WMOD00293 - Laserfiche WebLink

Home Browse Search

Six experiments are described in detail, representing all aircraft sampling period during which valley- released AgI (silver iodide) was transported to both Plateau-top and aircraft altitudes. Embedded convection was present during five of the experiments which probably enhanced vertical AgI transport. Cloud bases were generally a few hundred meters below the Plateau top with one exception which had bases above the Plateau. The valley-released AgI consistently had a pronounced vertical gradient above the Plateau. Concentrations of AgI ice nuclei, effective at -20 oC, were typically at least an order of magnitude less at lowest aircraft altitudes than found on top ofthe Plateau. Silver iodide was seldom detected as high as 1 km above the Plateau top. . . No evidence was found for enhanced ice particle concentrations at aircraft altitudes during the three experiments with sampling zone temperatures of -9 oC and above. However, suggestions of increased ice particle concentrations were found when sampling zone temperatures were colder. Silver iodide ice nuclei estimates, effective at prevailing supercooled liquid cloud temperatures, were lower than believed desirable for effective seeding during the three experiments with warmer cloud at aircraft sampling altitudes. The results presented are believed to indicate that current operational cloud seeding in Utah produces insignificant increases in ice particle concentrations and snowfall rates during warmer storm episodes. The main problem appears to be that concentrations of effective ice nuclei are too low for the typical mildly supercooled liquid water clouds reached by the Agl. Seeding appeared to markedly increase the ice particle concentration in colder supercooled clouds, and precipitation observations suggested associated limited snowfall. increases during some cases. A number of recommendations are made for increasing ice particles concentrations in the warmer supercooled liquid clouds. SUMMARY AND CONCLUSIONS Six early 1991 seeding experiments were examined in which AgI, released from a network of eight valley generators, was observed over the Wasatch Plateau of central Utah. These experiments, corresponding to aircraft sampling periods over the Plateau, were selected as all available cases with valley-released AgI detected at aircraft altitudes over the Plateau. Therefore, these cases are not representative of valley seeding in general. Valley-released AgI was not transported to Plateau top and aircraft altitudes during some other early 1991 sample periods (e.g., Super and Holroyd, 1994; Super, 1994). All the storm periods reported herein had SL W cloud over the Plateau as measured by microwave radiometer and aircraft, although amounts were very limited in the coldest experiment and the SL W was well above the Plateau top during another experiment. Five of the experimental periods had embedded convection present. The sixth period, the coldest storm sampled during 1991, experienced passage of a mesoscale feature which may have aided transport over the Plateau. Silver iodide IN were measured by NCAR acoustical IN counters in an aircraft, in a 4-wheel drive vehicle and at a fixed site near the head of a major canyon. Concentrations of AgI IN, effective at ;.20 oC in each NCAR counter's cloud chamber, were often hundreds per liter at the fixed site and along the upwind highway following the Plateau top's windward edge for a 6.7 km north-south distance. Concentrations at lowest aircraft sampling altitudes were typically over an order of magnitude less than along the Plateau top. These measurements indicate a consistent, rapid decrease in valley-released AgI IN concentrations with height above the Plateau. Little AgI IN were found as high as 1 km above the Plateau top, and the aircraft sometimes overflew the plume while within 600 m of the Plateau top. 62