Utilizing Surface Observations and Modeling with HYSPLIT to Assess Low-Level Stability for the Targeting of Cloud Seeding Material

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 thermodynamic stability can significantly impact transport of cloud seeding material released from valley or foothill locations over higher elevation target areas. A two-surface-site (2SS) method has been used to estimate stability in the atmospheric layer between a potential lower or mid-elevation seeding site and a crest height elevation, using available surface temperature and dew point data. The method yields approximations of integrated stability through the layer, and can be expressed in terms of the amount of low-level warming, or upper-level cooling, required to yield a neutral lapse rate (well-mixed environment). These stability indications have been further classified in terms of their likely impact on the dispersion of cloud seeding material. The 2SS method is useful in locations lacking nearby rawinsonde data, or where such data may not be representative of the location or time period of interest. The NOAA HYSPLIT (Hybrid Single Particle Lagrangian Intregrated Trajectory) model, used in conjunction with North American Model (NAM) 12-km meteorological analysis and forecast data, has been utilized for similar purposes. HYSPLIT simulates the trajectory and dispersion of plumes of airborne material released from a point or multiple points, and can be configured to represent actual or potential seeding material releases from existing ground-based sites.

Comparisons during numerous storm periods in Utah have shown generally good correspondence between stability estimates using the 2SS method and the simulated behavior of plumes in HYSPLIT. Each type of analysis has its strengths and weaknesses in terms of the ability to accurately represent low-level thermodynamic stability and its potential impact on seeding operations. The HYSPLIT model takes wind data into account, which the 2SS analysis does not, and clearly presents a much greater degree of sophistication. However, the HYSPLIT model is limited by the input meteorological data set, with the primary limitations being related to the data resolution, including representation of terrain features and near-surface boundary conditions. The NAM data set is very limited in regards to its representation of basin-specific cold pools and temperature inversions, which can have a substantial impact on the HYSPLIT output. Both the HYSPLIT modeling and 2SS analyses can be used to estimate the degree of low-level stability real-time, in support of operations, or post-hoc, for the purpose of examining the storm-period climatology of thermodynamic stability for an area of interest. Crest-top ice detector data was available in some areas where analyses were conducted, aiding in the selection of storm periods with cloud seeding potential for both the real-time and post-hoc analyses.

Estimations were made of the climatology of low-level thermodynamic stability in various portions of Utah, based on the 2SS and HYSPLIT methodologies, specifically during storm periods believed to have cloud seeding potential. Results suggest that low-level stability is not a major constraining factor for ground-based cloud seeding operations in most areas, with some exceptions. In the mountainous western U.S., low-level thermodynamic stability during stormy periods is quite variable geographically, and is greatly influenced by the terrain profile of a given area. Closed basins, as well as areas that lie just to the south of significant mountain barriers, particularly barriers oriented in an east-west direction (e.g., the Uinta Range in northeastern Utah), appear to be the most problematic in terms of low-level stability during the winter season. The seasonal period from December through the first half of February is the primary period of concern regarding low-level temperature inversions and basin cold pools in Utah.