MONITORING 89GHZ RADIOMETRIC CHANNEL FOR SYNERGISTIC ATMOSPHERIC RETRIEVALS WITH COLOCATED GROUND-BASED CLOUD RADAR OBSERVATIONS

D. Rossi1,2, M. Montopoli^3,2, S. Di Fabio², A. Balotti^1,2, R. Lidori², V. Rizi^1,2, M. Iarlori^1,2, D. Cimini^4,2

¹Dipartimento di Scienze Fisiche e Chimiche (DSFC), Università dell’Aquila, L’Aquila, Italy, ²Centro di Eccellenza in Telerilevamento E Modellistica Previsionale di eventi Severi (CETEMPS), Università Dell’Aquila, ³Consiglio Nazionale delle ricerche, Istituto di Scienze dell’Atmosfera e del Clima (CNR-ISAC), ⁴Institute of Methodologies for Environmental Analysis (CNR-IMAA)

Radiometer calibration plays a key role in ensuring the reliability of the inferred atmospheric parameters such as, for example, integrated water vapor (IWV) and liquid water path (LWP). The accuracy in estimating such quantities is crucial for several topics spanning from NWP validation to operational meteorology and climatology. A niche but important aspect where IWV and LWP also play a relevant role is the quantification of the absorption effects that cause the signal loss (also known as path integrated attenuation, PIA) in active systems. As general rule, more liquid water leads to greater PIA. Thus, accurate quantifications of LWP would in principle lead to a better estimation of total PIA and, consequently, a compensation of signal losses in active system can be efficiently achieved. Obviously, such a LWP and PIA relationship would allow for synergistic atmospheric retrievals with colocated ground-based cloud radar observations.

A striking example of colocated ground based active and passive channels where accurate LWP would be useful for PIA calculations, is given by the W-band (RPG‑FMCW‑94) cloud radar that incorporates an active 94GHz radar system together with a 89GHz radiometric channel. Such configuration is common within the Aerosol, Clouds and Trace Gases (ACTRIS) European research infrastructure, making the LWP estimation goal of broader interest.

However, although estimates of LWP from single channel is challenging, the accurate estimate is primarily driven by absolute calibration. The 89GHz radiometric channel is typically calibrated using a cold target, at liquid nitrogen boiling temperature, through an off-line procedure that is performed every few months (six, as suggested by the instrument manufacturer).

This work explores a procedure for calibration monitoring of the (RPG‑FMCW‑94) 89GHz radiometric channel using colocated routine radiosounding observations (RAOBS). RAOBS are processed through the Passive and Active Microwave radiative TRAnsfer forward model (PAMTRA) to obtain reference simulated brightness temperature in clear sky (TBsim).

The dataset used comes from the Casale Calore observatory of the Department of Chemical and Physical Sciences of University of L’Aquila (L’Aquila, Italy) and managed by CETEMPS (L’Aquila, Italy). A RPG‑FMCW‑94 cloud radar is working in Casale Calore since 2022, where radiosondes are launched routinely usually twice per month. This allows for a dataset collection of 55 available cases from 21st March, 2023, to 26th June, 2025. Time colocation between measured TBs (TBmeas) at 1-second sampling and RAOBS during the balloon flight is accomplished by averaging TBmeas in a 1-hour time window starting from the RAOB launch time. To avoid cloud contamination, cases for which the maximum standard deviation of TBmeas calculated for each 10-minute time range inside the 1-hour window is higher than a given threshold are discarded.

The results overall show a systematic bias between simulated and observed brightness temperatures, with the radiometer tending to underestimate the brightness temperature. The derived correction factors are computed by means of linear regression and then provided as a starting point for the radiometer calibration, allowing for a better retrieval of LWP which is crucial for correcting the W‑band radar reflectivity measurements for path attenuation effects, effectively exploiting radar–radiometer synergy.

Keywords: RPG‑FMCW‑94 radar; 89 GHz radiometer; Casale Calore observatory; routine radiosonde launches; calibration monitoring; PAMTRA; radar–radiometer synergy