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 , 4:57 PM GMT on February 06, 2013
The 12Z surface analysis on January 30, 2013 (close-up analysis), indicated a "squall line" just ahead of a cold front slicing through the Ohio and Tennessee Valleys. Sure enough, the corresponding 12Z mosaic of composite reflectivity (below; larger image) displayed a very narrow line of relatively high reflectivity (formally, a narrow cold-frontal rainband...more in a few moments).
So this was not a squall line in any traditional sense, assuming that you accept the AMS definition hook, line and sinker (the AMS glossary defines a squall line as a continuous or broken line of active thunderstorms). Indeed, the lightning data from 11Z to 12Z on January 30 (courtesy of WSI) indicated that there weren't any cloud-to-ground strikes in the vicinity of this so-called squall line.
The 12Z mosaic of composite reflectivity on January 30, 2013, shows a classic narrow cold-frontal rainband (NCFRB) stretching from south-southwest to north-northeast across Ohio and Kentucky (focus your attention on the thin, red ribbon of roughly 50 dBZ reflectivity). Larger image. Courtesy of WSI and Penn State.
I'm wondering how literally the AMS requirement of thunder should be interpreted here. For the record, there are tornadic supercells that are bereft of CG (cloud-to-ground) and IC (in-cloud) lightning. So, if we follow the letter of the law, these tornadic supercells weren't even "thunderstorms" because they didn't produce lightning and thunder. Yet, dynamically speaking, they are, without reservation, supercells (deep, moist convection driven by rotating updrafts). Confused? I'm not surprised. Such aberrant supercellular storms develop in atypical environments (read more), where the microphysics in these convective clouds simply does not lend itself to a separation of charge necessary to spark lightning. The inner cores of hurricanes also have little to no lightning, yet they are typically described as large conglomerations of tropical thunderstorms.
The bottom line here is that the line of convection in Ohio and Kentucky on January 30, 2013, was not a squall line according to a strict interpretation of the AMS definition. In my view, however, this definition is ostensibly too strict. Why not substitute "deep, moist convection" for "thunderstorms?" I realize that "deep, moist convection" is a bit unwieldy to continually write or say, but, if the line of convection on January 30 doesn't qualify as a squall line, then I think there's a problem. That's because the term, "squall," generically refers to blustery, showery weather. So a "squall line" could be a line of blustery, showery weather. And that's exactly the kind of weather the line of convection produced on January 30, 2013 (note the reports of damaging winds from the Storm Prediction Center in southern Ohio, Kentucky, western Pennsylvania, etc.).
The 12Z NAM model analysis of 1000-mb streamlines on January 30, 2013. Note the confluence of streamlines (shift in wind direction) over Ohio and Kentucky. Larger image. Courtesy of Penn State.
The debate about the AMS definition of squall line aside, the line of convection on January 30 was, as I hinted earlier, a narrow cold-frontal rainband (NCFRB). As their name suggests, NCFRBs are noticeably narrow, with the distance measured perpendicular to the line as small as one to two kilometers. Narrow cold-frontal rainbands form along a surface cold front where there's a noticeable shift in wind direction. For confirmation, check out the NAM model analysis of 1000-mb streamlines at 12Z on January 30, 2013 (above; larger image). Updrafts in narrow cold-frontal rainbands can be intense, with speeds reaching 10 meters per second (about 22 mph) at relatively low altitudes (one kilometer or so). Despite these speedy updrafts in the lower troposphere, the depth of the convection is, on average, three kilometers.
To get a sense for the vertical structure of temperature in an environment conducive for a narrow cold-frontal rainband, check out (below) the 12Z skew-T at Wilmington, Ohio (KILN) on January 30, 2013, where the NCFRB passed at roughly this time (see meteogram). Note the "EL" on the left of the skew-T (circled in red)...it stands for the equilibrium level, which is the altitude at which the temperature of a rising air parcel (lifted from the surface) equals the temperature of its environment. In other words, the equilibrium level is the altitude where a rising air parcel becomes neutrally buoyant and stops accelerating upward. Put another way, the equilibrium level is a proxy for the tops of convective clouds. So, yes, the depth of convection associated with narrow cold-frontal rainbands is relatively shallow.
The 12Z skew-T at Wilmington, Ohio (KILN), on January 30, 2013. The temperature sounding is red, and the dew-point sounding is green. Note the altitude of the equilibrium level (EL), which is a proxy for the tops of the convective clouds associated with the narrow cold-frontal rainband over southwest Ohio at this time. Courtesy of the University Center of Atmospheric Research.
