UW-AOS Colloquium - November 4, 2024 - Fiaz Ahmed

Speaker: Fiaz Ahmed, researcher, Dept. of Atmospheric and Oceanic Sciences, UCLA

Title: Tropical Convection and Dynamics within the ITCZ

Abstract: Tropical convection affects us all. Interactions between tropical convection and its environment at the kilometer-scale can have consequences at the very largest scales. However, these interactions are confounded by fast timescales (a few hours) and a tight coupling with the dynamics. In this talk, I first present a data-driven approach that leverages precipitation measurements to build a simple physical model of tropical convection. This model identifies (and helps construct) a cloud buoyancy measure from environmental thermodynamic variables. This buoyancy measure is shown to be the dominant thermodynamic control on precipitation across tropical land and ocean.

In the second part of talk, I use simple models to highlight two ways in which the buoyancy control of precipitation leaves its imprints on the inter-tropical convergence zone (ITCZ). First, the influence of water vapor on convection implies a moist-dry partitioning of the tropics. This partitioning is used to understand why the ITCZ narrows under global warming. Second, interactions between the buoyancy control on convection and spatial SST patterns are set up in a layered atmospheric model. These interactions help understand how top- and bottom-heavy vertical velocity profiles respectively emerge in the West and East Pacific ITCZs.