ESTIMATION OF ATMOSPHERIC LIQUID WATER BY COMBINED OBSERVATIONS FROM GROUND-BASED SINGLE-CHANNEL MICROWAVE RADIOMETRY AND GNSS

G. Elgered¹, J. L. Davis², P. Forkman¹

¹Chalmers University of Technology, ²Lamont-Doherty Earth Observatory of Columbia University

We assess the accuracy of a one-channel ground-based microwave radiometer (MWR) co-located with a GNSS station of geodetic quality with the goal of having a simple inexpensive MWR design to determine the integrated liquid water (ILW) in the atmosphere. The contributions to the sky brightness temperature observed by the MWR are mainly the integrated water vapour (IWV) and the ILW. Two additional and significant contributions are from oxygen and the cosmic microwave background. These two contributions can however be modelled with high accuracy using surface meteorological observations. Therefore, for an isolated MWR at least two channels, at separated frequencies, with different sensitivity to the IWV and the ILW are needed. With the launch of global navigational satellite systems (GNSS) it has been shown that a ground-based receiver station is able to provide estimates of the signal delay due to the atmosphere which includes the effect caused by water vapour. This equivalent zenith wet delay (ZWD) is approximately proportional to the amount of water vapour in the atmosphere and can be estimated from GNSS with an accuracy which is comparable to that of an MWR. Thus, a GNSS station co-located with a single-channel MWR can provide the information needed to separate the contributions from the IWV and the ILW in the atmosphere. Furthermore, during conditions with no clouds containing liquid water, the GNSS observations will offer a method to determine the sky brightness temperature and can then be used to calibrate the MWR.

We use 24 days of MWR data from June 2018 to assess this new approach. The MWR has two independent channels with centre frequencies at 21.0 GHz and 31.4 GHz. The latter channel is used together with GNSS data to estimate the ILW. In order to obtain reference values of the ILW the channels were calibrated using elevation scans, also known as tip curves. The GNSS station ONS1 is part of the IGS network and the data and the estimated total propagation delays are publicly available. When using GNSS data to account for the water vapour contribution to the sky brightness temperatures observed by the MWR we need to subtract the so called hydrostatic signal delay resulting in a time series of equivalent ZWD. The pressure observations needed are obtained from a calibrated barometer located close to the GNSS station.

The results indicate that a one-channel MWR together with a GNSS station is comparable in terms of accuracy to a two-channel MWR. Given the proliferation of GNSS stations and the relative low-cost of single-channel MWRs, this method may represent an improvement on inferring IWV by GNSS data only. We present our result and discuss possible methods further to improve the technique.