#11 Postby wjs3 » Wed Sep 06, 2006 11:28 pm
not quite.
A tropical cyclone (a well developed one) is low pressure (convergence--air coming together)) at the surface, combined with high pressure (divergence--air spreading apart) aloft.
A ULL is low pressure aloft (just the opposite). Think of it as a pocket of cold air aloft. This cold air, simplifying the explanation a great deal, leads to an area where air tend to converge at the upper levels of the troposphere...
This leads to a couple of things...first is that the cold air aloft can make the atmosphere more unstable...more prone to convection. If that happens and the convection can sustain itself, then in certain circumstances--and these are pretty rare, but they happen,--a ULL can transform itself into a surface low, and, eventually even a tropical cyclone.
Thinking more generally about it, though...a ULL (like any low pressure) is an area of convergence (air wants to go form high pressure to low pressure, so in a ULL, air at high level is heading toward the ULL). Convergence, aloft, in a ULL, tends to make the air column heavier (more and more air is piling into the column from above), and pressures at the surface, therefore, tend to be high. This is just the opposite of what you need--low pressure at the surface--for a TC.
ULLs can also be nasty because of the shear they impart to tropical cyclones--or not. It's a funny thing...get a ULL close enough to a TC and it shears the TC. Get it far enough away, and it can actually enhance outflow and make a TC stronger. You could write a book about it really.
Hope this helps.
0 likes
