Good, insightful discussions, as usual.
Yep. It appears that insufficient thunderstorms (quantity and strength) conspired to keep this little invest out of the record books. Actually, we can blame it on the insufficient SST's.
More than most people realize, other things being equal, the intensity and depth of oceanic thunderstorms is closely correlated with the temperature of the underlying sea surface.
When I found that the SST's near 92L were far too cold, I knew the likelihood of true (pure) tropical cyclogenesis was nearly nonexistant.
It really does take SST's of nearly 80 F (27 C) to have enough 'vapor pressure' to produce the 'kind' of thunderstorms (very rich in moisture), which, when sufficiently organized, can release enough 'latent heat of condensation' over a large-enough area and over a long-enough time span to actually WARM the middle layers of the atmosphere sufficiently enough for significant mid-level pressure falls to occur.
And the whole system, through a 'positive convective-feedback mechanism' (more, stronger, deeper and even better organized thunderstorms) continue until surface pressures head south of the border. Eventually, the converging, cyclonic windfield strengthens (because of the lowering central pressures and a 'tightening' pressure gradient) to the crucial threshhold of 30 Knots and Viola! A depression is born. Thanks, essentially, to a bunch of VERY intense thunderstorms acting in concert over a large enough area and over a long enough time scale to generate a self-sustaining warm core. Tropical cyclogenesis in a nutshell.
(Please realize that the above 2 paragraphs do not apply to 92L, as it was already a well-developed cyclone of NON-tropical origins, and already had a well-established cyclonic wind field of tropical-storm-strength, BUT resulting from 'baroclinic' (cold core) 'extratropical' processes.)
Bottom line: To acheive Tropical Depression (true warm-core cyclone) status requires a LOT of sufficiently deep and strong thunderstoms to impart a sufficient amount of 'latent heat' to the mid-atmosphere to begin lowering pressures and get the whole thing started.
And only SST's of at least (around) 80F can produce copious amounts these types of deep and extremely moist thunderstorms. We're not talking your average, garden-variety thunderstorm. These are truely the tropics' "hot towers", for the amount of heat they release into the middle and upper layers of the atmosphere, with cloud tops colder than -80 C and 'convective turret penetrations' well into the stratosphere, at more than 60,000 feet!
Ultimately, it was the laws of physics (vapor-pressure temperatures of the sea surface water), that precluded the possibility of the type of sustained, deep thunderstorm activity needed to establish that all-essential warm core.
Although 92L certainly had strong enough winds and low enough central pressure to qualify as "Tropical Storm Force", but it never quite acquired the *essential warm core* required to qualify as a tropical system. Not with those sniggling SST's.
Moral of the story: Appearances can be deceiving. We've all watched many non-tropical systems which "look" quite striking, even hurricane-like, even sometimes with an eye-like feature. But an impressive-looking swirl of clouds is *sometimes* just that. Extratropical yes, but decidedly not the same animal as a TC or Sub-TC, despite appearances.
One should look into the thermodynamic processes occuring in the mid-levels of the low, to determine if a warm-core transition is occuring (or not), or even likely to occur (or not), based on the degree/extent/moisture content/depth/organization of the thunderstorms, which in turn, are 'based' on the underlying SST's.
Take away: Vapor Pressure Temperature increases *dramatically* around 80F, so the air can hold *much more* water vapor, the very fuel needed to spawn and sustain true tropical cyclones.
Ah, that essential 27 C isotherm. The Encroaching 27 C Isotherm Coming soon to a coastline near you.
So, you may ask: What's in a name? A Warm Core! (Or sometimes neutral. See below post!)
Edited by CoconutCandy (Fri Jun 05 2009 11:02 AM)
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