Downstream control of blocking and Rossby wave breaking by extratropical cyclones
Lead Research Organisation:
University of Reading
Department Name: Meteorology
Abstract
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.
| Description | Atmospheric blocks are a type of weather event characterized by stationary high pressure systems. They occur frequently in Europe and cause heatwaves in summer and extreme low temperatures in winter. Forecasting blocking events as far in advance as possible is therefore important. In this work we identified ways that forecasts of atmospheric blocking can be improved. We first showed that forecasts of atmospheric blocking, and the upper-level jet stream, can be improved by running forecasts with a model that has a better dynamical core (the part of the model that solves the equations of motion that govern the weather). Forecasts with the improved model were able to predict blocking events further in advance. We then showed that poor forecasts of atmospheric blocks forming over Europe are often linked to extratropical cyclone development over the Atlantic ocean: the location and intensity of the cyclone in the forecast 2-4 days prior to block development had a large impact on the block that develops. This suggested that a way to improve block forecasts may be found by improving extratropical cyclone forecasts. To investigate this, in the final part of work for this award, we ran forecast experiments where we modified the structure of a cyclone in a forecast and identified the impact on the block forecast. We showed changing the development of the cyclone did change the forecast of the atmospheric blocking event. |
| Exploitation Route | Predicting atmospheric blocking events remains a challenge and there are still occasions when blocking events are missed in forecasts of just a few days. The impacts that blocking events can have, e.g. extreme heat waves, means that it is important for society to reduce errors in block forecasts. We identified processes important for block development in forecasts and highlighted potential ways in which the forecasts could be improved. More work is needed to establish if our findings are robust for many different blocking events and in different forecast models. If they are, further work could use the findings to reduce the error in block forecasts. |
| Sectors | Agriculture, Food and Drink,Energy,Leisure Activities, including Sports, Recreation and Tourism |
| Description | Met Office CASE partnership |
| Organisation | Meteorological Office UK |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | Regular meetings to discuss research directions and visits to the Met Office, Exeter, to perform experiments. |
| Collaborator Contribution | Input into research direction and technical help. Contact with wider science community at the Met Office. |
| Impact | N/A |
| Start Year | 2015 |