AN INVESTIGATION OF CIRRUS CLOUD PROPERTIES USING AIRBORNE LIDAR

Abstract

The impact of cirrus clouds on the Earth's radiation budget remains a key uncertainty in assessing global radiative balance and climate change. Composed of ice, and located in the cold upper troposphere, cirrus clouds can cause large warming effects because they are relatively transmissive to short-wave solar radiation, but absorptive of long wave radiation. Our ability to model radiative effects of cirrus clouds is inhibited by uncertainties in cloud optical properties. Studies of mid-latitude cirrus properties have revealed notable differences compared to tropical anvil cirrus, likely a consequence of varying dynamic formation mechanisms. Cloud-aerosol lidars provide critical information about the vertical structure of cirrus for climate studies. For this dissertation, I helped develop the Airborne Cloud-Aerosol Transport System (ACATS), a Doppler wind lidar system at NASA Goddard Space Flight Center (GSFC). ACATS is also a high spectral resolution lidar (HSRL), uniquely capable of directly resolving backscatter and extinction properties of a particle from high-altitude aircraft. The first ACATS science flights were conducted out of Wallops Island, VA in September of 2012 and included coincident measurements with the Cloud Physics Lidar (CPL) instrument. In this dissertation, I provide an overview of the ACATS method and instrument design, describe the ACATS retrieval algorithms for cloud and aerosol properties, explain the ACATS HSRL retrieval errors due to the instrument calibration, and use the coincident CPL data to validate and evaluate ACATS cloud and aerosol retrievals. Both the ACATS HSRL and standard backscatter retrievals agree well with coincident CPL retrievals. Mean ACATS and CPL extinction profiles for three case studies demonstrate similar structure and agree to within 25 percent for cirrus clouds. The new HSRL retrieval algorithms developed for ACATS have direct application to future spaceborne missions. Furthermore, extinction and particle wind velocity retrieved from ACATS can be used for science applications such as dust transport and convective anvil outflow. The relationship between cirrus cloud properties and dynamic formation mechanism is examined through statistics of CPL cirrus observations from more than 100 aircraft flights. The CPL 532 nm lidar ratios (also referred to as the extinction to backscatter ratio) for cirrus clouds formed by synoptic-scale uplift over land are lower than convectively-generated cirrus over tropical oceans. Errors in assuming a constant lidar ratio can lead to errors of ~50% in cloud optical extinction derived from space-borne lidar such as CALIOP. The 1064 nm depolarization ratios for synoptically-generated cirrus over land are lower than convectively-generated cirrus, formed due to rapid upward motions of tropical convection, as a consequence of differences in cloud temperatures and ice particle size and shape. Finally, the backscatter color ratio is directly proportional to depolarization ratio for synoptically-generated cirrus, but not for any other type of cirrus. The relationships between cirrus properties and formation mechanisms determined in this study can be used as part of a larger global climatology of cirrus clouds to improve parameterizations in global climate models and satellite retrievals to improve our understanding of the impact of clouds on weather and climate.