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Since 2012 the Met Office has been running a short-range convective-scale ensemble prediction system over the UK, known as MOGREPS-UK. In this paper we consider MOGREPS-UK in its past, present and future configurations. We describe the evolution of the system during its first few years as an operational model and explain the rationale behind its development. The operational configuration of MOGREPS-UK is evaluated using neighbourhood verification techniques which allow the comparison of ensemble and deterministic forecasts in a probabilistic sense. We compare the performance of MOGREPS-UK to that of the higher resolution UK deterministic convective-scale model, the UKV, and show that over a three-month long trial MOGREPS-UK performs better for all variables considered. Plans of future upgrade options of MOGREPS-UK that take advantage of the increased computing capacity at the Met Office are discussed. Three different options were compared: increasing the domain size (now implemented), decreasing the horizontal grid-spacing, and increasing the number of ensemble members. Objective verification results from month-long winter and summer trials show that all options have their benefits, with the most improvement seen with the increase in ensemble size, particularly for precipitation.

The Madden Julian Oscillation (MJO) has been diagnosed in the World Weather Research Program (WWRP)/World Climate Research Program (WCRP) Sub-seasonal-to-Seasonal prediction project (S2S) database using the Wheeler and Hendon index (Wheeler and Hendon 2004) over the common hindcast period 1999–2010. The S2S models display skill to predict the MJO between 2 and 4 weeks. The majority of S2S models tend to produce a weaker MJO than in ERA Interim, with a phase speed decreasing with lead time. All the S2S models produce realistic patterns of MJO teleconnections at 500 hPa, with an increased probability of positive North Atlantic Oscillation (NAO) following an active MJO over the Indian ocean and of negative NAO following an active MJO over the west Pacific. However, the amplitude of the MJO teleconnection patterns are significantly weaker than in ERA Interim over the Euro Atlantic sector and are often too strong over the western North Pacific. Models with lower horizontal resolution tend to produce weaker teleconnections. In the lower stratosphere, several S2S models produce teleconnections which are too strong compared to ERA Interim. These results suggest that although the S2S models display significant skill in predicting the MJO propagation beyond two weeks, all the S2S models do not fully exploit the predictability associated to the MJO in the Northern Extratropics, particularly over Europe.

Analyses of 1-min data from automatic weather stations (AWSs) in the Gulf of Carpentaria region are used to study southerly nocturnal bores over northeastern Australia. The data cover the time period 1 August to 31 December in the three years 2012-2014. A total of 12 days on which a bore passed over the region of interest were found. Ten of the disturbances occurred between August and mid-October in the presence of synoptic-scale ridging across the continent behind a frontal trough. The other two occurred in early November in the presence of localized ridging behind an inland trough separate from a frontal trough. Six of the bores were followed some hours later by a clear air mass change. Four of the 12 cases are examined in detail wherein interpretations of the AWS data are aided by analysis fields from the European Centre for Medium-range Weather Forecasts (ECMWF). The results indicate that the longevity of the bore is related to the time of day that the cold front interacts with the stable maritime layer to the north of it. The ECMWF analyses are able to capture the frontal troughs as well as the timing of their passage through the AWS network. In the analyses, the troughs are seen as narrow strips of elevated cyclonic vorticity at 950 mb. Vertical cross-sections intersecting the troughs showed structural characteristics of a shallow cold front, but the horizontal resolution of the analyses is inadequate to capture the generation of bores ahead of the fronts.

Convective-scale ensemble prediction systems (EPS) are often initialized with downscaled initial condition perturbations (ICPs) from a global coarser EPS. Although downscaled ICPs have been shown to have a positive impact at short ranges, they cannot represent the uncertainty at small scales. Hence, there is a spin-up of around 9-12 hours until the forecast perturbations develop realistic small-scale structures. On the other hand, ensemble data assimilation (EDA) is a common approach to obtain initial perturbations at all scales resolved by the numerical model. However, the high computational cost of EDA systems severely limits their size and their resolution. An alternative cheaper method to derive small-scale ICPs is considered here, based on a random sampling of the model background-error covariances. This paper provides an evaluation of random and EDA-based IC perturbation methods against the baseline downscaling approach, in the framework of the pre-operational convective-scale EPS developed at Meteo France with the AROME-France model at a 2.5km horizontal resolution. Small-scale IC perturbation methods are shown to significantly improve the short-range EPS performance for surface weather variables. For 2-meter temperature and 10-meter wind speed random ICPs give as good results as the EDA, owing to the very short spin-up of random perturbations. Precipitation forecasts are also strongly improved during the first six forecast hours, especially when initial humidity perturbations are included . The sensitivity of the EPS performance to the EDA size, horizontal resolution and representation of model errors is discussed. It is found that a large fraction of the initial uncertainty can be properly described with an EDA of reasonable size and at a slightly coarser resolution than the EPS.

