Short Catalog of MWISP Lidar Measurements
Luc Bissonnette, Gilles Roy, Gilles Vallée, and Christian Bastille
Defence Research Establishment Valcartier
2459 Pie-XI Blvd. North
Val-Bélair, QC G3J 1X5
Canada
POC: Luc Bissonnette
Tel: 418-844-4000 Ext. 4437
FAX: 418-844-4511
Email: luc.bissonnette@drev.dnd.ca
1.0 Multiple-Field-of-View (MFOV) Lidar
1.1 Lidar Source:
-
Nd:YAG laser
Wavelength: 1.06 micrometers
Repetition frequency: 100Hz
Pulse duration: 12 ns
Beam diameter at transmitter: 25 mm
Beam divergence: 0.5 mrad
Polarization: linear
1.2 Receiver:
-
200-mm diameter, 760-mm focal length, off-axis parabolic mirror
The receiver field of view (FOV) is continuously scanned at the repetition
frequency (100 Hz) of the laser source by means of an aluminized rotating
glass disk at the periphery of which apertures of different sizes have been
etched. The disk is placed in the image plane of the parabolic mirror so that
each aperture defines a different FOV. Two disks were used. In the first case,
the most frequently used configuration during MWISP, the disk has 32
circular apertures defining FOVs between 0.1 and 12 mrad full angle. Of
these, only 16 are usable for cloud parameter retrieval, i.e. only 16 of them
define FOVs greater than the beam divergence. The full sequence of all usable
FOVs is completed in 32/100 s. In the second case, there are 4 identical
groups of 8 annular FOVs ranging from 0.5 to 12 mrad. In this configuration,
a full FOV sequence is completed in 8/100 s.
1.3 Scanner:
Both the outgoing laser pulse and the received backscattered radiation pass
through a 350 mm x 225 mm elliptical, 45-degree incidence plane mirror that can
be rotated about a horizontal axis to allow scanning in the vertical plane
perpendicular to the rotation axis. In the MWISP program, the laser beam was
mechanically prevented to drop below 15 degrees at both ends.
1.4 Data Recording:
The backscattered signal is split into its parallel and perpendicular polarization
components with respect to the outgoing laser pulse, log-amplified to compress
the dynamic range, and digitized (8-bit) at a selectable frequencies of 25, 50 or
100 MHz. The maximum achievable dynamic range is 4.5 decades or 32000. The
digitization frequencies define range bins of 6, 3 and 1.5 m, respectively. The
intrinsic lidar spatial resolution based on pulse length is 1.8 m.
1.5 Data Analysis:
1.5.1 Direct Backscatter:
Lidar returns are proportional to the cloud or precipitation backscatter
coefficient at the range position corresponding to the recorded time
following the emission of the laser pulse. In turn, the backscatter
coefficient is proportional to the droplet density. Hence, the raw
backscatter results provide a qualitative picture of the cloud/precipitation
structure. However, the signal strength is subjected to the inverse range
squared dependence, and is attenuated by both scattering and absorption.
Therefore, care must be exercised in interpreting the spatial and temporal
maps constructed from the raw backscatter signals.
1.5.2 Depolarization:
The ratio of the two polarization components of the received signal allows
discrimination between the liquid, crystal and mixed phase of the cloud or
precipitation. Large raindrops are aerodynamically squashed and give rise
to depolarization but in a manner that, under most circumstances, can be
easily differentiated from crystal depolarization. Hence, lidar
depolarization gives temporal and spatial information on cloud phase and
precipitation type.
The lidar backscatter strength is heavily weighted by the small droplets
because of their large number and their scattering efficiency at the
operating wavelength of the lidar; this must be kept in mind in interpreting
lidar depolarization in comparison with radar depolarization. Being a ratio,
the depolarization values are not affected by range and extinction as long
as the signal strength in either polarization component is above the noise
level.
