Does Inertial Instability Facilitate Tropical Cyclogenesis?

In early studies of tropical cyclogenesis, some authors have cited the release of upper-tropospheric
inertial instability as a necessary condition for the development of a pre-existing disturbance.
Although inertial instability is no longer considered a necessary condition for tropical cyclogenesis,
its presence may lead to favourable interactions between the tropical cyclone (TC) outflow layer
and its environment by facilitating divergent outflow. So how beneficial is upper-tropospheric
inertial instability to the formation and intensification of tropical cyclones? We seek to answer this
question via an observational and modelling approach.

Using the IBTrACS best-track dataset, we identify the initial date and location of all tropical
cyclones in the East Pacific and North Atlantic basins during 1979–2014. For each storm, the
ECMWF ERA-Interim reanalysis is used to measure the spatial extent of instability at 250 hPa over
a 1000 km squared sampled domain. For the East Pacific basin, 679 out of 717 TCs are associated with
instability, with a maximum of 45% of the sampled domain unstable in one case. For the North
Atlantic basin, 490 out of 567 TCs are associated with instability, with a maximum of 39% of the
sampled domain unstable. This degree of widespread instability suggests that instability is not the
result of deep convection, but is associated with the environmental flow.

To evaluate the effect of these unstable regions during tropical cyclogenesis, idealised simulations
of a 3D hurricane are conducted using the CM1 model. The results of two simulations are
presented. The first simulation is the model's default 3D hurricane case, which is used as a control.
The second simulation examines how tropical cyclogenesis in the control simulation is affected by
instability due to anticyclonic shear associated with a subtropical jet on the poleward side of the
control TC. With these simulations, we test the hypothesize that tropical cyclogenesis associated
with upper-tropospheric inertial instability leads to quicker intensification of an incipient vortex.