Observations of water vapour absorption using airborne microwave radiometers at 89 and 157 GHz

In 1996 a new five channel humidity sounder, the Advanced Microwave Sounding Unit-B, will be be launched on the NOAA-K polar orbiting satellite. In support of the data, interpretation from this instrument, including the retrieval of water vapour profile information, an airborne millimetre wave radiometer has been developed to validate the gaseous absorption model. In particular the so called water vapour continuum absorption has been measured and compared with predictions from models based on laboratory data. The two channel scanning microwave radiometer (MARSS) with channels at 89 and 157 GHz has been operating on the C-130 aircraft of the UK Meteorological Office for five years. The MARSS results have been compared with algorithms specific to microwave and millimetre wavelengths based on independent laboratory measurements and also general purpose line by line models (GENLN2 and FASCOD3). At 89 and 157 GHz the results are broadly similar and all show good agreement except for very dry and very moist atmospheres. Averaging over all the aircraft data these three models give biases of less than 1 K for both channels and RMS errors of under 3.5 K at 89 GHz and 7.0 K at 157 GHz. Best agreement was found in middle latitudes with a model overestimate for arctic atmospheres and a model underestimate for tropical atmospheres. The Liebe MPM-89 model was generally the best, particularly for the middle latitude dataset.