arXiv subjects

Zachary McGraw

Publications and source records attributed to Zachary McGraw.

3 recordsLinked to original sources

arXiv2026

Reconciling the Lack of a Robust Anvil Cloud Amount Response to Warming

The stability-iris hypothesis proposes that tropical anvil cloud amount decreases with warming as anvils rise into environments of greater static stability. Here we show that there is no clear theoretical or simulated link between increasing static stability and anvil amount. We demonstrate analytically that identified sensitivities of static stability and pressure velocity to surface warming arise from decreasing air density aloft, which enters pressure-coordinate formulations but does not represent a process that dynamically constrains anvil formation. An ensemble of RCEMIP simulations verifies that the stability increase is robust yet arises from the air density reduction, and hence is not accompanied by a robust response in anvil amount, convectively driven water convergence into anvil levels, or the efficiency with which this convergence produces cloudiness. Overall, these findings dispel the expectation of a systematic reduction in anvil amount with warming arising from basic physical arguments.

physics.ao-ph↗

arXiv2026

No Reduction of Tropical Convection with Warming Expected from Theory or Models

Theoretical arguments have for decades anticipated substantial weakening of tropical convection as the surface warms, yet convection-resolving models show no robust change in column-integrated convective mass transport and an increase in transported volume. Here we reconcile this apparent discrepancy by demonstrating that the two main theoretical arguments identify a redistribution, rather than a straightforward reduction, of convective motions. A commonly invoked hydrological argument accordingly captures a weakening at a single altitude that is offset by strengthening aloft as the troposphere expands upward. Further, reduced convective mass flux following atmospheric structures that rise with warming simply reflects decreasing air density aloft. Convective motions and transported water are shown to be highly invariant to surface warming along isotherms, and the ability to predict convective responses at individual altitudes is a manifestation of this invariance. Together, these findings recast and simplify long-standing expectations of how the tropical atmosphere will respond to warming.

physics.ao-ph↗

arXiv2025

Direct Radiative Impacts of Stratospheric Aerosols on the Tropical Troposphere: Clouds, Precipitation, and Circulation in Convection-Resolving and Global Simulations

A concern for stratospheric aerosol injection (SAI) is that stratospheric aerosols could inadvertently alter rain and winds through mechanisms independent of the intended surface cooling. We here use a multi-model framework to investigate how the tropical troposphere responds to SAI when sea surface temperatures are held fixed. By performing convection-resolving simulations in small-domains and in mock-Walker setups, and contrasting these with global climate model simulations, we trace how stratospheric aerosols radiatively heat the troposphere, and in turn alter convection, clouds, and rainfall. Our simulations show an SAI-induced reduction in tropical mean precipitation, yet decreased cloud radiative heating moderates this effect and complicates its predictability. Regional rainfall anomalies within the tropics can be substantial. However, surface-temperature-independent effects on tropical circulation are found to be negligible, indicating that stratospheric aerosols do not inherently alter the tropical overturning circulation as previously suggested. These results clarify the mechanisms governing SAI hydroclimate impacts and show that key uncertainties arise from cloud processes that models are unable to constrain. Consequently, near-term SAI deployment would carry the risk of being implemented without the ability to reliably predict its hydroclimate impacts.

physics.ao-ph↗