[1] Atmospheric water vapor and surface humidity strongly influence the radiation budget at the Earth's surface. Water vapor not only absorbs solar radiation in the atmosphere, but as the most important greenhouse gas it also largely absorbs terrestrial longwave radiation and emits part of it back to the surface. Using surface observations, like longwave downward radiation (LDR), surface specific humidity (q) and GPS derived integrated water vapor (IWV), we investigated the relation between q and IWV and show how water vapor influences LDR. Radiation data from the Alpine Surface Radiation Budget (ASRB) network, surface humidity from MeteoSwiss and GPS IWV from the STARTWAVE database are used in this analysis. Measurements were taken at four different sites in Switzerland at elevations between 388 and 3584 m above sea level and for the period 2001 to 2005. On monthly means the analysis shows a strong linear relation between IWV and q for all-sky as well as for cloud-free situations. The slope of the IWV-q linear regression line decreases with increasing altitude of the station. This is explained by the faster decrease of IWV than of q with height. Both q and IWV are strongly related with LDR measured at the Earth's surface. LDR can be parameterized with a power function, depending only on humidity. The estimation of LDR with IWV has an uncertainty of less than 5% on monthly means. At lower altitudes with higher humidity, the sensitivity of LDR to changes in q and IWV is smaller because of saturation of longwave absorption in the atmospheric window.Citation: Ruckstuhl, C., R. Philipona, J. Morland, and A. Ohmura (2007), Observed relationship between surface specific humidity, integrated water vapor, and longwave downward radiation at different altitudes,

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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.

Abstract. The STARTWAVE (STudies in Atmospheric Radiative Transfer and Water Vapour Effects) project aims to investigate the role which water vapour plays in the climate system, and in particular its interaction with radiation. Within this framework, an ongoing water vapour database project was set up which comprises integrated water vapour (IWV) measurements made over the last ten years by ground-based microwave radiometers, Global Positioning System (GPS) receivers and sun photometers located throughout Switzerland at altitudes between 330 and 3584 m. At Bern (46.95 • N, 7.44 • E) tropospheric and stratospheric water vapour profiles are obtained on a regular basis and integrated liquid water, which is important for cloud characterisation, is also measured. Additional stratospheric water vapour profiles are obtained by an airborne microwave radiometer which observes large parts of the northern hemisphere during yearly flight campaigns. The database allows us to validate the various water vapour measurement techniques. Comparisons between IWV measured by the Payerne radiosonde with that measured at Bern by two microwave radiometers, GPS and sun photometer showed instrument biases within ±0.5 mm. • E, 366 m), which is located on the south side of the Alps, the bias is +1.9 mm. The sun photometer at Locarno was found to have a bias of −2.2 mm (13% of the mean annual IWV) relative to the data from the closest radiosonde station at Milano. This result led to a yearly rotation of the sun photometer instruments between low and high altitude stations to improve the calibrations. In order to demonstrate the caCorrespondence to: J. Morland (june.morland@mw.iap.unibe.ch) pabilites of the database for studying water vapour variations, we investigated a front which crossed Switzerland between 18 November 2004 and 19 November 2004. During the frontal passage, the GPS and microwave radiometers at Bern and Payerne showed an increase in IWV of between 7 and 9 mm. The GPS IWV measurements were corrected to a standard height of 500 m, using an empirically derived exponential relationship between IWV and altitude. A qualitative comparison was made between plots of the IWV distribution measured by the GPS and the 6.2 µm water vapour channel on the Meteosat Second Generation (MSG) satellite. Both showed that the moist air moved in from a northerly direction, although the MSG showed an increase in water vapour several hours before increases in IWV were detected by GPS or microwave radiometer. This is probably due to the fact that the satellite instrument is sensitive to an atmospheric layer at around 320 hPa, which makes a contribution of one percent or less to the IWV.

