Retrieval of two-dimensional absorption coefficient structure from a scanning radiometer at 23.GHz

In this work the retrieval of the structure of the atmospheric specific absorption coefficient through a tomographic acquisition is discussed. Brightness temperatures have been collected daily at 23.8 GHz using a radiometer in tipping curve mode regardless of the sky condition, except for rainy days, for about 60 elevation scans. Each scan has been processed to retrieve the two-dimensional structure of the underlying atmosphere through the (x-z) values of the specific absorption coefficient. The forward model is inverted after binning in space and linearization around a reference model. The scanned vertical plan is modeled by allocating M bins spanning in both the vertical and the horizontal direction, whereas the measurements have been collected at 12 elevation angles between 23° and 90°. Singular value decomposition allows identifying an orthogonal basis for the data space (12 elevation angles) and for the model space (M bins). It indicates also that only five eigenvalues are such that the condition number of the matrix representing the linearized problem is acceptable (?20), so that only five components of the model space may be resolved. A higher number would imply unacceptable noise contamination. In spite of these limitations, the retrieved panel of specific absorption coefficient reveals the horizontal structure of the atmosphere. The scale of the experiment is of the order of kilometers. Therefore, because the vertical structure of the atmosphere dominates over the horizontal, a vertical detrending is needed to reveal horizontal structures. An example shows the typical problems encountered in the inversion and the remedies that have been adopted.