[1] Atmospheric water vapor is a key parameter for the analysis of climatic systems (greenhouse gas effect), in particular over high latitudes where water vapor displays an important seasonal variability. The sparse spatial and temporal sampling of atmospheric water vapor observations across Canada needs to be improved. A series of instruments and methods including a 940-nm solar absorption band radiometer (R) and radiosonde (S) analysis from a numerical weather prediction model and a ground-based bi-frequency Global Positioning System (GPS) were used to evaluate the integrated atmospheric water vapor (IWV) at various sites in Canada and Alaska from a multiyear database. The IWV-R measurements were collected within the framework of the North American Sun Radiometry network (AERONET/AEROCAN). Intercomparisons between [IWV-GPS and IWV-S], [IWV-R and IWV-GPS], and [IWV-R and IWV-S]show root mean square (RMS) differences of 1.8, 1.9, and 2.2 kg m À2 , respectively. GPS meteorology appears to be the easiest approach to calibrate the solar radiometer water vapor band owing to its flexibility, and it allows us to overcome the Sun radiometry limitation in high-latitude areas like the Arctic. The sensitivity of the GPS retrieval to various parameters like GPS satellite constellation and meteorological data are discussed. The classical linear relationship between the surface temperature and the integrated weighted mean temperature profile needed for IWV-GPS retrieval may be significantly different for Arctic air masses compared with midlatitude air masses in the case of tropospheric temperature profile inversion. An ever-expanding multiyear (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) North American summer water vapor climatology, derived from AERONET/Canadian Sun Radiometer Network, is presented and analyzed, showing a mean value of 19.8 ± 6.1 kg m À2 and variations from 17 kg m À2 in Alaska to 23 kg m À2 in southeastern Canada. The results in Bonanza Creek, Alaska, show significant interannual variations with a peak in 1997, which may be linked to an El Niño event that occurred in the same year. Such a database may also be useful for climate model validation as shown for the Canadian Global Environmental Model (RMS difference of 3.4 kg m À2 ). In the end we show that, even if data are selected only for cloud-free atmospheres, there are no significant differences as compared with radiosonde climatology at Canadian Northwestern sites ( 3% relatively to Bonanza Creek summer mean value).

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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,

A study of the validation and calibration process for integrated water vapor (IWV) measurements derived from sun radiometry at the 940-nm solar absorption channel employed in the Aerosol Robotic Network (AERONET) Aerosol Canada (AEROCAN) is presented. The sun radiometer data are compared with GPS meteorology records used as a reference. Three Canadian sites from different climatic regimes covering the period 2000–04 are considered. The observations from five different sun radiometers (IWV-SUN) were processed using the initial AERONET IWV retrieval procedure (V1) whereas GPS-derived IWV (IWV-GPS) was retrieved using “GPSpace” software developed by the Geodetic Survey division of Natural Resources Canada. A sensitivity study is carried out to highlight the influence of both central wavelength and signal amplitude on the 940-nm filter characteristics, which are instrument dependent and can drift due to aging. The comparison between IWV-SUN (V1) and IWV-GPS shows an average rmse of 0.23 ± 0.11 g cm−2 (22%) and a mean bias of −0.09 ± 0.16 g cm−2 (9%). Furthermore, it is shown that the use of GPS for determining the 940-nm channel calibration constants for the solar radiometers improves IWV retrievals (rmse reduced by about 35% and bias by a factor of 3–10) without any knowledge of the 940-nm filter characteristics. These results are discussed within the context of the new AERONET IWV processing procedure (V2), which accounts for solar 940-nm region filter characteristics. The GPS receiver technique appears to be a powerful calibration tool because of its continuous observation capability, its robustness, and its operational simplicity.