Comments on “How Much Does the Upward Advection of the Supergradient Component of Boundary Layer Wind Contribute to Tropical Cyclone Intensification and Maximum Intensity?”

In a recent paper, Li et al. (2020) carried out an extensive ensemble of axisymmetric numerical simulations to examine the “the importance of supergradient winds in TC (tropical cyclone, our insertion) intensification”, claiming that this topic “is still under debate”. In their Introduction they state: “One view is that the spinup of the eyewall occurs by the upward advection of high tangential momentum associated with supergradient winds from the boundary layer. The other view argues that the upward advection of supergradient winds by eyewall updrafts results in an outward agradient force, leading to the formation of a shallow outflow layer immediately above the inflow boundary layer”. One might ask why these are considered to be “separate views”? One could argue that they are part of the same picture, irrespective of the degree to which the ascending air is supergradient. If the air that exits the boundary layer is supergradient, it must surely move outwards. What other force would make the air move inwards against the positive agradient force (which includes, of course, the radial pressure gradient force)? Their paper seems to be motivated by arguments presented by Schmidt and Smith (2016) and Montgomery and Smith (2017), who did indeed argue that the spinup of the eyewall occurs by the upward advection of high tangential momentum associated with supergradient winds from the boundary layer, a result that Li et al. seem to regard as debatable. However, much of their paper is based on a misinterpretation of these arguments that leads them to carry out an ensemble of numerical