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OcrOBER 1988 ARLIN B. SUPER AND JAMES A. HEIMBACH, JR. 1155 Rosemount icing rate detector data were obtained during 659 h of January 1985 (85 h data are missing due to equipment malfunctions). Icing events were re- corded during 9% of all hours with data (the corre- sponding percentage values for February and March 1985 were 8% and 7%, respectively). The January me- dian calculated SL W content for hours with icing on the Main Ridge was 0.035 g m -3. As discussed in Part I, observations from this type of sensor are often underestimates. Main Ridge temperatures ranged from - 3 0 to -11 oC during icing events, with a median of -8.50e. Most January icing occurred when winds were west- erly at the Crest Observatory. However, during these periods, light and either northerly or variable winds were usually observed at the seeding site. This suggests either a general dispersion of the AgI prior to transport over the Main Ridge and/ or southward drift of portions of the plume. Either would result in enhanced hori- zontal dispersion. Six successful in-cloud research flights were flown in a wide variety of SL W conditions: one on 10 Jan (in limited SLW), two on 15 Jan (in abundant SLW), another on 19 Jan (with SLW only above the AgI plume), and the final two on 28 Jan (in virtually all- ice clouds). "The afternoon mission of 15 Jan is presented in detail to illustrate evaluation techniques and results. Other missions, although analyzed in similar fashion, are dis- cussed more briefly. Section 5 summarizes the results of all six missions and presents for each mission sup- porting information, including atmospheric stability, cloud top height and temperature, and Main Ridge temperature. Further supporting information on winds is given in section 6. a. 15 Jan, p.m. At 50 kPa a low was centered over northern Hudson Bay, and the associated trough extended southward to Florida. A second deep low layover the Baja Peninsula. The resulting flow aloft over the Bridger Range was northwesterly, with weak positive vorticity advection. Snow began early in the morning over the Bridger Range and gradually decreased during the afternoon. A pibal released from the seeding site at 1223 was lost within cloud only 100 m aboveground as it drifted to- ward the Main Ridge. The obscured ceiling over the seeding site gradually improved to overcast by the end of the mission. The Main Ridge crest remained in cloud for the mission duration. The aircraft was also in cloud over both the BR T A and Main Ridge up to 3.6 km altitude, the highest flown during this mission. A complete up- wind sounding was precluded by other air trafffic, but cloud tops were observed at 4.6 km (-21 OC) over the BRTA on the morning mission. Mean hourly SLW values of 0.04-0.05 g m-3 were derived from the icing rate detector atop the Main Ridge during this mission, greater than all but one Jan- uary day. Winds at the Crest Observatory were 8-10 m S-1 from 2700, and the temperature was -90e. Ac- cording to SH, significant nucleation should have oc- curred within the AgI plume over the Main Ridge in these conditions, and the resulting ice crystals, along with any remaining AgI, should have been transported toward the BR T A. After taking off at 1241, the aircraft climbed around the south end of the Bridger Range to Livingston (Fig. 1). Winds of 2900 at 10 m s -1 were observed at the seeding generator altitude (2.2 km). The wind in- creased to 14 m S-I from 2850 at 3.0 km. Slight stability was indicated by dOe/ dz of 0.8 K km -1 through this layer, where Oe is the equivalent potential temperature. Supercooled liquid water of about 0.05 g m -3 was encountered on climbout from 2.2 km altitude and 9 km south of the seeding site until leveling out at 3.3 km over the south end of the BR T A. Amounts were then generally 0.1 to 0.2 g m -3 approaching the Liv- ingston area. The first N-S pass over the BR T A began at 1258 at 3.3 km altitude and -l30e. Almost continuous SLW in the range 0.05-0.15 g m -3 was found from 20 km north to 16 km south of the seeding site latitude (SSL) shown in Fig. 1. Limited zones of similar magnitude were found further north. The only significant ice was encountered south of the SSL in concentrations less than 4 L -I, and no AgI was detected. However, the seeding generator had been on for only 0.5 h, which was probably inadequate for the plume to have traveled the 17 km downwind. Pairs of passes were next flown at 3.0 and 2.85 km altitudes. Nine additional passes followed at 2.7 km between 1350 and 1515, as will be discussed. A sounding was made in the Livingston vicinity about 1520. Cloud base was at 2.8 km, and SLW was recorded from that level to the top of the sounding at 3.6 km. The sounding was stable, as indicated by a mean lapse rate dOe/ dz of 2.3 K km -1 through the layer. As much as 0.20-0.25 g m -3 SL W was measured between 3.3 and 3.5 km, which may have resulted in part from upslope flow against the mountains SE of Livingston. The aircraft returned to base along the SSL at 3.6 km altitude until 9 km east of the seeding site, where descent commenced. From 45 to 26 km east of the seeding site, SL Won the order of 0.05-0. 10 g m -3 was encountered, after which no SL W was found until18.5 km east, almost to the N-S sampling track over the BRTA. Marked downdrafts on the order of 2-4 m S-I were found in this dry region just to the lee of the Bangtail Ridge. A second dry region existed from 10 km east of the seeding site to the Main Ridge, probably also associated with lee subsidence. Between the dry regions, i.e., over the windward slope and top of the