5.3
Multiple Remote Sensor Observations of Gust Front to Bore Transition
Kevin Knupp, University of Alabama, Huntsville, AL
On 21 July 2002, a nocturnal gust front was observed by a network of special sensors that were set up in the Oklahoma panhandle as part of the International H2O experiment (IHOP-2002). As the gust front moved over the IHOP domain during the 0300-0600 UTC period, the nocturnal boundary layer became increasingly stable, and the gust front transformed to a turbulent, and then laminar bore.
This paper will provide an integrated analysis summary of comprehensive measurements of this transition, and the corresponding changes in ABL properties that promoted the gust front to bore transition. Special measurements were provided by the NCAR S-Pol dual polarization radar, the UAH Mobile Integrated Profiling System (915 MHz profiler, 2 kHz sodar, 12-channel microwave radiometer, lidar ceilometer), a suite of profiling instruments set up 14 km east of S-Pol (NCAR ISS, the UW AERI, NASA Raman and GLOW lidars), and surface instrumentation, including a north-south array of NCAR surface flux stations.
The NCAR S-Pol radar provided detailed measurements on the evolution of a vigorous gust front that propagated eastwards into an air mass that was becoming increasingly stable as the nocturnal boundary layer formed. The S-Pol data provide details on the evolution of the gust front, and reveal that even by 0300, the cloud field associated with the gust front was about 30 km wide. With time, the radar fine line transitioned from a wavy structure to a more linear and wider feature as the bore developed immediately ahead of the gust front. During passage over the MIPS and other profiling instruments, the bore was in a formative stage, immediately in advance of the gust front. The combined disturbance exhibited a turbulent structure in which the cloud updraft (0.5 m s-1) was laminar, transitioning to a turbulent structure with 2-4 m s-1 small-scale updrafts/downdrafts as the core of the disturbance passed over two 915 MHz wind profilers. A significant water vapor perturbation preceded the surface wind shift. Integrated water vapor after the bore passage remained at levels that exceeded the integrated water vapor values before arrival of the disturbance, even though low level drying was measured at low levels by surface instrumentation, the 12-channel microwave radiometer, and the AERI. Within about one hour after bore passage, P-Pol and surface data indicate that the disturbance evolved to a laminar bore with one primary updraft, and only minor oscillations in low level wind.
.Session 5, Observational Field Experiments
Tuesday, 11 January 2005, 11:00 AM-12:00 PM
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