The Role of the Lower-Free Troposphere in Tropical Cyclone Quasi-Equilibrium

Tropical convection is sensitive to measures of conditional instability. In particular, a measure of lower-tropospheric entraining buoyancy (B_L) appears tightly related to tropical precipitation. This B_L measure can be decomposed into contributions from lower-tropospheric instability (CAPE_L) and lower-tropospheric subsaturation (SUBSAT_L); B_L increases in response to increases in CAPE_L and decreases in SUBSAT_L. The latter effectively measures the effects of lower-tropospheric entrainment. We investigate the relationship between these buoyancy components and precipitation associated with tropical cyclones (TCs). We use reanalysis (ERA5) data to compute the buoyancy components, rainfall from TRMM 3B42, and TC tracks from the TempestExtremes feature tracker, for the time period from 2002-2013. We also examine how the components of B_L vary as a function of distance to the storm center. A ±1 degree box around the storm center is used to isolate the interior of the storm. A ±5 degree box around the storm center—excluding the storm interior—is used to identify the storm environment. We find that the tropical precipitation-B_L relationship also holds when conditioned on TCs, much like the rest of the tropical regions. This is true for both the storm center and its environment.

We find that B_L values are close to zero within both the TC interior and its environment (the TC interior has a slightly larger B_L), implying that TCs are in a state of convective quasi-equilibrium (QE). However, this state of QE is attained through different pathways within the storm interior and its environment. The storm interior has lower CAPE_L values than its environment at higher intensities. This is because the TC’s warm core strengthens the vertical stratification, plausibly due to subsidence within the TC eye. Quantitatively this is shown using equivalent potential temperature (θ_e) variations. Both the boundary layer averaged θ_e (θ_eB) and the lower-tropospheric saturated θe (θ_eL^sat) increase as one moves from the TC environment to its interior, but θ_eL^sat increases faster than θ_eB with intensity. The storm interior is also more moist than its environment in the lower troposphere (smaller SUBSAT_L values; closer to saturation), with average θ_e in the lower-free troposphere (θ_eL) increasing faster than θ_eL^sat as one moves from the storm environment to its interior. The high moisture-low CAPE_L values with the TC interior, and the relatively low moisture-high CAPE_L values in the TC environment thus enforce a state of near-zero B_L, both within and outside the storm. We also found that TCs with higher intensity had lower SUBSAT_L values in their interior, relative to their environment.