楼主#更多 发布于：2014-11-16 14:20
Final Thoughts on Supertyphoon Haiyan
Like the outbreak of tornadoes over the Middle West on November 17, I have some final thoughts about Supertyphoon Haiyan (Yolanda), which ravaged the Philippines on November 8. Specifically, my general goal here is to update readers on the science of the eye of hurricanes/typhoons and the processes by which tropical cyclones intensify.
The 0657Z color-enhanced infrared image of Supertyphoon Haiyan from the MTSAT geostationary satellite on November 7, 2013. Courtesy of the Naval Research Laboratory.
I'll focus on Haiyan's eye primarily because of the striking satellite presentation of the supertyphoon as it bore down on the Philippines...check out, above, the color-enhanced infrared image from the MTSAT geostationary satellite just before 07Z On November 7 (07Z is 3 P.M., PHT). Then I'll generalize my discussion to update the most current science regarding the eyes of relatively strong tropical cyclones as well as the connection to tropical cyclogenesis. For the record, tropical cyclogenesis is the process by which tropical cyclones form and intensify (this blog will focus on the latter). Finally, given that the explanation relying on Conditional Instability of the Second Kind (CISK) is now outdated, I'll elaborate on WISHE (Wind-Induced Surface Heat Exchange), which is much more widely accepted as a theory for tropical cyclogenesis than CISK (which is now mostly viewed as outdated and, in some circles, just plain incorrect).
The 2235Z microwave image (91 Gigahertz) of the then Tropical Storm Haiyan on November 4, 2013 (microwave data were measured by the Special Sensor Microwave Imager/Sounder (SSMIS) on the DSMP F-18 satellite). Courtesy of the Naval Research Laboratory.
Haiyan's satellite eye (sometimes different from the aircraft eye...the definition of an aircraft eye appears in this recently published paper) first became apparent on microwave imagery on November 4...check out the 2235Z 91-Gigahertz image (above) from the Special Sensor Microwave Imager/Sounder (SSMIS) on the DSMP F-18 satellite. Recall, from a previous blog, that microwave imagery has the capability of revealing the structure of the core of a tropical cyclone even when it's shrouded by high clouds. To see my point, check out the standard 2230Z visible image (here's the zoomed version) and the 2313Z infrared image on November 4. So, on standard satellite imagery, Haiyan's eye structure was not discernible from space around the time as the microwave image above.
The cross section of temperature differences between the Supertyphoon Haiyan's core and the storm's environment, in Kelvins (essentially Celsius), on November 7, 2013 (data were measured by the Advanced Microwave Sounding Unit (AMSU) on the NOAA-18 polar-orbiting satellite). Courtesy of NOAA and CIMSS.
Okay, take a look (above) at the cross section of temperature differences between the storm environment and Haiyan's center (in Kelvins) around 07Z on November 7 (the same time as the infrared satellite image I showed at the start of my blog). The data were measured by the Advanced Microwave Sounding Unit (read more) on the polar-orbiting NOAA-18. What jumps out at me is the strength of Haiyan's warm center (compared to the surrounding storm environment)in the mid- to upper troposphere. Such warm anomalies led to an extremely low surface pressure at this time (911 millibars) and winds of 150 knots (the pressure later dipped to 895 millibars and max sustained winds jumped to 170 knots).
For all practical purposes, the eye of Haiyan was, on average, hydrostatic, meaning that slowly subsiding air in the eye was consistent with the vertical pressure gradient (acting upward) essentially was balanced by gravity. In hydrostatic environments, the surface pressure is simply a proxy for the weight of the air column over the storm's center that extends from the ocean surface to the top of the atmosphere (the column of air is assumed to have unit cross-sectional area). An exceptionally warm air column, like the one over Haiyan's center on November 7, has a dramatically low average air density, which, in turn, translates to a very low column weight and a very low surface pressure. Of course, in an hydrostatic environment, decreasing barometric pressure ultimately means that mass must be removed from the air column (in other words, mass divergence must occur).
