Airborne Observations of a Front near a Col during FASTEX

Abstract

An analysis of an oceanic front situated near a col defined by the surface pressure field is presented. There have been few observational examples of this type of front presented in the literature. The primary source of information for this study was data recorded by an aircraft equipped with a Doppler radar. The front was approximately two-dimensional and the cross-frontal scale at low levels was 30?40 km. A prefrontal low-level jet was identified in the high-resolution analyses and was shown to be supergeostrophic. Surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance (TWI) in the alongfront direction. Large negative values of TWI (the vertical shear is less than predicted for the given horizontal temperature gradient) were located near a region of frontogenesis. The strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in geostrophic balance at the time of the observations.