Numerical Study of Precipitation Core-Gap Structure along Cold Fronts

The mechanism responsible for the core-gap structure of precipitation along narrow cold-frontal rainbands (NCFRs) is investigated through analyses of idealized cloud-resolving simulations of cold fronts. The control simulation, in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy (CAPE) of ∼60 J kg−1, reproduces the typical alongfront variability of observed NCFRs. The simulated NCFR is broken up into regularly spaced, ellipsoidal cores oriented at a clockwise angle to the cold front. While horizontal-shear instability (HSI) has frequently been proposed as a mechanism leading to the alongfront variability of NCFRs, no characteristic features of HSI are recognized in the simulated vertical vorticity field at the leading edge of the cold front. The alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front. The wave phase lines are oriented nearly perpendicular to the direction of mean vertical shear, with enhanced (suppressed) precipitation occurring at the wave updrafts (downdrafts). An analysis of the eddy kinetic energy budget indicates that the wavelike disturbance derives most of its energy from the mean vertical shear and the buoyancy. Sensitivity experiments reveal a systematic relationship between the alongfront variability of NCFRs and the stability of the prefrontal thermal stratification. Simulated precipitation cores remain essentially parallel to the cold front when the prefrontal environment is absolutely stable or almost neutral to surface parcel ascent. The typical alongfront variability of NCFRs is reproduced for weakly unstable environments with small amounts of CAPE (≤140 J kg−1). On the other hand, simulations with sufficiently unstable environments produce precipitation cores oriented counterclockwise to cold fronts.