Implementation and Validation of a Retrieval Algorithm for Profiling of Water Vapor From Differential Attenuation Measurements at Microwaves

The knowledge of the water vapor (WV) distribu-tion in the Earth’s atmosphere is of great importance for weatherprediction. Meteorological models, in particular, the so-calledlimited area models, can assimilate humidity measurements,increasing the reliability of the simulated atmospheric dynamics.An important improvement can be achieved, for instance, if weare able to provide the total column with a sufficient precisionand accuracy. In this paper, the novel normalized differentialspectral attenuation (NDSA) approach is applied to retrievethe vertical profile of WV—and thus the total column—from measurements of differential attenuation signals at microwaves.A forward model (FM) has been used to simulate the ray-tracing of a microwave signal from a transmitter to a receiverin the atmosphere by using the 3-D atmospheric parameters asprovided by a numerical weather prediction (NWP) model. Fromthe NDSA measurement, the integrated WV (IWV) content canbe directly derived. A further retrieval code is able to invertthe measurements of IWV along the path length, providing thevertical humidity profile, which is directly related to the totalvertical column assimilated by weather prediction models. In thispaper, we show that the values of the total column can beretrieved with a precision and accuracy up to about 0.6% and2.1%, respectively, which could have a positive impact on NWPmodels at short time scale.