wxmann_91 wrote:senorpepr wrote:mempho wrote: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.