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S. B. Vosper

Abstract

Results from a series of numerical simulations of three-dimensional stably stratified flows past conical orography with unit slope are presented and are compared directly with laboratory results from a stratified towing tank. The simulations are conducted with a finite-difference model (configured to simulate flows in the towing tank) based on the inviscid nonhydrostatic equations of motion in σ (normalized pressure) coordinates. A free-slip lower boundary condition is implemented. The flows studied have values of the Froude number, F h = U/Nh, between 0.1 and 0.8, where U is the mean flow speed, N is the buoyancy frequency, and h is the mountain height.

To excite unsteadiness in the simulations an asymmetric perturbation is applied to the initial potential temperature (θ) field. The resulting variation of the temporally averaged drag coefficient with F h is found to compare reasonably well with the laboratory measurements and there is a general trend for the drag coefficient to decrease as F h increases. For many of the simulations the temporal evolution of the drag is highly unsteady: when F h ⩽ 0.3 the unsteadiness is quasiperiodic and is due to vortex shedding in the lee of the orography. The nondimensional vortex shedding frequency is similar to that measured in the laboratory. Simulations conducted without the initial θ perturbation do not exhibit vortex shedding and in this case the drag is significantly reduced. The vorticity generated in both the perturbed and unperturbed flows is shown to be largely perpendicular to the isentropic surfaces and hence potential vorticity anomalies are present. These anomalies appear early on in the simulations and are caused by internal dissipation within the vortices due to numerical viscosity. Simple tests in which the free-slip lower boundary condition is replaced with one containing a surface friction parametrization show that one of the main effects of friction is to suppress the vortex shedding. Further, the results indicate that in the laboratory an inviscid mechanism (in which vertical vorticity is generated via the tilting of baroclinically generated horizontal vorticity) dominates over the generation of vertical vorticity in the boundary layer.

At F h = 0.4 a local maximum occurs in the temporally averaged drag and this corresponds to the occurrence of wave breaking in the lee of the mountain. The wave-breaking process itself is highly unsteady and after continuous growth of the wave amplitude (and drag) convective instability eventually leads to a complete collapse of the overturning region and a significant fall in the drag.

Further unsteadiness occurs at higher values of F h when a rigid-lid upper-boundary condition is enforced: for 0.45 ⩽ F h ⩽ 0.6 the evolution of the simulated drag is quasiperiodic and this is shown to be caused by the generation of an unsteady wave motion upstream. Comparison with existing linear theory indicates that this is due to the existence of a wave mode whose horizontal group velocity is small but has a nonzero frequency.

The effects of blockage, due to the presence of the towing-tank side walls, are investigated by enforcing radiative boundary conditions at the spanwise lateral boundaries. As found in the towing-tank experiments, blockage effects are shown to significantly increase the drag coefficient and nondimensional shedding frequency at low Froude numbers (F h ≲ 0.4) and also alter the Froude number at which wave breaking occurs.

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Stephen D. Eckermann, Andreas Dörnbrack, Harald Flentje, Simon B. Vosper, M. J. Mahoney, T. Paul Bui, and Kenneth S. Carslaw

Abstract

The results of a multimodel forecasting effort to predict mountain wave–induced polar stratospheric clouds (PSCs) for airborne science during the third Stratospheric Aerosol and Gas Experiment (SAGE III) Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO 2000) Arctic ozone campaign are assessed. The focus is on forecasts for five flights of NASA's instrumented DC-8 research aircraft in which PSCs observed by onboard aerosol lidars were identified as wave related. Aircraft PSC measurements over northern Scandinavia on 25–27 January 2000 were accurately forecast by the mountain wave models several days in advance, permitting coordinated quasi-Lagrangian flights that measured their composition and structure in unprecedented detail. On 23 January 2000 mountain wave ice PSCs were forecast over eastern Greenland. Thick layers of wave-induced ice PSC were measured by DC-8 aerosol lidars in regions along the flight track where the forecasts predicted enhanced stratospheric mountain wave amplitudes. The data from these flights, which were planned using this forecast guidance, have substantially improved the overall understanding of PSC microphysics within mountain waves. Observations of PSCs south of the DC-8 flight track on 30 November 1999 are consistent with forecasts of mountain wave–induced ice clouds over southern Scandinavia, and are validated locally using radiosonde data. On the remaining two flights wavelike PSCs were reported in regions where no mountain wave PSCs were forecast. For 10 December 1999, it is shown that locally generated mountain waves could not have propagated into the stratosphere where the PSCs were observed, confirming conclusions of other recent studies. For the PSC observed on 14 January 2000 over northern Greenland, recent work indicates that nonorographic gravity waves radiated from the jet stream produced this PSC, confirming the original forecast of no mountain wave influence. This forecast is validated further by comparing with a nearby ER-2 flight segment to the south of the DC-8, which intercepted and measured local stratospheric mountain waves with properties similar to those predicted. In total, the original forecast guidance proves to be consistent with PSC data acquired from all five of these DC-8 flights. The work discussed herein highlights areas where improvements can be made in future wave PSC forecasting campaigns, such as use of anelastic rather than Boussinesq linearized gridpoint models and a need to forecast stratospheric gravity waves from sources other than mountains.

