Summertime tropospheric water vapor measurements above the Arctic Circle by Raman lidar and comparison to weather model reanalyses
Résumé
Original summertime time series of water vapor profiles in the polar atmosphere have been obtained by a Raman lidar system during the Pollution in the ARCtic System (PARCS) campaign held May 12th to 27th, 2016 in Hammerfest, Norway (70°40’N) as part of the French Arctic initiative. Such vertical profiles are of paramount importance to improve our knowledge of the Arctic atmospheric evolution under anthropic pressure for both climate and air quality purposes. Indeed, in this pristine region, atmospheric moisture plays several key roles in the climate and atmospheric chemistry: i) on sea ice melt onset, ii) on multiphasic chemistry via the interactions between water vapor, aerosols and cloud cycles, and iii) on photolysis rates influencing the tropospheric oxidation capabilities during summer. Moreover, ground-based lidar measurements appear as very relevant because the ongoing positive moisture anomaly observed during summertime due to global warming is concentrated at low altitude. It presents strong spatiotemporal gradients typical of the region, which are not well resolved by satellite observations or weather models.
The ground/air measurement strategy for Arctic pollution in PARCS, including the Water Vapor & Aerosol LIdar (WALI) as well as an airborne Raman system and sonde, will be described. We will present lidar measurements of the water vapor mixing ratio performed by WALI embedded in the mobile atmospheric station of our laboratory. The vertical profiles of water vapor are obtained day-round with an uncertainty below 10% for a vertical resolution of 100 m and a temporal resolution of 30 minutes, up to 2500m during daytime. The precision of the lidar calibration via the airborne meteorological sonde has been evaluated and will be discussed. Lidar-derived water vapor profiles confirm the strong spatio temporal variability highlighted by previous studies, in terms of vertical gradients and sudden moist fronts. They will be compared to profiles of reanalyses used in mesoscale transport models, as given by the WRF & ECMWF models.