In this work, a comparison between soil moisture products derived from satellite and land model data was performed; in particular, the soil moisture retrievals of SMOS and ASCAT were compared with those of the ERA-Interim/Land model, produced by the ECMWF in a timeframe of 3 years. Subsequently, for a limited period of time, the product from the SMAP radiometer was joined to SMOS, ASCAT and ERA-Interim model data as a fourth dataset. In both cases, the whole H-SAF region of interest, which includes Northern Africa and Europe, was analysed. In order to validate the products, the Triple Collocation technique was applied to estimate the independent error standard deviation of three systems that observe the same target parameter. When more than three datasets were available, the Quadruple Collocation technique was used to jointly estimate the error standard deviation of four sources. Moreover, when the SMOS and SMAP radiometer products were considered, the Extended Collocation was adopted in order to evaluate the error variances of the systems, taking into account the possible presence of an error cross-correlation between the radiometer retrievals.
The spaceborne scatterometer is a microwave radar that provides high precision radiometric measures of the normalized radar cross section σ0 of the ocean surface. The backscatter is affected by 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 σ0 and the surface wind filed 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 σ0 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 obtained wind speeds are then correlated to the σ0’s acquired over these six TCs.
The results obtained in this work support the need to extend the CMOD4 model.
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