A Numerical Study of the Effects of Differential Cloud Cover on Cold Frontal Structure and Dynamics

Abstract

The effects of sensible heating and momentum mixing on the low-level structure and dynamics of a two-dimensional cold front are studied with a hydrostatic primitive equation model. Effects of inhomogeneous heating arising from a contrast in low-level cloud cover across the front are emphasized. The relative importance of grid resolution and the choice of method for parameterizing planetary boundary layer (PBL) processes in the model are also examined. Frontal updraft dynamics are studied in terms of the following inquiries: (a) the relative importance of turbulent momentum transport, differential sensible heating, and the reduction in static stability in the heated region ahead of the front; (b) the nature of the interaction between the adiabatic, semigeostrophic frontal circulation and the thermally forced circulation; and (c) possible roles played by dry symmetric instability and density current dynamics. The terms in the frontogenesis and divergence budget equations are computed to determine the relative roles played by the various physical and dynamical processes in generating the frontal secondary circulation system. A strong, narrow updraft jet forms in the presence of uniform sensible beating across the front. Although the greatest impact on frontogenesis occurs as a response to the reduction in static stability resulting from uniform sensible heating, additional forcing results from the nonlinear interaction between the adiabatic frontal circulation and the thermally forced circulation arising from a cross-front gradient in heating (due to the introduction of an overcast low cloud deck behind the front). The relative importance of inhomogeneous heating, however, increases with the grid resolution and the use of a multilevel treatment in place of bulk mixed-layer PBL models. Numerical experiments reveal that symmetric instability does not create the updraft jet, despite the development of negative potential vorticity ahead of the surface cold front. Highly unbalanced dynamics and a density current-like “feeder flow” behind the cold front are strongly indicated in the presence of sensible heating effects. Budget analyses show that the frontogenetical effect of sensible heating is only indirectly important through its strengthening of the confluence (convergence) field. The nonlinear and unbalanced ageostrophic vorticity terms in the divergence budget equation exert the strongest controls on the development of the updraft jet when sensible heating is nonuniform. These results suggest that differential cloud cover across cold fronts may promote the development of frontal squall lines. Nonhydrostatic models that include explicit prognostic equations for microphysics and use improved parameterization of boundary layer fluxes in the presence of clouds are needed to more fully address this possibility.