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
ABSTRACT
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 // eloranta@lidar.ssec.wisc.edu