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[1] Measurements of integrated water vapor (IWV) density using precision filter radiometers (PFRs) at two high-alpine, mid-latitude stations in the Swiss Alps are reported. ) and a large negative bias at JFJ (À1.5 kg m À2 ). Microwave radiometer IWV at JFJ agreed well with PFR IWV suggesting that GPS IWV suffers from unmodeled effects. Linear IWV trend analysis indicated no significant trend at either Davos or JFJ for clear-sky periods. The GPS-IWV time series is at present too short to determine the trend for all-weather conditions. Citation: Nyeki, S., L. Vuilleumier, J. Morland, A. Bokoye, P. Viatte, C. Mätzler, and N. Kämpfer (2005), A 10-year integrated atmospheric water vapor record using precision filter radiometers at two high-alpine sites, Geophys. Res. Lett., 32, L23803,

In this paper an integrated assessment of the vertically integrated water vapor (IWV) measured by radiosonde, microwave radiometer (MWR), and GPS and modeled by the limited-area mesoscale model of MeteoSwiss is presented. The different IWV measurement techniques are evaluated through intercomparisons of GPS to radiosonde in Payerne, Switzerland, and to the MWR operated at the Institute of Applied Physics at the University of Bern in Switzerland. The validation of the IWV field of the nonhydrostatic mesoscale Alpine Model (aLMo) of MeteoSwiss is performed against 14 GPS sites from the Automated GPS Network of Switzerland (AGNES) in the period of 2001–03. The model forecast and the nudging analysis are evaluated, with special attention paid to the diurnal cycle. The results from the GPS–radiosonde intercomparison are in agreement, but with a bimodal distribution of the day-to-night basis. At 0000 UTC, the bias is negative (−0.4 kg m−2); at 1200 UTC, it is positive (0.9 kg m−2) and the variability increases. The intercomparison of GPS to MWR shows better agreement (0.4 kg m−2), with a small increase of the daytime bias with 0.3 kg m−2. The intercomparison of MWR to the radiosonde gives a bimodal distribution of the bias, with an increase in the standard deviation at the daytime measurement. The relative bias is negative (−3%) at 0000 UTC and is positive (3%) at 1200 UTC. Based on this cross correlation, it can be concluded that the bimodal distribution is a result of radiosonde humidity measurements. Possible reasons are the solar-heating correction or sensor errors. The monthly bias and standard deviation of aLMo exhibit a strong seasonal dependence with a pronounced dry bias during the warm months of May–October 2002. The diurnal IWV cycle in 2001 shows good model performance between 0000 and 0900 UTC but IWV underestimation by up to 1.5 kg m−2 for the rest of the day. In 2002 the diurnal cycle shows a systematic dry bias in both the analysis and forecast that is more pronounced in the analysis. This substantial underestimation of IWV was found to correlate with overestimation of aLMo precipitation, especially light precipitation up to 0.1 mm (6 h)−1 in 2002. There is strong evidence that this underestimation can be related to the dry radiosonde bias in midday summer observations. The aLMo dry bias is about 1.0–1.5 kg m−2 greater in the nudging analysis as compared with the forecast initialized at 0000 UTC.

[1] The 31 stations in the Global Positioning System (GPS) network of Switzerland span an altitude range of 330 to 3584 m. The highest station in the network, Jungfraujoch, suffers from a constant negative bias in the Integrated Water Vapor (IWV) due to a protective radome. We compared Jungfraujoch GPS IWV measurements with coincident Precision Filter Radiometer (PFR) observations and showed that the bias in the GPS is fairly constant with respect to the time of year and to the PFR IWV value. A correction was developed for the GPS data and validated by comparison with coincident Raman lidar observations. The IWV observations from nine GPS stations, including Jungfraujoch, were then compared with the IWV field of the ECMWF 40 year reanalysis (ERA40) data. Altitude differences between the ERA40 surface level and the GPS stations resulted, as expected, in a positive bias in the ERA40 IWV. A fairly linear relationship, with an intercept of À0.3 mm, was found between this bias and the difference between the ERA40 surface pressure and the surface pressure at the GPS station. The ERA40 reanalysis captured water vapor variations on timescales of several days very well, as evidenced by an r 2 correlation greater than 0.9 where the altitude difference between ERA40 and the GPS station was less than 1000 m. A comparison between ERA40 and GPS at Davos showed that the reanalysis underestimates IWV during winter temperature inversions.