Originally posted by Chris Rozoff
Actually, as a clarification to some of your poitns, the driving force of Emanuel's engine is the flux of energy from the ocean (caused by the vast wind speeds near the water surface). Later papers from Emanuel disavow the argument from the radiative forcing, as that is a relatively trivial player in the engine.
I never said that sensible heating drove the hurricane - I said latent heating drove the huricane - this latent energy comes from the sun warming the ocean surface and producing ample water vapor. As the tropical disturbance intensifies, the latent heating of a column of air leads to pressure drops at the surface, this leads to pressure gradients the drive the winds, and these winds increase the latent heat flux from the ocean surface feeding into the active convection which leads to a positive feedback process. The maximum potential intensity for a hurricane is directly related to the difference in the temperature of the ocean surface to that at the tropopause - a.k.a. a heat engine.
If anything, stratiform precipitation acts as a barrier to the efficiency of the engine, by causing the outflow to occur at warmer temperatures (esp. if stratiform precip is near the eyewall).
Stratiform precipitation, by definition, is caused by forced ascent. There is very little buoyancy within the hurricane even if you look at slantwise ascent, so most of the precipitation is not convective in nature - except in some of the outer rainbands and select "hot towers" within the eyewall, yet even there parcels only become buoyant after symmetric instability has lifted them out of the boundary layer.
Ok...you are right, but how does anyone seriously claim that this was important in Alex? At best, we know that turbulence is mixing down higher winds.
It should hold true for all weather sytems - Alex would not be exempt. There have been several obs studies, such as HAL, that have looked at these processes.
Winds are not zero in the eye.
Winds are forced by pressure gradients - so if there is no pressure gradient in the eye, then the winds would be calm.
The original argument seemed to place too much emphasis on convection as a direct cause of winds (presumbly through vertical momentum transport)..... Similarly the RMW is not a really caused of convective downdrafts.
I guess I didn't see this as the original claim - absolutely the mean hurricane circulation is not driven by convective downdrafts - but clearly the strongest winds within a hurricane are not at the surface - they are at the top of the boundary layer. So, convective downdrafts will be able to more effectively transport this higher wind speed air toward the surface, and will lead to much stronger wind gusts in areas where there is active convection.
Back to forecasting, the track of Alex has been favorably consistent with the warmest surface waters -
http://www.osdpd.noaa.gov/PSB/EPS/SST/data.../natlanti.c.gif
It would seem that the observed curvature has outpaced the model forecasts consistently - keeping the system from crossing north of the Gulf stream.
Glen
