Raman lidar and HSRL measurements of aerosol and water vapor variability

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Description

Observations by surface, airborne, and satellite sensors have noted significant changes in aerosol properties in proximity to clouds. Some of these changes are due to hygroscopic growth of the aerosols. We use Southern Great Plains (SGP) Raman lidar aerosol and water vapor measurements and NASA Langley Research Center airborne High Spectral Resolution Lidar (HSRL) measurements acquired during the Cumulus Humilis Aerosol Processing Study (CHAPS-June 2007) and Routine ARM Aerial Facility (AAF) Clouds with Low Liquid Water Depths (CLOWD) Optical Radiative Observations (RACORO-June 2009) campaigns to investigate aerosol hygroscopicity and variations in aerosol properties in the daytime boundary layer. The Raman lidar measurements of relative humidity are compared with coincident measurements from the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter and SGP radiosondes. Aerosol humidification factors are derived using the high (10-sec) temporal resolution Raman lidar profile measurements of aerosol backscatter and relative humidity at or near the top of the daytime boundary layer. These humidification factors are compared with similar factors measured by the surface Aerosol Observing System (AOS) at SGP. Preliminary results from the CHAPS campaign show that the humidification factor f(RH=85%/60%) derived from the Raman lidar measurements can vary widely but is on average consistent with AOS values. Coincident airborne HSRL measurements of aerosol backscatter, depolarization, and extinction are used along with the Raman lidar profile measurements of relative humidity to investigate how both aerosol extensive and intensive optical properties vary with relative humidity and distance from clouds. Raman lidar measurements and airborne CIRPAS Twin Otter aircraft in situ measurements from June 2007 show relative humidity was about 5–10% higher in proximity to clouds. Preliminary results using the Raman lidar and HSRL data from CHAPS show that aerosol backscatter and extinction decreased by approximately 25–40% moving 1–2 km away from clouds. The variations in aerosol optical thickness are smaller, typically around 10–15%, since the changes in aerosol backscatter and extinction were generally confined to the top of the boundary layer. Variations in the coincident HSRL aerosol depolarization and backscatter wavelength dependence measurements also suggest that aerosol nonsphericity and size changed in response to variations in relative humidity. These changes are due in part to hygroscopic aerosol growth. This presentation will discuss these preliminary results as well as ongoing investigations using data acquired during the RACORO campaign.