INFLUENCE OF STRONG GRADIENTS OF WIND AND HUMIDITY ON FRONTAL AGEOSTROPHIC CIRCULATIONS

The impact is studied of strong gradients of wind and humidity on the ageostrophic circulation developed around a conceptual cold-frontal sy stem. The simulated basic wind and temperature fields are chosen to be st represent the frontal observations made during the Fronts 87 field experiment and, in particular, to take account of the presence of stro ng low-level jets. The numerical solution of the diagnostic Sawyer-Eli assen equation in both its adiabatic and diabatic forms permitted us t o test the impact of the wind and humidity structure on the correspond ing ageostrophic circulation. In particular, strong negative shear abo ve the low-level jet core enhanced subsiding motions on the warm side of the front, which restricted the horizontal extent of the cloud cove r. On the other hand, the presence of well-marked horizontal humidity gradients aloft, at the rear edge of the cloud cover, enhanced the asc ending motions ahead of the frontal surface, creating locally a second ary maximum of upward motions. The degree of stability within the pref rontal boundary layer was found to be of major importance with regard to the intensification of the upright convection near the leading edge of the front. Moreover, the incorporation of a parametrization scheme , representing the Ekman pumping at the top of the boundary layer, enh anced the diagnosed circulation ahead of the front. A comparison with observations made during two case-studies of Fronts 87 seems to confir m these results.