WMOD00485 - Laserfiche WebLink

Home Browse Search

OcrOBER 1988 ARLIN B. SUPER AND JAMES A. HEIMBACH, JR. 1159 the entire seeded zone was 0.06 mm h -I, while that of the controls was 0.03 mm h -1, suggesting that seeding increased the snowfall by about a factor of two. ~ I c. 10 Jan Ridging along the Pacific Coast and a deep trough over the central states placed the project area in strong NW flow. The sky was obscured most of the day with light snowfall at the seeding site. A pibal released at 0934 indicated WNW flow before it disappeared into the clouds about 300 m above the seeding site at 2.5 km altitude. The seeding generator was turned on at 1040 and operated continuously until 1447. At 1212 the aircraft scientist observed cloud all along the Bridger Main Ridge and lenticular clouds east of the Main Ridge below 5 km. The icing rate detector at the Crest Ob- servatory indicated the presence of SL W during most of the flight. The first sensor cycle of the day was at 1205. Thereafter, hourly mean SLW contents of 0.03- 0.04 g m -3 were observed. At 1444 the aircraft scientist visually observed a liquid water cap cloud on the Main Ridge below the aircraft, which was flying at 3.4 km on its return to base. A zone ofSLW (Maximum 0.05 g m -3) of about 1 km east-west extent was detected at the same approximate time and altitude 2 km east of the Main Ridge. These observations confirm the ic- ing rate detector indications that some SL W was pres- ent over the Main Ridge during this flight. Single N-S passes were made at 3.9, 3.6 and 3.0 km and two were made at 3.3 km. No SLW was detected over the BRTA on any of these passes. Essentially, no AgI was detected by the acoustical counter during the passes from 3.9 km down to and including 3.0 km. The maximum IPC was less than 5 L -1 throughout the 3.9-3.0 km layer. The first pass at 2.85 km detected a weak AgI plume a few kilometers south of the SSL. This position was appropriate for the 2950 wind direction at that altitude and location; however, no SLW was detected and al- most no ice particles were observed. A series of four passes was then made at 2.7 km. Each detected limited zones ofSL W with peak amounts near 0.1 g m -3. The mean value of Dr was 14 ~m for the wettest nonseeded 1 km of each pass with corre- sponding mean droplet concentration of 250 cm -3 . Markedly enhanced IPC was found downwind of the seeding site on each pass. Peak IPC values ranged from about 20 to 40 L -I , and the enhanced IPC zone widths were approximately 5-8 km. Two final passes were made at 2.85 km, which had total counts of 48 and 23 from the acoustical counter, about 10%-20% of the values detected 150 m lower. With the exception of one very narrow zone of ice particles peaking at 20 L -1 , the IPC was no more than a few per liter. Liquid water contents were also very low. Therefore, the effects of the AgI seeding were gen- ~~ ,. \~ ,. erally limited in altitude to the lowest flight level pos- sible. The IPC in both control zones was 0.5 L -I with an estimated mean precipitation rate of only 0.01 mm h -1, In contrast, the IPC in the subdivided seeded zone ranged from 3 to 7 L -1 with precipitation rates from 0.02 to 0.05 mm h -1. Zone C-S in the center of the seeded zone had the highest IPC and precipitation rate. Most of the enhanced IPC was due to hexagonal and spherical (compact) crystals less than 0.6 mm across. However, the increased snowfall rate in the seeded zone was primarily due to particles larger than 1.0 mm. d. 19 Jan Northwesterly flow persisted aloft, as a slow-moving arctic cold front approached from northeast of the pro- ject area. Though aircraft-observed cloud tops were -190C at 5.0 km altitude just west of the Bridger Range, satellite imagery showed tops less than -450C only 45 km east of it. Liquid water of 0.05-0.20 g m-3 was found on aircraft climbout from 1.7 to 4.2 km altitude, with additional ice cloud above. The highest SLW contents of the month (0.05-0.06 g m-3) were recorded at the Crest Observatory prior to 1300 with west winds. Thereafter no SL W was detected as the wind direction became variable and the wind speed decreased. Dry arctic air behind the front was overridden by moist northwesterly flow aloft, while the 500 m layer immediately above the BRTA was very stable. North-south passes from 3.9 to 2.7 km altitude were made over the BRTA beginning at 1254. Some SLW was found along the entire 3.9 km pass, but only well north of the BRTA on subsequent 3.6, 3.3 and 3.0 km passes. No SLW was detected on any of six 2.7 km passes. The AgI plume was detected only at the 2.7 km level (no passes were made at 2.85 km). Total counts per pass were high, ranging from 240 to 1860, sug- gesting very limited vertical extent. Meandering of the plume within the stable layer resulted in more variable plume width estimates than on the other days, ranging from 0.3 to 12.5 km. Reduced AgI plume spreading and increased meandering are both characteristic of strong atmospheric stability (Holroyd et al. 1988). Rather uniform IPCs were recorded at 2.7 km, usu- ally from 2-10 L -(, but occasionally higher. No dif- ferences were noted between high AgI zones and nat- ural cloud sampled crosswind. The lack of an IPC en- hancement was probably due to the dearth of SLW within the stable arctic airmass. Considering the strong stability, it is remarkable that the AgI was even trans- ported over the Main Ridge and the BRTA. e. 28 Jan. a.m. A trough embedded in zonal flow aloft was entering Montana. Satellite imagery showed the storm extending