WMOD00540 - Laserfiche WebLink

Home Browse Search

518 8 1640-1850 GMT 20 FE8 79 6 ! ~4 .. :r 2 120 80 40 o 40 X (kml 80 120 160 200 FIG. 5. Horizontal wind components (m. s-')parallel to the bar- rier on 20 February 1979 under stable orographic flow. Dashed line shows flight path of cloud-physics aircraft. Wind flow is into the plane of the paper (from Parish, 1982). and distributed over a large area, providing the opportunity to target the seeding effects to the upper elevations of the ARB as additional snowfall for spring runoff. This finding is in agreement with those of other programs that have been conducted over mountain barriers (Ludlam, 1955; Mielke et aI., 1970; Hobbs, 1975; Rauber and Grant, 1986). Seeding opportunities in orographic clouds can be forecast through the use of animated satellite infrared imagery sensitive to cloud-top temperature changes (Reynolds et aI., 1978), which allows the operational lead time necessary to ready the aircraft and other equipment for a seeding mission. On the other hand, SL W in convective clouds tends to be confined to newly formed convective cells, where its lifetime tends to be short, say 10 to 15 minutes. b. Air motions in Sierra Nevada storms Winds over mountains are complicated. However, they must be documented and understood if an effective and reliable seeding strategy is to be developed. Observations at the ground reveal little about winds aloft. Wind observations on SCPP have been derived mostly from the navigation systems of the research aircraft and from ra- winsonde stations distributed across the barrier. During four winters additional detailed wind data were obtained from Doppler radar sets operated by the National Center for At- Vol. 67, No.5, May 1986 ~ TABLE 3. Number of days between I January and 31 March meeting suspension criteria. Year No. of days 1979/80. 23 1980/81 stand-down 1981/82 15 1982/83 49b 1983/84 31' 1984/85 2 . Used only river and reservoir criteria (no snowpack). b From 26 February through the end of the observational period project in suspension because of excess snowpack water equivalent. , All of January under suspension due to high water level of Lake Tahoe. mospheric Research and by the National Oceanic and At- mospheric Administration. The most interesting finding so far is that a barrier jet oc- curs over the foothills during many winter storms. Although low-level jets have been observed immediately in advance of cold fronts (Hobbs, 1978), numerical simulations of the SCPP situation have indicated a strong topographic in- fluence on low-level winds well in advance of the approach- ing cold front (Parish, 1982). The barrier jet blows from the southeast toward the northwest parallel to the mountain crest (Fig. 5). It can be thought of as a flat ribbon about 1 km deep with its center about 1 km above the terrain, and extend- ing from the valley floor well up toward the crest (-100 km width). Speeds up to 25 m' S-l are common; in some cases the barrier jet has been observed to exceed 40 m . s -I. The barrier jet occurs when a large-scale low-level flow of stable air impinges on a two-dimensional mountain barrier of significant height. It can occur in both the deep and shal- low orographic clouds and is a significant factor in attempts to target seeding effects. 6. Suspension criteria During the first years of SCPP, detailed studies were con- ducted to develop a set of criteria to be used for suspending seeding operations during potentially hazardous situations. Even though actual precipitation increases might be quite small, the Bureau of Reclamation wished to avoid even the appearance of contributing to any weather-related losses. TABLE 4. Blue Canyon precipitation total for 12-month period ending 30 June and ranking for the'85-year record. The annual average is 1717 mm (67.58 in.). Precipitation Year mm in Rank Comment 1977 685 26.98 85th Driest year on record. 1978 2232 87.87 14th 1979 1527 60.10 45th 1980 2179 85.77 16th 1981 1074 42.30 70th No field operations. 1982 2936 115.60 1st Wellest year on record. 1983 2747 108.15 2nd Record snowpack water content. 1984 2075 81.71 19th Fourth wellest July-Dec on record, driest Jan on record.