Monday, 3 June 2002
Microphysical Interpretation of Cirrus Measurements with LidaróComparision to a coupled optical-microphysical model
The development of high-latitude (67.9oN) cirrus cloud systems has been observed with a ground-based polarization Raman lidar in January 1997. The experimental results can be summarized as follows. Firstly, a strong correlation is found between the particle optical properties, specifically depolarization ratio d and extinction-to-backscatter ratio S, for ambient cloud temperatures warmer than ~ -45oC (d < ~ 40%). An anti-correlation is found for colder temperatures (d > ~ 40%). Secondly, over the length of each measurement (4-7.5 hours), the particle properties vary systematically. Initially, d » 60% and S » 10 sr are observed. Then, with decreasing d, S first increases to ~ 27 sr (d » 40%) before decreasing to values around 10 sr (d » 20%). Thirdly, the particle optical properties distinctly depend on the ambient temperature. Based on ray-tracing computations of particle scattering properties, these data may be interpreted in terms of size, shape and growth of the cirrus particles. Near the cloud top in the early stage of the cirrus development, light scattering by small hexagonal columns with aspect ratios close to one is dominant. Over time, as the cloud base extends to lower altitudes with warmer temperatures, the ice particles grow and get morphologically diverse (the scattering contributions of hexagonal columns and plates are roughly the same for large S and depolarization values of ~ 40%). Toward cloud base, light scattering is predominantly by plate-like or complex ice particles. Since no in situ data of particle microphysical properties are available for comparison, we test our hypothesis for consistency with a coupled microphysical and optical cirrus model. We look at the development of the cirrus clouds over time. In our approach, we use a size-distribution resolving cirrus model with explicit microphysics for generation of the cloud microphysical properties. Temperatures are taken from radiosonde ascents during the lidar observations. The vertical wind speed and the humidity profile are deduced from sensitivity tests where values are selected which yield best agreement between the observed and modeled temporal evolution of the cirrus geometrical properties (cloud height and vertical extent). The particle are assumed to be hexagonal columns or plates. The simulated microphysical data are then converted to cloud optical properties by use of optical data obtained with a ray-tracing model, and under the assumption that aspect ratio and maximum dimension of the ice crystals are correlated. Finally, synthetic and measured optical properties are compared. One cirrus event has been studied so far. Good agreement between lidar and theoretical data is found for all measurement periods considered. The lidar ratio, which is more sensitive to particle size than to crystal shape, is well simulated. If columnar hexagonal cirrus particles are assumed, synthetic depolarization ratios match the observed values at and near the cloud top, but tend to be larger at lower cloud altitudes. This effect is probably due to the fact that the cloud model does not account for height-dependency of particle habits. Calculated and measured extinction coefficients agree well. These results support the proposed microphysical interpretation of cirrus measurements obtained with lidar.