WMOD00402 - Laserfiche WebLink

Home Browse Search

2 The initial results from the SF6 tracing program showed that on very convective days with little wind, the tracer gas was able to be located within about one mile of the release point. When. wind was present the SF6 was found several hundred feet above the ground in convective conditions, but in a comparatively narrow band. When the tracer flowed around a modest (1500 ft high) hill, a clear change in direction of the flow after passing the hill occurred. This may have been due to wind funneling down an upstream valley. All of these results point to the complexities of air flow across mountainous terrain and the need for additional new research and detailed measurements in areas where seeding aerosol releases are to be made. This particularly applies to the need for accurate and continual surface flow measurements. Details of the instrumentation and the flight records and data are given under Section I of this report. The study of convection over land, below and in stratus clouds was completed and the results published in 1986 in Atmospheric Research, 20, 87-100. It shows that the vapor flux from the evaporating snow surface can drive convection and maintain a supercooled water cloud layer without the assistance of heat flux from the surface, or entrainment or radiative cooling at cloud top. Because the saturation vapor pressure over water is higher than that over ice, the base of the supercooled water cloud has a lower limiting height. When the cloud base is lowered to this height, the air at the bottom of the convective layer is just saturated with respect to ice and the eva- poration of snow stops, as does the vapor-driven convection. This limiting cloud base height varies with snow-surface temperature. . The lower the snow temperature, the higher the cloud base height limit for continued convec- tive transfer from the surface. As an example, if the snow surface tempera- ture is -50C, there could be vapor-driven convection, provided the cloud base height is more than 80 m above the surface. Whereas, if the snow temperature is -lOoC the convection would stop if the cloud base height above the surface became less than 150 m. Section II. Precipitation Chemistry and Stable Isotope Proqraa: The goalS of this work were to measure the trace chemical composition of snow collected at the HALO site and relate this to its stable oxygen isotopic composition and observed ice crystal habit and structure, so that estimates could be made of: (a) the elevation range over which supercooled liquid water is captured by falling ice crystals1 (b) the extent to which AgI and possibly 1n203 are captured by warmer and colder habit ice crystals1 and (c) the extent to which these techniques might be developed for covar- iate analysis of future randomized seeding experiments1 also (d) in cooperation with the SCPP management it was decided that specially-tagged AgI aerosols using Cs1 would be used from aircraft seeding to test the GUIDE model predictions for targeting seeding effects into the HALO site under the fixed target experiment criteria. I I I I I I I I I I I I I I I I I I I