Abstract. We describe Global Atmosphere 6.0 and Global Land 6.0 (GA6.0/GL6.0): the latest science configurations of the Met Office Unified Model and JULES (Joint UK Land Environment Simulator) land surface model developed for use across all timescales. Global Atmosphere 6.0 includes the ENDGame (Even Newer Dynamics for General atmospheric modelling of the environment) dynamical core, which significantly increases mid-latitude variability improving a known model bias. Alongside developments of the model's physical parametrisations, ENDGame also increases variability in the tropics, which leads to an improved representation of tropical cyclones and other tropical phenomena. Further developments of the atmospheric and land surface parametrisations improve other aspects of model performance, including the forecasting of surface weather phenomena. We also describe GA6.1/GL6.1, which includes a small number of long-standing differences from our main trunk configurations that we continue to require for operational global weather prediction. Since July 2014, GA6.1/GL6.1 has been used by the Met Office for operational global numerical weather prediction, whilst GA6.0/GL6.0 was implemented in its remaining global prediction systems over the following year.

A very high resolution numerical weather prediction model is nested inside the Met Office's main United Kingdom forecast model to investigate whether further enhancements to resolution provide any benefit for fog forecasting. The London Model shows similar performance to its lower resolution equivalent at short lead times, but improved performance at longer lead times and an improved frequency bias of forecast fog events. Differences in the model cloud parametrization are the key reason for the differing behaviour, leading to systematically less cloud, colder night time minimum temperatures and therefore more fog in the London Model. Benefit of the enhanced resolution is also found, via an improved representation of how orographic variability enhances turbulence in the stable boundary layer.

A series of idealised numerical simulations is performed to investigate the effect of wind direction on the pressure forces exerted on a high elliptical mesoscale ridge in the presence of Coriolis effects. At the Rossby number considered here (Ro ∼ 13), rotational effects have a significant impact on the flow fields, however the primary effect of rotation on the drag is to provide the asymmetry required to initiate vortex shedding when the flow is perpendicular to the mountain ridge. It is found that linear theory, although not valid for such high mountains, provides a useful scaling for the variation of drag with wind direction. For a large range of wind directions, the flow is in a high-(super-linear) drag state and wave breaking, vortex shedding and upstream flow blocking are observed. However, when the flow is close to being parallel to the major axis of the mountain ridge, the drag becomes sub-linear, and none of the above processes are seen. We show that the change from a high-drag state to a low-drag state can be explained in terms of the aspect ratio of the mountain, that is the ratio of the across-flow mountain length to the along-flow length. Finally we demonstrate that the results found for the idealised elliptical mountains also apply to a real mountain of similar dimensions.

A lee wave forecast system has been run operationally at the UK Met Office since 2006. The forecasts are produced by a numerical model for flow over complex terrain (3DVOM) which is run for five separate hilly regions across the UK. These regions cover Dartmoor (southwest England), Snowdonia (north Wales), Cumbria and the Pennines (northern England), the Grampians (Scotland) and the Mourne and Sperrin mountains (Northern Ireland). Examples of verification of the model forecasts against aircraft and satellite observations are presented. Three years of forecast data for these regions have been used to generate a lee wave climatology for the UK. The model predicts large geographical differences, with lee waves occurring least frequently over Dartmoor and most frequently over Snowdonia and the Grampians. Large amplitude waves, with peak vertical velocities exceeding 3 m s −1 at 700 hPa or above, are more common in forecasts for the Grampian region than others. Lee waves occur more frequently in forecasts during winter months than in summer. The most favourable conditions are those in which there is little turning of the lower tropospheric winds and analysis suggests that the waves are typically trapped in the lower troposphere. The influence of the lee waves on the near-surface flow has also been investigated. Large accelerations and flow deflections can occur beneath the waves. It is suggested that the latter correspond to turbulent lee wave rotors. Preferred locations for this behaviour have been identified in the model forecasts for the Grampians and Pennines.

ABSTRACT:The extent to which the drag due to flow blocking by mountains is affected by height variations in the static stability is examined using a series of numerical simulations. The results are used to investigate how best to estimate the depth-averaged upwind static stability for the purposes of parametrizing the drag.

This study focuses on the accuracy of simple methods used in parametrization schemes for predicting the drag due to orographically excited gravity waves (mountain waves). Linear and nonlinear model simulations of flow over a long, low two-dimensional ridge are used to evaluate the importance of internal wave reflection and nonlinearity. A long ridge with a small non-dimensional mountain height and a gentle slope is used so that, in the absence of vertical variations in the background profile of wind and stability, the mountain-wave drag is accurately predicted by linear theory. Simulations conducted for simple idealised profiles in which the background stability has a two-layer (troposphere-stratosphere) structure show that whilst the drag is accurately predicted by linear solutions, interference effects due to partial wave reflection can alter the drag significantly. Estimates of the drag which are based solely on low-level measurements of wind and stability, such as those in current operational mountain-wave parametrizations, cannot account for this effect. Results from simulations based on more complex realistic profiles, obtained from both radiosondes and a forecast model, show that the linear and nonlinear drag predictions can differ significantly. This implies that linear solutions can be inaccurate even when they are calculated for the full atmospheric profile (rather than being based on low-level average quantities). It is hypothesised that, in this case, the nonlinearity is due to resonant triad interactions which occur when there are oscillations in the Scorer parameter with a wavelength half that of the dominant vertically propagating mountain wave.

SUMMARYThe impact of rotation on the orographic drag experienced by air flowing around a wide mountain is investigated. The work builds on numerical modelling studies performed byÓlafsson and Bougeault, who investigated the effect of rotation and surface friction on the drag experienced by flow around a single elongated mountain perpendicular to the direction of the flow. The region of parameter space they explored is extended by performing a series of idealized model experiments with a larger range of ridge lengths. The drag force in these simulations is compared with the predictions of a heuristic flow-blocking model devised by Shutts. The results show that Shutts's model overestimates the effect of rotation upon the drag force. However we find that Shutts's model predicts both the drag force exerted on the upstream side of the ridge and the upstream features reasonably well. Finally the implications of the results for NWP parametrizations of subgrid-scale orographic drag are discussed.

Helicopter-triggered lightning is a phenomenon which affects operations over the North Sea during the winter. It is thought that the presence of the helicopter triggers the majority of lightning strikes, since there is generally little or no natural lightning activity in the area in question prior to or following the strike, and strike rates are much higher than would be expected if due purely to chance. However, there has been little progress to date in the ability to predict triggered lightning strike occurrence with NWP data. Previous attempts have resulted in forecasts which are insufficiently discriminating (i.e. high false alarm rate) to be of practical use.In this study, previous work on triggered lightning is reviewed and case studies are examined in order to identify common meteorological conditions for helicopter-triggered lightning strikes. Using forecast data from the Met Office Unified Model, an algorithm for triggered lightning risk was produced based on outside air temperature and precipitation rate. Evaluation against past helicopter strike cases has demonstrated that the new algorithm successfully forecasts lightning risk on 80% of occasions when triggered lightning occurred. In addition, the algorithm correctly forecast 8/9 natural lightning strikes which were observed in the operating area during winter 2010-2011. The areas of risk highlighted are usually small, which should allow helicopter operators to plan flights around high risk regions. The information in this study can also be used to inform helicopter operators of the likely conditions in which triggered lightning strikes occur.