Cloud macrophysical and optical properties derived from micropulse and Raman lidar over SGP site

Description

Active remote sensing such as lidars or radars can be used with other data to quantify the cloud properties at regional scale and at global scale (Dupont et al. 2009). Lidar remote sensing is sensitive to very thin and high clouds but has a significant limitation due to signal attenuation in the ability to precisely quantify the properties of clouds with a cloud optical thickness larger than 3. In this study, 10 years of backscatter lidar signal data are analyzed by a unique algorithm called STRAT (Morille et al. 2006). We apply the STRAT algorithm to data from both the collocated Micropulse lidar (MPL) and a Raman lidar (RL) at the ARM SGP site between 1998 and 2009. Raw backscatter lidar signal is optimized, and dead-time, overlap, and after-pulse corrections are taken into account. The cloud properties for all levels of clouds are derived, and distributions of cloud base height (CBH), top height (CTH), physical thickness (CT), and optical thickness (COT) from regional statistics are compared. The goal of this study is (1) to establish a climatology of macrophysical and optical properties for all levels of clouds observed over the ARM SGP site and (2) to estimate the discrepancies induced by the two remote sensing systems (pulse energy, sampling, resolution, etc.). Our first results tend to show that the MPLs, which are the primary ARM lidars, have a distinctly limited range of usable lidar signal, especially during summer daytime period (only 50% of usable lidar signal at an altitude of 7 km). Consequently, the MPL-derived annual cycle of cirrus cloud base (top) altitude is biased low, especially for daylight periods, compared with those derived from the RL data, which ranges from 7.5 km in winter to 9.5 km in summer (from 8.6 to 10.5 km). The optically thickest cirrus clouds (COT>0.3) reach 50% of the total population for the Raman lidar and only 20% for the Micropulse lidar due to the difference of pulse energy and the effect of solar irradiance contamination. A complementary study using the cloud fraction derived from the Micropulse lidar for clouds below 5 km and from the Raman lidar for cloud above 5 km allows for better estimation of the total cloud fraction between the ground and the top of the atmosphere. The study presents the diurnal cycle of each cloud fraction for each season in comparisons with the Long et al. (2006) cloud fraction calculation.