Abstract
Abstract. Integrated Water vapour (IWV) has been measured since 1994 by the TROWARA microwave radiometer in Bern, Switzerland. Homogenization techniques were used to identify and correct step changes in IWV related to instrument problems. IWV from radiosonde, GPS and sun photometer (SPM) was used in the homogenisation process as well as partial IWV columns between valley and mountain weather stations. The average IWV of the homogenised TROWARA time series was 14.4 mm over the 1996–2007 period, with maximum and minimum monthly average values of 22.4 mm and 8 mm occurring in August and January, respectively. A weak diurnal cycle in TROWARA IWV was detected with an amplitude of 0.32 mm, a maximum at 21:00 UT and a minimum at 11:00 UT. For 1996–2007, TROWARA trends were compared with those calculated from the Payerne radiosonde and the closest ECMWF grid point to Bern. Using least squares analysis, the IWV time series of radiosondes at Payerne, ECMWF, and TROWARA showed consistent positive trends from 1996 to 2007. The radiosondes measured an IWV trend of 0.45±0.29%/y, the TROWARA radiometer observed a trend of 0.39±0.44%/y, and ECMWF operational analysis gave a trend of 0.25±0.34%/y. Since IWV has a strong and variable annual cycle, a seasonal trend analysis (Mann-Kendall analysis) was also performed. The seasonal trends are stronger by a factor 10 or so compared to the full year trends above. The positive IWV trends of the summer months are partly compensated by the negative trends of the winter months. The strong seasonal trends of IWV on regional scale underline the necessity of long-term monitoring of IWV for detection,understanding, and forecast of climate change effects in the Alpine region.
Highlights
Water vapour is the strongest natural greenhouse gas
This paper presents Integrated Water vapour (IWV) measured by the TROpospheric WAter RAdiometer (TROWARA) microwave radiometer in Bern, Switzerland for 1994–2007
The original TROWARA time series was corrected with the help of the changepoint magnitudes identified by the homogenisation techniques as well as comparison with simultaneous radiosonde, Global Positioning System (GPS) and sun photometer (SPM) data
Summary
Water vapour is the strongest natural greenhouse gas. Under the assumption of constant relative humidity, the Clausius Clapeyron equation predicts a water vapour increase of 6% per degree Kelvin increase in atmospheric temperature. Ross and Elliott (1996) analyzed surface to 500 mb Integrated Water Vapour (IWV) calculated from North American radiosonde data. A similar study carried out for the whole of the Northern Hemisphere for the 1973 to 1995 period (Ross and Elliott, 2001), showed that the radiosonde IWV trends for Europe were smaller, often negative and not significant at the 95% level. Morland et al.: Tropospheric water vapour above Switzerland over the last 12 years minute or less) They offer an independent source of atmospheric information from both radiosonde and analysis data. Water vapour observations from microwave radiometers and other instruments have been collected by the US Atmospheric Radiation Program (ARM) since the mid 1990s (Revercomb et al, 2003). 5, IWV trends calculated from the homogenized TROWARA dataset are compared with those obtained from radiosonde and ECMWF data
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