Abstract ID: 211
Predictability and windows of sub-seasonal forecast opportunity for North Atlantic-European weather regimes
Lead Author: Christian M. Grams
Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT), Germany
Keywords: physical mechanisms leading to predictability on sub-seasonal timescales, Prediction studies, Model evaluation and diagnostics, weather regimes,
Abstract: Weather regimes are large-scale atmospheric flow patterns that govern the character of weather in the midlatitudes over continent-size regions. In the North Atlantic-European region weather regimes explain most of the multi-day atmospheric variability and are of particular interest for sub-seasonal weather forecasting, e.g. in the energy industry. Several weather regimes are characterized by blocking anticyclones which are thought to provide predictability due to their persistence. However, current sub-seasonal numerical weather prediction models have hardly skill for daily and weekly forecasts of weather regimes beyond 10 days. At the same time slower components of the climate system, such as the stratosphere or the Madden-Julian-Oscillation (MJO), are well-known potential sources for predictability in the midlatitudes on sub-seasonal time scales.
In this presentation we shed light on the predictability and windows of sub-seasonal forecast opportunity for North Atlantic-European weather regimes. First, we briefly discuss the general forecast skill of weather regimes in reforecasts from the S2S database. We then elucidate how physical and dynamical processes related to synoptic weather systems affect regime life cycles and thereby complicate regime forecasts. We focus on the upper-tropospheric divergent outflow due to latent heat release in ascending air streams, so-called warm conveyor belts (WCBs). Such diabatic outflow often amplifies the waviness of the jet stream, resulting in ridge building and the onset of blocking anticyclones. We find evidence that a misrepresentation of diabatic WCB outflow at regime onset is likely the cause for vanishing regime skill on sub-seasonal time scales. At the same time results suggest that a correct representation of WCB activity might be a window of forecast opportunity for regimes. Finally, we discuss how the occurrence of regimes is modulated by the state of the winter stratosphere and the MJO, which provide another window of forecast opportunity for weather regimes on sub-seasonal time scales. Interestingly, we find again that WCB activity related to synoptic-scale weather systems modulate the MJO teleconnections towards North America and Europe. We conclude that knowledge about physical and dynamical processes on synoptic scales is key for exploiting the potential windows of forecast opportunity for weather regimes on sub-seasonal time scales.
Co-authors:
Büeler, Dominik (Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT), Germany; now at: Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland)
Pickl, Moritz (Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT))
Quinting, Julian F. (Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT))
Wandel, Jan (Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT); now at: Deutscher Wetterdienst, Offenbach, Germany)
