Altimeter microwave surface observations in tropical cyclones

Surface measurements in tropical cyclones (TCs) are difficult to obtain and only microwave sensors below 15 Ghz can penetrate the deep convective clouds surrounding the areas where surface winds are the largest (Quilfen et al., 2007). These kind of measurements are crucial to complement the Dvorak TC intensity analysis as obtained from infrared/visible measurements (Velden and Coauthors, 2006). Moreover, multi-frequency measurements are needed to separate wind and rain effects in intense rainy areas. We analyze in this study the ability of Jason-1 dual-frequency altimeter measurements to provide estimates of surface winds, waves, and rainfall rate in the case of two cat-5 hurricanes. Isabel and Wilma are particularly interesting to present very different structures (eye size and visibility, asymmetries), although scaled at the same intensity at the time of Jason-1 overflight. This offers a good test bed to show that the altimeter can be used to distinguish such features. Variability of the surface wind speed and rainfall rate is well characterized along the altimeter ground track. As expected from the eyewall conical shape, the maximum rainfall rate occurs slightly more outward than the maximum surface wind speed (MSWS). Contrary to Wilma showing an eyewall replacement cycle at the altimeter observation time, Isabel MSWS and rain rate structures are very symmetric. Altimeter measurements in Isabel are compared with the Stepped Frequency Microwave Radiometer (SFMR, Uhlorn et al., 2007) measurements and with the Hurricane Research Division (HRD) wind analysis, showing good agreement of the wind profiles but underestimation of the SFMR and altimeter winds by comparison with the HRD ones. In the Wilma case, there are no direct surface measurements to compare with the altimeter. Altimeter MSWS estimate at 62.7 m s−1 may not coincide with the peak wind estimated at 67 m s−1 from flight level-based measurements in the best track reanalysis. However, estimated altimeter MSWS values are suggested to be biased low because of the rather large footprint (~8 / 9 kms) in high sea state conditions and because the wind speed retrieval empirical model may underpredict the radar cross section at the high wind speed encountered in the rainy areas of the TC eyewalls. The availability of more than 15 years of dual-frequency altimeter measurements, together with the continuous improvement of TC intensities estimates, is a motivation to revisit the original work performed by Young (1993) to better exploit the modern altimeter dual-frequency capabilities.