Raman lidar profiling of water vapor and aerosols over the ARM SGP site

We have developed and implemented automated algorithms to retrieve profiles of water vapor mixing ratio, aerosol backscattering, and aerosol extinction from Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) Raman lidar data. This Raman lidar system is unique in that it is turn-key, automated system designed for unattended, around-the-clock profiling of water vapor and aerosols. These Raman lidar profiles are important for determining the clear-sky radiative flux, as well as for validating the retrieval algorithms associated with the MODIS and MISR sensors on the EOS Terra satellite platform.

Because relative humidity is a key parameter affecting aerosol scattering and extinction, we have also developed and implemented routines to simultaneously retrieve profiles of relative humidity. These routines utilize the water vapor mixing ratio profiles derived from the Raman lidar measurements together with temperature profiles derived from a physical retrieval algorithm that uses data from a collocated Atmospheric Emitted Radiance Interferometer (AERI) and the Geostationary Operational Environmental Satellite (GOES). These aerosol and water vapor profiles (Raman lidar) and temperature profiles (AERI+GOES) have been combined into a single product that takes advantage of both active and passive remote sensors to characterize the clear sky atmospheric state above the CART site.

Aerosol extinction, water vapor, and relative humidity profiles were computed using Raman lidar data acquired after April 1, 1998. The high resolution Raman lidar water vapor measurements and AERI temperature measurements provide a much more detailed representation of the atmospheric state than can be achieved using radiosondes alone. These measurements depict in great detail the rapid atmospheric changes associated with the passages of dry lines and cold fronts over the SGP site.

We have computed aerosol optical thicknesses (AOT) by integrating the Raman lidar aerosol extinction profiles. These values agreed within 5% with simultaneous measurements of AOT made by a Cimel Sun photometer. Average aerosol extinction profiles were computed as a function of optical thickness to characterize the vertical distribution of aerosols. These profiles show the occurrence of high aerosol extinction values at altitudes of 3-6 km above the surface. In particular, aerosol extinction profiles acquired during May and August-September 1998 show episodes when high aerosol extinction was measured throughout several kilometers in the lower troposphere over several days. The Raman lidar aerosol extinction profiles derived during May 13-21, 1998 were most likely associated with the smoke from fires in Central America. Profiles from this event display large variability in the both the magnitude and vertical distribution of aerosol extinction during these periods.

We have also begun using these lidar aerosol and water vapor profiles to investigate lidar aerosol extinction/backscattering ratio, and the relationships among water vapor mixing ratio, relative humidity, aerosol extinction, and aerosol extinction/backscatter ratio for hygroscopic aerosols. The average aerosol extinction/backscattering ratio (355 nm) was between 55-60 sr for altitudes between 1-4 km, with a small decrease above 4 km. The lidar data show that often the aerosol extinction and aerosol extinction/backscattering ratio increased significantly when the relative humidity increased above 60-70% near the top of the boundary layer.