Meso‐β‐scale circulations in realistic fronts. II: Frontogenetically forced basic states

Abstract

AbstractThis paper deals with the mesoscale circulations that may appear in a frontogenetically forced flow. Two mechanisms are studied: conditional symmetric instability (CSI) and transient adjustment of a finite momentum anomaly. the study is focused on two situations selected from frontal events that were documented during the MFDP/FRONTS 87 field experiment.In Part I of this paper, a successful numerical simulation of CSI during the second intensive observing period (IOP2) was presented. However, the growth rate of the perturbation was very small (≈ f/4) and 42 hours were needed before vertical velocities reached their maximum value ≈5 cm s−1. Therefore, interactions between CSI and the large‐scale frontogenetic forcing are expected; these are investigated in this Part. CSI is released (without significant increase of the growth rates) for small forcing only since confluence is found to shrink the area fulfilling the pure CSI criterion rapidly. It is thus not likely to occur in active fronts.The IOP7 is then investigated. This very anisotropic front was embedded in a strong large‐scale confluence. the creation and maintenance of the observed ‘Wide Cold‐Frontal Rainband’ (WCFR) (some 100 km wide) above the sloping frontal surface and behind the surface cold front are investigated. Using a hierarchy of threedimensional (3D) and two‐dimensional (2D) versions of the model, this band is shown to be produced by the inertial, transient adjustment of low pseudo‐angular‐momentum air that was advected into the mid‐troposphere by the frontal convective jet. the 3D, as well as the 2D, forced simulations produce a negative equivalent potential vorticity chimney‐like area which matches well with the observational data. This area evolves on the same time‐and space‐scales as the roll‐like circulation associated with the WCFR. This mechanism bears similarities with mature CSI and their distinction in diagnostic studies may be difficult. However, the presence of a momentum source (the frontal convection) is found here to be the important criterion.