Climatology and forcing mechanisms of funnel clouds in Alaska

Abstract

There are no forecasting systems for funnel clouds for Alaska. The inability of forecasting is problematic because funnel clouds pose a threat to aviation, which serves as Alaska’s main form of transportation. Motivated by the lack of research on the formation of funnel clouds in Alaska, this research investigated characteristics of funnel clouds and atmospheric conditions under which funnel clouds form using operational Doppler weather radars and radiosonde soundings as well as synoptic weather maps. In Alaska, funnel clouds usually occur during the summer months May to September with a maximum of occurrence in July and around 1500 Alaska Daylight Time as shown in the funnel cloud observational data. The observed funnel clouds are usually not associated with severe thunderstorms and do not occur with strong synoptic scale forcing. As such, it was hypothesized that local effects from sea breeze fronts and orographic circulations might be the main forcing. Operational soundings indicate that some, but not all funnel cloud events occurred under large Convective Available Potential Energy (greater than 500 J) and strong lowlevel wind shear. Funnel clouds were difficult to identify in routine operational Doppler weather radars because the funnel clouds display small cross-sectional area compared to the radar resolution. An algorithm to retrieve similar vertical profiles from the entire radiosonde data than those observed during documented funnel cloud events was developed. By using similarity between radiosonde profiles of days of the observed funnel clouds and the similar radiosonde profiles scanned over the years, an idea of funnel cloud or severe storm occurrence can be ascertained. The mechanisms for funnel cloud formation differ by region. In Interior Alaska, the Alaska Range’s katabatic slope winds and the Tanana Valley wind create the needed vorticity. Along the west coast of Alaska, air-sea interaction plays a role. In Cook Inlet, topography and land-sea play a role. All funnel cloud events have weak synoptic scale forcing.