Precipitation
Cloud
ice and small precipitation particles were imaged with a PMS optical array
probe (OAP) providing two-dimensional shadow images of particles with a
vertical window height of 0.8 mm (2D-C).
This probe produces shadow images of precipitation-size particles with
0.2 mm resolution.
A
brief discussion of the probe is given in Detwiler and Hartman (1991). This
probe performed very well during the project.
An example of the recordings for the flight 781 on
Flight 813 – 2DC Images
Figure 1: 2DC images recorded during flight 813 (
Figure 2: Particle size distribution for the buffers shown in figure 1.
Larger
precipitation particles were imaged and counted by the custom-built Optical
Array Hail Spectrometer, sensitive to particles between 0.9 mm and 12 cm in
diameter. The automated counting and
sizing circuitry includes only particles in the size range 4.5 mm to 4.5
cm. The probe performed well during the
project, except during descent. It is
not de-iced, and descent from cooler to warmer layers of the atmosphere caused
condensation on the windows and loss of data.
Additional
information on large particles can be inferred from the recorded videotape with
the camera mounted under the right wing of the airplane. If the video camera housing window is not
iced-over, the video can be used to view precipitation particle impacts and
some judgement can be made about the character of the precipitation (liquid,
soft ice, hard ice, etc.). In addition, the windscreen microphone, recorded on
one of the two audio tracks, can be used to verify when hail is striking the
windscreen. Mushy hailstones produce muffled sounds, while hard hailstones sound
almost like a hammer hitting the windscreen.
The
HVPS (High Volume Particle Sampler) measures the size, shape and concentration
of precipitation size particles. It
operates by taking a two-dimensional shadow digital picture of the particles
that pass through a 4.5 x 20.3 cm laser light plane that is normal to the
direction of aircraft flight. The sheet of light is imaged onto a linear array
of 256 photo-elements, so that the HVPS has 200 mm pixel resolution when
measured across the array. In order to
keep the pixel resolution along the flight path constant, the system clock
frequency is adjusted according to the detected True Air Speed (TAS) by the
data system itself.
The maximum clock frequency for the HVPS is 240 kHz. The HVPS data recorded during this project
has 400 mm
resolution along the flight path. An
example of the data obtained with the HVPS instrument is presented below
(Flight 813
The above
figure shows a typical display of the software developed in IDL to display the
T-28 HVPS data
for each flight. On the lower right
corner, you can see a display of the particle size distribution, while
on the lower left corner the track of the T28 and the position of the buffer
displayed above are shown.