Winds and the Weather: Evaluating Three Dimensional (3D) Stratospheric / Tropospheric Jet Relationships and Their Roles in Extreme Cool-Season Weather Events

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

The stratosphere is a layer of the atmosphere where the increase of temperature with height discourages vertical motions, thereby barring entry to the clouds, storms, and frontal weather systems that form in the troposphere below. Nevertheless very long waves can propagate from the troposphere into the stratosphere and influence its circulation, most dramatically by disrupting its circumpolar vortex and causing a sudden stratospheric warming (SSW) event. Conversely, the stratospheric circulation can have a substantial effect on tropospheric weather, albeit through indirect pathways which are not entirely understood. In particular several studies have shown that cold air outbreaks (CAOs) are more likely when the stratospheric circumpolar vortex is weak, or disrupted by an SSW, and the condition of the stratospheric vortex can also influence the tropospheric jet streams in ways that can in turn affect surface weather. Interactions between the stratosphere and troposphere are thus of practical as well as scientific importance and are a focus of substantial research effort. Research under this award uses novel methods to characterize the three-dimensional structure of stratosphere-troposphere interactions. The work complements previous studies which have emphasized spatially aggregated quantities like the mean strength of the stratospheric vortex averaged over the polar cap and the mean speed of tropospheric jets averaged around circles of latitude. A hemispheric average can suffice to identify an SSW but it may conceal differences in the structure of the event which matter for its interactions with the troposphere. For instance the stratospheric vortex can be weaker in a hemispherically-averaged sense because the vortex center has moved away from the pole, or because the vortex has in fact split into two distinct vortices. A split vortex and a displaced vortex are likely to influence the troposphere in different ways. Their influence on jet streams may also depend on longitudinal variations in jet speed that would be obscured by averaging around a latitude circle. The work consists primarily of analysis of stratosphere-troposphere interactions identified in two datasets developed by the Principal Investigators. One is called JETPAC, for JEt and Tropopause Products for Analysis and Characterization, which identifies the locations of jet stream maxima on a three-dimensional grid including the troposphere and lower stratosphere and also identifies the tropopause (or multiple tropopauses) in each grid column. The second is developed using an algorithm called CAVE-ART, for Characterization and Analysis of Vortex Evolution using Algorithms for Region Tracking. CAVE-ART uses methods from computer vision to identify the three-dimensional structure of the stratospheric polar vortex. CAVE-ART is able to track separate pieces of the vortex in the event of a splitting SSW event. The work has broader impact due its relevance to significant weather events such as CAOs. The datasets generated for the project are made available to the research community in order to maximize their value for scientific discovery. Web-based outreach materials are developed and used to conduct outreach to the public, and the work supports two students at New Mexico Tech. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.