times, and horizontal and vertical resolutions. These runs are then compared to re-analysis data. The main conclusions from this work are: (a) objectively identified cyclone tracks are represented satisfactorily by most hindcasts; (b) sensitivity to vertical resolution is low; (c) cyclone depth is systematically under-predicted for a coarse resolution of T63 by both climate models; (d) no systematic bias is found for the higher resolution of T127 out to about three days, demonstrating that climate models are in fact able to represent the complex dynamics of explosively deepening cyclones well, if given the correct initial conditions; (e) an analysis using a recently developed diagnostic tool based on the surface pressure tendency equation points to too weak diabatic processes, mainly latent heating, as the main source for the under-prediction in the coarse-resolution runs. Finally, an interesting implication of these results is that the too low number of deep cyclones in many freerunning climate simulations may therefore be related to an insufficient number of storm-prone initial conditions. This question will be addressed in future work.

European windstorms are a high-impact weather phenomenon, regularly inflicting substantial damages, both human and economic. This study examines a set of objectively selected intense European windstorms from the 1979-2015 period using re-analysis and forecast products from the European Centre for Medium-Range Weather Forecasts (ECMWF). The storms are first categorized with respect to their diabatic relative to baroclinic contribution to deepening using the pressure tendency equation, and secondly with respect to their track relative to the jet stream as the large-scale element controlling storm deepening and propagation. As expected, baroclinic processes dominate the majority of storms, such that deepening is closely related to warm advection ahead of the cyclone centre. Contributions from diabatic processes vary strongly and exceed those from horizontal temperature advection in 10 out of 58 cases, with values of up to 60%. Remarkably, in several cases, planetary waves in the stratosphere appear to facilitate cyclogenesis but can also act to oppose deepening in a few cases. The diabatic contribution is significantly correlated with the time a given storm spends on the equatorward side of the jet, where there is greater potential for diabatic processes in the warm, moist air. In terms of forecast quality and consistent with previous studies, the storm core pressure is generally underestimated and storms tend to be too slow and shifted south in the forecast, particularly for longer lead times. These biases, however, reduce markedly with the improvement of the operational system over time. There is no systematic dependence of forecast behaviour on the diabatic contribution or track relative to the jet. In the future, some of these analyses should be repeated with homogeneous reforecast data to substantiate these findings better.

The Met Office runs an operational high-resolution convection-permitting model over Tropical Africa to support National Meteorological Services across the continent. This article evaluates the prediction of convective storms in two convection-permitting versions of the Met Office Unified Model over the Lake Victoria basin region in East Africa. Two model configurations are compared for a case of severe convection: the first was tuned for tropical regions and the second was based on the operational UK model configuration. The tropical configuration compares better with satellite-derived rainfall observations in terms of domain-average rain-rate and the distribution of rain-rates is better, particularly for the more intense rain-rates. However, both configurations generally produce too much rain, too many small storms, and a lack of light rain, which is compensated for by heavy rain in the cores of the storms. The tropical configuration was then assessed over two rainy seasons. In both seasons, the onset time of night-time precipitation over Lake Victoria is delayed and the amount of precipitation is underestimated, which suggests the model may miss night-time storms that occur over the lake. During the first 12-24 hr of the simulations, the precipitation field is still spinning up from the lower-resolution global model fields used to initialise the convection-permitting model. This leads to large overestimates in the domain-average precipitation during this period and highlights the need to run convection-permitting models for at least two days to reduce the impact of cold-starting from a non-convective-permitting analysis. Despite the spin-up issues, fractions skill score analysis shows that forecast skill decreases with increasing lead time. This implies that there is inherent skill in the early stages of the model forecast and underlines that short-term forecast (0-48 hr) improvements in the model are likely to be achieved when using an improved analysis for initialisation. K E Y W O R D S convection, convection-permitting models, precipitationThis article is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.

Seasonal climate forecasts have the potential to support planning decisions and provide advanced warning to government, industry, and communities to help reduce the impacts of adverse climatic conditions. Assessing the reliability of seasonal forecasts, generated using different models and methods, is essential to ensure their appropriate interpretation and use. Here we assess the reliability of forecasts for seasonal total precipitation in Sahelian West Africa, a region of high year-to-year climate variability. Through digitizing forecasts issued from the regional climate outlook forum in West Africa known as Prévisions Climatiques Saisonnières en Afrique Soudano-Sahélienne (PRESASS), we assess their reliability by comparing them to the Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) project observational data over the past 20 years. The PRESASS forecasts show positive skill and reliability, but a bias toward lower forecast probabilities in the below-normal precipitation category. In addition, we assess the reliability of seasonal precipitation forecasts for the same region using available global dynamical forecast models. We find all models have positive skill and reliability, but this varies geographically. On average, NCEP’s CFS and ECMWF’s SEAS5 systems show greater skill and reliability than the Met Office’s GloSea5, and in turn than Météo-France’s Sys5, but one key caveat is that model performance might depend on the meteorological situation. We discuss the potential for improving use of dynamical model forecasts in the regional climate outlook forums, to improve the reliability of seasonal forecasts in the region and the objectivity of the seasonal forecasting process used in the PRESASS regional climate outlook forum.