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Last modified
7/28/2009 2:38:57 PM
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4/18/2008 9:59:01 AM
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Weather Modification
Title
Corrections of Surface Particle Probe Measurements for the Effects of Aspiration
Date
8/4/1988
Weather Modification - Doc Type
Report
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<br />AUGUST 1988 <br /> 110.0 <br /> 100..0 <br /> 90.0 <br />~ 80.0 <br />z <br />~ <br /><l: 70.0. <br />0: <br />I- <br />Z <br />W <br />u <br />z 60.0. <br />8 <br />~ 50.0. <br />N <br />a <br />w <br />I- <br />z 4<l.o. <br />::J <br />0 <br />L <br />'" <br />U 30.0. <br />::J <br />0: <br />I- <br /> <br />TERR Y DESHLER <br /> <br />553 <br /> <br /> <br />y= 0.42 X <br />CorrelatIon Coeffc.ent = <br /> <br />20.0. <br /> <br />10.0. <br /> <br />0..0 <br /> <br />+ <br /> <br />0.0. 10.0 20.0 30.0. 40.0. 50..0 60.0 70..0 80..0 90.0. j 00.0 11 0..0 <br /> <br />ASPIRATED 2[)-c CONCENTRA TlON a:') <br /> <br />FIG. 6. Simultaneous ice particle concentration measurements at the surface by two <br />2D-C probes, one aspirated (abscissa) and the second carried on a truck in an aircraft <br />configuration (ordinate). The correlation coefficient and regression equation are shown <br />at the top of the plot. The regression equation is obtained by minimizing the perpen- <br />dicular distance from each point to the best fit line and is forced through zero. <br /> <br />Figure 6 presents IPC measured by the two probes <br />for the 82 comparison periods. The measurements have <br />a correlation coefficient of 0.90. A scatter plot of av- <br />erage particle diameter is shown in Fig. 7. The mea- <br />surements are reasonably well correlated, but the as- <br />pirated probe overestimates IPC by a factor of 2.4 and <br />underestimates average diameter by a factor of 0.68. <br />The amount by which IPC are overestimated is the <br />same as that found by Holroyd (1986) and is in agree- <br />ment with the oil-slide comparisons. <br />Comparing size distributions measured by the two <br />instruments for these 82 sample periods resulted in <br />distributions typified by the one shown in Fig. 8. As <br />shown by the distribution, the aspirated probe over- <br />counts particles < 1 mm diameter and undercounts <br />particles > 1 mm. These size distributions are reason- <br />ably consistent with the data presented earlier for com- <br />parison of an airborne and aspirated 2D-C. The dif- <br />ference is that the magnitude of distortions in the shape <br />of the size distribution is less, and the particle size sep- <br />arating over- and undercounting has increased from <br />0.6 to 1.0 mm. However, these data are not consistent <br />with the size distribution presented by Holroyd (1986). <br />Holroyd's comparisons were conducted somewhat dif- <br />ferently since only one 2D-C probe was available. His <br /> <br />technique consisted of collecting consecutive data <br />samples in near uniform snowfall as the probe was <br />driven and then aspirated. Holroyd's measurements <br />indicated that the shape of the size distribution was <br />the same for both types of operation, truckborne and <br />aspirated; with the only difference a fairly uniform in- <br />crease in crystal concentration by a factor of 2.4. <br />Holroyd's sampling was conducted in near calm <br />conditions and he argues that the differences in size <br />distribution measured during the aircraft experiments <br />(Deshler et al. 1986) may be because comparisons with <br />the aircraft data were conducted in a crosswind. To <br />check this suggestion, average winds were calculated <br />for each of the 82 comparison periods covered in Fig. <br />6, and size distributions were examined to find spectral <br />comparisons similar to the one presented by Holroyd. <br />Five such spectra were found, and they all occurred at <br />winds ~ 1.1 m S-I. However, 24 other samples were <br />found collected at winds < 1 m S-I and all of these <br />conformed with the tendency suggested by Fig. 8. An <br />example is shown in Fig. 9, collected at 0.1 m s-t. Thus, <br />these data indicate that ice particle counting errors <br />caused by aspiration are a function of ice particle size, <br />and will occur even in calm conditions. These mea- <br />surements are in agreement with Norment's (1985) <br /> <br />....G <br />
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