Tracking a tropical cyclone with ERS-SCAT: a CMOD model review

The spaceborne scatterometer is a microwave radar that provides high precision radiometric measures of the normalized radar cross section (σ) of the ocean surface. The backscatter is related to the superficial roughness that is in turn related to the local wind. Since microwave wavelengths are used the scatterometer, at first order, can be meant as an instrument which provides measurements independent of clouds and sun illumination therefore it is able to observe the internal structure of a Tropical Cyclone (TC). The relationship between the σ and the surface wind field is described by a geophysical model function (GMF). The model used in the ERS scatterometer processing is the well-known semi-empirical model CMOD4. Unfortunately this model is not tailored for high wind speeds, such as the case of TCs. This fact causes a poor quality in the wind field estimated through the scatterometer data acquired over a TC. In this paper we describe a study in view of a possible extension of the CMOD4 for high wind speeds. The study has been based on the ERS-2 σ measurements relevant to six selected TCs and the corresponding wind speeds obtained by employing the Holland model. We have selected six TCs and for each one we have developed a 3D wind speed pattern making use of the wind speed available through the NHC (National Hurricane Center) warnings. The wind speed so obtained is then correlated with the σ acquired over these six TCs. The results obtained in this work support the need to extend the CMOD4 model.