On The Retrieval of IWV From Microwave Attenuation Measurements: Experimental Results

The knowledge of the water vapor distribution in the Earth's atmosphere is of great importance for weather prediction. Meteorological models, including the so called limited area models, can assimilate humidity measurements, increasing the reliability of the simulated atmospheric dynamics. Existing atmospheric remote sensing instruments are still insufficient to cover global scales with systematic WV measurements in the lowest part of the troposphere (5-6 km): this would indeed help to improve both climate modeling and numerical weather prediction capabilities at short time scale. In order to solve this problem, a new approach (the Normalized Differential Spectral Attenuation-NDSA) capable of estimating the integrated water vapor (IWV) through attenuation measurements in the Ku/K band along microwave is assessed. Nevertheless, the developed technique needs to be demonstrated with real measurements before moving toward space applications. A recent project (SWAMM-Sounding Water Vapor by Attenuation Microwave Measurements) had as main goal to develop low-cost instrumentation to perform the first NDSA measurements. The instrument was designed to operate with the transmitter located on the ground and the receiver placed on a moving platform (e.g. airborne, stratospheric balloon, HAPS), even though, as a preliminary test, the instrumentation was tested in a ground- to-ground link configuration. An instrument prototype, which includes a synthesized microwave transmitter and a Software Defined Radio (SDR) microwave receiver has been designed, assembled and tested in laboratory. Laboratory tests confirm that the transmitter generates two tones, respectively at 18.8 GHz and 19.2 GHz, with very high stability in both amplitude and frequency, and the receiver measures the amplitude (and its fluctuations) of the received signals with enough accuracy (< 0.1 dB) over a wide dynamic range. The differential attenuation is ±0.02dB then far below the theoretical value of the minimum sensitivity required to measure IWV. The instrument was then tested on a ground-to-ground link during an experimental campaign, which was carried out in the period June-October 2018. In order to obtain a transect in the troposphere of tens of km, (thus providing a value of differential attenuation high enough to be measurable by the instrument), the transmitter was installed at sea level and the receiver on the top of a mountain (around 2165m a.s.l.). In detail, the receiver and the transmitter were installed at the CNR laboratories, respectively in Bologna (central Italy) and Cimone mount in the Apennines; as a result, the length of the transect is 62 km. Data are acquired continuously for four months well covering different atmospheric conditions occurred in the summer/autumn seasons. Differential attenuation exhibits temporal variations, at both weekly and monthly scales, which can be correlated to the IWV. As first validation we compared the IWV calculated by NDSA measurements with the same quantity calculated, under the assumption of plane parallel atmosphere conditions, by using different hygrometers placed along the transect and daily radiosoundings of a nearby WMO station. The comparisons show that data are generally in good agreement as both absolute values and temporal variations. However, further investigations are necessary to better interpret the results also in terms of a few discrepancies found. Finally, Sentinel- 3 OLCI L2- IWV product will be used as additional term of comparison for the retrieved IWV over the same transect. A detailed analysis will be presented at the conference.