Calibration and validation of water vapour lidar measurements from Eureka, Nunavut, using radiosondes and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer

The Canadian Network for the Detection of Atmospheric Change and Environment Canada DIAL lidar located at the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Nunavut, has been upgraded to measure water vapour mixing ratio profiles. The lidar is capable of measuring water vapour in the dry Arctic atmosphere up to the tropopause region. Measurements were obtained in the February to March polar sunrise during 2007, 2008 and 2009 as part of the Canadian Arctic ACE (Atmospheric Chemistry Experiment) Validation Campaign. Before such measurements can be used to address important questions in understanding dynamics and chemistry, the lidar measurements must be calibrated against an independent determination of water vapour. Here, radiosonde measurements of relative humidity have been used to empirically calibrate the lidar measurements. It was found that the calibration varied significantly between each year's campaign. However, the calibration of the lidar during an individual polar sunrise campaign agrees on average with the local radiosonde measurements to better than 12%. To independently validate the calibration of the lidar derived from the radiosondes, comparisons are made between the calibrated lidar measurements and water vapour measurements from the ACE satellite-borne Fourier Transform Spectrometer (ACE-FTS). The comparisons between the lidar and satellite-borne spectrometer for both a campaign average and single overpasses show favourable agreement between the two instruments and help validate the lidar's calibration. The 39 nights of high-Arctic water vapour measurements obtained offer the most detailed high spatial-temporal resolution measurement set available for understanding this time of transition from the long polar night to polar day.