Two Dimensional Optical Array Probes

2D Cloud and Precipitation Probes

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.

3. Sensor Specifications

3a. General Information

Manufacturer: Particle Measuring Systems Inc., Boulder, Co.

RAF Resident Expert: Darrel Baumgardner

(303) 497-1054

darrel@ncar.ucar.edu

Typical Mounting

Location: Pylons on fuselage or wings

Calibration Method: Monodispersed glass beads and spinning disk with etched dots

Range: 25 mm - 800 mm (2D-C)

200 mm - 6400 mm (2D-P)

Accuracy: Diameter: Function of particle size, shape and orientation

Concentration: Function of particle size

3b. Primary Output

RAF Parameter Name Plain Language Name Description

SDWC1 2D-C Shadow Or Total count of all particles passing through the laser beam of the 2D-C

SDWP1 2D-P Shadow Or Total count of all particles passing through the laser beam of the 2D-P

The raw shadow information is maintained in a compressed format and is left to the user to analyze at their own discretion. Software is available from the RAF to assist in this processing but is not a routine option.

3c. Derived Output

RAF Parameter Name Plain Language Name Description

CON2C? Concentration # of particles per unit volume from the 2D-C probe - number per liter

CON2P? Concentration # of particles per unit volume from the 2D-P probe - number per liter

where r is the particle radius, W is the diode array width (800 mm and 6400 mm for the 2D-C and 2D-P, respectively), v is the particle velocity and t is the sample period.

4. Data Interpretation

The electronic response time of the 2Ds impose some limitations on the minimum detectable size. A photodiode is registered as shadowed when its output is sensed as changing by at least 50%. The edges of particles will oftentimes be missed and particles in the lower end of the size range can pass undetected when the velocity of a particle through the beam exceeds the response of the probe. At 100 ms^-1 this imposes a lower size threshold of 30-40mm on the 2D.

The 2Ds are particle imaging instruments, not liquid water content probes. The 2Ds are able to capture a lot of information about a particle just from its shadow, however, if the water content of ice particles is desired, some fairly loose assumptions must be made with regard to the phase, habit, and density of the particles. These assumptions may lead to significant errors in derived liquid water content. A number of pattern recognition algorithms have been developed for analyzing 2D data; however, none of them work very efficiently for any but the most simple of partice shapes.

The sample volume of these instrument is relatively small with respect to the normally low concentrations typically encountered in clouds. This imposes a limitation on the minimum sampling time if a statistically significant measurement is to be made.