Aerosol indirect effect studies at Southern Great Plains during the May 2003 Intensive Operations Period

Using almost 10 years of observations of clouds and aerosols from the US Southern Great Plains (SGP) atmospheric observatory and the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) in China, the impact of aerosols on single-layer overcast clouds over continental land for different regimes were investigated. Atmospheric conditions at the two sites were first compared in an attempt to isolate the influence of aerosols on cloud properties from dynamic and thermodynamic influences. Cloud types and amounts are similar at the two sites. The dominant aerosol types at the SGP and SACOL sites are sulphate and dust, respectively, with greater aerosol optical depths (AODs) and absorption at the SACOL site. Aerosol first indirect effect (FIE) ranges from 0.021 to 0.152 and from −0.078 to 0.047 at the SGP and SACOL sites, respectively, when using the AOD below cloud base as CCN proxy. Although differences exist, the influence of meteorological conditions on the FIE at the two sites is consistent. FIEs are easily detected under descending motion and dry condition. The FIE at the SGP site is larger than that at the SACOL site, which suggests that the cloud albedo effect is more sensitive under relatively cleaner atmospheric conditions and the dominating aerosol at the SACOL site has less hygroscopicity. The radiative forcing of the FIE over the SGP site is −3.2 W m−2 for each 0.05 increment in FIE. Cloud durations generally prolong as aerosol loading increases, which is consistent with the hypothesis of the aerosol second indirect effect. The negative relationship between cloud duration time and aerosol loading when aerosol loading reaches a large value further might suggest a semidirect effect.

Download Full-text

Complexities in the First Aerosol Indirect Effect over the Southern Great Plains

10.5194/acp-2016-289 ◽

2016 ◽

Cited By ~ 1

Author(s):

Sam Pennypacker ◽

Allison L. Steiner

Keyword(s):

Droplet Size ◽

Great Plains ◽

Optical Thickness ◽

Indirect Effect ◽

Meridional Flow ◽

Ice Clouds ◽

Southern Great Plains ◽

Aerosol Loading ◽

Cloud Optical Thickness

Abstract. The aerosol first indirect effect (FIE) is typically characterized by a reduction in cloud droplet size and an increase in cloud optical thickness in the presence of high concentrations of condensation nuclei. Past studies have derived observational evidence of the FIE in specific locations and conditions, yet critical uncertainties in the validity of this conceptual model as it applies to a range of cloud types and meteorological settings remain unaddressed. We utilize five years of surface aerosol measurements and Moderate Resolution Imaging Spectroradiometer (MODIS) observations of cloud properties to discern the FIE in springtime cloud statistics over the Southern Great Plains region of the United States. We extend this analysis to explore the role of three confounding factors: cloud phase, observational uncertainty and the role of regional meridional flow. While high aerosol days are dominated by smaller average droplet size in liquid clouds, the response of cloud optical thickness is variable and is dominantly a function of cloud water path. Ice clouds experience more variability in their response to high aerosol loading and satellite retrieval uncertainty thresholds. Finally, the direction of meridional flow does not play a large role in stratifying the cloud response to different aerosol loading. Overall, these observations show that much of the classical theory for liquid clouds is supported. Higher aerosol loadings are correlated with a reduction in effective radius and generally higher cloud optical thickness, and this relationship dominates over any driving influence from the low-level jet. However, for ice clouds we see a variable response that may be driven by aerosol composition and cold cloud microphysics. These observations provide further insight into the importance of considering deviations from the classic FIE in understanding regional variability in aerosol-cloud interactions in a continental setting.

Download Full-text