Rain-Induced Sea Surface Winds: What Can We Learn from Satellite Scatterometer Data?

Abstract:

Recent work shows that, in contrast to global numerical weather prediction models, space-borne scatterometers are able to resolve the increased wind variability near convection areas. This unique capability is essential for climate applications, since wind variability directly impacts air-sea fluxes and, as such, air-sea interaction. Although scatterometer-derived wind quality is known to be degraded by rain contamination effects (notably for Ku-band systems), recent results show that for C-band scatterometers, such as the Advanced Scatterometers (ASCAT-A and ASCAT-B) onboard the Metop satellite series, the wind quality degradation under rain conditions is mainly due to the increased sub-cell wind variability. Moreover, ASCAT winds are of comparable quality to buoy winds while they provide a substantially larger coverage. Since convective systems rapidly evolve in time, ASCAT A and B acquisitions (overlapping in the tropics with only 50 minutes difference) provide a unique opportunity to carry out in-depth analyses of the dynamics associated to rain events.

In this study, Meteosat Second Generation (MSG) rain rate time series (every 15 minutes) around the Metop-A and B satellite overpasses are used to evaluate both the presence of rain and the maturity stage of the convective systems. Also, several wind-variability sensitive parameters derived from ASCAT data, such as the inversion residual, the measurement noise (Kp) and the singularity exponents (an estimation of the wind decorrelation between an observation cell and its neighboring cells), are used to assess both the quality of the derived winds and the potential presence of gust fronts. Moreover, divergence and curl fields are derived from ASCAT A and B winds to provide insight on the wind convergence/divergence and vorticity processes associated with convection and related downdrafts/downbursts.

Although much affected by rain, a similar analysis will also be carried out for Ku-band scatterometers, such as Oceansat-2 and the upcoming Rapidscat (if calibrated data are available in time). Intercomparison of C-band and Ku-band winds under rain conditions will certainly provide new insights on how to improve scatterometer processing to better resolve rain-induced dynamics.