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 June 11, 2002, is shown in Figure 1, and the particle size distribution for the buffer shown is displayed in Figure 2.

Flight 781 – 2DC Images

Figure 1: 2DC images recorded during flight 781 (June 11, 2002). The red arrow shows the portion of the T-28 flight track where the shown 2DC buffer was collected.

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 High-Volume Particle Sampler (HVPS) instrument collects shadow images of hydrometeors with sizes ranging between 0.2 and 4.5 mm, with 0.2 mm resolution. It was specially modified by attaching 2 segmented plates between the main arms of the probe. Signals from these plates can be used to estimate electric charge on hydrometeor, but for this project charge information recording was disabled.

The HVPS 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 0.2 mm pixel resolution when measured across the array. In order to maintain constant 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.

The above figure shows a typical display of the software developed in IDL to display the T-28 HVPS data for each flight.