| Paper: | FR-A1.9 |
| Session: | Instruments and Calibration (Posters) |
| Time: | Friday, March 30, 09:00 - 10:20 |
| Presentation: |
Poster
|
| Topic: |
Advanced radiometer techniques: |
| Title: |
EXPLOITING SUN-TRACKING MICROWAVE RADIOMETERS FOR TESTING RADIATIVE TRANSFER MODELS OF PRECIPITATING CLOUDS |
| Authors: |
Marianna Biscarini; Sapienza University of Rome | | |
| | Mario Montopoli; ISAC CNR | | |
| | Luca Milani; Sapienza University of Rome | | |
| | Klaide De Sanctis; Himet | | |
| | Saverio Di Fabio; CETEMPS | | |
| | Frank S. Marzano; Sapienza University of Rome | | |
| | George Brost; Air Force Research Laboratory | | |
| Abstract: |
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 radio-propagation 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. The use of RTM simulations driven by numerical weather forecasts paves the way to new approaches based on the prediction of radio-propagation parameters for specific target areas and temporal periods (as opposed to common prediction schemes based on stationary path attenuation models statistics). |
