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550 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 5 However, as a function of size the aspirated instrument overcounts particles less than 0.5 mm and undercounts particles greater than 0.7 mm. b. Comparison of oi/-coated slides and aspirated 2D-C Measurements using oil-coated slides were made during four storms: January-March 1986 and Decem- ber 1986, near Donner Summit at 1.8 km in the central Sierra Nevada. Altogether, 95 slides were successfully exposed. Typical conditions for the exposures were temperatures of -10 to -2 oC, winds light, and snowfall 1-4 mm h -I. Snowfall during three of the four storms consisted of a wide range of habits: needles, sheaths, small graupel, dendrites, branched plates, and aggre- gates of needles, dendrites, and branched plates. During one storm the snowfall was limited to small graupel, needles, sheaths, and aggregates of needles and sheaths. A scattergram of IPC measured simultaneously by the oil-coated slides and the 2D-C is shown in Fig. 2. Also shown are results from the four aircraft passes. The equations at the top of the plot, describing the relationship between the X (abscissa) and Y(ordinate) 120.0 data, were obtained using the method of minimizing the least-square error of the perpendicular distance from each point to the best fit line. The regression is forced through zero. Measurements by the oil-coated slides are in general agreement with the aircraft data, indicating that the aspirated 2D-C overestimates IPC by a factor of 2 to 3. c. Comparison of truck-mounted and aspirated 2D-C Several experiments similar to the one performed by Holroyd (1986) were conducted also at 1.8 km in the central Sierra Nevada. In November and December 1986, 5.5 h of snowfall were simultaneously sampled with one probe aspirated and the second carried on a truck in an aircraft configuration. In conjunction with these experiments, 3.5 h of measurements were also collected with both probes aspirated. Comparison of the aspirated measurements is pre- sented first to establish a basis for comparison when one of the probes is carried on a truck. The two aspi- rator horns were identical, and the airspeed in the sam- pling region between the tips of the 2D-C probe was . measured at 13.5 m S-I in both aspirators, using a hot + OlL SLIDES y = 0.41 X CorrelatiOn Coeff;c;ent = 0.60 Number of POints = @ AJRBCRNE 2D-C Y = 0.31 X 100.0 Correlation CoeffiCieni = 0.78 Number of POinis = ,,~ ~ eo.O ~ w ~ a: UJJ) 4:. + gs ~ Vi 4OJ) --' a 2OJ) 0.0 0.0 20.0 40.0 ASP1RA TED 2D-C CONCENTRA TlON a.:') + + + + ~ 60.0 100.0 120.0 80.0 FIG. 2. Simultaneous ice particle concentration measurements by oil-coated slides and an aspirated 2D-C. Concentration measurements from the comparison of airborne and aspirated 2D-C are also shown. Correlation coefficients and regression equations. are shown at the top of the plot. The regression equations are obtained by minimizing the perpendicular dista~ce from each point to the best fit line and are forced through zero.