Aerosol-induced modification of organised convection and top-of-atmosphere radiation

Nidhi Nishant,Olivier Geoffroy,Steven C. Sherwood

doi:10.1038/s41612-019-0089-1

Nidhi Nishant, Olivier Geoffroy + Show 1 more

Open Access

https://doi.org/10.1038/s41612-019-0089-1

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Abstract

Aerosol effects on cloud cover and precipitation could affect the global climate but have proven difficult to verify, because cloud and rain amounts are so strongly influenced by local meteorological conditions. Here model and observational evidence is presented that an increase in CCN concentration slightly invigorates mixed-phase convective clouds and narrows tropical convergence and rain bands, while expanding associated cloud cover particularly at mid-levels. A suite of model simulations with various approaches indicates a 4 ± 3.8% decrease in the rain-to-cloud area ratio per doubling of the CCN concentration, an effect also detected in satellite observations. Idealised numerical experiments suggest the area ratio change is due to the invigoration-induced static stability increase. Although the invigoration and cloud amount changes are much smaller than suggested in some studies, in simulations the latter cool the planet by 0.71 ± 0.25 W/m2 in deep convective regions, suggesting a global effect of order 0.2–0.5 W/m2, per aerosol doubling. The contribution to present-day anthropogenic forcing is even harder to quantify but could compare to that of the direct aerosol radiative forcing. These results indicate a previously unrecognised pathway for aerosols to indirectly cool the climate by altering convective clouds dynamically.

Highlights

It has been hypothesized that aerosols can affect deep convective clouds via a process, here called “Aerosol–cloud invigoration,” where an increase in aerosol loading deepens the convective clouds due to the strong coupling between the cloud microphysics and cloud system dynamics.[17,18]
Stronger updraft velocities (Supplementary Fig. 2) occur when aerosol effects are represented in the model, irrespective of whether the aerosols are represented as the proxy perturbations to latent heating (PERT20 and PERT2) or as high concentration nuclei (CCN) concentration (POLL2)
Precipitation and cloud-area data came from the University of Utah Tropical Rainfall Measuring Mission (TRMM) satellite feature database,[64] which reports raining and cloud areas based on the TRMM Microwave Image (TMI) and Visible and Infrared Sensor npj Climate and Atmospheric Science (2019) 33

Summary

Introduction

Aerosol-induced changes in cloud structure have clear implications for cloud radiative effects and precipitation[1,2,3,4,5] via aerosol–cloud interactions.[6,7] Until now most attention has focused on low clouds (i.e., non-precipitating or warm-rain), where reduction of cloud droplet size is well established[8,9,10,11,12] but the net effect on the albedo and lifetime of the low clouds remains controversial.[13,14,15,16] For deep-convective clouds, which contain frozen and liquid hydrometeors, these effects are extremely complex and poorly understood due to the complicated dynamics, thermodynamics and microphysics of these clouds.It has been hypothesized that aerosols can affect deep convective clouds via a process, here called “Aerosol–cloud invigoration,” where an increase in aerosol loading deepens the convective clouds due to the strong coupling between the cloud microphysics and cloud system dynamics.[17,18] Numerous observation-based studies have reported higher cloud tops and cloud cover with greater aerosol loading;[19,20,21,22] difficulty in discriminating aerosol–cloud interaction from meteorological covariations[23] and uncertainties in satellite retrievals[24,25,26,27,28] are major limitations for quantifying these effects.[29]. Experiments with proxy heating perturbations and high CCN/cloud droplet concentrations are denoted as PERT and POLL respectively, followed by number 2 or 20 denoting the spatial resolution.

Results

Conclusion