1.5.3 Liquid Water Content and Effective Droplet Diameter:
The MFOV technique allows measurement of the FOV dependence of the
multiple small angle forward scatterings induced by cloud and
precipitation droplets or crystals. At the lidar operating wavelength, small
angle forward scatterings are mostly the results of diffraction. From
diffraction theory, we know that the angular width of the resulting forward
scattering pattern is inversely proportional to the cross-section diameter of
the scatterers. Thus, by properly modeling the scattering processes, we can
retrieve from MFOV lidar measurements a good estimate of the effective
droplet diameter defined as the ratio of the third to the second order
moments of the size distribution. Combining the size information and the
backscatter strength, and assuming a functional form for the size
distribution (e.g. a modified gamma function for clouds), we can infer
liquid water content (LWC).
Both the LWC and the effective droplet diameter are derived as functions
of radial range with the same range bin resolution as for the raw
measurements. However, because of propagation effects, the diameter
retrieval, and hence the LWC, is subject to averaging over a sliding axial
length equal to one scattering mean free path.
The lidar system and the retrieval method were designed for water droplet
clouds. In the case of precipitation and ice crystal clouds, the MFOV
method still returns values of droplet diameters and LWC. From
preliminary analysis, these values appear reasonable but, at this stage of
the analysis, we cannot determine their accuracy. There is probably a bias
toward smaller raindrops or crystals, and in the case of ice crystals a
totally different method of calculation of the equivalent LWC will have to
be developed.
1.6 MWISP Lidar Measurements
Three types of lidar measurements were performed in the MWISP program.
Type 1 – Quick single-FOV elevation scans
The FOV was set at a single intermediate value of 4 mrad and the elevation
scanner was operated at a continuous constant speed of about 2 degrees/s. The
scan plane was vertical and aligned in the azimuthal direction of the summit
observatory, i.e. approximately 87 degrees from true north. The scan is from 16degree elevation looking east toward the summit to 15 degrees looking west.
These scans provide a qualitative picture of the spatial structure of cloud base and
precipitation, and give a good indication of their liquid, solid or mixed phase. A
sample scan image showing a nearly uniform water cloud base on top of a lower
ice cloud (or precipitation) that curls down the mountain slope is reproduced in
Fig. 1.
Type 2 – Step-by-step MFOV elevation scans
These measurements were made at multiple FOVs for subsequent retrieval of
droplet diameter and LWC. Since the FOV recordings are sequential, the scanner
can not move during a least a complete set of FOVs. Therefore, the scanner was
stopped at discrete elevation angles, and bursts of 1000 lidar shots were recorded
at each scanner position. For most cases reported here, the scans were made from
an elevation angle close to the vertical to 16 degrees looking east toward the
summit, by steps of 2 degrees. Typically, it took 12 to 15 minutes to complete
these scans. Since this is rather long, the step-by-step scans were performed only
when conditions were sufficiently stable for the derived profiles to be
representative of the spatial cloud and precipitation structure. An example of such
measurements during a snow shower is shown in Fig. 2.
The planned analysis in these cases is the retrieval of the elevation-range vertical
profiles of LWC and effective droplet diameter up to the penetration depth of the
lidar pulses.
Type 3 – MFOV stare mode measurements
The aim of these measurements is to derive the temporal evolution of the vertical
profiles of LWC and effective droplet diameter in the clouds and precipitation
above the Cog Railway Base site. For most of these measurements, the lidar was
pointed vertically and bursts of 1000 MFOV lidar shots were fired at intervals of
one minute for periods ranging from 10 to 60 minutes. An example of raw
measurements is illustrated in Fig. 3. In a few cases, the elevation angle was set at
16 degrees looking toward the summit. However, the vertical direction was
preferred because it allowed greater penetration depths through the cloud vertical
structure.
Figures 4 to 13 give the time lines delimiting each of these lidar measurement
types for the 20 days of valid results. The lower line is for measurements of type
1, the middle line for measurements of type 3, and the upper line for
measurements of type 2. The date is identified at the top of each graph and the
time is given in UTC.
Figure 1: Single FOV elevation scan recorded on 16 April 1999 at 23:54 UTC.
