Downstream control of blocking and Rossby wave breaking by extratropical cyclones

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Skillful weather and climate forecasts on timescales from hours to centuries are vital for protecting lives and livelihoods and managing the effects of climate change. The Observing System Research and Predictability Experiment (THORPEX) of the World Meteorological Organization grew out of a recognition that, despite continued improvements in numerical weather prediction forecasts (THORPEX specifically considered 1-14 day lead times), further improvements needed to be made. Errors in these forecasts arise due to errors in the initial conditions (the chaotic `butterfly effect'), boundary conditions (e.g. sea surface temperature errors in a weather forecast) and errors in the model formulation (so-called model error). Errors occurring at tropopause level propagate downstream due to their influence on the development, propagation and breaking of the planetary-scale Rossby waves associated with meanders of the upper-level (~10 km high) jet stream. Ridges and troughs in the jet stream are the major driver of the development of extratropical cyclones (a.k.a winter storms) associated with strong surface winds and rain as well as blocking events associated with e.g. summer heatwaves and health impacts due to the trapping of pollutants. One type of systematic error is that due to the mis-representation of diabatic processes such as clouds and radiation in extratropical cyclones, resulting from the necessity to parametrize convection and other moist processes in global (and most regional) weather forecast models. The influence of errors in the representation of these processes in a given storm are propagated downstream and so can affect the forecast of future developing storms or blocking episodes, leading to so-called 'forecast busts'. Identification of such systematic errors is the first step to determining model improvements to reduce them. The CASE partner for this project is the Met Office and our collaboration is with both the 'ensemble forecasting' and 'model evaluation and diagnostics' research groups. In a seamless model prediction system, such as the Met Office's operational climate and weather forecast model (known as the Unified Model), identification and reduction of systematic errors using one component of the system (here we use the weather forecasting component) can potentially improve forecasts across all forecast time horizons from hours to centuries. In this project we causally associate errors in tropopause structure caused by the mis-representation of diabatic processes in extratropical cyclones and determine their influence on downstream error in Rossby-wave breaking and blocking.