The dramatic warmth we observe in the mid- to upper troposphere on the AMSU cross section above comes from relatively gentle subsidence (downward-moving air) driven by deep convection (thunderstorms) in the eye wall. Recent research at Penn State (Part 1; Part 2) based on computer simulations of parcel trajectories shows that a large fraction of upper-tropospheric air parcels over the eye typically descend five to ten kilometers over a period of several days, all the while staying in the eye. Yes, you read this correctly. For a sufficiently strong tropical cyclone (maximum sustained wind speeds greater than 70 to 90 knots), Penn State researchers determined that most air parcels very close to the storm's center will remain inside the eye for a very long time...for several days or more. Such long resident times (several days) were insensitive to vertical wind shear (Penn State researchers looked at trajectories with bulk vertical wind shear up to about 20 knots, which forecasters categorize as moderate). With regard to these long resident times, Penn State results are consistent with the containment vessel theory put forth by Hugh Willoughby in 1998 (Willoughby stated that the air in the eye essentially stays in the eye).
What about air parcels in the eye that are close to the eyewall? Research has shown that the residence time for air parcels close to the eye / eyewall interface is much shorter (a few hours or so) because air parcels get mixed (stirred) into the eyewall much more readily. So the percentage of air parcels in the eye that get mixed into the eyewall increases dramatically with increasing distance from the storm's center. In summary, air parcels close to the center of the eye can remain in the eye for several days, while air parcels close to the eye / eyewall interface can get stirred (mixed) into the eyewall within a few hours or so.
Let's get back to air parcels residing close to the storm's center (long residence times). If air parcels in the upper troposphere over the storm's center descended a full ten kilometers, they would warm by roughly 100 degrees Celsius (over 200 degrees Fahrenheit)! That's just not realistic, folks. The eye of an intensifying hurricane / typhoon simply can't warm up that much.
That's because potential temperature is NOT conserved. For the record, potential temperature is the temperature that an air parcel would reach if it descended adiabatically to a specified pressure level, all the while warming by compression to the tune of roughly 10 degrees Celsius per kilometer of descent.
Rather, air parcels descending in the eye of a hurricane / typhoon are likely to be surrounded by, and eventually mixed with, cooler air. Thus, this local mixing with cooler air reduces the potential temperature of descending air parcels, thereby explaining why the eyes of hurricanes don't warm up to unrealistic levels (which they would if pure adiabatic descent was solely at work).
The bottom line here is that air parcels descending in the eye from the upper troposphere toward the top of the boundary layer (near 850 mb) warm by compression, but they also mix with surrounding cooler air, thereby preventing air temperatures in the eye from becoming unrealistically high. Still, the eye is dramatically warmer than its environment (revisit the cross section through Haiyan). Thus, the mean column air density inside the eye is low because the mean column temperature is high. As a result, the weights of air columns in the eye are relatively low, and, assuming the the environment in the eye is hydrostatic (no large vertical acceleration...just gently subsiding air, on average), the surface air pressure is low (as the 895 millibars in the eye of Supertyphoon Haiyan attests). Again, in a hydrostatic environment, decreasing barometric pressure ultimately means that mass must be removed from the air column (in other words, mass divergence must occur).
The skew-T of Hurricane Jimena's eye at 2058Z on September 23, 1991. The temperature sounding is outlined in red, and the dew-point sounding is green. The data were measured by dropwindsondes released from an aircraft flying at approximately 600 mb (roughly four kilometers). Courtesy of NOAA.
The skew-T above, whose data were measured by dropwindsonde in the eye of Hurricane Jimena in 1991 (track), shows the footprint of subsidence in the eye. Indeed, note the wide separation between the temperature sounding (in red) and the dew-point sounding (in green) roughly between 850 mb and 600 mb (the dropwindsondes were released from an aircraft near a pressure altitude of 600 mb (approximately four kilometers above the ocean). Also note the subsidence inversion near 850 mb, which offers more supporting evidence of subsidence in the eye (check out this flash animation showing how a temperature inversion forms aloft in response to subsidence; courtesy of, and copyright by, Penn State's online program in weather forecasting)
Hopefully, you now have a good grasp on how the central barometric pressure in the eye of a hurricane / typhoon can get so low. Moreover, you now have an appreciation for why "popular" explanations such as "Rising motion causes low pressure" easily qualify as bad science (I'll have more to say about this falsehood in a future blog).
