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TERRY DESHLER 551 Y = 13.0 c.. 10. X = 14.2 c.. 11. Y = 0.93 X CorTelatiOll CoeffiCient = 0.92 Number of POints = 219 + + :t + .. 0.0 0,0 20.0 30.0 ASP1RA TED 2D-C CONCENTRATION (I.:') 10.0 + + + + 40.0 60.0 70.0 50.0 ao.o FIG. 3. Simultaneous ice particle concentration measurements by two aspirated 2D- C probes. Humphries' (1985) system is shown on the abscissa and Holroyd's (1986) system is shown on the ordinate. The correlation coefficient and regression equation are shown at the top of the plot, along with averages of the X (abscissa) and Y (ordinate) data. The regression equation is obtained by minimizing the perpendicular distance from each point to the best fit line and is forced through zero. wire anemometer. The clock rate for both probes was set for an airspeed of 8 m S-I. Simultaneous measure- ments were collected on three separate days for 16, 46, and 168 min. The probes were located approximately 10 m apart. Snowfall ranged from <1 to 4 mm h-1, winds from 0 to 2 m S-l, and the temperature from -2 to 20C. The snow crystal types sampled covered a wide range, including needles, graupel, dendrites, plates, sheaths, and broad-branched plates. Crystals were usually light to moderately rimed and aggregation occurred at times. Data from the two probes were averaged over 1 min intervals providing 219 points for comparison. A scatter plot of IPC measured by each instrument is shown in Fig. 3. The measurements have a correlation coefficient of 0.92. A scatter plot of average particle diameter is shown in Fig. 4. Average particle diameter is defined as the arithmetic mean diameter of all particles sampled within an averaging interval, typically 1 min. For each particle, the longest dimension is used for the diameter. Size distributions measured during these comparisons were almost identical. An example is shown in Fig. 5. These comparisons indicate that both probes produce equivalent measurements when aspirated. On the five days used for intercomparison, the truck- mounted probe was driven back and forth along a 1.3 km straight stretch of road approximately 30 m to the north and centered on the location of the aspirated probe. Weather conditions were similar to those ob- served when both probes were aspirated, except that winds covered a broader range and snow crystal habits were more limited. On three of the days snowfall was primarily composed of needles, sheaths, aggregates of these, and some small graupel. On one of the days small graupel was the primary habit with some needles, while on the other day the habits were needles, dendrites, aggregates of these, and small graupel. For comparison, time periods were selected from the truck-mounted system when the probe was being driven at a reasonably constant speed of 11 to 13 m S-I. Each pass of the truck past the fixed probe provided I to 1.5 min of data for comparison. These time periods were then used to select equivalent sampling periods from the fixed probe. Altogether, 82 comparison pe- riods were obtained from 5.5 h of operation during five days of sampling. Sample volumes from the two probes were nearly identical since the truck speed was close to the airspeed in the aspirator. -,..