A Mixed-Layer Model of the Diurnal Dryline

Abstract

This study examines the diurnal behavior of the dryline system using a mixed-layer model to represent the cool moist air capped by an inversion to the east of the line. This inversion is referred to as the dry front, and the intersection of this dry front with the terrain is the dryline. The results indicate that boundary layer heating is sufficient to drive the dryline and explain its diurnal variation. The daytime eastward propagation of the model dryline of 200 km agrees well with other numerical studies and is in approximate agreement with dryline observations. The present model results also indicate a nearly vertical inversion slope up to a height of 2 km in the early afternoon. Model simulations with sloping terrain consistently yield a nocturnal low-level jet between 0000 local time (LT) and 0100 LT, with a speed of 20?25 m s?1, located below the inversion. The effect of each mixed-layer process, such as entrainment, surface heat flux, and nighttime cooling, is examined. Entrainment tends to steepen the slope of the dry front near the dryline but has little impact on its eastward advance. The dryline advance is most sensitive to the amplitude of the surface heat flux relative to the depth of the mixed layer and the strength of the inversion. Large heat fluxes, in combination with a shallow mixed layer and a weak inversion, produce the greatest dryline advance. The westward surge of the dryline at dusk is most sensitive to the amplitude of the nighttime cooling: larger cooling produces a larger surge. The model simulations consistently predict a local maximum in the inversion height (called a spike) near the dryline at dusk associated with entrainment and boundary layer convergence. This process may be one of the possible triggers for the deep convection often seen just to the east of the dryline.