Doppler-On-Wheels radar observations made during the Convective and Orographically-induced Precipitation Study (COPS) on 12 August 2007 showed that precipitating clouds only developed between the north–south orientated Murg and Nagold Valleys of the northern Black Forest. The clouds produced moderate precipitation. The Weather Research and Forecasting (WRF) model run at 700 m horizontal resolution in the inner domain was able to simulate the location of the precipitation. Insight is therefore gained into the physical mechanisms responsible for the initiation and development of the convection. Convergence lines resulting from thermally driven flows constrained the initial location of the convection within warm and moist cores produced by heating on elevated surfaces. The heaviest precipitation was later produced by secondary convection, which was initiated within the cores at the boundary between cold-pool outflows and thermal flows. Copyright © 2011 Royal Meteorological Society

There is only limited understanding of the processes driving year-to-year variability in European winter climate and the skill of seasonal forecasts for Europe in winter is generally low. The winter of 2005-2006 is a useful case-study because it was the coldest winter in large parts of western Europe for over a decade, and the coldest in central England since 1995-1996. Here, we present results of experiments with a range of general circulation models to investigate the importance of both the Atlantic Ocean and stratospheric circulation in producing the unusually cold winter of 2005-2006. We use models with different combinations of horizontal and stratospheric vertical resolution, allowing the sensitivity of the response to model formulation to be tested. The response to Atlantic sea-surface temperature (SST) anomalies is improved in a more recent model with higher horizontal resolution. The results show that both Atlantic SSTs and the January 2006 sudden stratospheric warming are likely to have contributed to the cold 2005-2006 European winter. © Crown Copyright 2008. Reproduced with the permission of HMSO. Published by John Wiley & Sons Ltd.

This paper presents results from simulations of trade wind cumulus with the Met Office Large Eddy Model (LEM) based on observed environmental profiles from the Rain In Cumulus over the Ocean (RICO) field experiment. Comparisons of updraught core parameters are made with the in situ data collected onboard the three research aircraft that participated in RICO. The default set-up of the LEM was unable to produce sufficient amounts of rainwater content when compared to measurements by the aircraft. As a main aim of RICO was to quantify the importance of precipitation in the trade cumulus regime, we test the sensitivity of the model to changes in the rain microphysics, the large-scale forcing, and horizontal resolution. By changing these model variables we are able to obtain reasonably good agreement with the observations of updraught core vertical velocity, cloud and rainwater contents. Furthermore, the LEM produces comparable surface precipitation rates to those derived from radar measurements during RICO in a previous study. This gives us some confidence in the ability of the LEM to simulate realistic precipitating trade cumulus. Copyright © 2007 Royal Meteorological Society

The period 1984–1985 has seen a dramatic improvement in the quality of the ECMWF operational forecasts for tropical regions. This improvement is discussed in terms of the model changes during this period. Revisions include the introduction of a parametrization of shallow cumulus convection, modifications to the parametrization of deep cumulus convection, a new cloud scheme, and an increase in horizontal resolution with a change in spectral truncation from T63 to T106. Impact of the various model changes is assessed through a set of forecast experiments and through parallel 10-day forecasts over a 20-day period. The results show that the improvements in tropical forecasts are mainly through the reduction of systematic errors in response to a more realistic tropical diabatic forcing. The impact of these changes on the analyses is also assessed. This is shown to be substantial, particularly for the thermal state.

The ability of the High-Resolution Global Environmental Monitoring (HiGEM) climate model to represent high-impact, regional precipitation events is investigated in two ways. The first focuses on a case study of extreme regional accumulation of precipitation during the passage of a summer extratropical cyclone across southern England on 20 July 2007 that resulted in a national flooding emergency. The climate model is compared with a global numerical weather prediction (NWP) model and higher-resolution, nested limited-area models. While the climate model does not simulate the timing and location of the cyclone and associated precipitation as accurately as the NWP simulations, the total accumulated precipitation in all models is similar to the rain-gauge estimate across England and Wales. The regional accumulation over the event is insensitive to horizontal resolution for grid spacings ranging from 90–4 km. Secondly, the free-running climate model reproduces the statistical distribution of daily precipitation accumulations observed in the England–Wales precipitation record. The model distribution diverges increasingly from the record for longer accumulation periods, with a consistent under-representation of more intense multiday accumulations. This may indicate a lack of low-frequency variability associated with weather regime persistence. Despite this, the overall seasonal and annual precipitation totals from the model are still comparable to those from ERA-Interim.