Ricerc@Sapienza

The effects of the scattering troposphere on propagating signals are important for several
microwave applications such as remote sensing and telecommunication. In particular, passive
remote sensing exploits ground-based radiometers to retrieve profile information of the
atmosphere. On the other hand, telecommunication applications (e.g., satellite communications)
require an accurate estimation of the atmospheric effects to minimize the outage probability of
the link.
Within this context, atmospheric effects can be described through the joint knowledge of the radiopropagation parameters: atmospheric brightness temperature TB and total path attenuation At
(also referred to as atmospheric extinction). These two quantities can be described by the radiative
transfer theory that formalizes the spatial evolution of the atmospheric radiance and is
implemented trough radiative transfer models (RTM). For successfully testing and validating RTM,
measurements of both TB and At are needed. A typical approach to get these two quantities is
to exploit combined measurements of satellite-beacon receivers (which provide measurements
of At) and ground-based radiometers (which measures the TB). The disadvantage of this approach
is that At and TB would be affected by different errors because they are derived using two distinct
measuring instruments, each of them with different calibration and accuracy. Moreover,
radiometers and beacon receivers typically work at different frequencies. This would imply that
a frequency scaling approach would be required before using At and TB pairs for quantitative
analysis. Actually, most of microwave radiometers are able to provide attenuation products as
result of retrievals approaches based on forward models. However, such retrievals can suffer of
large uncertainties in rainy conditions due to poor modeling of scattering.
The only instruments able to provide simultaneous measurements of At and TB in all-weather
conditions are ground based Sun-tracking radiometers (STR) that exploit the Sun as a stable
radiance source. STR performs the retrieval of At exploiting two nearly simultaneous
measurements of TB at the same elevation. This is accomplished by alternatively pointing the
receiving antenna toward-the-Sun and off-the-Sun during the Sun tracking.
The aim of this work is to exploit STR measurements to test and validate RTM simulations in
cloudy and rainy conditions. We have considered two different models. First, a Sky Noise
Eddington Model (SNEM): a 1D-model that gives an analytical approximation of the solution of
the radiative transfer equation. SNEM simulations provide a synthetic clouds dataset through
random generation of seasonal-dependent and time-decorrelated meteorological variables with
statistics driven by radiosounding profiles. Second: a pseudo-3D radiative transfer model based
on the Goddard Satellite Data Simulator Unit (G-SDSU, Matsui et al. JGR 2014) that is able to
produce synthetic radiances and path attenuation as measured by ground-based microwave
radiometers at several elevation angles and frequencies. In this work we use G-SDSU to convert
3D temporal profiles of meteorological variables, produced by Numerical Weather Forecasts, into
predicted TB and At.
RTMs are tested exploiting measurements from a STR installed at the Air Force Research
Laboratory in Rome, NY, USA. The STR has four receiving channels at K (23.8 GHz), Ka (31.4
GHz), V (72.5 GHz) and W (82.5 GHz) band providing measurements for the years 2015-2016
at elevation angles varying between 20° and 70° (due to the antenna pointing-switching for the
Sun tracking). The agreement between measurements and models highlights the reliability of the
produced radiation database that can be exploited to develop and update parametric prediction
models of attenuation. T