Question Box:

[bob rulz]

On this question...doesn't shear hurt the vertical development of overland storms? I know some of our hail producing storms can get very, very high. It's hard to imagine that shear would help those storms.

No... shear helps to vent the storm and introduce more dry air, which helps an overland tstm. Actually, during a tstm that is producing hail, usually the shear is VERY high. It helps in the hail development process.

The highest-sheared storms will be the ones that produce tornadoes, if I remember correctly.

Yet another question, what makes hurricanes larger or smaller? Is it just random chance or does something create the size?

There is a theory which a met presented on here that if a hurricane is exposed to shear in their formative stages, they might start out larger than the average cane, thus develop into bigger canes. Examining the satellite loops of Katrina and Rita, one can notice that at first, there are TUTT's near them that hinder them in the beginning, only to move away, but still provide good UL divergence. This allowed them to fan their clouds freely and become big and fat.

Time also has to do with it too. More time, the more time for fanning of the clouds, and the storm keeps getting larger during ERC's. Of course, 2005 being an exception. Ivan went through a ton of ERC's, allowing its windfield to expand each and every time, thus it became huge.

Speed of the storm, IMO, has something to do with it too. Think about it: if a hurricane travels at an extremely fast rate of speed, the storms on the outer fringes never develop because they will lag behind, sheared apart by the storm's outflow

(BTW, when I mention "fanning of the storm" I mean development of convection on the outer rainbands. That is key to bringing TS force winds to the sfc and thus expanding the windfield. How large a storm looks on sat is a poor indicator of the actual windfield.)

The surrounding atmospheric environment is important too. We had low atmospheric pressures across the basin in 2005, so thunderstorms easily developed on the outer fringes of the hurricane. If there were high pressure, the storm would be sort of suppressed, squeezed. Good example would be Andrew of 1992, which was kept small by a huge area of high pressure to the north (which of course drove Andrew into FL in the first place).

Just my thinking here.

Sat loop of Lenny: http://vortex.plymouth.edu/hur_dir/1999 ... 9_sum.html (scroll down to bottom)

Don't forget that eyewall replacement cycles may also lead to the hurricane expanding in size, as we saw with Katrina.

[wxmann_91]

On this question...doesn't shear hurt the vertical development of overland storms? I know some of our hail producing storms can get very, very high. It's hard to imagine that shear would help those storms.

No... shear helps to vent the storm and introduce more dry air, which helps an overland tstm. Actually, during a tstm that is producing hail, usually the shear is VERY high. It helps in the hail development process.

Shear helps tilt the updraft of the thunderstorm, thus separating the downdrafts and updrafts, and increasing the longetivity of the thunderstorm. Thus, with the updraft tilted and strong, separated from teh downdraft, large hail can result.

TC's are vertically stacked storms.

All this is definitely true, but there is an even more important effect that moderate vertical wind shear has on the organization of thunderstorm cells: dynamical pressure effects.

Basically, without getting too technical, a storm's updraft will interact with the changing speed and direction of the winds with altitude in the following two ways:

1) As the environmental wind speed increases with height, the updraft of the storm will be transporting lower-speed air from below into the higher wind speeds aloft. This has the effect of producing high pressure on the upstream side of the updraft, and low pressure on the downstream side of the updraft in the midlevels. This causes a preferential development of new updrafts on the downstream side of the old one and can help intensify and increase the longevity of the storm.

2) Rotation of the updraft. This is caused by the updraft tilting the horizontal vorticity present due to the environmental wind shear. The low pressure created in the rotating updraft at midlevels also leads to an enhancement of the updraft strength, due to an upward directed pressure gradient from the low-levels to the low pressure in the mid levels of the storm. Storms that have strong, deep, and persistent updraft rotation like this are called supercells.

These two effects combined can lead to a significant enhancement in the strength of the storm cell relative to another storm with the same buoyancy in a lower shear environment, simply due to these dynamic pressure effects.

I've glossed over a lot of stuff in this, but a full explanation can get rather technical and really requires diagrams to fully appreciate (at least for me). For a more thorough explanation, complete with nice diagrams and animations, check out:

http://meted.ucar.edu/topics_convective.php

and look for the topics entitled "Principles of Convection I, II, and III", particularly part III.

That's a great link Wthrman!! Thanks for sharing.

[wxmann_91]

The highest-sheared storms will be the ones that produce tornadoes, if I remember correctly.

Many times, correct. But instability's important too.

[Wthrman13]

No problem. I forgot to add that the above two effects are active even if there is only speed shear, but no directional shear. In such a case, with enough shear, you can get splitting storms, where both the left-moving and right-moving supercells are equally intense. However, in most cases, there is directional shear as well, and in the northern hemisphere, the wind vector usually rotates clockwise with height during conditions that are favorable for storm development. This causes the low pressure areas in the midlevels to line up in such a way so that the right-moving supercell is favored, and the left one is suppressed.