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A two‐dimensional numerical study of effects of vertical diffusion in frontal zones
Author(s) -
Håkansson Måns
Publication year - 2002
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.01.47
Subject(s) - parametrization (atmospheric modeling) , frontogenesis , turbulent diffusion , diffusion , smoothing , turbulence , context (archaeology) , mechanics , boundary layer , geology , geostrophic wind , adiabatic process , statistical physics , meteorology , mathematics , physics , paleontology , statistics , mesoscale meteorology , quantum mechanics , thermodynamics , radiative transfer
Parametrization of vertical diffusion tends to cause excessive smoothing of free‐troposphere frontal‐zone gradients in several forecast models. It has been observed that these models produce frontal structures that are diffuse, and that the dynamic stability of the frontal zones is higher in forecasts than in the corresponding analyses. Because of their inherent characteristics, frontal zones are sensitive to the way turbulent diffusion is parametrized in the model calculations. In several models, the estimation of vertical diffusion is based on the theory for mixing within boundary layers, assuming that the free‐tropospheric turbulent exchanges may be determined by an extension of boundary‐layer theory. This is an unrealistic assumption and consequently the observed model deficiency in describing upper frontal zones may to some significant degree be attributed to the crudeness of the vertical‐diffusion scheme. To address the problem, and in an effort to achieve further understanding of instabilities and turbulent fluxes associated with frontal dynamics, a two‐dimensional, semi‐geostrophic, nearly adiabatic model describing a vertical cross‐section through an idealized frontal zone is presented. The model utilizes the same kind of vertical‐diffusion scheme as numerous operational forecast models. Different types of instability generated in the model's interpretation of the frontogenesis process are presented and discussed in the context of vertical diffusion. Results show that turbulent diffusion as described by the parametrization scheme plays an important role in smoothing the frontal gradients, especially within the shear zone below the jet stream. Furthermore, the scheme shows sensitivity to the choice of parameters. Extreme values of wind shear and temperature gradient in frontal zones are likely to be weakened by the parametrized turbulent fluxes. These effects impact on short‐range forecasts but may also affect the results when modelling future climatological scenarios. Copyright © 2002 Royal Meteorological Society