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60 JOURNAL OF WEATHER MODIFICA nON Volume 38 Natural ice activation is simulated using a gener- alization of the Meyers et a1. (1992) formula: Ni= NIFNexp[12.96(SI- 1)]; T<-5C,' r >r. v S1 supersaturation with respect to ice, and T<-2C', r >r v 1W supersaturation with respect to water. The variable Nu''N is a forecast variable in RAMS which can vary both vertically and horizontally whenever such data are available. Nonnally we use measurements in field campaigns with the CSU con- tinuous flow diffusion chamber to infer NIFN. In the 'I . I I I I :! _ AJt101IUlW .,.,..nIt.u. ,,,',n.,.. - .. 2lIIfII. ..... ...........t .. __ nw ,.at'" == .. c-.. '.71 :.... .. .Ut ..,... .. ..,., ..... u...tIIt ..n == ...........,.,. .= ............. 0 _ PIit II l. 20IItIlIr ..... .141 IIIW'" ...... ,.. 1-11014f u 1-7JlIIft '"t~f3 ,",n-n IN n.QlIf g _1U~ UIWT 0 absence of those measurements NU'N is based on the estimates reported in Meyers et al. (1992) and al- lowed to drop off in concentration with height consis- tent with observed lapse in large aerosol concentra- tions. Recent measurements at the Storm Peak Labo- ratory. in the Park Range of Colorado by DeMott (personal communication) suggest that IFN concen- trations are probably lower than Meyers original es- timates. However, sensitivity experiments using these lower background IFN values did not change the re- sults appreciably. Secondary ice particle production by the rime- splinter mechanism following Mossop (1976) is also simulated. A seeding algorithm was added into the model based on sources of IFN from ground-based seeding generators. Figure 2 shows the activation data that we used to simulate IFN production at each generator site. AgI was then added as another prognostic IFN field. . " . t o IH10 t_ __ ... 1IttIo. a ... .. 1.. ., .. ..1. -- E - - - 15 et.oW ,.,..",.. jI('J - Reviewed - . . !! - - Figure 2. Calibrated AgI activity for the generators used by WWC. The dashed line labeled "1973 max," including the acetone- induced activation enhancement at warmer temperatures, is the fit used in the model. Provided by Larry Hjermstad. (WWC used a 4% AgI solution with sedium iodide as a carrier in acetone along with 1 % moth balls to improve nuclei activation between -2.50C and -8.00C.)