Aerosol effects on cloud cover as evidenced by ground‐based and space‐based observations at five rural sites in the United States

Abstract

Previous studies of the second aerosol indirect (lifetime) effect on cloud cover have estimated the strength of the effect without correcting for near‐cloud contamination and other confounding factors. Here we combine satellite‐based observations with a multiyear ground‐based data set across five rural locations in the United States to more accurately constrain the second indirect aerosol effect and quantify aerosol effects on radiative forcing. Results show that near‐cloud contamination accounts for approximately 40% of the satellite‐derived aerosol‐cloud relationship. When contamination is removed and the effect of meteorological covariation is minimized, a strong physical aerosol effect on cloud cover remains. Averaged over all stations and after correcting for contamination, the daytime solar and total (solar + IR) radiative forcing is −52 W/m2 and −19 W/m2, respectively, due to both direct and indirect aerosol effects for aerosol optical depths (τ) between 0 and 0.3. Averaged diurnally, the average total radiative forcing is +16 W/m2.

Response to Revewer #1

“…In addition, the direct aerosol radiative effect (i.e. the increase in reflected radiation due to aerosol loading in clear-sky conditions) in satellite observations is significantly larger (35-65 %) than that inferred from larger (greater than 20 km 2 ) cloud-free ocean regions where the aerosol retrievals are located farther from the clouds (Twohy et al, 2009). The extent to which these near-cloud aerosols influence the correlationbased statistics used as diagnostics for aerosol-cloud interactions in satellite observations is largely unknown at the global scale.…”

“…The two views (satellite zenith angles at ∼ 55 • forward and nadir) offered by the instrument provide nearsimultaneous (within 90 s) observations of the same location on the Earth. This provides the ability to separate the surface from the atmospheric signals in order to increase the accuracy of the 1 km aerosol retrieval (in version 4.01 the aerosol is reported at 1 km resolution) using the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm (Thomas et al, 2009). Cloud properties are also retrieved using the same inputs (e.g.…”

“…Despite the advances in satellite-based retrievals in recent decades, obtaining robust statistical relationships between aerosols and clouds remains difficult. Challenges associated with obtaining accurate passive satellite retrievals generally involve the following issues: (1) an artificially high aerosol optical depth (AOD) retrieval due to the presence of undetected cloud in areas assumed to be cloud-free (cloud contamination) (Remer et al, 2005); (2) radiation scattered by clouds that illuminate the aerosol field and are not accounted for in the 1-D radiative transfer model in satellite retrievals causing erroneously high AOD retrievals (3-D effects) (Varnái and Marshak, 2009), and (3) humidification causing aerosols to swell near clouds, thereby enhancing AOD without any increase in aerosol number concentration (humidification effect) (Twohy et al, 2009). Attributing the contribution of these mechanisms to the enhancement in AOD and particle size near clouds is difficult using passive sensing instruments from satellite-based observations alone.…”

Combining airborne in situ and ground-based lidar measurements for attribution of aerosol layers

“…Aerosols are tiny particles suspended in the atmosphere that severely affect human health (e.g. Tie et al, 2009;Apte et al, 2015;Pope III et al, 2015) and climate. Most of the particles have a direct cooling effect on climate by scattering solar radiation (e.g.…”

Response to Reviewer #2

“…Challenges associated with obtaining accurate passive satellite retrievals generally involve the following issues: (1) an artificially high aerosol optical depth (AOD) retrieval due to the presence of undetected cloud in areas assumed to be cloud-free (cloud Published by Copernicus Publications on behalf of the European Geosciences Union. contamination) (Remer et al, 2005); (2) radiation scattered by clouds that illuminate the aerosol field and are not accounted for in the 1-D radiative transfer model in satellite retrievals causing erroneously high AOD retrievals (3-D effects) (Varnái and Marshak, 2009), and (3) humidification causing aerosols to swell near clouds, thereby enhancing AOD without any increase in aerosol number concentration (humidification effect) (Twohy et al, 2009). Attributing the contribution of these mechanisms to the enhancement in AOD and particle size near clouds is difficult using passive sensing instruments from satellite-based observations alone.…”