Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

Daniel T Mccoy,Daniel P Grosvenor,Gregory S Elsaesser,Jonathan M Wilkinson,Adrian A Hill,Paul R Field,Anja Schmidt,Ben J Shipway,Frida A.-M Bender

doi:10.5194/acp-18-5821-2018

Abstract

Abstract. Aerosol–cloud interactions are a major source of uncertainty in inferring the climate sensitivity from the observational record of temperature. The adjustment of clouds to aerosol is a poorly constrained aspect of these aerosol–cloud interactions. Here, we examine the response of midlatitude cyclone cloud properties to a change in cloud droplet number concentration (CDNC). Idealized experiments in high-resolution, convection-permitting global aquaplanet simulations with constant CDNC are compared to 13 years of remote-sensing observations. Observations and idealized aquaplanet simulations agree that increased warm conveyor belt (WCB) moisture flux into cyclones is consistent with higher cyclone liquid water path (CLWP). When CDNC is increased a larger LWP is needed to give the same rain rate. The LWP adjusts to allow the rain rate to be equal to the moisture flux into the cyclone along the WCB. This results in an increased CLWP for higher CDNC at a fixed WCB moisture flux in both observations and simulations. If observed cyclones in the top and bottom tercile of CDNC are contrasted it is found that they have not only higher CLWP but also cloud cover and albedo. The difference in cyclone albedo between the cyclones in the top and bottom third of CDNC is observed by CERES to be between 0.018 and 0.032, which is consistent with a 4.6–8.3 Wm−2 in-cyclone enhancement in upwelling shortwave when scaled by annual-mean insolation. Based on a regression model to observed cyclone properties, roughly 60 % of the observed variability in CLWP can be explained by CDNC and WCB moisture flux.

Highlights

The degree to which the aerosol indirect effects that result from anthropogenic aerosol emissions have acted to increase planetary albedo and mask greenhouse gas warming is highly uncertain (Andreae et al, 2005; Carslaw et al, 2013; Boucher et al, 2014; Forster, 2016)
This work was motivated by a set of idealized convection-permitting experiments designed to examine how midlatitude cyclone properties change in response to cloud microphysics
Analysis of observed covariability between meteorology, warm cloud microphysics, and cyclone cloud properties is consistent with increasing cloud droplet number concentration (CDNC), leading to an increase in cyclone cloud liquid water path, fractional coverage, and albedo

Summary

Introduction

The degree to which the aerosol indirect effects that result from anthropogenic aerosol emissions have acted to increase planetary albedo and mask greenhouse gas warming is highly uncertain (Andreae et al, 2005; Carslaw et al, 2013; Boucher et al, 2014; Forster, 2016). Establishing how much the aerosol emitted during the 20th century has enhanced the liquid water amount and the albedo of midlatitude storm systems is a key step in constraining the climate sensitivity inferred from the observational record. McCoy et al.: Aerosol midlatitude cyclone indirect effects in observations the synoptic-scale atmospheric processes and much smallerscale cloud microphysical processes play a role in regulating the cyclone life cycle (Naud et al, 2016, 2017; Grandey et al, 2013; Lu and Deng, 2015, 2016; Thompson and Eidhammer, 2014; Igel et al, 2013; Zhang et al, 2007)

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Conclusion