A climatology of lee waves over the UK derived from model forecasts

Abstract

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.

Mountain Waves in High Resolution Forecast Models: Automated Diagnostics of Wave Severity and Impact on Surface Winds

“…• No simplifying (linear) approximations applied to the model equations of motion; the most hazardous mountain wave flows are highly non-linear, e.g., rotors and hydraulic jumps/wave breaking • Realistic initial and boundary conditions, data assimilation, representation of convergence and convection; problems can occur in 3DVOM when there is significant horizontal variation in conditions and atmospheric forcing (e.g., trough or low centre within the domain), since it is initialised by a single profile • Thorough representation of moist processes (noting that the U.K. has a very moist climate with cloud and rainfall common); 3DVOM is a dry model, but in reality, reversible latent heating (cloud formation and evaporation) effects favour flow over terrain rather than blocking, also affecting wave amplitude [58]; meanwhile, irreversible latent heating effects (e.g., upslope rainfall) modify the stability profile and, hence, the wave response [59][60][61][62]; further, any orographically-triggered deep convection will negate wave activity • More sophisticated boundary layer scheme; the boundary layer is known to impact lee wave generation and downwind decay [63][64][65], while the performance of the boundary layer scheme also decides the accuracy of forecast lee wave impacts on near-surface winds [13,44] • Direct simulation of the diurnal cycle through radiation, surface and boundary layer parametrisations, including for instance nocturnal stable boundary layers; boundary layer stability strongly affects wave propagation and lee wave decay [64] • Full and contiguous coverage of the U.K. (and eventually beyond, as future computing resources allow) • Lee wave impacts become prognostic; the interplay of lee waves with the atmospheric environment in which they form, including other weather phenomena, is represented • Access to a comprehensive, standardised set of diagnostics, long-term central archiving…”

“…More recently, the dynamic core of the model was replaced with a new version, ENDGame (Even Newer Dynamics for General atmospheric modelling of the environment) [51]. This core supports short wavelength gravity wave activity more readily than the previous dynamical core, which required the use of computationally-expensive (due to numerical instability issues) dynamics settings in order to simulate mountain waves with meaningful fidelity at 1.5-km resolution [44,52,53]. As a result, these settings could not be used operationally.…”

“…The model is described in detail elsewhere, along with some of the case studies that have been used for its validation, as well as the model's strengths and limitations [42,44]. In most cases, the model provides good guidance regarding lee wave activity, particularly in the moderate terrain of the U.K. where the constraint of linear dynamics has limited impact.…”

“…Furthermore, the incorporation of a parametrisation for boundary layer friction (see below) allows the impact of lee waves on the near-surface wind to be modelled. For instance, it is found that the model simulates particularly strong 10-m wind perturbations clustered in an area to the east of the Pennine hills in northern England, where aviation and high-sided ground vehicles are known to be particularly at risk from the effects of strong winds and turbulence, and another similarly-affected area near the Moray Firth in northeast Scotland [44]. Nevertheless, the linear assumption, absence of moisture processes and uniform background flow assumption can introduce inaccuracies at times.…”

Lee Waves on the Boundary-Layer Inversion and Their Dependence on Free-Atmospheric Stability

“…Unlike other types of mountain waves, trapped waves propagate horizontally (Smith, 1979): their phase lines are vertical and multiple wave crests can extend over several hundreds of kilometers downstream of an obstacle. If the atmosphere contains sufficient moisture, clouds may form at the wave crests (Houze, 2014) and give rise to a characteristic stripe pattern, frequently observed in satellite images close to mountainous areas, e.g., the Alps (Doyle et al, 2002), Pyrenees (Georgelin and Lott, 2001), Rocky Mountains (Ralph et al, 1997), and Pennines (Vosper et al, 2012).…”