Shallow-cloud impact on climate and uncertainty: A simple stochastic model

Eli A. Mueller,Samuel N. Stechmann

doi:10.1515/mcwf-2020-0002

Abstract

AbstractShallow clouds are a major source of uncertainty in climate predictions. Several different sources of the uncertainty are possible—e.g., from different models of shallow cloud behavior, which could produce differing predictions and ensemble spread within an ensemble of models, or from inherent, natural variability of shallow clouds. Here, the latter (inherent variability) is investigated, using a simple model of radiative statistical equilibrium, with oceanic and atmospheric boundary layer temperatures,ToandTa, and with moistureqand basic cloud processes. Stochastic variability is used to generate a statistical equilibrium with climate variability. The results show that the intrinsic variability of the climate is enhanced due to the presence of shallow clouds. In particular, the on-and-off switching of cloud formation and decay is a source of additional climate variability and uncertainty, beyond the variability of a cloud-free climate. Furthermore, a sharp transition in the mean climate occurs as environmental parameters are changed, and the sharp transition in the mean is also accompanied by a substantial enhancement of climate sensitivity and uncertainty. Two viewpoints of this behavior are described, based on bifurcations and phase transitions/statistical physics. The sharp regime transitions are associated with changes in several parameters, including cloud albedo and longwave absorptivity/carbon dioxide concentration, and the climate state transitions between a partially cloudy state and a state of full cloud cover like closed-cell stratocumulus clouds. Ideas of statistical physics can provide a conceptual perspective to link the climate state transitions, increased climate uncertainty, and other related behavior.

Highlights

Clouds have long been recognized as a leading source of uncertainty in future climate predictions [5, 7, 9, 41]
It is being used with boundary-layer-integrated quantities, q and Ta, which is outside its range of normal use, but allows a simple parameterization that maintains physical consistency between (i) the amount of latent heating and (ii) the amount of water converted between vapor and liquid
Given the earlier results that show the impact of shallow clouds on mean climate and climate uncertainty, we investigate whether shallow clouds may have an impact on climate sensitivity

Summary

Introduction

Clouds have long been recognized as a leading source of uncertainty in future climate predictions [5, 7, 9, 41]. Such a formula is commonly used in large-eddy simulations (LES) and cloud-resolving models (CRMs) [12, 13, 16] It is being used with boundary-layer-integrated quantities, q and Ta, which is outside its range of normal use, but allows a simple parameterization that maintains physical consistency between (i) the amount of latent heating and (ii) the amount of water converted between vapor and liquid. The chosen value for al and the relative values of al and al were motivated by the fact that longwave absorption is dominated by water vapor in the atmosphere, and, upon cloud formation, the boundary layer behaves as a nearly perfect black body in the infrared spectrum In terms of these uxes, the net radiative ux at the sea surface and boundary layer are given by. The longwave absorptivity was varied as a proxy for carbon dioxide (CO ) concentration in section 5, and many other changes to the parameterizations were explored in sections 6 and 7

Numerical Methods

Discussion of Additional Processes

Findings

C Number of equilibrium completely cloudy states

Conclusions