A Proposed Relationship Between Nocturnal Convection and Bores as Determined from Observations Taken During the IHOP_2002 Project

Researchers have long established that summer thunderstorms and convective precipitation are most frequent not in the afternoon, but rather after sunset over a broad region of the Great Plains of the United States and Canada. Researchers have long documented that these nocturnal convective systems have public safety implications, as for example, significant flash floods in the eastern 2/3rds of the U.S. typically occur at night. In addition, nocturnal mesoscale convective systems produce injuries or fatalities through torrential rains and flash floods, high winds, hail, intense electrical storms and even tornadoes. Recent numerical modeling studies motivated in part by this nocturnal maximum in convective activity have proposed that bores can maintain convection in the presence of stable boundary layers. This study goes beyond the case study approach to investigate the relationship between bores and nocturnal convection over the Southern Great Plains using a climatological approach to data taken during the 2002 International H2O Project (IHOP). Our analysis examined 50 gravity currents, bores, solitary waves and gravity waves events associated with convective systems during June 2002. The most important goal of this research is to predict the likelihood and behavior of atmospheric bores in order to provide guidance on forecasting nocturnal convection.

Our study suggests that the cold outflows of nocturnal convective storms often produce bore-like features in the presence of a low-level jet and a stable, nocturnal boundary layer that surge ahead of the convective system. In some cases, the bores initiate convection along their leading edge, while in other cases convection continues well behind the leading edge of the bores for several hours. The bore structure is revealed from 13 bore events observed by the vertically profiling remote sensors at the IHOP Homestead site including NCAR's MAPR spaced antenna wind profiler. The passage of the IHOP bores are found to be associated with significant net ascent near and above the height of the low-level jet with net displacements ranging from several hundred meters to well over 1 km. This net ascent is characteristic of bore-like disturbances rather than solitary waves and is found to destabilize the lower troposphere to deep convection by creating convective available potential energy (CAPE) and removing convective inhibition (CIN) through layer lifting. Another finding from this analysis is that the bore structures are surprisingly variable with differences in the net displacements, undular vs. solitary regions of vertical motion, and events with and without evidence of significant mixing.

Wave-like disturbances are tracked in the lower troposphere using satellite imagery, regional Doppler radar composites including the S-Pol radar, surface analysis from the National Weather Service Network (NWS), Automated Surface Observing Stations (ASOS), Oklahoma mesonet sites, and three-hourly soundings with 2-second sampling frequencies from 5 DOE ARM sounding sites. During June 2002, more than 100 different surface observations capture pressure, temperature and wind shift traces characteristic of gravity currents, bores, solitary waves and gravity waves, recording properties about the wave events such as: location, time, type, pressure perturbation, speed, distance, and displacement heights. One finding from the climatology using wind roses and supplemental scatter plots of the wind direction versus time suggests that density currents during this period preferred to propagate towards the south-southeast, with a dominant trend towards propagation in the south-southeasterly direction. Bores seemed to have a similar track as density currents, but surprisingly with more directional variability. Bores as well propagated most often towards the south-southeast.

Aside from dimensional characteristics, the Scorer parameter, a proxy for wave trapping, provides information about the dynamic and thermodynamic structures of the environment before and after it passes a sounding site. Scorer parameter results suggest the possibility for multiple wave ducts to simultaneously exist at different heights; MAPR signal-to-noise ratio values seem to agree with this finding as well. A few case-by-case examinations of the Scorer parameter suggest the directional preference of bores may be due to the strong directional dependence the wind has on the Scorer parameter. They also suggested like the wind roses that, typically, the most favorable ducting direction was parallel to typical orientations of the nocturnal low-level jet.

Because our climatology has also provided a general look into the behavior of wave-like nocturnal events, we are currently examining the bores and their environment in greater detail aiming to bridge the gap between theory and observations leading toward a better understanding of: the ducting mechanisms for long-lived events, the impact of lifting on CAPE and CIN, the ability of bores to initiate and/or maintain convection and the differences between the various cases observed by MAPR. A primary goal of this research is to develop a sounding-based method for determining the likelihood of bore occurrence and how the subsequent bore will impact the longevity of nocturnal convection. From examining the pressure, temperature and wind fluctuations at various ASOS, NWS and Oklahoma mesonet stations, one hypothesis we intended to test is that variations in bore structure are in part due to their life cycle, which tend to produce undular structures later in their lifetime. It has been suggested previously by various authors that general disturbances along an interfacial stable layer will degrade into amplitude-ordered wave packets, so a search for an innate life cycle of density current-generated bores is currently underway.