Allison McComiskey

Comparison of methods for deriving aerosol asymmetry parameter

Received 21 December 2004; revised 19 March 2005; accepted 7 June 2005; published 21 January 2006. [1] Values for Mie-equivalent aerosol asymmetry parameter (g) were derived using a variety of methods from the large suite of measurements (in situ and remote from surface and aircraft) made in Oklahoma during the 2003 aerosol Intensive Operations Period (IOP). Median values derived for dry asymmetry parameter at 550 nm ranged between 0.55 and 0.63 over all instruments and for all derivation methods, with the exception of one instrument which did not measure over the full size range of optically important aerosol. Median values for the ‘‘wet’’ asymmetry parameter (i.e., asymmetry parameter at humidity conditions closer to ambient) were between 0.59 and 0.72. Values for g derived for surface and airborne in situ measurements were highly correlated, but in situ and remote sensing measurements both at the surface and aloft did not agree as well because of vertical inhomogeneity of the aerosol. Radiative forcing calculations suggest that a 10% decrease in g would result in a 19% reduction in top of atmosphere radiative forcing for the conditions observed during the IOP. Comparison of the different methods for deriving g suggests that in computing the asymmetry parameter, aerosol size is the most important parameter to measure; composition is less important except for how it influences the hygroscopic growth (i.e., size) of particles.

Direct aerosol forcing: Calculation from observables and sensitivities to inputs

[1] Understanding sources of uncertainty in aerosol direct radiative forcing (DRF), the difference in a given radiative flux component with and without aerosol, is essential to quantifying changes in Earth’s radiation budget. We examine the uncertainty in DRF owing to measurement uncertainty in the quantities on which it depends: aerosol optical depth, single scattering albedo, asymmetry parameter, solar geometry, and surface albedo. Direct radiative forcing at the top of the atmosphere and at the surface is calculated at three locations representing distinct aerosol types and radiative environments. Sensitivities, the changes in DRF in response to unit changes in individual aerosol or surface properties, are also calculated for these conditions. The uncertainty in DRF associated with a given property is computed as the product of the sensitivity and typical measurement uncertainty in the respective property. Sensitivity and uncertainty values permit estimation of total uncertainty in calculated DRF and identification of properties that most limit accuracy in estimating forcing. Absolute total uncertainties in modeled local diurnally averaged forcing range from 0.2 to 3.1 W m � 2 for the ranges of properties examined here. Relative total uncertainties range from � 20 to 80% with larger values at higher latitudes, where fluxes are low. The largest contributor to total uncertainty in DRF is single scattering albedo; however, decreasing measurement uncertainties for any property would increase accuracy in DRF. Comparison of two radiative transfer models suggests the contribution of modeling error is small compared to the total uncertainty although comparable to uncertainty arising from some individual properties.

On the link between ocean biota emissions, aerosol, and maritime clouds: airborne, ground, and satellite measurements off the coast of California.

Surface, airborne, and satellite measurements over the eastern Pacific Ocean off the coast of California during the period between 2005 and 2007 are used to explore the relationship between ocean chlorophyll a, aerosol, and marine clouds. Periods of enhanced chlorophyll a and wind speed are coincident with increases in particulate diethylamine and methanesulfonate concentrations. The measurements indicate that amines are a source of secondary organic aerosol in the marine atmosphere. Subsaturated aerosol hygroscopic growth measurements indicate that the organic component during periods of high chlorophyll a and wind speed exhibit considerable water uptake ability. Increased average cloud condensation nucleus (CCN) activity during periods of increased chlorophyll a levels likely results from both size distribution and aerosol composition changes. The available data over the period of measurements indicate that the cloud microphysical response, as represented by either cloud droplet number concentration or cloud droplet effective radius, is likely influenced by a combination of atmospheric dynamics and aerosol perturbations during periods of high chlorophyll a concentrations.

Trends in sulfate and organic aerosol mass in the Southeast U.S.: Impact on aerosol optical depth and radiative forcing

Emissions of SO2 in the United States have declined since the early 1990s, resulting in a decrease in aerosol sulfate mass in the Southeastern U.S. of −4.5(±0.9)% yr−1 between 1992 and 2013. Organic aerosol mass, the other major aerosol component in the Southeastern U.S., has decreased more slowly despite concurrent emission reductions in anthropogenic precursors. Summertime measurements in rural Alabama quantify the change in aerosol light extinction as a function of aerosol composition and relative humidity. Application of this relationship to composition data from 2001 to 2013 shows that a −1.1(±0.7)% yr−1 decrease in extinction can be attributed to decreasing aerosol water mass caused by the change in aerosol sulfate/organic ratio. Calculated reductions in extinction agree with regional trends in ground-based and satellite-derived aerosol optical depth. The diurnally averaged summertime surface radiative effect has changed by 8.0 W m−2, with 19% attributed to the decrease in aerosol water.

New approaches to quantifying aerosol influence on the cloud radiative effect

The topic of cloud radiative forcing associated with the atmospheric aerosol has been the focus of intense scrutiny for decades. The enormity of the problem is reflected in the need to understand aspects such as aerosol composition, optical properties, cloud condensation, and ice nucleation potential, along with the global distribution of these properties, controlled by emissions, transport, transformation, and sinks. Equally daunting is that clouds themselves are complex, turbulent, microphysical entities and, by their very nature, ephemeral and hard to predict. Atmospheric general circulation models represent aerosol−cloud interactions at ever-increasing levels of detail, but these models lack the resolution to represent clouds and aerosol−cloud interactions adequately. There is a dearth of observational constraints on aerosol−cloud interactions. We develop a conceptual approach to systematically constrain the aerosol−cloud radiative effect in shallow clouds through a combination of routine process modeling and satellite and surface-based shortwave radiation measurements. We heed the call to merge Darwinian and Newtonian strategies by balancing microphysical detail with scaling and emergent properties of the aerosol−cloud radiation system.

