Low-level Atmospheric Stability During Icing Periods in Utah, and Implications for Winter Ground-based Cloud Seeding
In mountainous regions where winter season cloud seeding is conducted for the purpose of higher-elevation snowpack augmentation, the frequency and character of low-level atmospheric stability can significantly impact transport of cloud-seeding material released from valley and foothill locations over higher elevation target areas. A two-surface-site (2SS) method was developed to estimate stability in the layer from the valley/foothill surface to mountain-top height (approximately 700 mb) in Utah, using available surface temperature and dew point data. The method yields approximations of integrated stability in the layer, which were classified according to their likely impact on operational seeding, and can be expressed in terms of the low-level warming, or upperlevel cooling, required to yield a neutral lapse rate (well-mixed environment). The stability estimation method was applied to stormy periods during three winter seasons when mountain-top icing was documented via ground-based high elevation icing rate sensors, and when temperatures were adequately cold for activation of silver iodide particles as ice-forming nuclei. That partitioning method identifies periods when silver iodide seeding potential likely exists. The indications of the 2SS analysis method are that seeding material releases from most valley/foothill locations are likely to undergo timely and effective dispersion to mountain barrier crest height during a large percentage (~75%) of icing periods exhibiting apparent silver iodide seeding potential.
Comparisons of the 2SS method stability estimates to similar rawinsonde-derived estimates showed good correspondence in over 80% of the cases analyzed, providing some confidence in the utility of the 2SS method in the absence of available rawinsonde data. Comparisons were also made between 2SS stability estimates and modeled seeding plume behavior using the NOAA HYSPLIT (Hybrid Single Particle Lagrangian Intregrated Trajectory) model with NAM (North American Model) meteorological input data during icing periods. Agreement between modeled plume behavior and stability indications of the 2SS method was found in over 80% of the modeled periods. Results of these comparisons provide confidence in the overall stability climatology for icing periods as presented in this paper, as well as the real-time operational utility of the 2SS method in areas where other data (e.g., rawinsonde) are not available.
The analyses presented here comprise a portion of a more comprehensive study, based on data from several ice detector sites in Utah. Support for the establishment of these sites, and for analysis of the data, was provided by a consortium of Lower Colorado River Basin States.