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 , 8:02 PM GMT on November 08, 2013
Swirls of low clouds called mesovortices formed in the eye of Super Typhoon Haiyan as it churned westward toward the Philippine Islands yesterday (November 7). I annotated this 0515Z visible image from the Korean geostationary satellite, COMS-1, so you can see the mesovortices. The loop of visible images (below, larger loop) from COMS-1 below spanned from 2130Z on November 6 to 0815Z on November 7 (5:30 A.M. PHT to 4:15 P.M. PHT on November 7).
This animated gif of visible satellite images from the Korean geostationary satellite, COMS-1, shows swirls of low clouds called mesovortices forming in the eye of Super Typhoon Haiyan. The loop spans the daylight hours from 2130Z on November 6 to 0815Z on November 7 (5:30 A.M. PHT to 4:15 P.M. PHT on November 7). Larger loop. Courtesy of CIMSS. Please check out Scott Bachmeier's blog.
Mesovortices in the eyes of very strong tropical cyclones are typically meso-beta features, with a spatial scale on the order of 2 to 20 kilometers. Mesovortices don't produce really strong winds, but they help to explain why the eyes of intense tropical cyclones are not necessarily clear with "light" winds (turbulence associated with mesovortices can produce relatively strong winds inward from the eye wall...more in a moment).
Curiously, these mesovortices in the eyes of very strong hurricanes sometimes take on polygonal shapes (check out this 1812Z visible satellite image of Hurricane Isabel on September 13, 2003). Indeed, eyes with polygonal shapes ranging from triangular to hexagonal have been observed on high-resolution satellite imagery. Granted, the sides of the polygons sometimes don't exactly connect, but this imperfection should not dilute my message here. I'll have more to say about polygonal shapes just a bit later. But first, perhaps the notion of low clouds in the eyes of intense tropical cyclones is not intuitive, so I'll provide a little background.
Making Low Clouds in the Eye
The low clouds that we sometimes observe in the cores of hurricanes and typhoons typically form beneath the temperature inversion in the eye. This inversion is a stable layer that forms in response to subsiding air in the eye at roughly 850 mb (about 1500 meters). At these relatively low altitudes, there are several sources of water vapor that pave the way for low clouds to form.
Water vapor for low clouds forming in the eye of a hurricane comes from the evaporation of warm ocean water, the inward movement of rain-cooled outflow from eye-wall thunderstorms, and the downward transport of water vapor below the temperature inversion in the eye. Courtesy of, and copyright by, Penn State's online certificate program in weather forecasting.
Not surprisingly, water evaporating from the warm sea gets trapped beneath the inversion. Also, eddies can turbulently mix moist air inward along the edges of the inner eye wall, where subsidence (sinking air currents) in the eye advects (transports) moisture downward into the boundary layer (below the temperature inversion...check out the green arrows on the idealized schematic above). Also, air plummeting below cloud base in the downdrafts of eye-wall thunderstorms can sometimes get drawn into the eye at low levels...you can see the outflow of rain-cooled air getting drawn into the eye on the idealized schematic above (light blue arrows).
Any way you slice it, moisture injected into eye below the temperature inversion near 850 mb, coupled with weak upward motion below the inversion, set the stage for stratus and stratocumulus to sometimes form in the eye of hurricanes and typhoons.
There's little doubt that some of the stratus clouds that form in the eyes of hurricanes are more distinctive than others. Indeed, mesovortices (like the ones that formed in Super Typhoon Haiyan) will only spin up in the decks of low clouds that form inside the eyes of major hurricanes. Let's investigate.
Mesovortices and Turbulence in the Eyes of Hurricanes
Starting with some basics, let's agree that air parcels near the ocean's surface spiral cyclonically inward toward the center of a hurricane, accelerating as they attempt to conserve angular momentum. Not surprisingly, relatively small losses of wind speed to surface friction (on the order of ten percent) prevent a true conservation of angular momentum. Nonetheless, once the outward acting centrifugal force nearly matches the pressure gradient force, air parcels stop their inward spiral and rise to form the eye wall.
