A General Reference to Our Work:
W. P. Arnott, Y. Y. Dong, J. Hallett, and M. R. Poellot, 1994: Role of small ice crystals in radiative properties of cirrus: A case study, FIRE II, 22 Nov 1991. J. Geophys. Res. 99, 1371-1381.
REPLICATOR DATA:
Replicator data consists of 2 dimensional images of crystals embedded in a liquid plastic called Formvar. Ice crystal images obtained in this fashion are called replica. The substrate is standard 16 mm movie projector film. For data analysis, we view replica with a microscope using a video camera. A frame grabber card on a Macintosh Quadra 650 computer is used to digitize the output of the video camera for subsquent digital image analysis. Stepper motors are used for precise positioning of the film. We have an aircraft data file that provides for the conversion of film position to location in the cirrus cloud. The digital output from the frame grabber is used to pulse the stepper motors under computer control. The maximum cloud sample rate for the replicator is approximately 2 Li/sec, though the effective sample rate is determined by the choice of microscope objective and varies from 0.01 Li/sec to 0.1 Li/sec.
After the software moves the film to a region of interest, the person doing replicator analysis (operator) is presented with an image of the formvar coated film on the computer screen. Images typically contain both ice crystals and artifacts, so the operator selects ice crystals by pointing the mouse on them and clicking the mouse button. Then the operator points the mouse at 1 of 5 buttons that specify crystal habit. [We coursely separate crystal habits into 5 categories - Plates, Columns, Plate Polycrystals, Column Polycrystals, and Miscelleneous.] The software then grabs the particle, determines its maximum dimension, projected area on the film, and perimeter. All the crystals from each screen are binned and placed in an output file. Ice crystal mass is estimated from use of the mass dimensional relationship given in the above reference.
Ice crystal replica in the size range from 2-1000 microns can be obtained, though particles larger than about 200 microns can fracture upon impact. In the case of fracture, the operator selects a crystals portion that best resembles the maximum dimension the crystal would have in air (clearly a judgement call is necessary here). Artifacts on the film are only a problem below 30 microns. Below 15 microns, the number of artifacts is usually a factor of 1 to 10 greater than typical numbers of ice crystals. This is the signal to noise issue that the operator must be aware of to prevent the counting of artifact as ice crystals.
PMS 2DC DATA:
The Particle Measuring Systems 2DC probe is an electroptical instrument that images crystals by time slicing the output of a 1-D diode array. The method of analysis of 2DC data is described in the article cited above. Briefly, the entire-in technique is used where the only crystals counted are those that do not extend to the boundary of the diode array. The maximum dimension of the crystal is determined from the largest of the the crystal dimension along or perpendicular to the diode array. The sample rate of the 2DC probe varies from about 0.1 Li/sec for the smallest size to 7 Li/sec for the middle sizes. The size range of the 2DC probe used on the Citation aircraft was 66-1000 microns. The crystal projected area is determined directly from the number of diodes occluded. Crystal mass is estimated from use of the mass dimension relation cited in the above reference.
WHY AT LEAST TWO PARTICLE PROBES:
The replicator provides detailed habit information about crystals.
RETURN to replicator data selection