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J. D. Price, S. Vosper, A. Brown, A. Ross, P. Clark, F. Davies, V. Horlacher, B. Claxton, J. R. McGregor, J. S. Hoare, B. Jemmett-Smith, and P. Sheridan

During stable nighttime periods, large variations in temperature and visibility often occur over short distances in regions of only moderate topography. These are of great practical significance and yet pose major forecasting challenges because of a lack of detailed understanding of the processes involved and because crucial topographic variations are often not resolved in current forecast models. This paper describes a field and numerical modeling campaign, Cold-Air Pooling Experiment (COLPEX), which addresses many of the issues.

The observational campaign was run for 15 months in Shropshire, United Kingdom, in a region of small hills and valleys with typical ridge–valley heights of 75–150 m and valley widths of 1–3 km. The instrumentation consisted of three sites with instrumented flux towers, a Doppler lidar, and a network of 30 simpler meteorological stations. Further instrumentation was deployed during intensive observation periods including radiosonde launches from two sites, a cloud droplet probe, aerosol monitoring equipment, and an instrumented car. Some initial results from the observations are presented illustrating the range of conditions encountered.

The modeling phase of COLPEX includes use of the Met Office Unified Model at 100-m resolution, and some brief results for a simulation of an intensive observation period are presented showing the model capturing a cold-pool event. As well as aiding interpretation of the observations, results from this study are expected to inform the design of future generations of operational forecasting systems

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D. R. Jackson, A. Gadian, N. P. Hindley, L. Hoffmann, J. Hughes, J. King, T. Moffat-Griffin, A. C. Moss, A. N. Ross, S. B. Vosper, C. J. Wright, and N. J. Mitchell

Abstract

Gravity waves (GWs) play an important role in many atmospheric processes. However, the observation-based understanding of GWs is limited, and representing them in numerical models is difficult. Recent studies show that small islands can be intense sources of GWs, with climatologically significant effects on the atmospheric circulation. South Georgia, in the South Atlantic, is a notable source of such “small island” waves. GWs are usually too small scale to be resolved by current models, so their effects are represented approximately using resolved model fields (parameterization). However, the small-island waves are not well represented by such parameterizations, and the explicit representation of GWs in very-high-resolution models is still in its infancy. Steep islands such as South Georgia are also known to generate low-level wakes, affecting the flow hundreds of kilometers downwind. These wakes are also poorly represented in models.

We present results from the South Georgia Wave Experiment (SG-WEX) for 5 July 2015. Analysis of GWs from satellite observations is augmented by radiosonde observations made from South Georgia. Simulations were also made using high-resolution configurations of the Met Office Unified Model (UM). Comparison with observations indicates that the UM performs well for this case, with realistic representation of GW patterns and low-level wakes. Examination of a longer simulation period suggests that the wakes generally are well represented by the model. The realism of these simulations suggests they can be used to develop parameterizations for use at coarser model resolutions.

Open access
Rachel A. Stratton, Catherine A. Senior, Simon B. Vosper, Sonja S. Folwell, Ian A. Boutle, Paul D. Earnshaw, Elizabeth Kendon, Adrian P. Lock, Andrew Malcolm, James Manners, Cyril J. Morcrette, Christopher Short, Alison J. Stirling, Christopher M. Taylor, Simon Tucker, Stuart Webster, and Jonathan M. Wilkinson

Abstract

A convection-permitting multiyear regional climate simulation using the Met Office Unified Model has been run for the first time on an Africa-wide domain. The model has been run as part of the Future Climate for Africa (FCFA) Improving Model Processes for African Climate (IMPALA) project, and its configuration, domain, and forcing data are described here in detail. The model [Pan-African Convection-Permitting Regional Climate Simulation with the Met Office UM (CP4-Africa)] uses a 4.5-km horizontal grid spacing at the equator and is run without a convection parameterization, nested within a global atmospheric model driven by observations at the sea surface, which does include a convection scheme. An additional regional simulation, with identical resolution and physical parameterizations to the global model, but with the domain, land surface, and aerosol climatologies of CP4-Africa, has been run to aid in the understanding of the differences between the CP4-Africa and global model, in particular to isolate the impact of the convection parameterization and resolution. The effect of enforcing moisture conservation in CP4-Africa is described and its impact on reducing extreme precipitation values is assessed. Preliminary results from the first five years of the CP4-Africa simulation show substantial improvements in JJA average rainfall compared to the parameterized convection models, with most notably a reduction in the persistent dry bias in West Africa, giving an indication of the benefits to be gained from running a convection-permitting simulation over the whole African continent.

Open access