1. Introduction The Two dimensional optical array probes (2D-OAP), models 2D-C and 2D-P, are instruments developed by Particle Measuring Systems (PMS Inc., Boulder, Co) for the measurement of cloud and precipitation drop size distributions. These sensors are used primarily for the study of cloud microphysical processes, particularly the growth of cloud drops and ice crystals through aggregation, riming and coalescence into drizzle, rain drops, graupel or other forms of precipitation. 2. Operating Principles The 2Ds record the two dimensional shadows of hydrometeors as they pass through a focussed He-Ne laser beam (Fig. 1). The shadow is cast onto a linear diode array and the on/off state of these diodes is stored during the particle's passage through the laser beam. This informatio, along with the time that has passed since the previous particle, is sent to the data system and recorded for post-flight analysis. Information about a particle's shape and size is deduced from analysis of the recorded shadow with a variety of pattern recognition algorithms. Figure 2 illustrates some measurements by the 2D probe in several different types of clouds, ranging from rain drops to pristine ice crystals to more complex heavily rimed ice particles. Figure 3 is a photograph of the 2D-C in the canister that is normally mounted on an aircraft pylon. A complement of 2Ds is normally flown during a project to cover the size range of interest. The 2D cloud probe (2D-C) measures in the range from 25 mm to 800 mm and the 2D precipitation probe (2D-P) measures in the large size range from 200 mm to 6400 mm. Additional information available at "http://raf.atd.ucar.edu/Bulletins/B24/2dProbes.html" [Summary provided by UCAR]
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