Now the stage is set for subsidence (downward motion) to occur over the core region of the tropical cyclone. For the record, subsidence is likely caused by the combination of two processes. First, there is a forced response to the massive release of latent heat inside the eye wall, which promotes prolonged ascent in the hot towers (tall thunderstorms) surrounding the developing eye. As a forced response to local upward motion, compensating subsidence occurs inside (and outside) the eye wall. Second, there's an inward mixing of higher momentum from the eye wall via turbulence and/or breaking waves (mesovortices). In turn, this influx of higher momentum from the eye wall causes the air on the periphery of the eye to become slightly supergradient above the boundary layer (faster than the speed associated with gradient-wind balance). Here, air diverges outward into the eye wall, further promoting subsidence. Both of these processes represent non-hydrostatic pressure perturbations that pave the way for subsidence in the eye.
The above flash animation allows you to better grasp the result of the mixing of air that occurs at the inner eye-wall boundary of a hurricane. Before you embark, please check out this animation, which shows (without explanation) air parcels exiting the eye just above the boundary layer (focus your attention on air parcels sinking in the eye and then watch them "diverge" into the eye wall). This exodus just above the boundary layer boils down to the forces acting on individual air parcels and their struggle to attain gradient-wind balance. Courtesy of, and copyright by, Penn State's online certificate program in weather forecasting.
One of the upshots from this discussion is that mesovortices (sometimes called "sub-storms") help to mix momentum from the eye wall into the eye, thereby accounting for some of the observations that the eye is not necessarily the island of calm that's popularly advertised. So the question now becomes: What causes mesovortices in the eyes of major hurricanes?
How Do Mesovortices Form?
Recent research at Penn State suggests that these mesovortices grow in much the same way that their much larger cousins, mid-latitude cyclones, develop. Indeed, the development of mesovortices appears to be linked to the sharp baroclinic zone associated with temperature differences between the eye and eye wall (the growth of vortices in an environment where baroclinicity prevails is called baroclinic instability). Mesovortices likely owe their genesis to the large horizontal wind shear that sometimes exists along the inside of the eye wall of mature hurricanes. Here, large shear can generate cyclonic swirls that spin-up on the periphery of the eye and intensify via baroclinic instability.
A numerical simulation showing how the eye of a mature hurricane can evolve from an annular to a polygonal eye structure (color key). Courtesy of Dr. Wayne Shubert, Colorado State University. Feel free to check out Dr. Shubert's 1999 paper on mesovortices.
The notion of barotropic instability has also served as the basis of research regarding the genesis of mesovortices...check out the numerical simulation (above) of the evolution of the relative vorticity field inside a mature hurricane. The region marked in red indicates large values of cyclonic relative vorticity in the eye wall (large frictional convergence generates high values of cyclonic vorticity...picture an ice skater increasing her spin by drawing her arms inward toward her body). Note the large gradient of cyclonic relative vorticity on the inside of the eye wall and the outside of the eye. Although the physics and mathematics needed to adequately explain this evolution is beyond the scope of this blog, the results of this numerical simulation will at least give you an idea of how the evolution from an annular (shaped like a ring) to a polygonal eye structure might occur.
But the evolution of the eye's polygonal structure does not last long. Watch the numerical simulation unfold from 10 to 20 hours and then from 22 to 48 hours. To summarize, check out this 48-hour animated simulation). At 48 hours, note the single mesovortex near the center of the eye. When the simulation began, turbulent mixing caused high values of cyclonic vorticity to swirl into the eye, paving the way for mesovortices to form on the periphery of the eye. The vorticity associated with the mesovortices then diffused (spread out; became less concentrated). The remaining vorticity migrated inward, causing the center of the eye to spin up. Such "centrally located" mesovortices have been observed...check out the mesovortex near the center of the eye of Hurricane Katrina as the storm took aim at the Gulf Coast on August 28, 2005.
I hope this gave you a better sense for how mesovortices develop in the eyes of major hurricanes.
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