Improving the blend of multiple weather forecast sources by Reliability Calibration

Creating a forecast that is seamless across time yet is optimal at each forecast validity time is often achieved by blending forecasts from multiple Numerical Weather Prediction models (or using other forecast sources, such as an extrapolation nowcast). With the increasing usage of convection-permitting ensemble models at shorter lead times, the blending of these forecasts with longer range ensemble models with parameterised convection can lead to a clear transition from one forecast source to another. This is particularly noticeable when visualising the evolution of the gridded forecast. Calibrating the forecast sources with a common truth prior to blending provides a method of improving forecast skill whilst also unifying the characteristics of the forecasts to create a smoother blend throughout the evolution of the forecast.

This presentation aims to describe a non-parametric method, utilising tools from the Met Office’s IMPROVER codebase (https://github.com/metoppv/improver), for calibrating the reliability of the forecast without degrading the forecast resolution. This approach is assessed for its usability for gridded precipitation rate and total cloud amount forecasts. Reliability is markedly improved resulting in similar skill between forecast sources during the blending period and therefore extends the lead time range at which the forecast is more skilful than climatology. This approach is also presented as a step within a series of steps to improve forecast skill therefore highlighting that this approach can be complementary to other techniques without significant tuning. Further refinements to the Reliability Calibration technique removed artefacts in the gridded forecasts. Caveats, including a reduction in sharpness following calibration, are also presented.