MESO-BETA-SCALE CIRCULATIONS IN REALISTIC FRONTS .2. FRONTOGENETICALLY FORCED BASIC STATES

This paper deals with the mesoscale circulations that may appear in a frontogenetically forced now. Two mechanisms are studied: conditional symmetric instability (CSI) and transient adjustment of a finite momen tum anomaly. The study is focused on two situations selected from fron tal events that were documented during the MFDP/FRONTS 87 field experi ment. In Part I of this paper, a successful numerical simulation of CS I during the second intensive observing period (IOP2) was presented. H owever, the growth rate of the perturbation was very small (approximat e to f/4) and 42 hours were needed before vertical velocities reached their maximum value approximate to 5 cm s(-1). Therefore, interactions between CSI and the large-scale frontogenetic forcing are expected; t hese are investigated in this Part. CSI is released (without significa nt increase of the growth rates) for small forcing only since confluen ce is found to shrink the area fulfilling the pure CSI criterion rapid ly. It is thus not likely to occur in active fronts. The IOP7 is then investigated. This very anisotropic front was embedded in a strong lar ge-scale confluence. The creation and maintenance of the observed 'Wid e Cold-Frontal Rainband' (WCFR) (some 100 km wide) above the sloping f rontal surface and behind the surface cold front are investigated. Usi ng a hierarchy of three-dimensional (3D) and two-dimensional (2D) vers ions of the model, this band is shown to be produced by the inertial, transient adjustment of low pseudo-angular-momentum air that was advec ted into the mid-troposphere by the frontal convective jet. The 3D, as well as the 2D, forced simulations produce a negative equivalent pote ntial vorticity chimney-like area which matches well with the observat ional data. This area evolves on the same time- and space-scales as th e roll-like circulation associated with the WCFR. This mechanism bears similarities with mature CSI and their distinction in diagnostic stud ies may be difficult. However, the presence of a momentum source (the frontal convection) is found here to be the important criterion. The m omentum-adjustment process first exhibited by Holt and Thorpe is thus shown to be efficient in association with frontal convection in produc ing anafront-type mesoscale bands. The study indicates that such a phe nomenon is more likely to occur in fronts than pure modal CSI.