Calibrating ground‐based microwave radiometers: Uncertainty and drifts. Issue 4 (21st April 2016)

• Record Type: Journal Article
• Authors: Küchler, N.; Turner, D. D.; Löhnert, U.; Crewell, S.
• Is Part Of: Radio science. Volume 51:Issue 4(2016:Apr.)
• Page Start: 311
• Page End: 327
• Publication Date: 2016-04-21
• Subjects: calibration accuracy -- microwave radiometer -- liquid nitrogen -- tipping curve calibration -- remote sensing -- water vapor; Radio meteorology -- Periodicals; Radio wave propagation -- Periodicals; 621.38405
• DOI: 10.1002/2015RS005826
• Languages: English
• ISSNs: 0048-6604

Abstract

Abstract: The quality of microwave radiometer (MWR) calibrations, including both the absolute radiometric accuracy and the spectral consistency, determines the accuracy of geophysical retrievals. The Microwave Radiometer Calibration Experiment (MiRaCalE) was conducted to evaluate the performance of MWR calibration techniques, especially of the so‐called Tipping Curve Calibrations (TCC) and Liquid Nitrogen Calibrations (LN2cal), by repeatedly calibrating a fourth‐generation Humidity and Temperature Profiler (HATPRO‐G4) that measures downwelling radiance between 20 GHz and 60 GHz. MiRaCalE revealed two major points to improve MWR calibrations: (i) the necessary repetition frequency for MWR calibration techniques to correct drifts, which ensures stable long‐term measurements; and (ii) the spectral consistency of control measurements of a well known reference is useful to estimate calibration accuracy. Besides, we determined the accuracy of the HATPRO's liquid nitrogen‐cooled blackbody's temperature. TCCs and LN2cals were found to agree within 0.5 K when observing the liquid nitrogen‐cooled blackbody with a physical temperature of 77 K. This agreement of two different calibration techniques suggests that the brightness temperature of the LN2 cooled blackbody is accurate within at least 0.5 K, which is a significant reduction of the uncertainties that have been assumed to vary between 0.6 K and 1.5 K when calibrating the HATPRO‐G4. The error propagation of both techniques was found to behave almost linearly, leading to maximum uncertainties of 0.7 K when observing a scene that is associated with a brightness temperature of 15 K. Key Points: Calibrations of microwave radiometers are accurate within ±0.5 K New quality criteria for calibrations were developed and tested The absolute temperature of a liquid nitrogen‐cooled load was determined