The first aerosol indirect effect quantified through airborne remote sensing during VOCALS-REx

Abstract

Abstract. The first aerosol indirect effect (1AIE) is investigated using a combination of in situ and remotelysensed aircraft (NCAR C-130) observations acquired during VOCALS-REx over the southeast Pacific stratocumulus cloud regime. Satellite analyses have previously identified a high albedo susceptibitility to changes in cloud microphysics and aerosols over this region. The 1AIE was broken down into the product of two independently-estimated terms: the cloud aerosol interaction metric, and the relative albedo (A) susceptibility S R-τ = dA 3d ln τ LWP , with τ and N a denoting retrieved cloud optical thickness and in situ aerosol concentration respectively and calculated for fixed intervals of liquid water path (LWP).ACI τ was estimated by combining in situ N a sampled below the cloud, with τ and LWP derived from, respectively, simultaneous upward-looking broadband irradiance and narrow field-of-view millimeter-wave radiometer measurements, collected at 1 Hz during four eight-hour daytime flights by the C-130 aircraft. ACIτ values were typically large, close to the physical upper limit (0.33), with a modest increase with LWP. The high ACIτ values slightly exceed values reported from many previous in situ airborne studies in pristine marine stratocumulus and reflect the imposition of a LWP constraint and simultaneity of aerosol and cloud measurements. S R-τ increased with LWP and τ , reached a maximum S R-τ (0.086) for LWP (τ ) of 58 g m −2 (∼14), and decreased slightly thereafter. The 1AIE thus increased with LWP and is comparable to a radiative forcing of −3.2-−3.8 W m −2 for a 10 % increase in N a , exceeding previously-reported global-range values. The aircraftderived values are consistent with satellite estimates derived from instantaneous, collocated Clouds and the Earth's Radiant Energy System (CERES) albedo and MOderate resolution Imaging Spectroradiometer (MODIS)-retrieved droplet number concentrations at 50 km resolution. The consistency of the airborne and satellite estimates, despite their independent approaches, differences in observational scales, and retrieval assumptions, is hypothesized to reflect the ideal remote sensing conditions for these homogeneous clouds. We recommend the southeast Pacific for regional model assessments of the first aerosol indirect effect on this basis. This airborne remotely-sensed approach towards quantifying 1AIE should in theory be more robust than in situ calculations because of increased sampling. However, although the technique does not explicitly depend on a remotelyderived cloud droplet number concentration (N d ), the attimes unrealistically-high N d values suggest more emphasis on accurate airborne radiometric measurements is needed to refine this approach.

Dual‐FOV Raman and Doppler lidar studies of aerosol‐cloud interactions: Simultaneous profiling of aerosols, warm‐cloud properties, and vertical wind

“…For comparison, average values of airborne in situ observation for ACI N were close 0.55 [ Lu et al , ; Terai et al , ]. As an exception, Painemal and Zuidema [] found even larger values of 0.76–0.92. In these airborne field campaigns of marine layered clouds, flights were not sorted according to different flight levels within the cloud layer (base, center, and top level) and all these observations were then equally considered in the ACI studies, i.e., without distinguishing between the different height levels above cloud base.…”

Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign

“…The observed responses of warm low-level cloud properties to aerosols in a marine environment have been studied in recent years based on multiple observations, such as those from satellite-based remote sensing (Nakajima et al 2001;Menon et al 2008;Su et al 2010;Wang et al 2014;Dong et al 2014aDong et al , 2015, from surface-based remote sensing (Feingold et al 2001McComiskey et al 2009;Pandithurai et al 2009), and from aircraft (Zheng et al 2010;Painemal and Zuidema 2013;Twohy et al 2013). However, satellite remote sensing suffers from several inherent retrieval problems (Li et al 2009;Xi et al 2014).…”

Strong aerosol–cloud interaction in altocumulus during updraft periods: lidar observations over central Europe

“…The reason for the strong contrast between the ACI N values for clouds over land and sea is not clear but may be related to the fact that the observed cloud fields over oceans form at comparably simple meteorological and aerosol conditions. The studied short-lived cumuli fields or aged stratocumulus layers mostly develop within a well-mixed, undisturbed marine boundary layer at almost adiabatic-like stratification of the water content resulting in an height-independent CDNC from cloud base to top (Painemal and Zuidema, 2013). Effects of vertical motions (updrafts, turbulent mixing, and entrainment of drier air into the clouds) may then be comparably weak (Twohy et al, 2005;Terai et al, 2012;Werner et al, 2014).…”

“…An interesting approach (leading to a high study-mean ACI of 0.86) is presented by Painemal and Zuidema (2013). They combined airborne fast (1 Hz sampling) in situ measurements of N acc below the cloud with cloud optical depth and liquid-water path values obtained from simultaneous observations (also at 1 Hz resolution) with upward-looking broadband irradiance and narrow field-of-view millimeterwave radiometers.…”