Content area
Full Text
1. Introduction
The prelaunch calibration of the CloudSat cloud-profiling radar (CPR; Stephens et al. 2002), in-flight calibration, and stability over the period of operation has been very recently reported in Tanelli et al. (2008) and Stephens et al. (2008). This in-flight calibration relies on monthly comparisons of ocean backscatter measured at 10° incidence off-nadir using dedicated CloudSat maneuvers and the corresponding ocean backscatter predicted by different theoretical models. The rationale for using 10° is that at this incidence the ocean backscattering cross section becomes nearly independent of surface wind speed (Durden et al. 2003) and takes a value of about 7 dB. Direct comparisons of CloudSat measurements of ice cloud reflectivity and ocean backscatter with measurements gathered by an airborne cloud radar within the CloudSat beam have demonstrated that the calibration of CloudSat was accurate to within 0.5–1 dB (Protat et al. 2009, hereafter PAL09), which is better than the initial CloudSat specification of 2 dB (Stephens et al. 2008, 2002). This result has also been confirmed using statistical comparisons between continuous ground-based cloud radar observations over five different sites (PAL09). PAL09 also suggested that CloudSat could also now be used as a means to calibrate other ground-based or airborne uncalibrated radars. This idea is not new; it was proposed earlier for the lower-frequency spaceborne radar on board the Tropical Rainfall Measurement Mission (TRMM) in the pioneering works of Anagnostou et al. (2001) and Bolen and Chandrasekar (2000). In the present paper, this idea is developed further using two ground-based radars for which calibration problems are suspected by their operators and recent airborne Doppler cloud radar data taken in the Arctic along the CloudSat track. The paper is organized as follows: The principle of the statistical calibration using CloudSat as a reference is recalled in section 2, and some further refinements with respect to the PAL09 method are discussed. The calibration of the Barrow and Cabauw ground-based radars is discussed in sections 3 and 4. The calibration check of the airborne cloud radar is described in section 5. Conclusions are given in section 6.
2. The principle of the calibration technique
The principle of the statistical approach that is used to compare the calibration of a spaceborne radar with that of a ground-based radar has...