Abstract
Abstract. Aerosols can alter the macro- and micro-physical properties of deep convective clouds (DCCs) and their radiative forcing (CRF). This study presents what is arguably the first long-term estimate of the aerosol-mediated changes in CRF (AMCRF) for deep cloud systems derived from decade-long continuous ground-based and satellite observations, model simulations, and reanalysis data. Measurements were made at the US Department of Energy's Atmospheric Radiation Measurement Program's Southern Great Plains (SGP) site. Satellite retrievals are from the Geostationary Operational Environmental Satellite. Increases in aerosol loading were accompanied by the thickening of DCC cores and the expansion and thinning of anvils, due presumably to the aerosol invigoration effect (AIV) and the aerosol microphysical effect. Meteorological variables dictating these cloud processes were investigated. Consistent with previous findings, the AIV is most significant when the atmosphere is moist and unstable with weak wind shear. Such aerosol-mediated systematic changes in DCC core thickness and anvil size alter CRF at the top of atmosphere (TOA) and at the surface. Using extensive observations, ~300 DCC systems were identified over a 10 years period at the SGP site (2000–2011) and analyzed. Daily mean AMCRF at the TOA and at the surface are 29.3 W m−2 and 22.2 W m−2, respectively. This net warming effect due to changes in DCC microphysics offsets the cooling resulting from the first aerosol indirect effect.
Highlights
Aerosols can alter the radiative energy of the earth’s surfaceatmosphere system by directly attenuating solar radiation and/or by indirectly modifying cloud macro-physical and microphysical properties in many different ways
Following Li et al (2011) who revealed the impact of aerosols on cloud vertical development and precipitation frequency using ground-based measurements, this study aims to (1) identify factors governing the aerosol invigoration effect (AIV) and AIV-induced changes in cloud radiative forcing (CRF) under various thermodynamic and dynamic environmental conditions, and (2) estimate the long-term AIV-induced changes in the CRF at the top of the atmosphere (TOA) and at the surface using a combination of geostationary satellite retrievals and ground-based observations made at the US Department of Energy’s Atmospheric Radiation Measurement Program’s Southern Great Plains (SGP) site
Analyses of long-term cloud products from Atmospheric Radiation Measurement (ARM) measurements reveal that stratus and cirrus clouds are the major cloud types seen at the SGP site
Summary
Aerosols can alter the radiative energy of the earth’s surfaceatmosphere system by directly attenuating solar radiation and/or by indirectly modifying cloud macro-physical (areal coverage, structure, altitude) and microphysical properties (droplet size, phase) in many different ways. Studies have shown that high aerosol loadings can produce a large number of tiny cloud droplets (Twomey, 1977), which suppress the warm rain-forming process through indirect effects (Albrecht, 1989). This allows more cloud particles to ascend above the freezing level and convert to ice hydrometeors. The AIV is associated with the expansion of cloud anvil extent (Koren et al, 2010a) Such aerosolmediated changes in cloud parameters alter cloud radiative forcing (CRF), which is a component of aerosol radiative forcing (ARF). Among the aerosolinduced changes in radiative energy (IPCC, 2013), AMCRF
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