WMOD00310 (2) - Laserfiche WebLink

Home Browse Search

SEPTEMBER 1988 MARK F. HEGGLI AND ROBERT M. RAUBER 1013 TABLE 2. Summary of radiometer measurements of supercooled liquid water for all storm patterns that occurred from the 1983/84 through the 1986/87 field seasons. Typical supercooled liquid water amount (mm)/duration (h) Storm duration Storm category TN* /CD* Prefrontal Postfrontal (h) Zonal Developing 14/3 0.05-0.60/11-36 0,10-1.10/24-45 45-69 Occluding 12/6 0.05-0,20/6-13 0,05-0.60/11-32 24-48 Dissipating 12/5 0.05-0,60/3-13 0.10-0.40/6-19 12-24 Meridional Cutoff 7/5 0.05-0.60/3-30 0.05-0.20/8-17 21-48 Northerly 11/9 0,05-0.50/6-8 0.05-0.20/2-15 15-57 Neither (unclassifiable) 7/3 * Total number of storms in category, * Storms with complete radiometer data. In the cutoff storms, supercooled water measure- ments were greatest within the prefrontal region. Steady values of supercooled water ranged from 0.05 to 0.60 mm in the prefrontal orographic clouds. Embedded convection produced peaks of supercooled water in ex- cess of 0.70 mm. The duration of supercooled water varied from 3 to 30 h. The duration was sensitive to the trajectory of the cutoff. Storm centers that tracked directly over the project area had the longest durations. Storm centers that passed to the south had supercooled water present for shorter periods. Supercooled water from 0.05 to 0.20 mm usually existed in embedded convection from the passage of the front until the pas- sage of the cold core trough. The presence of supercooled water within rapidly digging systems in northerly flow depended on the tra- jectory of the storm. The project area was within a cold trough during storms with an offshore trajectory. The clouds over the project area were predominantly con- vective. Short-lived peaks of supercooled water some- times reached 0.50 mm. Sustained supercooled water was rarely measured. Storms with a trajectory that passed directly over the project area produced the larg- est sustained amounts of supercooled water ahead of the front. These values were typically 0.20-0.40 mm in an essentially nonprecipitating environment. The prefrontal segments of these storms were short, usually less than 6 h. Supercooled water diminished to 0.05- 0.10 mm within the frontal band. Postfrontal super- cooled water remained at these low amounts until the passage of the trough. Storms with an inland trajectory provided continuous supercooled water from 0.10 to 0.40 mm as long as clouds were present. b. Vertical distribution of water saturated cloud The vertical distribution study comprised the bal- ance of the analysis. When supercooled water was measured by the radiometer, saturation often existed within 1 km of the surrounding peaks. A saturated layer was encountered greater than 90% of the time in this lowest 1 km when supercooled water was present. The depth of the supercooled-water-bearing layers was greater than 0.5 km over 50% of the time, while some layers were as deep as 2.0 km. Saturated layers were found to exist most often between -80 and -lOoC, although they were often present over a much wider tempe:rature range. Saturation was occasionally indi- cated at temperatures as cold as -240C. 7. Comparison with other studies Previous SCPP studies on supercooled water distri- butions in winter storms occurring from 1978/79 to 1979/80 (Heggli et a1. 1983) showed that the largest supercooled water contents were associated with post- frontal convective radar patterns. Supercooled water was most prevalent 7-10 h after the passage of the 700 mb trough. Our study indicated that the largest instan- taneous quantities of supercooled water were associated with convection usually occurring in the postfrontal region. Heggli et a1. postulated that additional super- cooled water may have existed in the lowest 1 km above the terrain. Our study suggests not only that this is true but that these conditions can occur for long periods of time in the prefrontal as well as postfrontal environ- ments of particular storms. Radiometric measurements of supercooled water in storm systems over other mountain ranges of the west- ern United States have shown many features similar to the data presented here. For example, Rauber et a1. (1986) analyzed the supercooled water structure often storm systems that occurred over the Park Range of northwest Colorado. They noted an inverse relationship between supercooled water and precipitation rate in each of their case studies, a relationship also suggested in many SCPP storms. These authors also found that larger supercooled water measurements were associated with regions of storms where shallow clouds were pres- ent, a feature observed during SCPP.