Dryline on 19 June 2002 during IHOP. Part I: Airborne Doppler and LEANDRE II Analyses of the Thin Line Structure and Convection Initiation

Abstract

The evolution and finescale structure of a dryline that initiated a line of thunderstorms is presented. Both the along-line variability and mean vertical structure were examined using data collected by an airborne Doppler radar and a water vapor differential absorption lidar (DIAL). The initiation of convection appeared to result from the diurnally induced easterly flow in the maritime air east of the dryline that typically develops late in the day. This flow increased the low-level convergence and allowed rising parcels of air to reach the level of free convection. The along-line variability was largely attributed to numerous misocyclones distorting the thin line of radar reflectivity by advecting dry (moist) air across the dryline south (north) of the misocyclone. The misocyclones also influenced the location of the updrafts, with most of the peak values positioned north of the circulations. As a result, these updrafts were fortuitously positioned in regions of high mixing ratio where the first convective cells initiated

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

“…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.…”

“…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. Numerical models have struggled to accurately capture the daily cycle of the PBL (e.g.…”

Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

“…Many studies have demonstrated that misocyclones along air mass boundaries distort the boundaries and produce local maxima of low-level convergence and vertical velocity in their vicinity, which often could be preferred regions to form clouds and/or radar reflectivity cores (e.g., Wilson et al 1992;Atkins et al 1995;Lee and Wilhelmson 1997b;Friedrich et al 2005;Arnott et al 2006;Murphey et al 2006;Marquis et al 2007;Wakimoto and Murphey 2009). Although well-defined vortices with quasi-circular vorticity contours do not develop in the present simulations, it is suggested that in the U15-S2 and U20-S2 cases the concentrated vertical vorticity distorted the horizontal wind field and enhanced the low-level convergence in the vicinity of the vorticity maxima, leading to formation of the precipitation maxima north of the bow apexes.…”