The Strobolume

by Dr. Roland List*

One of the major problems of modern cloud physics is the description of the growth of precipitation particles (raindrops, graupels and hailstones). Theoretical computations show that the gas-phase contributes negligibly to the growth of the mass, as long as the growing particles show diameters of more than 2 mm. The determining factor for the growth of precipitation particles is, therefore, the accretion, the capture of cloud particles, either water droplets or ice particles with diameters from 10-100 M.

If we observe, for instance, the growth of ice particles falling in a cloud of under-cooled water droplets (temperature less than 0°C), we can determine the collection efficiency based on the growth of the mass of a test particle in form of icing. This can be done by careful determination of weight. The result, however, does not give us dependable information about the number of impinging particles because we do not know exactly how many of the particles are hurled back into the air stream. To determine this, stroboscopic photographs were taken with the General Radio Company Strob-olume during icing tests of the climate-controlled wind tunnel on the Weiss-

* Federal Institute for Snow and Avalanche Research, Weissfluhjoch-Davos (now with the Department of Physics, University of Toronto).

Figure 1. Icing of a 2-cm steel ball in an air stream (temperature —5°C).

fluhjoch, Switzerland. The Strobolume switch was in the high intensity position. Figure 1 shows the icing of a 2-cm steel ball in an air stream of a velocity of 20 m/s, at a temperature of — 5°C and an absolute humidity of approximately 4 g/m3. The average size of the water droplets is 50 ju. The photo was taken with an Alpa camera with normal lens and adaptor rings at//ll. Agfa, Isopan Record Film, with a sensitivity of 34—40°DIN (2000-8000 ASA) was used. The most important result obtained from this photographic observation is that, under existing icing conditions, no rebounding of impinging water droplets could be observed. Therefore, the number of impinging particles equals the number of captured particles (collision efficiency equals collection efficiency).

The following additional conclusions can be drawn from Figure 1: a) The direction of movement of droplets can be determined from their appearance in the photograph as streaks of light. Since the electronic flash of the Strobolume shows, directly following trigger, a defined intensity peak followed by a steady decay, the movement vector of a "particle streak" can be seen from

Figure 2. Icing of a 2-cm steel ball in an air stream (temperature — 25°C); diameter of the arriving ice particles or subcooled water droplets 50-100 /¿. Observe the "boundary layer" of the rebounding particles. For this photograph, a slot illumination (slot opening 2 mm) was used, necessitating a camera opening to {/1.6. The visible diffraction pattern (rains) results from reproduction of cloud droplets passing the steel ball at a distance of more than 2 cm in the direction of the camera.

Figure 2. Icing of a 2-cm steel ball in an air stream (temperature — 25°C); diameter of the arriving ice particles or subcooled water droplets 50-100 /¿. Observe the "boundary layer" of the rebounding particles. For this photograph, a slot illumination (slot opening 2 mm) was used, necessitating a camera opening to {/1.6. The visible diffraction pattern (rains) results from reproduction of cloud droplets passing the steel ball at a distance of more than 2 cm in the direction of the camera.

the brightness pattern. For a slot illumination, perpendicular to the camera axis and passing the center of the icing particle, the flow lines of the floating particles can be determined.

b) If the particles arriving at an obstacle are partly iced, the rebounding particles can be observed as such from their "streak direction" (Figure 2).

c) From the direction of the streak image, the velocity of cloud particles can be determined as a function of their location as long as the particles do not differ largely in size.

From these observations, it is apparent that the Strobolume is a useful device for research of the complex capture processes which, in a diversity of manner, play an important role in the formation of precipitation.

The observations described here were made within the framework of the research project No. 2071 of the Swiss National Fonds.

Commonwealth Armory, Boston November and 6, 1963

A cordial welcome awaits our New England friends at the GR booth. Drop around and see the new instruments you have been reading about in the Experimenter. We'll be glad to demonstrate them to you.

General Radio Company as. A-

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