Calibration of a Water Vapour Lidar using a Radiosonde Trajectory Method
Hicks-Jalali, S., Sica, R. J., Haefele, A., and Martucci, G.
by Atmospheric Measurement Techniques (AMT) at 2018-10-12, in review
Lidars are well-suited for trend measurements in the upper troposphere and lower stratosphere, particularly for species such as water vapour. Trend determinations require frequent, accurate and well-characterized measurements. However, water vapour Raman lidars produce a relative measurement and require calibration in order to transform the measurement into physical units. Typically, the calibration is done using a reference instrument such as a radiosonde. We present an improved trajectory technique to calibrate water vapour Raman lidars based on the previous work of Whiteman et al. (2006), Leblanc and Mcdermid (2008), and Adam et al. (2010) who used radiosondes as an external calibration source, and matched the lidar measurements to the corresponding radiosonde measurement. However, they did not consider the movement of the radiosonde. As calibrations can be affected by a lack of co-location with the reference instrument, we have attempted to improve their technique by tracking the air parcels measured by the radiosonde relative to the field-of-view of the lidar. This study uses GCOS Reference Upper Air Network (GRUAN) Vaisala RS92 radiosonde measurements and lidar measurements from the MeteoSwiss RAman Lidar for Meteorological Observation (RALMO), located in Payerne, Switzerland to demonstrate this improved calibration technique. We compare this technique to traditional radiosonde-lidar calibration techniques which do not involve tracking the radiosonde. Both traditional and our trajectory methods produce similar profiles when the water vapour field is homogeneous over the 30min calibration period. We show that the trajectory method more accurately reproduces the radiosonde profile when the water vapour field is not homogeneous over a 30min calibration period. We also calculate a calibration uncertainty budget that can be performed on a nightly basis. We include the contribution of the radiosonde measurement uncertainties to the total calibration uncertainty, and show that on average the uncertainty contribution from the radiosonde is 4%. We also calculate the uncertainty in the calibration due to the uncertainty in the lidar's counting system, caused by phototube paralyzation, and found it to be an average of 0.3% for our system. This trajectory method allows a more accurate calibration of a lidar, even when non-co-located radiosondes are the only available calibration source, and also allows additional nights to be used for calibration that would otherwise be discarded due to variability in the water vapour profile.
Hicks-Jalali, S., Sica, R. J., Haefele, A., and Martucci, G.: Calibration of a Water Vapour Lidar using a Radiosonde Trajectory Method, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2018-246, in review, 2018.