How does shear weaken a tropical cyclone?

[Ed Mahmoud]

I was doing a little light reading, and saw a reference to the theory from Gray (I assume Dr Gray from CSU, as a much younger man) in 1968 about shear pushing energy away from the storm as being challenged, with the newer theory being shear actually promotes stability on the upshear side, not allowing thunderstorms upshear.

I can't quite wrap my brain about that one, so I figured I'd ask someone to explain how shear could cause warming aloft on the up-shear side of a tropical system.

[Aslkahuna]

The strong winds associated with the shear could be bringing dry warm air aloft into the area if the flow originates from an upper high. For a quick look as to what shear can do look at current satellite imagery of either TS Iselle or the Invest further west in EPAC.

Steve

[Wthrman13]

From my understanding, the main reason that shear is detrimental to a tropical cyclone is that it disrupts the vertical continuity of the central warm core. The low pressure in the center of a TC at the surface is maintained hydrostatically by the warm column of air above the center (either in the eye, or if it doesn't have an eye, in the column of convection directly over the center). This warm column of air is maintained by the latent heat release in the central convection, or, in the case of a TC with a well-defined eye, by the compensating subsidence warming in the center of the eye, which is in turn driven by the eyewall convection. In a TC in an environment of little or no vertical shear, this warm column of air is more-or-less completely vertical, so that the hydrostatic pressure at the surface can maintain itself. However, when shear increases, it causes the warm core to tilt with height, so that the warmest air aloft is no longer directly above the low-level circulation center. This causes the pressure to rise at the surface in the center of circulation, weakening the storm. The weaker storm produces weaker and less convection, further decreasing the strength of the warm core, so to first order it's a positive feedback process. This is why strong shear can quickly disrupt and weaken even a strong TC.

However, there is some recent research that suggests that the subsidence warming aloft on the upshear side of the sheared TC can fight against this weakening trend somewhat, keeping the low-level warm core intact, even though the mid and upper-level warm core region is pushed aside. The reason there is subsidence on the upshear side to begin with is because of strong upper-level convergence of the TC's circulation with the strong upper-level winds. There are other complex feedbacks which makes this an active area of research.

Also, as Aslkahuna pointed out, vertical shear can also bring dry air aloft more effectively into the TC circulation, also contributing to weakening.

EDIT: I just realized that I also tried to answer this question of yours in the main forum, so I apologize if I've repeated myself!