Calibration of a 35 GHz airborne cloud radar: lessons learned and intercomparisons with 94 GHz cloud radars

Abstract

Abstract. This study gives a summary of lessons learned during the absolute calibration of the airborne, high-power Ka-band cloud radar HAMP MIRA on board the German research aircraft HALO. The first part covers the internal calibration of the instrument where individual instrument components are characterized in the laboratory. In the second part, the internal calibration is validated with external reference sources like the ocean surface backscatter and different air- and spaceborne cloud radar instruments. A key component of this work was the characterization of the spectral response and the transfer function of the receiver. In a wide dynamic range of 70 dB, the receiver response turned out to be very linear (residual 0.05 dB). Using different attenuator settings, it covers a wide input range from −105 to −5 dBm. This characterization gave valuable new insights into the receiver sensitivity and additional attenuations which led to a major improvement of the absolute calibration. The comparison of the measured and the previously estimated total receiver noise power (−95.3 vs. −98.2 dBm) revealed an underestimation of 2.9 dB. This underestimation could be traced back to a larger receiver noise bandwidth of 7.5 MHz (instead of 5 MHz) and a slightly higher noise figure (1.1 dB). Measurements confirmed the previously assumed antenna gain (50.0 dBi) with no obvious asymmetries or increased side lobes. The calibration used for previous campaigns, however, did not account for a 1.5 dB two-way attenuation by additional waveguides in the airplane installation. Laboratory measurements also revealed a 2 dB higher two-way attenuation by the belly pod caused by small deviations during manufacturing. In total, effective reflectivities measured during previous campaigns had to be corrected by +7.6 dB. To validate this internal calibration, the well-defined ocean surface backscatter was used as a calibration reference. With the new absolute calibration, the ocean surface backscatter measured by HAMP MIRA agrees very well (<1 dB) with modeled values and values measured by the GPM satellite. As a further cross-check, flight experiments over Europe and the tropical North Atlantic were conducted. To that end, a joint flight of HALO and the French Falcon 20 aircraft, which was equipped with the RASTA cloud radar at 94 GHz and an underflight of the spaceborne CloudSat at 94 GHz were performed. The intercomparison revealed lower reflectivities (−1.4 dB) for RASTA but slightly higher reflectivities (+1.0 dB) for CloudSat. With effective reflectivities between RASTA and CloudSat and the good agreement with GPM, the accuracy of the absolute calibration is estimated to be around 1 dB.

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

“…Hence, although Z does not exceed 25 dBZ, the embedded convective updrafts can be clearly identified as individual narrow plumes, which directly coincide with WCB air masses between 1 km and 6 km height. This qualitatively confirms results from previous studies (Crespo and Posselt, 2016;Oertel et al, 2019;Blanchard et al, 2020), although absolute values of radar reflectivity are difficult to compare since in strongly precipitating regions attenuation effects through Mie scattering have to be considered at shorter wavelengths (e.g., Ewald et al, 2019). This affects systems like the 94 GHz CloudSat radar (analysed by Crespo and Posselt, 2016) and the 95 GHz RASTA radar (analysed by Blanchard et al, 2020) more strongly than the here used MIRA-36, which uses a slightly larger wavelength.…”

“…The downward pointing Doppler radar operates at a frequency of 35.5 GHz and reaches an expected airborne sensitivity of −38 dBZ at 5 km distance (Mech et al, 2014), which reduces to ≈ −30 dBZ at 13 km distance (Konow et al, 2019). The measured along-flight radar reflectivity Z has been post-processed, offset-corrected and corrected for aircraft attitude (Ewald et al, 2019;Konow et al, 2019) and is obtained from Konow et al (2018).…”

Multi-layer Cloud Conditions in Trade Wind Shallow Cumulus – Confronting Models with Airborne Observations

“…larger raindrops show a higher reflectivity than smaller cloud droplets given the same mass mixing ratio. The HAMP radar is calibrated following Ewald et al (2019) and was operated at a vertical resolution of about 30 m with 1 Hz sampling. This sampling frequency corresponds to a surface footprint of about 136 m × 376 m at a cruising speed of about 240 m s −1 .…”

“…The instruments minimal detectable signal (MDS) in dBZ decreases with range r and is estimated by Ewald et al (2019) as MDS(r) = −39.8 + 20 log 10 r 5 km .…”

A unified data set of airborne cloud remote sensing using the HALO Microwave Package (HAMP)

“…Vaisala RD94 dropsondes (Busen, 2012) were deployed in all campaigns using the Airborne Vertical Atmospheric Profiling System (AVAPS, Hock and Franklin, 1999). Wang et al (2015) and Vaisala (2017) report that the measurement accuracy of these sondes for pressure is 0.4 hPa, temperature is 0.2 • C and relative humidity is 2 %.…”