WMOD00510 - Laserfiche WebLink

Home Browse Search

i . I e I' ~"l, , ,", ": '" ~ !.,V__,' ].(\NUARY 1978 55 HOLROYD, SUPER AND SILVERMAN 108 1012 . The measurement error was estimated in two ways. T~e first was to sum the rdative errors: dE dL dW dH dC dM -=-+~+-+-+-. (6) E L W H C M This usually results in dE> E for these experiments. While an upper limit of E+dEis reasonable, a lower limit of E- dE yields an absurd negative number. Noting that the upper limit is E(1+R) for R=dEIE, the relative error, a lower limit of E( 1 + R)-1 was chosen. The values of total relative error R are also given in Table 4. Typical values for individual relative errors, like dLI L, are also summarized separately for the American and Australian experiments. The second error estimate was to combine all extreme values in such a way as to achieve extreme effectiveness values: (L::l=dL) (W ::l=dW) (H ::l=dH) (C::l=dC) E~~~- . . (7) (MTdM) This latter method produces larger error bars than the former. It should be noted that these errors reflect only variances in the measurements used. Assumptions, such as those regarding the crystal distribution and mass of dry ice actually entering and subliming in the cloud, could make the actual errors larger than those quoted. These initial effectiveness calculations then need to be adjusted. As mentioned above, only part of the dry ice sublimes in the cloud ["-'30% after Meeand Eac\ie (1963); "'12% after Fukuta ei at. (1971) assuming no vertical air motions]. A final estimate of effectiveness, using the former sublimation corrections and the crystal counter normalization values (when known), is given in the last column of Table 4. Correct- ing for the Fukuta et at. (1971) sublimation rate instead will raise these values by a factor of 2.4. Corrections for updrafts and downdrafts are not attempted; vertical winds encountered in the seeded clouds were only a small fraction of pellet terminal velocity and were of variable direction. Corrections for the absolute counting efficiency are not applied because they are not known for any of the instruments. For e,xample, the highest counting instrument, the CSI-IPC, is known to count less than all the crystals; but the width of the sample volume, as determined by the laser beam, may be imprecisely known because of sidelobes. The values in the last column of Table 4 are arranged in Fig. 3 from highest to lowest. The American and Australian experiments are given different symbols because the Australian results have not been normalized to the American values. An average factor of 6 separates them. The error bars in Fig. 3 are determined from 1010 lO" II II . II . II . . I . .lULY 21 21 21 JULY 7. I 25 JUNE 7. I 23 JUNE TI I 7 DEC. 71 . DEC. 72 I III MARCH n I 7 DEC. 72 I'DEC.T2 21 . II II 108 1012 1010 10. DRY ICE EFFECTIVENESS, errotalo Q" FIG. 3. The dry ice effectiveness for each experiment, arranged in decreasing order. The numbers at the left are cloud identifica- tions. The values plotted are those from Table 4 and use the Mee and Eadie sublimation rate for dry ice within the supercooled cloud. Crystal counter normalization is applied for the oval symbols only. The error bars are from Eq. (7); errors resulting from assumptions may exceed these. Eq. (7); errors resulting from assumptions may exceed these. 4. General discussion Tables 4 and 5 and Fig. 3 describe all of the experi- ments in which turrets were penetrated for the purpose of counting crystals after dry ice seeding, whatever the success of sampling. Eight of the experiments, those whose jinal values are included in parentheses in Table 4, can be excluded from final consideration. They are useful only in being somewhat consistent with the better values. Five of them are for clouds that were only skimmed at the side or top edges by the sampling aircraft. While cloud 4 on 3 July 1976 was reasonably sampled, it was seeded by dumping the last of the dry ice supply out of the plane, possibly all at once. The pellets may have missed the tall, narrow, negatively buoyant cloud. The two clouds in Australia on 3 February 1973 can be excluded because a smaller adjacent cloud turret was found to have high natural crystal concentrations in excess of those found afterward in the seeded cloud. This suggests that ice multiplication was active that day. Natural ice crystals were not encountered in signif- icant concentrations in the Montana clouds on the pre-seeding passes, except as noted. A rate of seeding was chosen to try to ensure that the concentrations of crystal resulting from seeding greatly exceeded those that might be present naturally. On 23 June 1976 the highest average ice particle concentration measured for any cloud not previously seeded with dry ice was 0.4 i-I, All other area clouds, even those seeded at cloud base with AgI-NH41 showed 0.2 i-1 or less. i";u;;;;';" ~,i-~;