Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 3:14 PM GMT on September 23, 2013
Back in 2005, I wrote an essay in the Bulletin of the American Meteorological Society (BAMS) outlining the pitfalls of deterministic medium-range forecasts (this pdf file contains my essay). After my essay was published, BAMS published a response (pdf file) from several television broadcasters. In my opinion, their response seemed to defend "popular" expressions such as "warm air holds more water vapor" and "clouds act like a blanket at night." Both of these expressions are scientifically inaccurate, in my opinion, and I'll add the latter topic to my list of future blogs (here's Alistair Fraser's debunking of the former expression). To me, the most disconcerting aspect of the response by the broadcasting industry to my essay was that, by publishing the response, the American Meteorological Society seemed (at least to me) to tacitly endorse the validity of these two "popular" but scientifically inaccurate expressions.
So I was not surprised when I received an e-mail last week from a media professional, protesting my assertion that relatively strong vertical wind shear doesn't simply blow, advect, carry, transport (whatever similar verb you like) thunderstorms organized around the core of a hurricane away from the storm's low-level center of circulation (here's the relevant blog). Apparently, my argument about the time scale of core thunderstorms, which as you recall, was on the order of 30 to 60 minutes, didn't register with him. Time scales are important here because, by the time thunderstorms would be transported sufficiently far downshear (in the case of Humberto, roughly to the east) so that we would notice on satellite imagery, core thunderstorms would have long dissipated. No, the story is much more complicated than vertical wind shear simply transporting thunderstorms horizontally away from the core.
The 18Z enhanced infrared satellite image of Tropical Storm Humberto on September 16, 2013. At the time, relatively strong vertical wind shear disrupted the symmetry of Humberto's warm core, with the vortex associated with the low-level circulation decoupling from the upper-level vortex. Note high, cold cloud tops of tall thunderstorms downshear (roughly east of) the storm's low-level circulation. Larger image. Larger unannotated image. Courtesy of Penn State.
In my first Humberto blog, I pointed out that relatively strong vertical wind shear destroys the symmetry of the warm-core structure of a tropical cyclone. My goal today is to explain how relatively strong vertical wind shear weakens tropical cyclones in a slightly different way, drawing on satellite imagery so that readers can observe firsthand the point I'm trying to drive home. By way of review of Humberto, check out the 18Z analysis of deep-layer vertical wind shear on September 16, 2013, (at the time, Humberto was a tropical storm). Now focus your attention (above; larger image) on the corresponding enhanced infrared satellite image (18Z on September 16). Note the faint, grayish "doughnut" that indicates the relatively warm cloud tops in Humberto's low-level circulation. Also notice the high, cold cloud tops of tall thunderstorms that were separated from the storm's low-level circulation at this time. Contrary to television explanations that I heard, these storms were not transported eastward away from Humberto's low-level circulation.
For this case, a series of pictures is worth a thousand words. Check out (below) the animated gif of visible satellite images (courtesy of CIMSS; larger loop) showing the separation of the low-level circulation of Humberto from the deep thunderstorms that we typically observe around the core of a mature or developing tropical cyclone. The overarching message I want you to take away from this loop of visible satellite images is that the low-level circulation and the deep thunderstorms decoupled from each other. This decoupling is consistent with my previous argument that relatively strong vertical wind shear destroys the symmetry of the warm core of a tropical cyclone. Let's investigate.
A sequence of visible satellite images on September 16 showing the vortex housing Tropical Storm Humberto's low-level circulation decoupling from the upper-level vortex and the deep convection (thunderstorms) that formed downshear (roughly east of) Humberto's LLC. Larger loop. Courtesy of the Cooperative Institute for Meteorological Studies (CIMSS).
To explain this decoupling, I point out that relatively strong vertical wind shear over the core of a tropical cyclone means that the high-altitude outflow is constrained to a single channel. For example, westerly shear constrains high-level outflow on the western flank of the tropical cyclone (outflow on the western flank becomes limited or "constrained", the that the effective outflow channel lies on the eastern flank of the storm). At any rate, relatively strong winds at high altitudes advect (transport) cyclonic angular momentum over the core of the tropical cyclone. The arrival of cyclonic angular momentum is bad news for the upper-level high-pressure system (anticyclone) sitting atop the tropical cyclone. Indeed, imported cyclonic angular momentum weakens the upper-level high, which, in turn, disrupts (chokes off) the tropical cyclone's high-level anticyclonic outflow. This disruption of high-level outflow feeds back to the ocean surface, where barometric pressure increases, low-level convergence decreases, and eyewall convection weakens. With weakening convection in the eyewall, the vertical transport of cyclonic angular momentum also decreases (weakening updrafts), and the vortex in the lower troposphere eventually decouples from the upper-level vortex. I believe this decoupling is what you're seeing on the animated gif of visible satellite images above.
With regard to vorticity, vertical wind shear produces differential positive vorticity advection downshear of the tropical cyclone (in the case of Humberto, roughly to the east). By "differential" I mean positive vorticity advection at two different pressure levels (500 mb and 1000 mb, for example). This differential PVA promotes low-level convergence and upward motion downshear of the low-level circulation (in the case of Humberto, roughly east of the LLC). It is here that new showers and thunderstorms develop. They are not advected or carried downshear from the core. Rather, new thunderstorms develop in response to the differential positive vorticity advection and the displaced low-level convergence and upward motion.
Yes, I agree, it's very complicated, and I would have never offered such an explanation whenever I was on-air (believe it or not, I was on television for almost 20 years (Weather World at Penn State). And so I have empathy for on-air folks. So a fair question becomes: How would Grenci explain it on television? Given that large-scale convection in the Tropics develops most readily in weakly sheared environments (especially compared to convection in the middle latitudes), I'd be okay with relatively strong vertical wind shear "blowing off the tops of thunderstorms" around the core of a tropical cyclone and new convection developing downshear of the low-level circulation in response to displaced low-level convergence and stronger upward motion.
You might think that I'm nitpicking here, but I firmly believe my shortened explanation is closer to the scientific truth than the one that relies on core thunderstorms simply being transported away from the core by winds associated with relatively strong vertical wind shear.
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