Abstract
Abstract. In this paper, we use observations from the NASA ORACLES (ObseRvations of CLouds above Aerosols and their intEractionS) aircraft campaign to develop a framework by way of two parameterizations that establishes regionally representative relationships between aerosol-cloud properties and their radiative effects. These relationships rely on new spectral aerosol property retrievals of the single scattering albedo (SSA) and asymmetry parameter (ASY). The retrievals capture the natural variability of the study region as sampled, and both were found to be fairly narrowly constrained (SSA: 0.83 ± 0.03 in the mid-visible, 532 nm; ASY: 0.54 ± 0.06 at 532 nm). The spectral retrievals are well suited for calculating the direct aerosol radiative effect (DARE) since SSA and ASY are tied directly to the irradiance measured in the presence of aerosols – one of the inputs to the spectral DARE. The framework allows for entire campaigns to be generalized into a set of parameterizations. For a range of solar zenith angles, it links the broadband DARE to the mid-visible aerosol optical depth (AOD) and the albedo (α) of the underlying scene (either clouds or clear sky) by way of the first parameterization: P(AOD, α). For ORACLES, the majority of the case-to-case variability of the broadband DARE is attributable to the dependence on the two driving parameters of P(AOD, α). A second, extended, parameterization PX(AOD, α, SSA) explains even more of the case-to-case variability by introducing the mid-visible SSA as a third parameter. These parameterizations establish a direct link from two or three mid-visible (narrowband) parameters to the broadband DARE, implicitly accounting for the underlying spectral dependencies of its drivers. They circumvent some of the assumptions when calculating DARE from satellite products or in a modeling context. For example, the DARE dependence on aerosol microphysical properties is not explicit in P or PX because the asymmetry parameter varies too little from case to case to translate into appreciable DARE variability. While these particular DARE parameterizations only represent the ORACLES data, they raise the prospect of generalizing the framework to other regions.
Highlights
During the African burning season of August–October, a semi-permanent stratocumulus cloud deck off the southern African western coast is overlaid by a thick layer of biomass burning aerosols
We evaluate the radiative effects of those aerosols where the relationships found between direct aerosol radiative effect (DARE), aerosol optical depth (AOD), and albedo form the foundation of the parameterizations that capture the collective variability sampled from the viable cases from ORACLES 2016 and 2017
Rameters, using nine cases from the 2016 and 2017 ORACLES campaigns. This observationally driven link is expressed by two parameterizations of the shortwave broadband DARE, (1) in terms of the mid-visible AOD and scene albedo (PDARE) and (2) in terms of the mid-visible AOD, scene albedo, and aerosol single scattering albedo (SSA) (PXDARE)
Summary
During the African burning season of August–October, a semi-permanent stratocumulus cloud deck off the southern African western coast is overlaid by a thick layer of biomass burning aerosols. The spectral DARE in W m−2 nm−1 is determined from the difference between the net irradiance (Fλnet) with and without the aerosol layer: DAREλ = Fλn,eater − Fλn,ento aer Aircraft measurements, such as those collected during ORACLES, provide direct observations of the components necessary to calculate DARE. Observations of the dependence of flux changes on AOD help to develop confidence in radiative forcing calculations based on measured aerosol properties (Russell et al, 1999; Redemann et al, 2006). In this sense, the RFE in conjunction with Eq (2) provides closure to those calculations and constrains them from the radiative flux and DARE perspective.
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