Outflow Boundry's...good or bad?

[Pebbles]

Was looking at a hi rez visible loop and noticed outflow boundries coming off the northwest side of Ivan.... Is that a good or bad thing for Ivan? And would your explaination hold true for all hurricanes or does outflow boundries mean different things to different size hurricanes?

[Pigsnibble]

Good question...

I've heard many state that shape may be a good indicator of direction/shearing....

[Wthrman13]

Yes, a very good question.

My experience is that outflow boundaries that are near the periphery of a tropical cyclone are not usually detrimental to the development of the system. It's only when you see them racing out from near the core of the system, that it can become a problem, because then you are actually getting divergent flow away from the center of the vortex. However, the fact of the matter is that, with all the convection going on in a hurricane, there has to be some outflow at the surface, due to conservation of mass. In the core of a hurricane, the areas of surface outflow are highly organized and alternate with regions of inflow towards the center of the hurricane. This is essentially why rainbands exist. One way of looking at it is that they are due to intersecting outflow boundaries being organized into a spiral pattern associated with the larger-scale surface inflow of the hurricane. Towards the edge of the hurricane, the large-scale inflow is less pronounced, so the outflow boundaries are less organized. I have seen outflow boundaries near the edge of very strong hurricanes, moving away from the center, and they did not lead to any significant weakening of the storm.

Thus, the mere presence of outflow boundaries does not signal that a storm is weaker or weakening.

[calidoug]

" because then you are actually getting divergent flow away from the center of the vortex."

But the outflow is ALWAYS flowing away from the center. It's just at a much higher level in the atmosphere than the inflow.

[PurdueWx80]

We should also consider that there is massively sinking air around the periphery of hurricanes - generally over some of the outer rainbands. These raindbands are areas of localized convergence (and updrafts) but the sinking motion above introduces some drier air. Anytime you have dry air above moist, less dense air, you get thunderstorms that are capable of transferring the drier, cooler air towards the surface. This can also accelerate the motion, hence, the rapidly expanding surface outflows. This doesn't disrupt the inflow of air into the center of the hurricane, as that is taking place well w/i the boundaries of those outer rainbands.

[Wthrman13]

" because then you are actually getting divergent flow away from the center of the vortex."

But the outflow is ALWAYS flowing away from the center. It's just at a much higher level in the atmosphere than the inflow.

You completely missed my point. I wasn't talking about the upper-level outflow, but surface outflow induced by the downdrafts associated with the convection in the core of the hurricane. There IS outflow at the surface from these individual convective bands, it's just superimposed on a much larger-scale and more dominant inflow into the center of the hurricane. That's why there are rainbands and not just a simple uninterrupted, symmetrical inflow towards the center.

[Pebbles]

WOW...I actually understood all that... read it twice and then was able to picture it in my head! Thanks for the answers as this added to my understanding of the complex layering and movement of flows in these systems. Something I had slight trouble actually visualizing in the ol' nogger *points to her head and winks*

[schmita]

I can vouch for Purdue's explanation.
One of the outflow bands hit us.
Yesterday morning we had INTENSE lightning and thunder. MsBee's gauge measured 3.5" of rain.

The thunder was as if someone was shooting a gun next to your ears. It was quite mad.