Extreme Weather #19 - Modeling Tropical Cyclones
Maximum intensity questioned
In the last article I referenced Persing & Montgomery 2003:
This upper bound, for a given set of environmental conditions, is the maximum possible intensity, customarily referred to as maximum potential intensity, or MPI. If an MPI principle can be found, then the manifold [range] of possible intensities of the hurricane can be constrained.
Their paper does question this theory.
Because of these findings and the fundamental nature of the problem, we believe a reexamination and a more thorough testing of E-MPI theory is warranted..
..It is proposed that a new MPI should be formulated from first principles that incorporates this effect, whether as a novel approach to the problem or as a modification of E-MPI. It is proposed that further analyses of the thermodynamic structure of both idealized and real-case simulations of hurricanes be conducted in three dimensions.
The challenge for those of us not in the field of tropical cyclone research is making sense of the many papers.
How strong are the claims?
I was brought to this paper by Adam Sobel and co-authors who introduced the idea of MPI from Kerry Emanuel, noted that it was critiqued by Persing & Montgomery 2003 and Roger K. Smith and co-authors 2008, and then claimed:
..it nonetheless appears, under detailed scrutiny, to hold well enough to be useful, and it provides a strong theoretical underpinning for discussions of how TCs may change with climate
The “detailed scrutiny” is Bryan & Rotunno 2009 who don’t appear to really support the idea very strongly.
If you’re a casual reader who has just arrived at Science of Doom, sorry I don’t have a sound bite. Tropical Cyclones (TCs) are complicated. Making sense of the papers is a challenge.
For now, let’s look at a few claims in Persing & Montgomery 2003.
E-MPI is formulated on two-dimensional, axisymmetric principles, and neglects certain processes such as vertical wind shear, convective asymmetries, spiral arm bands, vortex Rossby waves, ocean spray, and wind-induced ocean cooling. Given these simplifications, one might anticipate that E-MPI can only serve as an approximate upper bound on real hurricanes that are three-dimensional and include all of these unmodeled processes..
..While E-MPI has been previously presented as a rigorous upper bound on storm intensity, at high spatial and temporal resolution the RE87 hurricane model pro- duces storms that greatly exceed E-MPI..
..The cause of superintensity is found to lie in the presence of high-entropy air in the low-level eye, which is entrained into the eyewall. The introduction of high-entropy air from the eye represents an additional source of heat to the eyewall of the storm and leads to a modified Carnot cycle that supports a stronger storm. Observational evidence points to the existence of this mechanism in real hurricanes and also in three-dimensional numerical models.
Conclusion
Kerry Emanuel came up with a theory in the late 1980s which used a few assumptions to make a simplified model (very common in physics, and sometimes very useful). This simplified model demonstrates that higher sea surface temperatures will lead to faster wind speeds in TCs - which is bad news.
This paper by Persing & Montgomery 2003 suggests that the simplifications lead to incorrect results. The other paper, Montgomery et al 2008, cited by Adam Sobel also suggests that the many simplifications lead to flaws in the Emanual MPI theory.
In the next article I’ll try and see how “it nonetheless appears, under detailed scrutiny, to hold well enough to be useful”.
References
Hurricane Superintensity, John Persing & Michael Montgomery, Journal of the Atmospheric Sciences 2003
Human influence on tropical cyclone intensity, Adam Sobel et al, Science 2016
The Maximum Intensity of Tropical Cyclones in Axisymmetric Numerical Model Simulations, Bryan & Rotunno, Monthly Weather Review 2009
A critique of Emanuel’s hurricane model and potential intensity theory, Roger K. Smith, Michael T. Montgomery & Stefanie Vogl, Q. J. R. Meteorol. Soc. 2008
Also worth considering factors other than just storm intensity
https://www.mattball.org/2023/08/the-horror-of-climate-change.html
Steve: In my ignorance, it isn’t clear that we should be concerned if MPI does rise with tropical ocean temperature unless we also know that the factors that prevent TC’s from achieving maximum intensity remain constant. IIRC, Wind sheer is predicted to increase on a warmer planet. I vaguely remember reading that the strongest hurricanes are limited because their winds bring up cooler water from below it front of the hurricane.
Some simple physics illustrates the pitfalls of over-simplifying potential energy/intensity. In space, gravitational potential is converted to velocity using mgh = 0.5*mv2. In the atmosphere, growing drag (friction with air) eventually negates the entire force of gravity, and velocity reaches a plateau (terminal velocity). Drag plays a more dramatic role on falling ping pong balls than falling golf balls.