Upper image is raw backscatter return; the color-coded scale is logarithmic but
the units are arbitrary. Lower image is depolarization ratio; the scale is linear,
white is 1% and black is 100%. The extended black regions in the depolarization
image are not real but correspond to regions where signal strength has reached
noise level. The radius grid size is 1500 m; scan is from 16 degrees looking east
toward the summit to 15 degrees looking west. East is to the right and west to the
left. The base of the water cloud is approximately at 2200 m above ground level.
Figure 2: 1.03-mrad FOV image of step-by-step elevation scan recorded on 15
April 1999 from 16:10 UTC to 16:23 UTC. Upper and lower images and color
code are as in Fig. 1. Total radius of pie shape display is 1350 m; scan is from 16
degrees looking east toward the summit to 72 degrees looking west. East is to the
right and west to the left. The base of the water cloud is approximately at 500 m.
The depolarization image shows nearly uniform snow below cloud base.
Figure 3: 1.03-mrad FOV image of a 1-hour long stare mode measurement event
recorded on 10 April 1999 between 17:59 UTC and 18:59 UTC. The lidar was
pointed vertically. Vertical scale is range expressed in microseconds from 0 to 8
(1 microsecond = 150 m); horizontal scale is elapsed time in millihours, from
17980 to 18980. Upper image is raw backscatter return; the color-coded scale is
logarithmic but the units are arbitrary. Lower image is depolarization ratio; the
scale is linear, white is 1% and black is 100%. The palettes are the same as in
Figs. 1 and 2. The extended black regions in the depolarization image are not real
but correspond to regions where signal strength has reached noise level. The
depolarization image indicates that precipitation is mixed rain and snow; at times
rain is denser than snow (blue regions). The cloud base is very clearly delimited
by the whitish band in depolarization image.
04-02-1999
MFOV stare mode
0.50
10
12
14
16
18
20
22
24
Time (UTC)
04-03-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 4
MFOV stare mode
20
MFOV elavation scan
22
24
04-04-1999
Single FOV elevation scan
10
12
14
16
18
MFOV stare mode
20
MFOV elevation scan
22
24
Time (UTC)
04-07-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 5
MFOV stare mode
20
MFOV elevation scan
22
24
04-08-1999
Single FOV elevation scan
10
12
14
18
16
MFOV elevation scan
20
MFOV elevation scan
22
24
Time (UTC)
04-10-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 6
MFOV stare mode
20
MFOV elevation scan
22
24
04-13-1999
Single FOV elevationscan
10
12
14
16
18
MFOV stare mode
20
MFOV elevation scan
22
24
Time (UTC)
04-14-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 7
MFOV stare mode
20
MFOV elevation scan
22
24
04-15-1999
Single FOV elevation scan
10
12
14
16
MFOV elevation scan
18
20
MFOV elevation scan
22
24
Time (UTC)
04-16-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 8
20
22
MFOV stare mode
24
26
04-17-1999
Single FOV elevation scan
10
12
14
16
18
20
MFOV stare mode
22
24
Time (UTC)
04-18-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 9
MFOV stare mode
20
MFOV elevation scan
22
24
04-19-1999
Single FOV elevation scan
10
12
14
16
18
20
MFOV stare mode
22
24
Time (UTC)
04-20-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 10
MFOV stare mode
20
MFOV elevation scan
22
24
04-21-1999
Single FOV elevation scan
10
12
14
16
18
20
MFOV stare mode
22
24
Time (UTC)
04-22-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 11
20
MFOV stare mode
22
24
04-24-1999
Single FOV elevation scan
10
12
14
16
18
20
MFOV stare mode
22
24
Time (UTC)
04-26-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 12
20
MFOV stare mode
22
24
04-27-1999
Single FOV elevation scan
10
12
14
16
18
20
MFOV stare mode
22
24
Time (UTC)
04-29-1999
Single FOV elevation scan
10
12
14
16
18
Time (UTC)
Figure 13
20
MFOV stare mode
22
24