Presented at the International Laser Sensing Symposium, Sept 6-8, 1999, Fukui, Japan
Measurements with the University of Wisconsin Volume Imaging Lidar
Edwin Eloranta
University of Wisconsin
1225 W. Dayton St.
Madison, WI USA 53706
eloranta@lidar.ssec.wisc.edu
Abstract
Boundary layer flow measurements have traditionally been acquired from
instruments mounted on towers, aircraft and balloons. Although much has
been learned from these observations, the measurements suffer from
severe sampling problems. This occurs because boundary layer flows include
coherent structures with a wide range of sizes and life times.
Temporal variations due to diurnal forcing and the movement of weather
systems make flow statistics measured with tower based instruments
non-stationary. Aircraft measurements yield more independent observations in
a shorter time. This reduces problems with temporal variation. However,
measurements must be performed over spatially uniform surfaces or the
spatial statistics measured will also be non-stationary. It is also very
difficult to derive the spatial structure and temporal evolution of coherent
flow structures from tower or aircraft measurements. Ideal boundary layer
measurements would provide the full 4-dimensional flow field. Active remote
sensors such as radar and lidar offer the only hope of such comprehensive
observations.
This paper describes measurements derived by mapping the spatial
distribution of naturally occurring aerosols with the University of
Wisconsin Volume Imaging Lidar (VIL). The paper provides an
overview of VIL measurement capabilities with an emphasis
on recent boundary layer measurements. Cirrus cloud observations with
the VIL will also be discussed briefly.
The Volume Imaging Lidar (VIL) is an elastic backscatter lidar
designed to image the 4-dimensional structure of the atmosphere. This
system couples an energetic (400 mJ), relatively high pulse repetition
rate (100 Hz) Nd-YAG laser with a large aperture receiver (0.5 m), and a fast
computer controlled angular scanning system (up to 100
degrees/s). High bandwidth data acquisition is sustained during
extended experiments by using a 7 gigabyte write once optical disk for
data storage. Data acquisition and system control are performed using
an Silicon Graphics Indigo 2 computer. Data analysis and real time
control of the system are facilitated by 2-dimensional and
3-dimensional displays of data.
A typical 2-minute volume scan with the VIL provides approximately 10
million measurements of aerosol backscatter in a pie shaped sector of
40 to 60 degrees in azimuth, 20 degrees in elevation and 18 km in
range. In a typical experiment, these scan patterns are continuously
recorded for periods of many hours. Images derived from these data
show boundary layer structures. Aerosol rich air is carried aloft by
convective plumes while cleaner air from above the boundary layer is
entrained into the boundary layer by downdrafts. Time-lapse video
images created from this data show both the spatial organization of
the convective elements and their temporal evolution. While the VIL
does not have Doppler velocity measurement capability,
cross-correlation of successive images provides precise area- and
time-averaged vertical profiles of the horizontal wind speed and
direction. It is difficult to determine the absolute accuracy of
these measurements because of other sensors which can measure average
winds over areas of approximately 100 square kilometers are not
available. However, measures of internal consistency between independent
estimates yield differences of less than 5 cm/sec in speed and 1 degree
in direction for averages of 1 hour. Comparisons with aircraft, tower and
balloon measurements all compare to within the expected statistical
variations expected between the the different sample volumes and
averaging times implicit in these measurements.
Recently, the VIL has been used to observe the flow of cold wintertime
air over warm water as part of a a project to improve Large Eddy
Simulation models of atmospheric flows. Time-lapse animations of
lidar data collected in this experiment show a variety of boundary
layer phenomena including: intense convection, a land-breeze case,
intricate spatial patterns of falling snow and complex gravity wave
patterns.
As part of this study, we applied our correlation wind algorithms to a
single horizontal plane at 5 m above the water surface which was
scanned at 12.5 second intervals. In this case, the scanned area was
divided into 250 m square areas and wind speed and direction was
computed in each square. Two data set have been analyzed at this
point. In the first, a ~6 minute average provided 576 wind vectors in
a 6 by 6 kilometer area. In second case, a ~40 minute average produced
960 wind vectors in a 6 by 10 km area. These wind fields show an
acceleration of wind speed and veering of direction as the wind
response to the decreased surface friction over the water. A plot of
average speed and direction as a function of distance from the shore
shows point to point fluctuations of less than 10 cm/sec in speed and
less than 1 degree in direction. The wind fields also show variations
in speed and direction downwind of shore features. The vector wind
fields are sufficiently precise to allow calculation of divergence and
vorticity fields with a noise level of approximately 0.001 1/sec;
these fields show structure associated with shore features and longitudinal
roll circulations.
Fast scanning and high sensitivity also allow the VIL to map
cirrus cloud structure. Under favorable conditions, overhead scans
obtained in less than 1 minute can image cirrus structure over a 120 km
interval. The application of these data to verification of satellite
cirrus cloud retrievals will be presented.
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eloranta@lidar.ssec.wisc.edu