Theoretical analysis of the frequency allocation of the hinge points around 22.235 GHz

Microwave ground- and satellite-based sensing of atmospheric precipitable water vapor (PWV) is commonly based on the measurement of down/up welling brightness temperature (Tb) in the spectral region around the water vapor rotational line at 22.235 GHz. The atmospheric absorption/emission at these frequencies is dependent on the thermodynamic state of the atmosphere, which is usually unknown. The dependence on atmospheric profile can be limited by choosing the channel frequency near the so called "hinge points". However, the frequency allocation of such points is not exactly determined. Therefore, we considered three databases of atmospheric thermodynamic sets of profiles (pressure, temperature, humidity) collected in three contrasting environments (Arctic, mid-latitude, tropical), and processed them with four commonly used microwave absorption models, to get simulated ground based measurements in the 19-26 GHz range. We evaluated a variety of different inversion methods for single-channel PWV retrieval. Thus, we show spectra of PWV retrieval uncertainty for typical cases, and we discuss our choice of the frequency location for the hinge points around 22.235 GHz.