How do cold pools influence the size of tropical cyclone embryos?

TL;DR

This paper develops a theoretical cloud chain model to quantify how cold pools influence tropical cyclone embryo size, integrating moisture dynamics and convective memory effects, and compares it with simulations.

Contribution

It introduces a novel cloud chain model linking cold pool dynamics to TC embryo size, incorporating moisture and convective memory effects, and couples it with the QG model for analytical insights.

Findings

The model captures the trend of cold pool influence on TC size.

Overestimation of embryo size suggests model simplifications.

Coupling with QG provides analytical expression for TC embryo size.

Abstract

The size of tropical cyclone (TC) embryos is an essential predictor of TC genesis. Recent studies have identified cold pools and planetary rotation as factors that increase and decrease TC embryo size. While the planetary rotation effect has been depicted using a quasi-geostrophic (QG) model, the cold pool effect still lacks a theoretical model. This paper presents a cloud chain model to derive the length scale regarding the influence of cold pools on the TC embryo vortex. Within the model, the amount of rain evaporation during a single convective event determines the wind speed and humidity at the cold pool edge, influencing the amount of sub-cloud moisture convergence for the next-generation convection and, therefore, the intensity of the next-generation cold pool. A perturbation analysis shows that cold pools exhibit a nonlocal dependence on air-column humidity, with the influence range determined by the cold pool size and a convective memory weight. The memory weight relies on the sum of the contributions of mechanical lifting and thermodynamic forcing to convective initiation. A crucial parameter is the ratio of rain evaporation to surface evaporation in a cold pool. By coupling the cloud chain model with the QG equation, an analytical expression for the TC embryo size is obtained. The theory captures the trend but overestimates the TC embryo size in cloud-permitting simulations. The deviation might be due to the oversimplification in estimating the fractional contribution of cold pools to convective initiation.