Impact of varying lidar measurement and data processing techniques in evaluating cirrus cloud and aerosol direct radiative effects

摘要

In the past 2 decades, ground-based lidar networks have drasticallyincreased in scope and relevance, thanks primarily to the advent of lidarobservations from space and their need for validation. Lidar observations ofaerosol and cloud geometrical, optical and microphysical atmosphericproperties are subsequently used to evaluate their direct radiative effectson climate. However, the retrievals are strongly dependent on the lidarinstrument measurement technique and subsequent data processingmethodologies. In this paper, we evaluate the discrepancies between the useof Raman and elastic lidar measurement techniques and corresponding dataprocessing methods for two aerosol layers in the free troposphere and fortwo cirrus clouds with different optical depths. Results show that thedifferent lidar techniques are responsible for discrepancies in themodel-derived direct radiative effects for biomass burning (0.05 W m−2at surface and 0.007 W m−2 at top of the atmosphere) and dust aerosol layers (0.7 W m−2 at surface and 0.85 W m−2 at top of the atmosphere).Data processing is further responsible for discrepancies in both thin(0.55 W m−2 at surface and 2.7 W m−2 at top of the atmosphere) and opaque (7.7 W m−2 at surface and 11.8 W m−2 at top of the atmosphere) cirrus clouds. Direct radiative effect discrepancies can be attributed to the larger variability of the lidar ratio for aerosols(20–150 sr) than for clouds (20–35 sr). For this reason,the influence of the applied lidar technique plays a more fundamental role inaerosol monitoring because the lidar ratio must be retrieved with relativelyhigh accuracy. In contrast, for cirrus clouds, with the lidar ratio being muchless variable, the data processing is critical because smoothing it modifiesthe aerosol and cloud vertically resolved extinction profile that is used asinput to compute direct radiative effect calculations.