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555 1.E.09 TERRY DESHLER I.E+OI AUGUST 1988 4.0 3.6 . 2.3;,....:.. 2.?: ..~. ...... '.... z.z .,.,. .1..8.... 2.2 1.E+08 ~ l.E+07 t5 i= ~ ~ I.E~ ....-.... '________n_o___' ............_n_ I. E+05 - N = 4.OE+07 exp( -3.28 D) nnno,non N = I.bE+07 exp( -2.19 D) l.Eo04 0.0 DIAMETER {mml ..,. ....,. .. ..~.7.. ,. ,. ,. ..... .... ,. ....., ,.0.6.. ....,. .. .. l.E+OO ~ I l.E-Ol. l:i o ~ lE-oZ Corc.= 10.2 L" Corc.= 4.1 L" Told # partICles = 1287 Told # partcles = 288 Wnd Speed . 0.1 m 5-1 1.0 2.0 FIG. 9. As in Fig. 8 except for average winds of 0.1 m S-I. .f' modeling work which also shows the errors introduced by aspiration to be a function of size. Why Holroyd's measurements do not agree with this tendency is not known. To explore roughly the influence of habit, average ratios (aspirated:truckborne 2D-C) of IPC by size bin were calculated for each day. Although several habits were observed during each snowfall event, the habits were reasonably stable through a sampling period and varied somewhat on the five days of sampling. The snowfall during Holroyd's measurements consisted of large (2 cm) dendritic aggregates and smaller particles. Dendritic aggregates were present on one of the five days when measurements were collected here, while needle aggregates were also present on this day and two other days. The results are presented in Fig. 10 and are listed in Table 2 to show the standard deviation and number of points in each measurement. The data on four of the five days are quite similar, while on the day with dendritic and needle aggregates values of the IPC ratios are higher. The tendency of the ratio to decrease as ice particle size increases is maintained, however, in contrast to Holroyd's measurements. The data were also stratified by windspeed. Average ratios (aspirated:truckborne 2D-C) of IPC by size bin are presented in Fig. 11 and are listed in Table 3 for specified windspeed windows. There is a tendency as winds increase for the ratio to decrease and for the size at which the aspirated 2D-C switches from overcount- ing to undercounting to move towards smaller sizes. The aircraft comparisons, where the crossover point is at 0.6 mm, are consistent only with this latter tendency. 4. Discussion Particle concentrations obtained from a 2D-C are based on measurements of particle flux through the sampling area. For ideal sampling conditions, the 2D- C sample area, target, would equal a free stream sample area, and particle speed through the target would be constant and equal to the airspeed. These conditions are satisfied by an instrument in motion on an aircraft or a truck, but the aspirated 2D-C deviates from the ideal on both counts. First, there is a convergence of streamlines from the mouth of the aspirator to the tar- get because of the parabolic-shaped horn, and second, particle speeds only reach a fraction of the airspeed at the target and are a function of particle mass. Nor- ment's (1985) calculations highlight these problems. Aspiration would not affect concentration measure- ments for infinitesimal particles since they follow the airflow. Then, convergence of the sample area is exactly offset by increased particle speed to maintain the same particle flux at the target as in the free stream. Trajec- tories of nonideal particles will deviate from the air streamlines based on particle momentum. Viscous forces will dominate smaller particles which will closely ..otiJ<i..