Assessment of Ground-Based Microwave Radiometer Calibration to Enable Investigation of Gas Absorption Models

Ground-based microwave radiometers are becoming more and more common for remotely sensing the atmospheric temperature and humidity profile, as well as path integrated cloud liquid water content. Several studies have been published, which compare radiosonde profiles with temperature profiles derived from microwave radiometer measurements and find biases of up to 1 K. The retrieved temperature profile is based on radiometric measurements and radiative transfer calculations. Once the accuracy of radiometer measurements is known, these can be used to validate existing gas absorption models. As the absolute accuracy of microwave radiometer measurements is determined by the quality of the calibration, this work investigates the uncertainty of two calibration techniques, which are commonly used with microwave radiometers. Namely, these are the liquid nitrogen calibration and the tipping curve calibration (Han and Westwater, 2000). Both methods are known to have open issues concerning systematic offsets and calibration repeatability. In this regard, this work focuses on the error assessment for the absolute calibration of the network suitable microwave radiometer HATPRO-G2 (Humidity And Temperature PROfiler - Generation 2), which makes up a significant part of the worldwide available systems (Rose et al., 2005). In order to capture dry high altitude conditions on the one side and mid-latitude, close to sea level conditions on the other side, the analysis is based on two deployments. Between August and October 2009, HATPRO-G2 was part of the Radiative Heating of Underexplored Bands Campaign - Part 2 (RHUBC-II) in Northern Chile (5320 m above mean sea level) conducted within the Atmospheric Radiation Measurement (ARM) program. Since 2010, it is part of the JOYCE (Jülich ObservatorY for Cloud Evolution) site located in Germany 92 m above mean sea level. For each of the deployments, a detailed error propagation for both techniques is performed. The uncertainty range of brightness temperature Tb measurements based on a single liquid nitrogen calibration is mainly caused by a reflective component from the liquid nitrogen surface of the cold calibration target. The overall calibration uncertainty is assessed for typical Tb values measured at each deployment. For RHUBC-II, the maximum uncertainty of $T_b$ has been determined to +-1.6 K in the K-band and to +-1.0 K in the V-band. For JOYCE, the maximum uncertainty is assessed to be +-1.5 K in the K-band and +-0.6 K in the V-band. When a standing wave phenomena at the cold calibration point is eliminated by averaging several calibrations, the uncertainty in the K-band can be reduced to +-0.8 K for both deployments. In the V-band, the uncertainties are reduced to values less or equal +-0.7 K for both deployments. Furthermore, the analyses of the liquid nitrogen calibration has revealed, that the pressure dependent boiling point correction for liquid nitrogen, originally used by HATPRO-G2, is only exact for standard pressure conditions. Therefore, the boiling point correction has been modified and is now valid for all altitudes. At the low pressure conditions of RHUBC-II (530 hPa), the improved boiling point correction shifts the cold target temperature compared to the previously used formulation by more than 1 K. HATPRO-G2 has seven channels in the K-band and seven channels in the V-band. At standard pressure conditions, only the K-band channels are transparent enough to be calibrated by the tipping curve calibration. However, at 530 hPa, the technique can be applied to two low opacity channels in the V-band as well. This offers the unique opportunity of an independent validation of the liquid nitrogen calibration in the V-band. The analysis shows, that the uncertainty in the tipping curve calibration is mainly due to atmospheric inhomogeneities. For RHUBC-II, the total uncertainty is assessed to be +-0.1 K to +-0.2 K in the K-band and +-0.6 K and +-0.7 K for the two V-band channels at 51 GHZ and 52 GHz. For the low altitude deployment at JOYCE, the total uncertainties for K-band channels are +-0.2 K to +-0.6 K. Finally, the well-characterized radiometer measurements are used to investigate current absorption models. The profiles of temperature, humidity, and pressure from 62 clear sky radiosondes are used for Tb simulations at zenith and compared to HATPRO-G2 measurements. Biases, outside the uncertainty range of the calibration can be ascribed to errors within the gas absorption coefficients. It is found that the results of the Atmospheric Model (AM)(Paine, 2012), which uses the most recent oxygen absorption parameters (Tretyakov et al., 2005, Makarov et al., 2011), are closest to RHUBC-II measurements

Polarimetric weather radar:from signal processing to microphysical retrievals

Accurate modelling of liquid, solid and mixed-phase precipitation requires a thorough understanding of phenomena occurring at various spatial and temporal scales. At the smallest scales, precipitation microphysics defines all the processes occurring at the level where precipitation is a discrete process. The knowledge of these microphysical processes originates from the interpretation of snowfall and rainfall measurements collected with various sensors. Direct sampling, performed with in-situ instruments, provides data of superior quality. However, the development of remote sensing (and dual-polarization radar in particular) offers a noteworthy alternative: large domains can in fact be sampled in real time and with a single instrument. The drawback is obviously the fact that radars measure precipitation indirectly. Only through appropriate interpretation radar data can be translated into physical mechanisms of precipitation. This thesis contributes to the effort to decode polarimetric radar measurements into microphysical processes or microphysical quantities that characterize precipitation. The first part of the work is devoted to radar data processing. In particular, it focuses on how to obtain high resolution estimates of the specific differential phase shift, a very important polarimetric variable with significant meteorological importance. Then, hydrometeor classification, i.e. the first qualitative microphysical aspect that may come to mind, is tackled and two hydrometeor classification methods are proposed. One is designed for polarimetric radars and one for an in-situ instrument: the two-dimensional video disdrometer. These methods illustrate the potential that supervised and unsupervised techniques can have for the interpretation of meteorological measurements. The combination of in-situ measurements and polarimetric data (including hydrometeor classification) is exploited in the last part of the thesis, devoted to the microphysics of snowfall and in particular of rimed precipitation. Riming is shown to be an important factor leading to significant accumulation of snowfall in the alpine environment. Additionally, the vertical structure of rimed precipitation is examined and interpreted