So how do all the pieces of the puzzle come together? How does WISHE (Wind-Induced Surface Heat Exchange) fit into the overall maintenance of tropical cyclones? Let's start with a preexisting disturbance (a group of showers and thunderstorms over tropical seas). As surface winds increase, there's an increase in surface energy fluxes (mostly latent) from the ocean. In simpler terms, rates of evaporation increase with increasing wind speed (spray from increasingly turbulent waves evaporates, etc.). The bottom line here is that equivalent potential temperatures increase in the lower troposphere. For the record, equivalent potential temperature, sometimes referred to as theta-e, is the temperature an air parcel would achieve if it brought down to 1000 mb dry adiabatically after it was lifted adiabatically (typically dry adiabatically and then moist adiabatically) until all of its the water vapor condensed. The equivalent potential temperature (theta-e) increases with increasing temperature and increasing amounts of water vapor. In a way, equivalent potential temperatures in the lower troposphere give forecasters a proxy for low-level instability. Theta-e values greater than 328, 340, and 360 Kelvins are considered marginally, moderately, and extremely high (respectively).
The secondary flow in a hurricane (characterized by upward motion where condensational heating takes place in eyewall thunderstorms, and compensating subsidence inward and outward from this heating. Courtesy of Willoughby, 1998.
At any rate, equivalent potential temperatures in the boundary layer of a developing disturbance increase above the environmental theta-e, setting the stage for more deep convection (thunderstorms) and enhanced condensational heating in the developing eyewall. If the complete truth be told, it's not really heating in the typical sense of the word...let's just say that rising air cools at a reduced rate. However you say it, eyewall temperatures become warmer than the environment (researchers describe this as a negative radial gradient). Given that the atmosphere works to mitigate gradients in temperature (and potential temperature), a secondary circulation of air ensues (above), with upward motion where there is condensational heating, and compensating subsidence inward (over the eye) and outward of the developing eyewall. And, of course, descent over the eye leads to lowering barometric pressure, as I have previously discussed in this blog.
In my experience, the scenario I describe above is the most current paradigm accepted by most tropical researchers. I would be amiss if I didn't mention that there are uncertainties related to this paradigm. For example, research has shown that the compensating subsidence to which I refer is actually strongest near the outer edge of the eye (just inward of the eyewall and NOT over the center of the eye). Indeed, descent near the eye/eyewall interface might be an order of magnitude larger than that at the center of the eye. Nonetheless, this eye/eyewall interface is generally not as warm as the center of the eye. That's because compressional warming during relatively strong descent is largely offset by cooling associated with evaporation / sublimation, mixing at the interface, etc.
I'm an old-school forecaster, so you won't be surprised to learn that I probably held onto the theory of CISK (Conditional Instability of the Second Kind) too long. In a nutshell, conditional instability and CAPE are at the heart of CISK, which boils down to a feedback between condensational heating (sometimes called convective heating) and frictional convergence of moisture in the boundary layer. Without reservation, CISK is still important for historical reasons, in my view, and CISK shares some common characteristics with WISHE (Wind-Induced Surface Heat Exchange), but most tropical cyclone researchers have dismissed CISK as a viable theory for the intensification and maintenance of tropical cyclones. WISHE, on the other hand, is widely accepted in the tropical-research community. WISHE is based on fluxes of heat energy and moisture (latent heat) from the ocean surface as the ultimate energy source for tropical cyclones. For the record, CISK neglects these fluxes of enthalpy (energy). Like CISK, however, WISHE is also a feedback...between wind-induced surface fluxes of enthalpy (essentially energy) and condensational heating in the eyewall.
I'll summarize the feedback loop that characterizes WISHE. I assume, for sake of argument, that there's a preexisting tropical disturbance.
(1) Surface winds increase the fluxes of heat energy and moisture (latent heat) from the ocean.
(2) Theta-e (equivalent potential temperature) increases above environmental values.
(3) Deep convection (thunderstorms) in the eyewall simultaneously warms (warm) the eye, lowering barometric pressure and increasing wind speed.
(4) Faster surface winds increase fluxes of heat energy and moisture from the ocean surface.
I have read discussions that offer both explanations, CISK and WISHE, as viable explanations for the maintenance and intensification of tropical cyclones, so you shouldn't be surprised that there are details about tropical cyclogenesis that are still in dispute. Nonetheless, I believe that the science of the the feedback between surfaces fluxes, deep convection, and winds is pretty much accepted as sound. There are dissenters, of course, which is why I always told students that all the problems in atmospheric science have not yet been solved...there's plenty of fodder for meteorologists to research.
At any rate, I was compelled to write about the eye of Supertyphoon Haiyan and to use it as a springboard to teach readers more about tropical cyclogenesis.
要注明这是 Weather Underground 一个博客的文章：http://www.wunderground.com/blog/24hourprof/final-thoughts-on-supertyphoon-haiyan