A Lagrangian Perspective on the Growth of Midlatitude Storms

TL;DR

This study uses a Lagrangian approach with ERA-5 data to analyze extratropical storm growth, revealing nonlinear relationships with baroclinicity and emphasizing the importance of storm growth time over maximum intensity.

Contribution

It introduces a Lagrangian perspective to quantify storm growth, revealing nonlinear effects of baroclinicity and proposing a correction to traditional linear models.

Findings

Storm growth rate increases monotonically with baroclinicity.

High baroclinicity reduces storm maximum intensity due to shorter growth time.

A simplified model reproduces the nonlinear relationship between baroclinicity and storm growth.

Abstract

Extratropical storms dominate midlatitude climate and weather and are known to grow baroclinicaly and decay barotropicaly. Traditionally, quantitative climatic measures of storm growth have been mostly based on Eulerian measures, taking into account the mean state of the atmosphere and how those affect eddy growth, but they do not consider the Lagrangian growth of the storms themselves. Here, using ERA-5 reanalysis data and tracking all extratropical storms (cyclones and anticyclones) from 83 years of data, we examine the actual growth of the storms and compare it to the Eulerian characteristics of the mean state as the storms develop. In the limit of weak baroclinicity, we find that baroclinicity provides a good measure for storm maximum intensity. However, this monotonic relationship breaks for high baroclinicity levels. We show that although the actual growth rate of individual storms monotonically increases with baroclinicity, the reduction in maximum intensity at high baroclinicity is caused by a decrease in storm growth time. Based on the Lagrangian analysis, we suggest a nonlinear correction to the traditional linear connection between baroclinicity and storms' activity. Then, we show that a simplified model of storm growth, incorporating the baroclinicity effect on the vertical tilt of anomalies, reproduces the observed nonlinear relationship. Expanding the analysis to include the mean flow's barotropic properties highlights their marginal effect on storm growth rate, but the crucial impact on growth time. Our results emphasize the potential of Lagrangianly studying storm dynamics to advance understanding of the midlatitude climate.