Originally posted by Chris Rozoff
I don't fully buy some of these arguments relating convection and observed winds at the surface. Consider the steady state hurricane. It is a vortex, of course maintained by convective forcing.
Better to say that a hurricane is a heat engine, and a relatively inefficient one at that. The sun warms the ocean surface, and evaporates water, that water vapor having latent heat that is later released as condensed clouds, and these clouds radiate energy back out to space, completing the engine cycle. I think using the term convective forcing is mis-leading - because when you say convection most people think of overturning in the atmosphere - but stratiform rain within a hurricane is much more widespread and likely a larger contributor to the overall heat budget.
So indirectly the convection leads to the winds. But the vortex structure determines the wind profile (through of course the pressure distribution). As tropical cyclones are warm core structures, the winds increase toward the ground (ocean really) to maintain thermal wind balance. The boundary layer turbulence, and ultimately the ground, exerts great drag on the wind, causing the winds to decrease toward the ground from the top of the boundary layer. The winds decrease with height above the boundary layer.
That heat energy is maximized at ~ 700 mb in the typical hurricane, but heating occurs throughout the cloud-bearing layer where there is ascent occuring. Peak rotational winds will occur closer to top of the boundary layer height - and decreasing gradually with increasing height, as you mentioned because of the thermal wind relationship for a warm core system, the horizontal pressure gradient, which is what really drives the winds, is much greater near the surface than aloft. As mentioned, surface friction acts to slow the winds within ~ 500 m of the surface owing to turbulent eddies.
See
http://www.nhc.noaa.gov/gifs/JLF_Fig1.gif
Because of turbulence in the boundary layer, higher winds can be mixed down toward the ground, providing wind gust measurements.
Actually, I think there is some clear evidence for convective enhancement of the downward momentum transport - a.k.a. strong gusty winds at the surface accompanying squalls. The precipitation drag aids in the enhancing downdrafts, which are interspersed among bands of ascent. So there is some truth to the notion that active
local convection can increase the likelihood of the higher winds at the top of the boundary layer making it to the surface without mixing out towards the mean wind profile.
Winds are not zero in the eye. They increase from 0 (relative to the hurricane motion) at the center to the maximum eye wall wind speeds in a roughly linear fashion. Winds tend to be concentrated in the convection and convective rainbands because of the very efficient PV production from convection. The radius of maximum winds occurs near the eyewall due to angular momentum conservation principles.
Chris
By PV, do you mean potential vorticity? That's probably a bit elevated a subject for here, but since convection causes latent heat release, and the thermal perturbations are a form of potential vorticity anamoly, then you get into a bit of a chicken or the egg argument. Horizontal winds are potentially variant within rainbands, but could be owing to variations in vertical momentum transport, etc... but I don't think PV bands could easily be tied to specific surface wind gusts. The scales are too far separated IMO. Instead, PV bands can enhance the probability of local convective development in the form of squalls/bands, and convective winds wthin this individual convective elements can enhance vertical momentum transport.
Ok, this got really long - sorry - this whole conversation really belongs in weather and forecasting - not the Target thread - but I don't have the authority to move it. Maybe a moderator could step in and do so.
Glen