Measurement of Ice Water Content in Cirrus with the HSRL

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Poster--ARM Science team meeting, 19-23 March 2001, Atlanta, GA.
High Spectral Resolution Lidar Measurements of Ice Water Content: Approach and Initial Progress

E. W. Eloranta, R. E. Kuehn and R. E. Holz
University of Wisconsin

More accurate methods to measure the ice water content of cirrus clouds are needed. We propose to attempt this difficult measurement using the University of Wisconsin High Spectral Resolution Lidar (HSRL) and radar observations. The HSRL provides calibrated measurements of the scattering cross section. Because cirrus particles are large compared to the 532 nm wavelength transmitted by the lidar, the scattering cross section is just 2 times the total area of particles per unit volume. Because ice crystals are non-spherical and often oriented by the aerodynamic forces it is important to note that this is the particle area projected on a plane perpendicular to the lidar beam. We are currently working to measure the effective radius of the ice particles (again projected on a plane perpendicular to the lidar beam) by observing multiple scattering in the lidar beam. Theory shows that the multiple scattering is robustly related to the angular width of the diffraction peak in the scattering phase function and this width is a directly related to particle area. Together these measurements provide the cross sectional area per particle and the total number of particles. The fall velocity of ice crystals is dependent on the horizontal projection of the particle area and on the particle mass. Thus, Doppler fall velocities measured by radar coupled with HSRL measurements offer the potential to determine ice water mass. Our poster ( ~5 megabyte pdf, 48" x 36") provides an overview of the proposed approach and describes progress on this recently initiated project.

Some Initial Results excerpted from the Poster
The University of Wisconsin High Spectral Resolution Lidar (HSRL) has been modified to record data with two new adjustable field-of-view data channels. Variations in the signal backscattered by molecules as the field-of-view is varied provides information on the forward scattering phase function for cirrus ice particles. The multiple field-of-view measurements and simultaneous HSRL measurements of the scattering cross section profile provide input to a lidar multiple scattering model. Fitting the observed multiple field-of-view data with the model provides particle size information. The recovery of particle size information is demonstrated using data acquired on 22-Feb-01. Time-Altitude images show backscatter cross section and depolarization ratios observed on this day. Large depolarizations in the cloud indicate that it is comprised of ice crystals.

An example of multiple field-of-view data and the model fit to this data shows a good fit for an assumed particle radius of 75 microns.

The sensitivity of the multiply scattered molecular signal to variations in particle size and receiver field of view can be shown with model computations for the cloud observed on 22-Feb-01. The lidar multiple scatter equation used for these computations is shown in the poster. It assumes a log-normal distribution of particles sizes. However these initial computations were made with gamma=0, (ie. the distribution width has been set to zero). The lidar multiple scatter model is an extention of the one presented in A Practical Model for the Calculation of Multiply Scattered Lidar Returns, Applied Optics 37, 2464-2472.

UW Lidar // April 5, 2001 //