An Analysis of the Conditional Instability of the Tropical Atmosphere

TLDR

The ice phase is incorporated into CAPE calculations for many tropical soundings, both on land and at sea. Finite CAPE coexists with infrequent deep convection in the tropics because convective inhibition energy (CINE) and mixing destroy the positive buoyancy of nascent cloud parcels. The latent heat of fusion in the ice phase more than offsets the negative buoyancy from water loading, and the tropical atmosphere’s departure from moist neutrality exhibits a wet‑bulb threshold of 22–23 °C, corresponding to a sea‑surface temperature near 26 °C, suggesting that conditional instability plays a key role in hurricane formation.

Abstract

The ice phase is included in thermodynamic calculations of convective available potential energy (CAPE) for a large number of soundings in the tropical atmosphere, at both land and ocean stations. It is found that the positive-buoyancy contribution to CAPE resulting from the latent heat of fusion more than offsets the negative-buoyancy contribution due to water loading in the reversible thermodynamic process. The departure from moist neutrality in much of the tropical atmosphere exhibits a threshold in boundary-layer wet-bulb potential temperature of 22°–23°C. The corresponding sea surface temperature is approximately 26°C, close to the empirical threshold for hurricane formation, which suggests that conditional instability plays an important role in the latter phenomenon. The simultaneous presence of finite CAPE and infrequent deep convection in the tropics is tentatively attributed to the convective inhibition energy (CINE) and to the mixing process that destroys positive buoyancy in incipient cloud parcels.