Moist thermodynamic constraints on large-scale tropical circulations

Ángel F. Adames Corraliza

University of Wisconsin-Madison

Tuesday, Nov 03, 2026, 2:15 pm MT
DSRC Room 2A305

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Abstract

The constraints on tropical motions arising from lower tropospheric convective quasi equilibrium (CQE), the weak temperature gradient (WTG) approximation, and entraining convection are examined. Linear solutions to a highly idealized model show that the high cloud density of the lower free troposphere (LFT, 600-950 hPa) allows gravity waves to quickly adjust the temperature of the cloud-free environment to that of the clouds. As a result, a form of LQE arises in which buoyancy is quasi-conserved as it follows the horizontal flow. Motivated by recent studies on tropical convection, this form of LQE is used with the zero-buoyancy plume framework under the limit in which the bulk entrainment is an inverse function of the saturation deficit (q*-q) to better understand tropical motions. This framework reproduces features of CQE found by previous work along with several new insights. First, it is found that convection quickly responds to balance the large scale LFT saturation deficit sources. This balance constrains the column process —the sum of vertical advection and diabatic sources and sinks of moist static energy (MSE) — of the subcloud layer and LFT to be equal, with their strength proportional to the amplitude of non-WTG adiabatic vertical motions. This result implies that the column process approximately vanishes in slow tropical motions that strongly obey the WTG approximation, allowing horizontal MSE advection to govern their evolution. Second, convective mass fluxes become more top-heavy and exponentially stronger as the saturation deficit decreases, leading to an exponential moisture-precipitation relation, as in observations.

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