Monitoring the Absolute Calibration of a Polarimetric Weather Radar

Abstract

The absolute calibration of a dual-polarization radar of the German Weather Service is continuously monitored using the operational birdbath scan and collocated disdrometer measurements at the Hohenpeissenberg observatory. The goal is to measure the radar reflectivity constant Z better than ±1 dB. The assumption is that a disdrometer measurement close to the surface can be related to the radar measurement at the first far-field range bin. This is verified using a Micro Rain Radar (MRR). The MRR data fill the gap between the measurement near the surface and the far-field range bin at 650 m. Using data from the first half of the warm season in 2014, a bias in radar calibration of 1.8 dB is found. Data from only stratiform precipitation events are considered. After adjusting the radar calibration and using an independent data sample, very good agreement is found between the radar, the MRR, and the disdrometer with a bias in smaller than 1 dB. The bias in is not captured with the classic one-point calibration, which is performed twice a day using a built-in test signal generator. This is attributed to the fact that the characterization of the transmit and receive path is not accurate enough. Solar interferences during the operational scanning are used to characterize the receiver. There, the bias found is small, about 0.2 dB, so that bias based on the comparison of the radar with external sensors is attributed to the transmit path. The representativeness of the disdrometer measurements are assessed using two additional disdrometers located within 200-m distance.

Can liquid cloud microphysical processes be used for vertically pointing cloud radar calibration?

“…For both initial DSDs, the variability determined from bootstrapping is minimal for γ = 0.0 and W = 0.25 m s −1 , and we conclude that Z Frisch et al (1995) for distinguishing between drizzle-free and drizzle-containing clouds. Given the idealized setup, we likely underestimated the uncertainties of Z γ =0 e and Z W =0.25 e and estimate the uncertainty to be at least 3 dB.…”

“…For example, waveguide corrosion of the MilliMeter wavelength Cloud Radar (MMCR) of the US Department Of Energy (DOE) Atmospheric Radiation Measurement (ARM) program at the North Slope of Alaska (NSA) site in Utqiaġvik (Barrow), Alaska, caused a 9.8 dB calibration offset in 2008 (Protat et al, 2011). Also, a liquid film on the radome or radar antenna caused by precipitation can temporarily lead to up to 4 dB of additional two-way attenuation (Frech, 2009). From an engineering perspective, radar calibration is complicated by the fact that radar returns span several orders of magni-tude in power and -particularly for pulsed radars -the span between the transmitted and received power is even larger.…”

“…Several studies have suggested calibrating radars by comparing their measurements of rainfall with integrated drop size distributions from ground-based disdrometers (Joss et al, 1968;Ulbrich and Lee, 1999;Frech et al, 2017). Tridon et al (2017) proposed using self-consistency checks of retrievals from simultaneous radar observations at multiple frequencies to identify calibration problems.…”

Monitoring the differential reflectivity and receiver calibration of the German polarimetric weather radar network

“…Z DR monitoring methods that use solar radiation are now commonly employed in operational weather radar networks (Huuskonen and Holleman, 2007;Holleman et al, 2010;Figueras i Ventura et al, 2012;Frech, 2013;Huuskonen et al, 2016;Frech et al, 2019). The sun can be considered an unpolarized source of radiation (i.e., the H and V powers are equal) also emitting at radar frequencies.…”

“…A goal of this paper is to identify the source of this Z DR temperature dependence. Similar to Hubbert (2017), solar scans are employed for a systematic analysis using the Hohenpeißenberg research radar (Frech et al, 2017). Each solar scan takes about 4 min and is repeated every 10 min.…”