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5. DISCUSSION OF MOUNTAINTOP ICING DATA The ice detector is a simple I-in. rod, vibrating at a known frequency. \Vhen supercooled liquid water freezes to the rod, the frequency changes. 'Vhen about 0.08 g of ice accumulates, a heater is activated and melts the ice in about 90 s. This apparatus is refelTed to as a "trip." Knowing the average wind speed and the number oftrips in a 1-h period, the liquid water content can be approximated. LOREP data shO\'\- that Red Hill has many more hours of icing than Mills Peak (fig 2.1). This icing occurs because Red Hill is a more prominent peak than is :\:Iills Peak, inducing rapid vertical ascent as moist air passes over the peak. :\It. Hough has similar orography to Red Hill. It is almost 300 m higher, and thus below 0 "C more often. This elevation produces more icing than at Red Hill. Tables 5.1 and 5.2 summarize icing episodes for :\It. Hough and Red Hill, respectively, for the 1993-94 winter season. The L \VC (liquid water concentration) from the icing rate meter was derived using the technique of Hindman (1986) from averages taken O\"er individual storms. The Red Hill wind speed detector failed on December 9 and could not be repaired until January 20. The Red Hill site battery failed on ::Ylarch 5. The Mt. Hough data set was complete for the entire season. The .Mills Peak site had no heat to the wind sensors until January 28. This site experienced very little icing, which is typical. Therefore, the data from this will not be discussed. Figure 5.1a shows yearly averages for four different parameters derived using the Red Hill data set. Values are plotted using the water year, i.e., 1990-91 water year is plotted as 1990. The value plotted for 1994 is the mean for the 7-yr data set for each parameter. .Although 1993 had almost 2 mo of missing data, :\Iarch had very little icing as indicated by :\It. Hough. April had 70 h of icing at Mt. Hough, and thus, based on past comparisons between these two sites, as many as 40 h of icing may have been missed at Red Hill. These lost data would bring the number of hours up to 242, or about the same as the drought year of 1991. These results contain several key points. First, the number of hours of icing is consistent at about 250 to 300 per season. The mean of 286 for Red Hill is close to the 300 h that was used in the design of the LOREP. However, liquid water concentrations and mean temperature at which icing occurs indicate that not all of these hours provide suitable conditions for seeding. Kote during the water years 1991 and 1993, critically dry years, the liquid water contents are small at 0.05 g m-3. Also, the average temperature at which icing occurred during 1991 and 1993 was warmer than -2 oc. In fact, for the 7-yr record, these 2 }T are the warmest and have the least liquid water ofthe 7. Unfortunately, these 2 yr make up two-thirds of the LOREP seeding experiment. \vnen seeding is conducted at these relatively warm supercooled temperatures and with relatively low liquid water amounts, seeded crystals will take much longer to grow to sufficient mass to be considered meaningful precipitation particles. . The upper histogram on figure 5.1a shows the ratio of icing hours to hours of precipitation measured at Plumas Eureka State Park. In every year, more hours of precipitation occurred than hours of icing. On average, this ratio is about 0.70, or 70 pct of the hours of precipitation have icing. This ratio indicates that Red Hill, at 1936 m t6400 ft), can be at or above freezing during 30 pct of the precipitation events. 26