CEDAR: Polar Mesospheric Cloud Research Using the Sondrestrom, Greenland Lidar

Sri International, Menlo Park CA

Investigators

Abstract

The investigators will study the aspherical nature of polar mesospheric cloud (PMC) particles and the interaction of PMC particles with the mesospheric sodium layer. The shape of PMC particles affects the growth-sedimentation-sublimation cycle associated with visible cloud formation. For example, the fall rate of a PMC particle depends on its shape as well as its size; an aspherical particle experiences different aerodynamic forces that could cause it to reside longer in the supersaturation region and thus could enhance its growth. The aspherical nature of PMCs can affect the characterization of PMC particle size distributions and the interpretation of PMC microphysics. Lastly, the presence of PMCs may affect sodium concentrations and the greater surface area-to-volume ratio of an aspherical particle may enhance the interaction of PMC particles with the mesospheric sodium species. Owing to the submicron size of PMC particles and the visible wavelengths used to probe them, Mie-Rayleigh scattering theory has predominantly been applied to study PMCs. However, these studies have assumed that the PMC particles are spherical -- a fundamental assumption with little experimental support. An aspherical particle causes incident linearly polarized light to depolarize when scattered. Lidar observations have shown nonzero PMC depolarization ratios in lidar measurements from Norway. Recently the Sondrestrom, Greenland, lidar system has also measured nonzero depolarization ratios at visible wavelengths. The results indicated that PMC particles can at times be aspherical. The goal of this research is to evaluate the aspherical nature of PMCs at Sondrestrom. Another new and emerging research area is the study of the interaction of PMCs with the mesospheric metal species. Sondrestrom sodium and Rayleigh lidar measurements have indicated a reduction in sodium density at the lower boundary of the sodium layer when PMCs are present. These new observations suggest a connection between PMCs and sodium that involves either heterogeneous chemistry in the presence of ice or the physical uptake of sodium onto existing ice particles. The investigators will study seasonal changes of sodium at high latitudes, including the effect of ice on the sodium distribution, by using a seven-year database of Sondrestrom lidar measurements and by using a sodium chemistry model. The research plan includes an analysis of PMC depolarization measurements made by the Sondrestrom lidar in the summers of 2003, 2004, and 2005 to address the question about the aspherical nature of the PMCs. The investigators will develop a model based on aspherical particle scattering to address the question of PMC shape and size. In addition, six years of coincident PMC and sodium density measurements will be compiled and the relationship of PMC occurrence to sodium density depletion will be detailed. In addition, new observations from 1064 nm PMC backscatter measurements in the summers of 2005 and 2006 will be used. The combination of 532 nm and 1064 nm backscatter measurements, with depolarization measurements at 532 nm, will be used to better constrain the PMC model and improve the shape and size parameter estimates. The seasonal variability in the sodium layer observed with the lidar will be used to test sodium chemistry models. The PMC depolarization measurements since 2003 and the modeling effort will provide a reasonable database that will be used to characterize basic PMC properties. PMCs are believed to be an indicator of global change. Thus, it is important to understand the physical and chemical mechanisms by which they are formed.