If you look closely at KILN's skew-T, you'll see an "M" at 500 mb. The "M" marks the local moist adiabat that an air parcel "follows" as it rises above the lifting condensation level (LCL). Obviously, a little background for my readers is in order here. For the record, the lifting condensation level is the altitude at which net condensation is set to begin inside a rising parcel of air. In other words, a rising air parcel becomes saturated at the LCL. Above the LCL, where net condensation is ongoing and clouds develop (see photograph), the air parcel still expands and cools as it rises, but the release of latent heat of condensation now reduces the parcel's cooling rate. The moist adiabats on a skew-T correspond to these reduced cooling rates (dashed green curves; revisit the primer flash animation that I showed you in an earlier blog). Not surprisingly, the rate of cooling associated with moist adiabats is formally called the moist adiabatic rate. As you can tell from the various slopes of the moist adiabats, the moist adiabatic lapse rate is not a constant.
At any rate, I drew, in black on this zoomed-in version, the local moist adiabat that rising parcels followed between the LCL and EL over Wilmington, Ohio, at 12Z on January 30, 2013. In this case, rising air parcels were slightly negatively buoyant (parcels were slightly colder than their environment). As a general rule, the typical environment conducive to a cold frontal rainband is nearly neutrally stratified. The bottom line here is that CAPE, the convective available potential energy, is zero (or nearly so) for environments where narrow cold-frontal rainbands form. Note that CAPE was zero over KILN at 12Z on January 30, 2013 (along the top of the skew-T).
With little or no CAPE, how do updrafts in narrow cold-frontal rainbands get so strong? Recall that low-level updrafts can reach 10 meters per second = 22 mph. As it turns out, the updrafts of NCFRBs are primarily driven by vertical gradients of pressure perturbations, a topic that's highly mathematical and beyond the scope of this blog.
The 1430Z base reflectivity from Pittsburgh on January 30, 2013, shows the core-and-gap structure of the narrow cold-frontal rainband over eastern Ohio and western Pennsylvania. Larger image. Courtesy of NCDC.
One of the telling characteristics of narrow cold-frontal rainbands is their tendency to develop kinks and breaks that are sometimes described as a core-and-gap structure. To see what I mean, check out the 1430Z base reflectivity from the radar at Pittsburgh (above; larger image) on January 30, 2012 (2.5 hours after the radar image at the top of this page). These gaps result from meso-gamma vortices (horizontal swirls of air whose spatial scale is on the order of several kilometers) that fracture the updrafts along the surface cold front. These fractures create the alternating precipitation cores and gaps in the narrow cold-frontal rainband shown on the Pittsburgh radar above.
Where do these meso-gamma vortices come from? Good question. As it turns out, a narrow cold-frontal rainband lies in a zone of strong horizontal wind shear (a change in wind speed over a specified horizontal distance) associated with a low-level jet stream that lies ahead, and nearly parallel to, the surface cold front. Check out, below, the NAM model analysis of 850-mb streamlines and isotachs (in knots) at 12Z on January 30, 2013 (larger image). Note that the low-level jet stream paralleled the surface cold front and that 850-mb wind speeds exceeded 60 knots over Ohio and Kentucky. The dramatic change in wind speed over the horizontal distance between the western edge and the core of the low-level jet stream created horizontal wind shear and meso-gamma vortices (see my idealized schematic). You can see the footprint of the horizontal wind shear associated with the low-level jet stream in the 12Z NAM model analysis of 850-mb vorticity (a measure of the rate of spin) on January 30, 2013.
The 12Z NAM model analsis of 850-mb streamlines and 850-mb isotachs (in knots) on January 30, 2013, shows a low-level jet stream just ahead of, and parallel to, the surface cold front. Larger image. Courtesy of Penn State.
These meso-gamma vortices are likely the ones being amplified to tornado strength whenever narrow cold-frontal rainbands spawn a twister. Tornadoes in NCFRBs are relatively rare. Indeed, the most serious threat posed by narrow cold-frontal rainbands is damaging straight-line winds.
I hope you learned something new. Narrow cold-frontal rainbands are not discussed very often on television because the term, squall line, is routinely used instead. That's a shame because "squall line" just doesn't do a narrow cold-frontal rainband any justice, in my humble opinion.
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