the sensitivity of CSCAT radar backscatter to rain substantially varies with rain rate,wind speed,radar polarization and incidence angle.The rain attenuation effect dominates at low wind speeds and high incidence angles(?>45°),while the rain intensification effect dominates at high winds and relatively low incidence angles(?<35°).Compared to the vertically-polarized beam,the measurements at horizontal polarization are more sensitive to rain and incidence angle.As such,the high winds acquired by CSCAT are generally unbiased at low and moderate rain rates due to the compensated rain effects at relatively low and high incidence angles,while the high winds are significantly underestimated under extreme rain conditions.Base on a specific Typhoon case,a simple simulation method is used to clarify the relation between the retrieved wind speed and the dependency of radar rain effects on the incidence angle.It is found that the backscatter measurements at low incidence angles,which are generally underestimated at high winds and heavy rainy conditions,have a larger influence on the wind inversion minimization,leading to much lower retrieved wind speeds than the“true”winds.Similar approach is used to study the SST effects on CSCAT measurements.The results show that the backscatters measured by vertical polarization are underestimated at low SST conditions,and the bias increases as the increase of incidence angle or the decrease of wind speed.However,the horizontal polarization does not show significant dependency on the incidence angle.As a result,the bias of the retrieved CSCAT wind speed(with respect to the reference“true”winds)is negative at low SST region and positive at high SST region.Moreover,the wind speed bias at low wind(outer swath)conditions is more remarkable than that at high winds(inner swath),which is in line with the?~0 dependency on SST.Finally,a geophysical model function(GMF)model accounting for the SST effects is developed in order to further improve the retrieved wind quality of CSCAT.