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 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 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 grav ity 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 images are distorted by the airflow into a highly elliptical shape, whereas the horizontal arm data show rou ghly circular images. These data (Fig. 2) show that the true shape of the raindrops is an oblate spheroid. The difference between using the oblate spheroid assumption versus the standard equivalent circle diameter overestim ates the drop diameter estimate 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.

Last modified: 10/9/96