A complete long-term series of integrated water vapour from ground-based microwave radiometers

Abstract

Integrated water vapour (IWV) is the vertical column density of atmospheric water vapour. IWV can be determined from microwave radiances measured by multi-channel radiometers on the ground or in space. Both quality and quantity of IWV measurements have rapidly increased during the past 10 to 20 years. Global maps and time series of IWV give evidence for a strong spatio-temporal variability of atmospheric water vapour, playing a key role in weather prediction and climate-change research. We analyse the relationships between microwave radiances and IWV using long-term observations of two radiometers at Bern, Switzerland. The first radiometer (Tropospheric Water Vapour Radiometer; TROWARA) measures 21 and 31 GHz radiances and permits the accurate retrieval of IWV. The long-term series of the TROWARA have some data gaps that possibly influence the trend analysis. On the other hand, the series of 142 GHz radiance of the second radiometer (Ground-based Millimeter-wave Ozone Spectrometer; GROMOS) are almost complete. The 142 GHz radiance is more affected by integrated cloud liquid water (ILW) than the 21 and 31 GHz radiances. The coincident radiometer data of GROMOS and TROWARA are utilized for exploration of the relationship between 142 GHz radiance, IWV and ILW. The IWV is calculated from the 142 GHz radiance of GROMOS when TROWARA data are not available. Thus, we can derive a complete series of IWV above Bern from 1994 to 2009. The combination of both series and the trend analysis are performed by means of multiple linear regression and bootstrapping. The observations indicate a positive trend up to +10% decade−1 of IWV in summer and a negative trend of about −15% decade−1 in winter.