The response of bora-type flow to sea surface temperature

SUMMARY A mesoscale numerical model is used to study the atmospheric boundary-layer (ABL) response to nonlinear orographic forcing with Coriolis effect, f , over a mountain with length (the cross-wind component) comparable to the Rossby radius of deformation, L R . The orographic-wave breaking occurring for Froude number Fr < 1, affected by f > 0, intensifies on the northern flank for westerly flows, as also found in other recent studies. A cumulative effect occurs as the Coriolis force lifts the northern ABL top and generates a stronger low-level jet (LLJ) than on the southern side. A differential layering also appears, since the specific humidity is higher in the lower southern ABL than in the related northern ABL, and vice versa. By contrast, there are higher values of the turbulent kinetic energy and humidity in the upper northern ABL.The breaking of flow symmetry around the orography due to f changes both the vertical vorticity and horizontal divergence field, (ζ, D), it modulates eddies and turbulence leading to the differential layering of the ABL. The stronger northern LLJ and its weaker southern counterpart, both meandering, together with the asymmetric wave breaking, induce strong lee-side fluctuations of the (ζ, D) field in the presence of f . The enhanced (ζ, D) production due to wave breaking over the distance ≈L R , the primary atmosphere-orography resonance occurs mainly in the vertical, while the 'f -enhancement' occurs in the horizontal plane. In this way, the initial mesoscale forcing may extend its effects over the synoptic scale.

Section: The Numerical Modelmentioning

confidence: 99%

Section: Simulations (A) Backgroundmentioning

confidence: 99%

Section: Components (U V )mentioning

confidence: 99%

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Boundary‐layer variations due to orographic‐wave breaking in the presence of rotation

Grisogono

Enger

2004

“…A grid mesh of 50 × 50 × 10 m at the source and a model dimension of 50 × 10 × 5 km are used. An advection scheme presented in Enger and Grisogono (1998) solves the prognostic equations. In the present study a time step of 2 s has been used, giving a typical Courant number less than 0.02.…”

Section: Numerical Methods and Boundary Conditionsmentioning

confidence: 99%

The role of advection of fluxes in modelling dispersion in convective boundary layers

Abiodun

Enger

2002

Self Cite

SUMMARYA Eulerian three-dimensional higher-order closure dispersion model is presented. The model uses mean wind and turbulence quantities from a second-order atmospheric boundary-layer model. The dispersion model is validated against results from tank and field experiments and compared to results from Lagrangian dispersion models. The results show quite good agreement with experiment and Lagrangian modelling results for point source dispersion in a convective boundary layer (CBL). Sensitivity studies with the model help to identify the role played by advection and horizontal transport terms in the equations for the fluxes in simulating the essential features of pollutant dispersion.Results from the sensitivity tests show that the characteristic features of dispersion from point sources in the CBL-with an ascending plume during ground-level release and a descending plume from a lifted point source-are caused by an imbalance between the advection and diffusion terms in the equation for the vertical flux. Furthermore, it is shown that there is also a tendency for the plume to split horizontally, which is similarly caused by an imbalance between the advection and diffusion terms in horizontal fluxes. The simulated lateral standard deviations of distance x from a point source, for the vertically integrated concentration, are proportional to x close to the source and to x 1/2 far away from the source, if and only if the advection and diffusion terms are included in the equations for the turbulent fluxes of concentration.

“…KoraÏ cin and Enger 1994;Grisogono 1995;Enger and Grisogono 1998), coastal ows (Tjernström 1999;Tjernström and Grisogono 2000), dispersion calculations (Enger and KoraÏ cin 1995) and atmospheric chemistry (Svensson 1998;Svensson and Klemm 1998). The original model has a statistical bulk-liquid water cloud parametrization (Tjernström 1988), which has been used to study the turbulent structure of a cloud-topped marine boundary layer (Tjernström and KoraÏ cin 1995;Svensson et al 2000).…”

Section: Introductionmentioning

confidence: 99%

A numerical model of the cloud‐topped marine boundary layer with explicit treatment of supersaturation–number concentration correlations

Svensson

Seinfeld

2002

SUMMARYA numerical model of the cloud-topped marine boundary layer is presented. The model, based on ensembleaveraged conservation equations and containing detailed aerosol microphysics, differs from previous models in one major respect: the mean supersaturation-cloud drop number concentration correlation term, generally neglected in previous numerical mean cloud models, is parametrized using a traditional boundary-layer uxgradient relationship. The present work highlights the importance of this correlation term and shows how a simple representation of this correlation can lead to dramatically different, and improved, simulations of cloud behaviour. Simulations are presented for a well-studied marine boundary-layer stratocumulus case. Inclusion of the supersaturation-cloud drop number concentration correlation leads to more physically realistic cloud drop spectra and cloud drop concentrations throughout the height of the cloud. Although the parametrization presented must be viewed as a rst approximation, the crudeness of the approach should be seen as a justi cation and starting point for future work.