What is cooling the tropopause above tropical cyclones?

The tropopause layer (~17 km altitude) above tropical cyclones (TCs) exhibits anomalously low temperatures, primarily during intensification and on spatial scales ~1000 km. These temperatures (here referred to as 'tropopause layer cooling', TLC for short) have been hypothesized to be the result of:

1) Adiabatic expansion and cooling in cloud tops that overshoot their level of neutral buoyancy.

2) Diabatic cooling from long-wave radiation near cloud top, especially in the TC core where deep convection was observed to reach higher altitudes than in the rest of the tropics.

3) Adiabatic cooling from convectively generated gravity waves.

Given the complex relationships between the thermal structure of the upper troposphere and the TC secondary circulation, determining which mechanisms are at play is paramount. TLC is also expected to destabilize the upper troposphere to convection and allow clouds to reach higher altitudes. Lastly, low temperatures near the tropopause can lead to in situ formation of cirrus clouds, which impact the radiative budget in the tropical tropopause layer.

Due to their small size and ephemeral nature, overshooting cloud tops (mechanism 1) are difficult to observe. We also expect mechanism 1 to be compensated by subsidence warming on spatial scales ~1000 km. We therefore set aside mechanism 1 and examine mechanisms 2 and 3.

The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) provides finely resolved, highly accurate temperature retrievals in all weather conditions. Using these retrievals, we estimate a total temperature tendency of order -1 K/day during maximum intensity at and above the tropopause. The total tendency is then compared to radiative heating rates calculated from liquid and ice water content estimates from the CloudSat Profiling Radar (CPR), CALIPSO’s CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization), and the Moderate Resolution imaging Spectrometers (MODIS) onboard Terra and Aqua. We find that cloudy-sky long-wave radiation produces cooling 2–100 times smaller than the total cooling, suggesting that mechanism 2 alone does not explain TLC. We conclude that mechanism 3 must play an important part in cooling the tropopause.

Mechanism 3 corresponds to a hydrostatic adjustment to the tropospheric 'warm core’; in order to fulfill the constraint that isobars be unperturbed at sufficiently large altitude above TCs, a compensating cooling must occur somewhere in the atmospheric column. Gravity waves generated by fast latent heat release propagate upward and outward from the convective source, triggering a wave-like response aloft. The perturbations associated with these waves have previously been observed to locally maximize and produce cooling near the tropopause. High-resolution, axisymmetric TC simulations from the Cloud Model 1 (CM1, a cloud-resolving model capable of explicitly resolving gravity waves) provide a temperature tendency budget near the tropopause, allowing us to determine the role of eddy terms in cooling the tropopause. The simulations are also used to characterize the hypothesized feedbacks on TC structure and dynamics associated with TLC.