Tuesday, 11 February 2003: 11:15 AM
Raman lidar: A versatile remote sensing instrument for water vapor and cirrus cloud studies
The Department of Energy’s Atmospheric Radiation Measurement (ARM) program has operated a Raman lidar at the Southern Great Plains site in northern Oklahoma nearly continuously since April 1998, collecting over 20,000 hours of data in all seasons under a wide range of atmospheric conditions. This Raman lidar measures the elastic backscatter from molecules and aerosols at the laser wavelength (355 nm), as well as the Raman backscattered light at 387 and 408 nm due to nitrogen and water vapor molecules, respectively. The elastic return is separated into both co-polarized and cross-polarized returns (with respect to the polarization of the outgoing laser beam). These signals are then processed to yield profiles of water vapor mixing ratio, relative humidity, aerosol extinction, aerosol backscatter, linear depolarization ratio, and cloud boundaries. Raman lidars are able to measure the aerosol extinction and backscatter directly, and thus no assumptions are required (as for single wavelength lidars) to retrieve these products.
The physical properties of clouds (such as the location, vertical distribution, particle size and shape, etc.) dictate the radiative impact of the clouds on climate. Cirrus cloud properties are difficult to observe, given their typical atmospheric altitude, and thus there is large uncertainty in the radiative feedback of these clouds. The Raman lidar measurements are being used to quantify the properties of these clouds. The Raman lidar is able to measure upper tropospheric humidity with approximately 5% absolute accuracy, and thus the linkage between ice supersaturation and cirrus formation and persistence can be examined. The ratio of the aerosol backscatter to aerosol extinction in clouds is related to the backscatter phase function of the cloud particles. Using this ratio in conjunction with the linear depolarization ratio, there is potential for ascertaining the ice crystal shape from lidar measurements. In this presentation, we will illustrate the Raman lidar capability and apply this capability to the study of cirrus cloud properties at the Oklahoma ARM site.