The role of heating and cooling associated with ice processes on tropical cyclogenesis and intensification

Abstract

A recent idealized numerical study of tropical cyclogenesis and subsequent intensification using warm‐rain‐only microphysics is extended to examine the modifications brought about by a representation of ice processes. It is found that the time taken to reach cyclogenesis is more than twice that in the equivalent warm‐rain‐only simulation. The subtle reasons for the difference in the length of the gestation period are discussed. A mid‐level vortex forms during the early gestation period when ice processes are present, but not when warm‐rain‐only processes are present. Axisymmetric balance calculations show that the spin‐up of this mid‐level vortex is related to the different spatial distribution of diabatic heating rate in the presence of ice, which leads to a system‐scale radial influx of absolute vorticity in the middle troposphere. The tropical‐cyclone vortex that forms in the simulation with ice is similar to that in the warm‐rain‐only simulation, with the strengthening frictional boundary layer exerting a progressively important role in focusing inner‐core deep convection. This vortex develops in situ on a much smaller scale than the mid‐level vortex and there is no evidence that it is a result of the mid‐level vortex being somehow carried downwards, as has been suggested previously by some researchers. Some implications of the results in relation to previous theories of tropical cyclogenesis are discussed.

The Evolution and Role of Midtropospheric Cyclonic Vortex in the Formation of Super Typhoon Nepartak (2016)

“…The enhancement of the midlevel vorticity usually corresponded to the decrease of the low‐level vorticity, suggesting that the development of midtropospheric vortex was unfavorable for the strengthening of the low‐level circulation. This is consistent with the idealized numerical simulations performed by Kilroy et al (), showing that the time taken to reach cyclogenesis in the experiment with midlevel vortex developed is more than twice that in the equivalent warm‐rain‐only simulation without midlevel vortex. Overall, the vorticity reductions were smaller than the positive vorticity increments in the lower and middle troposphere during the gestation of Nepartak.…”

“…Although the midtropospheric cyclonic vortex in TC genesis has been paid lots of attention, a consensus is still lacking on the role of midlevel vortex in tropical cyclogenesis. Recently, Kilroy et al (2017Kilroy et al ( , 2018 showed that the tropical cyclogenesis can occur without the existence of a midtropospheric vortex and the rapid organizations of the low-level vorticity in the experiments with or without midlevel vortex are similar. In this work, the formation of Super Typhoon Nepartak (2016) is to be investigated with the focus on the evolution and the role of midtroposheric vortex during the TC genesis.…”

An idealized numerical study of tropical cyclogenesis and evolution at the Equator

“…There are, of course, differences in detail between corresponding experiments associated with the stochastic nature of deep convection (Nguyen et al, ). As in all our recent idealized studies of tropical cyclogenesis (Kilroy et al, ; ; ), deep cumulus convection and the convectively‐induced vertical vorticity distribution are highly nonaxisymmetric, especially before and during much of the early RI phase. As a result, the emerging vorticity monopole retains a degree of relatively strong asymmetry during this stage.…”

“…In the idealized simulations by Kilroy et al . (; ; ) and Steenkamp et al . (), genesis was considered to have occurred at the end of a gestation period in which the maximum azimuthally‐averaged tangential wind speed, , increased only slowly and when this wind speed began a sharp increase.…”

Understanding the vertical structure of potential vorticity in tropical depressions

“…However, there is disagreement about the role of and processes responsible for the mid-level vortex in TC spin-up, with some recent studies showing that spin-up occurs more rapidly in a simulation where mid-level vortex formation was suppressed by eliminating ice processes (e.g. Lussier et al, 2014;Kilroy et al, 2018) and other studies reporting the absence of TD formation when the latent heat of fusion due to depositional ice growth is removed (Cecelski and Zhang, 2016).…”