Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 3:27 PM GMT on August 12, 2013
As a follow-up to my last blog regarding the hail-producing HP supercell in northwest Nebraska on August 3, 2013 (I recommend reading The Case of the Unwitnessed Hailstorm before you proceed), the storm that I presented on this 12Z mosaic of composite reflectivity (here's a closer look with counties) transitioned in a little over an hour to a bowing line segment of thunderstorms. For confirmation, check out, below, the 1330Z mosaic of composite reflectivity on August 3 (larger image).
The 1330Z mosaic of composite reflectivity on August 3, 2013. Larger image. Courtesy of Penn State.
Before you jump to any conclusions, supercells transitioning to bowing line segments is a fairly common evolution. And this kind of transition in storm mode (type) typically occurs when the outflow of cool air associated with the rear-flank downdraft (RFD) is both relatively strong and deep. In other words, such transitions occur when there's a strong RFD surge.
By way of review, there are two downdraft regions associated with the structure of supercells (see idealized schematic below). The rear-flank downdraft forms at the rear of the storm (obviously, not rocket science) in the vicinity of the rotating updraft (the mesocyclone) in response to drier air entraining (mixing) into the backside of the updraft. This entrainment of drier air promotes evaporative cooling and, in turn, increases negative buoyancy, which favors downward accelerations. Moreover, when hailstones and other icy hydrometeors (snow and graupel) get tossed into the rear-downdraft region, melting and sublimation of ice hydrometeors also help to cool the air and thereby increase negative buoyancy (melting of ice requires energy, which the surrounding air supplies).
An idealized schematic of a supercell thunderstorm. Courtesy of, and copyright by, the online Penn State Certificate Program.
Although I really can't say for sure what exactly caused the RFD surge on the backside of the HP supercell in northwestern Nebraska on August 3, I'm betting that the partial (or complete) melting of some hailstones probably played a role in enhancing the depth and strength of the rear-flank cold pool. The bottom line here is that the leading edge of the surging deep and strong RFD outflow initiated new thunderstorms, thus paving the way for the HP supercell to transition into a bowing line segment of thunderstorms early on August 3.
Below is the 1216Z image of storm-relative velocities (larger image; twin image of base reflectivity) I showed in my previous blog. This time I circled the footprint of the rear-flank downdraft...greens indicate negative (inbound) velocities. While speeds near 30 knots might not seem impressive, remember that the HP supercell was approximately 64 nautical miles from the radar site, placing the footprint of the RFD at roughly 6000 feet. Most cold pools have a depth of one kilometer, although cooler air flowing outward from thunderstorms have been as shallow as a few hundred meters and as thick as four kilometers (the depth of a cold pool largely depends on the longevity of the parent convection). Nonetheless, in the case of August 3, the footprint of the RFD at roughly 6000 feet (about two kilometers) was sufficiently large to convince me that rear-flank winds at lower attitudes were much stronger (keep in mind that the radar beam tilts upward and that the radar routinely misses strong low-level winds when the storm is relatively far from the radar site).
The 1216Z image of storm-relative velocities from the radar at North Platte, Nebraska (KLNX), on August 3, 2013. The white circle indicates the footprint of the rear-flank downdraft of the HP supercell that allegedly produced hail as large as four inches in diameter (or greater). Larger image. Courtesy of NOAA.
So, based on this image of storm-relative velocities, I believe there was an RFD surge associated with the HP supercell on August 3, 2013, and that the surging rear-flank gust front initiated new thunderstorms that paved the way for the supercell to transition into a bowing line segment of thunderstorms in a little over an hour.
Here endeth the lesson.
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