Use of In Situ Data to Test a Raman Lidar–Based Cloud Condensation Nuclei Remote Sensing Method

Abstract

A method of retrieving vertical profiles of cloud condensation nuclei (CCN) concentration from surface measurements is described. Surface measurements of the CCN concentration are scaled by the ratio of the backscatter (or extinction) vertical profile to the backscatter (or extinction) at or near the surface. The backscatter (or extinction) profile is measured by Raman lidar and is corrected to dry conditions using the vertical profile of relative humidity (also measured by Raman lidar) and surface measurements of the dependence of backscatter (or extinction) on relative humidity. The method assumes that the aerosol composition and the shape of the aerosol size distribution at the surface are representative of the vertical column. Aircraft measurements of aerosol size distribution are used to test the dependence of the retrieval on the uniformity of the shape of the aerosol size distribution. The retrieval is found to be robust for supersaturations less than 0.02% but breaks down at higher supersaturations if the vertical profile of the shape of the aerosol size distribution differs markedly from the shape of the distribution at the surface. Such conditions can be detected from the extinction/backscatter ratio.