National Aeronautics and Space Administration

Abstract:
Numerical weather prediction notoriously suffers from a drop in skill beyond two weeks. This is because of chaotic error growth making accurate forecast for local weather in the extratropics impossible on subseasonal to seasonal (S2S) time scales. Still, weather regimes, explain most of the atmospheric variability on time scales of 5-20 days in the North Atlantic-European region. Furthermore, teleconnections arising from slower components of the climate system, such as the stratosphere or tropical convection provide flow-dependent windows of forecast opportunity. In this seminar I will discuss how physical and dynamical processes on synoptic scales affect weather regime life cycles and might provide windows of subseasonal forecast opportunity. A focus lies on the representation of ascending air streams, so-called warm conveyor belts (WCBs) in the IFS ensemble forecasting system. WCBs inject air into the upper troposphere thereby amplifying the waviness of the jet stream, resulting in ridge building and ultimately in blocked weather regimes. With a focus on applications I will show examples of how regime diagnostics can aid S2S prediction, including findings about the role of synoptic activity in “predictability barriers” for forecasts of recent extreme events, including the Pacific Northwest heat wave in June 2021. I will close with a discussion of the concept of so-called “event-prone regimes” which might be a promising tool for extending the lead time for early warnigs of extreme events.