In the spirit of Hurricane Irene east of Florida right now…
I’ve never posted these ideas before, but I feel these are worth thinking about, for all the hurricane lovers. None of these, as far as I know, have been discussed extensively in the literature. Any, all, or none of these may have any merit….I do not have enough physics knowledge yet to say.
…in general, inversely proportional the the amount of atmospheric instability. Hurricanes with large eyes and more annular characteristics form in relatively low-instability and low-SST environments, such as the East Pacific or the Atlantic. Pinhole eyes tend to be more common in late October-November, since the lapse rates tend to be better later on in the season (the air temperature responds faster to a change of season than water temperature). They are also more common in regions of warmer water, such as the Tropical West Pacific or the Caribbean.
Another possibility is that anomalously low or high environmental pressures correlate to smaller and larger eyes, respectively. Pressures in most of the East Pacific and Atlantic basins are higher due to the strength of the subtropical high. However, in the West Pacific, the southeastern East Pacific, and the Caribbean, occasional monsoon troughs or westerly wind bursts lower the surrounding atmospheric pressures… and in these areas pinhole eyes are more common.
Eyewall Replacement, Initiation
From what I’ve observed, eyewall replacement cycles (ERCs) may not be completely random. Rather they seem to be responses to external perturbations in the environment. Factors below.
1) Diurnal maximum. Again, since water cools slower than air, lapse rates increase at night over the open ocean, which increases atmospheric instability. This increased instability may allow outer rings of convection to intensify and consolidate. Once this occurs, the inner eyewall gets choked of its inflow and the eyewall replacement essentially begins. (See Felix 2007 for a good example of a nocturnal initiation of an ERC.)
2) Dry air entrainment/shear. If they can penetrate far enough, these factors can destabilize the inner core. In a quasistatic hurricane I would believe that the inner eyewall is strong enough to induce localized subsidence around its periphery, preventing consolidation of an outer eyewall. Destabilizing the inner eyewall, thus, would allow outer eyewalls to consolidate more easily.
Large hurricanes tend to be very vulnerable to this. First, they have more outer rainbands that can form an outer eyewall. Also, more thunderstorms = more inflow drawn into the storm = greater susceptibility from pockets of dry air due to downsloping from a neighboring landmass or a stable airmass. Many Caribbean Cruisers go through multiple ERCs, and I hypothesize that the downsloping off Hispaniola and S. America may be to blame. For the same reason W. Pacific typhoons always seem to enter a stage of ERCs when they reach a certain latitude, when the surrounding environment becomes significantly more stable.
It’s the same idea for supercell updrafts, in fact. The more marginal the environment, the more susceptible the parent updraft is to cycling. The most powerful updrafts in the most intense environments remain quasisteady for a long time because the subsidence around the parent updraft kills any nascent updrafts in the flanking line. (For example, many tornadoes on 4/27 traveled over 100 mi – that’s takes 1.5 hr for a storm motion of 70 mph.)
3) Land. Many hurricanes seem to start ERCs just as they make landfall. This may not be coincidental. Air around the periphery of the hurricane slows down because land has more friction than water, so the air piles up and converges, which aids in lift. Besides the drier land air destabilizing the inner core, this frictional convergence can help initiate the development of an outer eyewall just as the hurricane nears/moves onshore.
4) Stability of Eyewall. This IS in the literature I think… the circumference of eyewalls increase proportional with eye radius. Pinhole eyes have fewer thunderstorms due to the small eyewall circumference, making them susceptible to even small perturbations. Moreover, the close proximity of individual thunderstorms within the eyewall increases the probability of negative interaction between them. Small eyewalls are just inherently unstable and generally will collapse with time.
Eyewall Replacement, Completion
The better the environment, the faster they get done. The factors that destabilize an inner eyewall do more damage on a more exposed, weaker fledgling outer eyewall. And as long as the outer eyewall remains weak, the inner eyewall still has life. So the storm seems to keep going on an endless ERC, when in fact neither eyewall obtains dominance because of dry air, shear, or cooler water.
Land can actually help finish an ongoing ERC. Frictional convergence affects outer rings of convection first, namely the outer eyewall – so a disorganized outer eyewall can get an extra boost, close off, contract, and kill off the inner eyewall just as the storm comes ashore. In these instances it can seem like the storm is strengthening on land, when in fact frictional convergence has simply helped connect loose ends. This temporary boost doesn’t last long though. (A similar morphology can sometimes be seen in storms that have weak, loose cores – see Fay/Ike 2008, among others.)
An Anti-Beta Effect
The Coriolis force varies with latitude (its stronger at the poles), so hurricanes have an impetus to move north of due west when other steering factors are absent. This is known as the Beta Effect. But are there instances when the tendency is to resist poleward acceleration?
I think so. Consider the relative inertias of differently-sized hurricanes. I suspect larger hurricanes will be harder to turn poleward just by Newton’s 2nd Law – even if the Coriolis force is fictitious. When a steering influence is weak or highly temporal, that may be the difference between a recurvature and a turn back west, though I have no direct evidence to substantiate that claim.
I should really include more of these to scatter throughout blog posts…