Description: A unique set of relaxation experiments with a forecast model initialized during December-January 1999-2019 is used to explore tropical influence on the Northern Hemisphere polar stratosphere and stratosphere-troposphere coupling, to quantify predictability benefits due to the perfect knowledge of the tropical variability. On average, predictability of the polar stratosphere, as represented by the 50-hPa geopotential height anomalies (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub-seasonal timescales, a statistically significant improvement in weekly mean skill scores can be demonstrated in 14-20% of individual forecast ensembles, mostly in cases when the skill of the corresponding control forecast is worse than average. In these forecasts, root mean square errors and forecast spread of Z50 during forecast weeks 3-5 are decreased by 10-15%. Stratospheric improvements are detected during periods of both vortex strengthening and vortex weakening, including most major Sudden Stratospheric Warmings that occurred during the study period, via modulation of the upward wave activity fluxes. An active Madden-Julian Oscillation is found in most of these events with MJO phase 5-7 preceding vortex weakening and MJO phase 3-4 preceding vortex strengthening. Forecasts with improved stratospheric circulation also have improved tropospheric circulation during and after the periods when improvements are detected in the stratosphere. However, stratosphere-troposphere coupling is limited to the Atlantic sector and overall plays only a secondary role in these tropospheric improvements. More likely, these improvements are due to tropospheric tropical teleconnections.

Description: While models often struggle to directly predict extreme events in the subseasonal range, especially those characterised by small spatial scales such as extreme precipitation, prospects are better for predicting large-scale circulation patterns. Fortunately, many extreme events are closely coupled to the large-scale flow: for precipitation, large-scale wave activity determines moisture transport and availability, especially in the mid-latitudes where frontal rainfall is dominant. We present a framework for identifying the precursor patterns that ‘set the scene’ for extreme events, and using them to augment direct model output to produce more skilful hybrid forecasts. Implementation and extension of this framework is supported by a new open-access Python package, Domino, which reduces such analyses to only a few lines of code. Specifically we consider the predictability of European regional daily rainfall extremes at lead times of several weeks. We will discuss how using information about large-scale precursors, in combination with direct model output, allows extra skill to be extracted from our existing models, and allows us to make the most of the high dimensionality and high data volumes produced by modern forecast systems. We will discuss the potential of this flexible framework to be applied to other geographical regions, different kinds of extreme events and to different time-scales, and plans for a semi-operational implementation of this approach using the IFS forecast model.

Description: In June 2021, the western North American continent experienced an intense heat wave with unprecedented temperatures and far-reaching socio-economic consequences. The magnitude of the heat wave was substantially underestimated by probabilistic weather forecasts for lead times beyond seven days. The record-breaking temperature anomaly coincided with a far northward extending upper-level ridge that was unambiguously linked to the intensity of the heat wave. During the 10 days preceeding the heat wave, the upper-level ridge was continuously fed by air masses originating to a substantial fraction from the lower troposphere that ascended in the West, Central, and East Pacific. We analyze the role of these strongly ascending airstreams, so-called warm conveyor belts (WCBs), for this extreme event using the operational ensemble forecasts from the European Centre for Medium-Range Weather Forecasts. Our analysis illustrates how anomalous WCB activity across the North Pacific - which is also associated with above-normal precipitation at the Meiyu-Baiu-Front - limited the predictability horizon of this extreme event. The results suggest that a mis-representation of WCB-related synoptic activity across the West and East Pacific in the ensemble forecasts results in an erroneous ridge position over the North American continent and a concomitant underestimation of the heat. We conclude that this chain of synoptic events which was essential for the upper-level ridge position and amplitude constituted a predictability barrier for the magnitude of the heat wave.