DROP DISTORTIONS NEAR LARGE AIRCRAFT
AS OBSERVED BY 2-D PARTICLE IMAGE
PROBES
Principal Investigator:
Robert A. Black
Collaborating scientist(s):
- Robert Rauber (Univ. of Illinois)
- Kenneth Beard (Univ. of Illinois)
Objective: To determine the cause and extent of the distortion of raindrops around the WP-3D
2-D probe mounts, and to devise means of minimizing and/or compensating for it.
Rationale: Giant raindrops (nominal mean diameter > ~.5.mm) are relatively rare in nature, yet
their influence upon the radar reflectivity and rain rate is substantial, thus it is important to
determine the circumstances in which they are observed. Most observations of such drops have
been made by small aircraft in the tropics, most notably in Hawaii and in south Florida. However,
only large aircraft have the range to properly survey most oceanic convection, and only they can
safely study hurricanes far from land. Since 1977, we have collected hundreds of flight-hours of
rainfall data from two NOAA WP-3D aircraft (NOAA-42 and NOAA-43) in numerous hurricanes.
In all that time, we found very few raindrop images whose equivalent circle diameter (ECD) was >
4 mm. Furthermore, large (> 1 mm in diameter) raindrop images were being distorted into ellipses
when viewed with the probe arms vertical. Because of this, we had a suspicion that the larger
drops were in fact breaking up before we could sample them.
Method: Obtain 2-D particle image data from all probe mounts, with the probes in all possible
orientations in order to obtain the best estimate of the true drop shape for all drop diameters. Use
these data to model the airflow around the WP-3D probe mount and determine the best mounting
positions for various purposes. Asses the ability of the NOAA WP-3D aircraft to observe large
drops.
Accomplishment: Drop images (Fig. 1) have been obtained from 2-D probes mounted in both
2-D probe canisters on the WP-3D aircraft (since 1993, NOAA-43 has a slightly different shape
2-D mount than NOAA-42) and in both the horizontal and vertical arm positions. When the probe
arms are horizontal, the diode array is parallel to the gravity vector, thus particle fallspeed is
expected to deform the images by 10% or less. Figure 1 shows that with the probe arms vertical,
the largest drop images are distorted by the airflow into a highly elliptical shape, whereas the
horizontal arm data show roughly circular images. In contrast, while needle-shaped ice crystal
images show a preferred orientation, other ice particle images show no distortion or preferred
orientation. These data (Fig. 2) show that the raindrops are deformed by flow parallel to the wings
into an oblate spheroid only when they are very near the aircraft. The difference between using the
oblate spheroid assumption versus the standard equivalent circle diameter overestimates the drop
diameter by 20% for a 200 pixel image.
Additional data (Fig. 3) obtained in 1995 has proved that at least 1 WP-3D (NOAA-42) can
observe large drops. Work is continuing to obtain drop image data from all positions. These data
will enable us to better estimate the effects of probe resolution, orientation, and aircraft probe
mount on the precipitation size spectra.
Key references:
Black, R. A., and P. T. Willis, 1996: Giant Drops observed from large aircraft. Proceedings,
12th International Conference on Clouds and Precipitation, Zurich, Switzerland, Vol. 1, pp.
27-30.
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Last modified: 11/19/97