An Investigation of Raman Lidar Aerosol Measurements and their Application to the study of the Aerosol Indirect Effect


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Physics, Atmospheric

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The problem of the increasing global atmospheric temperature has motivated a large interest in studying the mechanisms that can influence the radiative balance of the planet. Aerosols are responsible for several radiative effects in the atmosphere: an increase of aerosol loading in the atmosphere increases the reflectivity of the atmosphere and has an estimated cooling effect and is called the aerosol direct effect. Another process involving aerosols is the effect that an increase in their concentration in the atmosphere has on the formation of clouds and is called the aerosol indirect effect. In the latest IPCC report, the aerosol indirect effect was estimated to be responsible for a radiative forcing ranging between -0.3 W/m2 to -1.8 W/m2, which can be as large as, but opposite in sign to, the radiative forcing due to greenhouse gases. The main goal of this dissertation is to study the Raman lidar measurements of quantities relevant for the investigation of the aerosol indirect effect and ultimately to apply these measurements to a quantification of the aerosol indirect effect. In particular we explore measurements of the aerosol extinction from both the NASA Goddard Space Flight Center Scanning Raman Lidar (SRL) and the US Department of Energy (DOE) ARM Climate Research Facility Raman Lidar (CARL). An algorithm based on the chi-squared technique to calculate the aerosol extinction, which was introduced first by Whiteman (1999), is here validated using both simulated and experimental data. It has been found as part of this validation that the aerosol extinction uncertainty retrieved with this technique is on average smaller that the uncertainty calculated with the technique traditionally used. This algorithm was then used to assess the performance of the CARL aerosol extinction retrieval for low altitudes. Additionally, since CARL has been upgraded with a channel for measuring Raman liquid water scattering, measurements of cloud liquid water content, droplet radius and droplet number density using this new capability have been studied. Some discrepancies are found between the CARL and AERI measurements of liquid water path and droplet effective radius and they need to be studied in more detail when a larger dataset is available. To study the correlation between aerosol presence and cloud microphysics the calculations of IE, introduced by Feingold as a parameterization of the aerosol indirect effect, has been performed here for the first time using exclusively Raman lidar data. The work shown here is an indication that the combined measurements of aerosol extinction, cloud liquid water content, droplet radius and droplet number density with a Raman lidar represents an interesting new technique for the study of the aerosol indirect effect.