Moist Convection at a Surface Cold Front

Abstract

The motion of a surface cold front in an environment that is unstable to moist convection is studied with the aid ofboth hydrostatic and nonhydrostatic two-dimensional models. Simulations with the hydrostatic model essentially extend the work reported by Ross and Orlanski. It is shown that when deep convection occurs, Coriolis turning of the flow into the convective line creates a poleward low-level jet ahead of the front. It is also shown that after the generation and decay of the first convective element, another line develops on the order of a day later. It is found that the intensity of this line increases significantly ira north-south gradient of moisture is specified. The periodicity in convective activity at the front is explained in terms of an inertial gravity oscillation in the low-level convergence. The first convective system, which decays when the subeloud layer is dried out by the convection, forces a geostrophic imbalance in the surface front and the surrounding environment. In returning to geostrophic balance after the decay of the first system, the front and surrounding environment tend to oscillate as inertial gravity waves propagate away from the region of imbalance. It is shown that the convergence at low levels ahead of the front oscillates with a period of approximately 12 hours and that ascent returns to the frontal zone 6 hours after the decay of the first system.. The second system then develops when this low-level convergence destabilizes the atmosphere. The discrepancy between the inertial gravity wave period and the period between convective line generation obtained in the hydrostatic model (20-27 h) is explained by the crude representationof moist convection in the hydrostatic, filtered model. A nonhydrostatic model with horizontal resolution of 2.8 km is used to study the same system and a periodicity in convective activity is again found, this time with the second system commencing some 7 hours after the decay of the first. Finally, observations of cold fronts in the Midwest of the United States are analyzed to explore the importance of convective oscillations in the motion of surface cold fronts.