Observations of tropical cirrus by elastic backscatter lidars and the development of a cloud and aerosol retrieval algorithm for Raman lidars
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Tropical cirrus cloud properties from elastic backscatter lidars--- namely the Atmospheric Radiation Measurement (ARM) program's ground-based micropulse lidars (MPL) and the spaceborne Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) lidar--- are examined. The MPL detects significantly less cirrus clouds relative to CALIPSO, particularly during the daytime. However, the MPL samples enough cirrus at night to provide similar statistics of macrophysical and optical properties as CALIPSO. Both sets of lidar observations are supplemented with cloud radar observations to calculate radiative heating rate profiles from a ground-based and spaceborne perspective. The inferred radiative effect of clouds is much smaller when using the ground-based data, mostly due to the lack of cirrus detected by the MPL. The relatively new and more advance ARM Raman lidar (RL) is shown to be more sensitive to cirrus than the ARM MPL and detects a similar amount of cirrus as CALIPSO. Daytime measurements using the RL elastic channel are relatively unaffected by the solar background and are therefore suited for checking the observed diurnal cycles from the MPL and CALIPSO. Comparisons with RL observations show that the geometrical thickness of cirrus from the MPL and CALIPSO datasets are biased thin during the daytime due to increased noise. Various upgrades since its conception have made the ARM RL a viable tool for cloud studies as demonstrated by this thesis. Since the ARM RL was not originally designed for cloud observations, the current automated processing algorithms do not identify all clouds nor attempt to retrieve cloud extinction profiles. Therefore an improved Feature detection and EXtinction retrieval (FEX) algorithm is developed. The approach of FEX is to use multiple quantities to identify features (clouds and aerosols) using range-dependent context-sensitive detection thresholds. The use of multiple quantities provides complementary depictions of cloud and aerosol locations. The extinction profiles are directly retrieved using the Raman method, which are supplemented by other retrieval methods developed for elastic backscatter lidars. A classification of feature type is made guided by the atmosphere's thermodynamic state and the feature's scattering properties. The contribution of multiple scattering, which is significant for hydrometeors, is explicitly considered for each of the ARM RL channels. The FEX framework is also suitable for other advance lidars, i.e. high spectral resolution lidars (HSRL). The continuously operated, automated ARM RLs paired with FEX provide an enormous wealth of water vapor, temperature, aerosol and cloud data unmatched by other remote sensing systems.
- Atmospheric sciences