Upper-Tropospheric Static Stability in Tropical Cyclones: Observations and Modeling

Upper-tropospheric thermodynamic processes play an important role in tropical cyclone (TC) structure and evolution. Until recently, however, few observations existed within this upper-level region of TCs. High-altitude dropsonde observations from two field campaigns – the Office of Naval Research Tropical Cyclone Intensity Experiment (TCI) and the NASA Hurricane and Severe Storm Sentinel (HS3) – have provided datasets of unprecedented resolution in the upper troposphere and lower stratosphere. These observations reveal large static stability variations with both space and time in the upper levels of TCs.

The upper troposphere within TCs tends to be characterized by small static stability, with temperature lapse rates close to dry-adiabatic and squared Brunt-Vӓisälä frequency (N²) on the order of 10^-5 s^-2. Just above the cold-point tropopause, however, exists a distinct static stability maximum, with N² in some places exceeding 10^-3 s^-2. This lower-stratospheric stable layer is particularly pronounced within the TC cirrus canopy. A secondary, typically weaker, static stability maximum also can appear 1-2 km below the tropopause in the vicinity of TCs.

The variability observed in the dropsondes is simulated in an idealized, axisymmetric framework using Cloud Model 1 (CM1). As the simulated hurricane intensifies, static stability strengthens in the lower stratosphere, forming a shallow layer of particularly large N² just above the cold-point tropopause. A budget analysis of N² reveals that the strengthening of this lower-stratospheric stable layer is related to the upward growth of the TC circulation, which introduces strong vertical gradients of potential temperature tendencies due to radiative cooling and turbulent mixing. The secondary stability maximum 1-2 km below the tropopause appears to arise due to differential potential temperature advection within the upper-level outflow layer, combined with strong localized turbulent mixing in a shallow region of particularly strong vertical wind shear.