The effects of a frontal passage on fine-scale nocturnal boundary layer turbulence

It is accepted that atmospheric boundary layer (ABL) turbulence plays a role in limiting the contraction of a surface frontal zone. However, very little is known about the nature of turbulence in a frontal zone, especially at the dissipative scales. Without explicit knowledge of the turbulence in a frontal zone, accurate models of surface fronto-genesis are compromised. To address this problem, high-frequency measurements from sonic, hotwire and hotfilm anemometers are used to analyze the fine-scale turbulence in two synoptic cold fronts observed in the MICROFRONTS and CASES-99 field experiments. To quantify the turbulence in these fronts, fluxes, dissipation rates, velocity spectra and turbulent kinetic energy (TKE) budgets are calculated using these data. New results include the following. Dissipation rate increases by an order of magnitude during a frontal passage, compared to prefrontal values typical for a slightly stable ABL. A dissipation length scale commonly used by modelers is found to obey Monin-Obukhov similarity scaling near the ABL surface. The normalized first and second moments of the one-dimensional velocity spectrum conform to the scaling suggested by Kolmogorov's equilibrium hypotheses, even during the intense turbulence of the frontal passage. Calculations of TKE budgets were attempted as functions of time through the fronts. The dominant terms were found to be shear production and dissipation (shear production larger), although the magnitude of the residual term can also be significant. These results can be used in assessing the effects of turbulence in traditional semigeostrophic models of frontal collapse. The dissipation length results may be of particular use to modelers.