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 Ϫ1 , 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.

Multi-platform Observations Characterizing the Afternoon-to-Evening Transition of the Planetary Boundary Layer in Northern Alabama, USA

“…In the presence of an existing convergence feature, the gradual increase in the flow atop the surface layer could aid a parcel's ability to reach its level of condensation or free convection. While it is impossible to say this study confirms the diurnal dryline process examined by Jones and Bannon (2002) as a mechanism for the delay in convective initiation along an existing boundary until the afternoon-to-evening transition, our observations indicate the plausibility of a generalized low-level convergence enhancement resulting from AET effects. Related ongoing work includes multi-platform observational analysis and numerical simulation of observed AET convective enhancement and initiation case studies along pre-existing boundaries.…”

“…Summer has a decreased likelihood of migration activity (i.e., more random flight directions of biota are more likely), thus it is possible our summer results here are more robust. The increase in mean horizontal wind speed observed above the surface layer during the transition may play a role in enhancing existing horizontal wind convergence, as found along boundaries such as drylines, and could promote the invigoration of convective elements, similar to that shown by Jones and Bannon (2002) and Murphey et al (2006).…”

“…Still, in the mean a steady convergence increase from −0.85 × 10 −3 s −1 at 3 h before sunset to + 0.69 × 10 −3 s −1 at 2 h after sunset occurs and is statistically significant at the 0.01 probability value. This signature may relate to an increase in pre-existing low-level convergence along drylines shown by Jones and Bannon (2002) and could act in specific circumstances to aid in convective initiation or enhancement, akin to the Murphey et al (2006) case study.…”

“…As mentioned above, previous work has indicated a potential for increases to existing horizontal wind convergence atop the surface layer during the AET period, and a resulting enhancement effect on existing convective elements (Jones and Bannon 2002;Murphey et al 2006;Bluestein 2008). To help address this possibility, we consider the mean vertical motion, profile of horizontal wind magnitude, and radar-derived meso-γ scale convergence for the clear-sky, light wind conditions that best display the hallmark signatures of the afternoon-toevening transition.…”

“…Bluestein (2008) noted the common delay in convective initiation along southern USA Plains drylines until near sunset, coining the term "6:00 magic." Simulations described by Jones and Bannon (2002) show that the location and propagation of drylines in the southern Plains are highly dependent on the magnitude and changes of the surface heat flux, with a peak in inversion height and increase in low-level convergence ahead of the boundary occurring at sunset. Analysis by Murphey et al (2006) presents an example of dryline convective initiation triggered by enhanced low-level convergence in the AET period.…”