Effect of gradients in biomass burning aerosol on shallow cumulus convective circulations

This study examines the effect of spatial gradients in biomass burning (BB) aerosol on mesoscale circulations and clouds in the Amazon through high-resolution numerical modeling over areas of 30 km to 60 km. Inhomogeneous horizontal distribution of BB aerosol results in differential surface heat fluxes and radiative heating of the air, which generates circulation patterns that strongly influence cloud formation. The influence on air circulation and cumulus cloud formation depends on the BB aerosol loading, its vertical location, and the width of the plume. Plumes that reside at higher altitudes (~1.5 km altitude) produce monotonic responses to aerosol loading whereas the response to plumes close to the surface changes nonmonotonically with plume width and aerosol loading. Sensitivity tests highlight the importance of interactive calculations of surface latent and heat fluxes with a coupled land surface model. In the case of the plume residing at higher altitude, failure to use interactive fluxes results in a reversal of the circulation whereas for the plume residing nearer the surface, the interactive surface model weakens the circulation. The influence of the BB aerosol on heating patterns, circulations, surface fluxes, and resultant cloud amount prevails over the BB aerosol-cloud microphysical influences.

ARM’s Aerosol–Cloud–Precipitation Research (Aerosol Indirect Effects)

The conception of the Department of Energy’s (DOE’s) Atmospheric Radiation Measurement (ARM) Program over 20 years ago demonstrated prescience on the part of a number of astute scientists, many of whose words fill the pages of this monograph. The early years focused on a handful of cloud and radiation measurements and activities of relatively limited scope. The intervening decades have seen these efforts expanded to some of the finest instrumentation in theworld tomeasure aerosol, clouds, radiation, and precipitation, accompanied by a substantial modeling effort. Together these have allowed theUnited States and international communities to tackle one of the thorniest problems associated with climate change, namely the influence of aerosol particles on cloud microphysics, precipitation, and cloud radiative properties (aerosol indirect effects).ARMresearchwas at the forefront of aerosol indirect efforts from the outset, but because instrumentation was not readily in place and retrieval methodologies were still in their infancy, these were necessarily modeling efforts (e.g., Ghan et al. 1990; Feingold and Heymsfield 1992; Kim and Cess 1993) that addressed subsets of the problem. These early endeavors joined other key studies highlighting the climate forcing potential of tropospheric aerosol (e.g., Charlson et al. 1992) in setting the stage for a research effort that is, to this day, one of the cornerstones of ARM and the Atmospheric System Research Program (ASR). The goal of this chapter is to summarize ARM and ASR efforts in this realm. Because this chapter deals with measurement and modeling capabilities pertaining to each of the components of the aerosol–cloud– precipitation–radiation system, it rests heavily on other chapters that deal more specifically with each individual component. At the outset we note that the term ‘‘aerosol indirect effects’’ is often used loosely to include all aspects of aerosol–cloud interactions, whereas, by definition, the indirect effect is the radiative effect or forcing associated with these interactions. We will therefore introduce the term aerosol–cloud interactions (ACI) when we refer primarily to the microphysical/dynamical aspects of the problem and reserve ‘‘indirect effects’’ for the radiative forcing. ARM’s early focus on atmospheric radiation measurements, followed by years of refinement of microphysical retrievals, has placed it in an excellent position to address both ACI and the associated indirect effects. ACI or aerosol indirect effects are often used to convey a few underlying microphysical processes. The first is the ‘‘albedo effect’’ (Twomey 1977), which states that an increase in the number of aerosol particles results in more cloud condensation nuclei (CCN), a higher droplet concentration, and, all else being equal (particularly liquid water content), smaller drops and a more reflective cloud. It is a fundamental expression of the ability of aerosol particles to generate a larger drop surface area to volume ratio. The second, the ‘‘lifetime effect’’ (Albrecht 1989), proposes that aerosol suppression

A Bird’s-Eye View: Development of an Operational ARM Unmanned Aerial Capability for Atmospheric Research in Arctic Alaska

AbstractThorough understanding of aerosols, clouds, boundary layer structure, and radiation is required to improve the representation of the Arctic atmosphere in weather forecasting and climate models. To develop such understanding, new perspectives are needed to provide details on the vertical structure and spatial variability of key atmospheric properties, along with information over difficult-to-reach surfaces such as newly forming sea ice. Over the last three years, the U.S. Department of Energy (DOE) has supported various flight campaigns using unmanned aircraft systems [UASs, also known as unmanned aerial vehicles (UAVs) and drones] and tethered balloon systems (TBSs) at Oliktok Point, Alaska. These activities have featured in situ measurements of the thermodynamic state, turbulence, radiation, aerosol properties, cloud microphysics, and turbulent fluxes to provide a detailed characterization of the lower atmosphere. Alongside a suite of active and passive ground-based sensors and radiosondes deploy...

A Novel Approach to Atmospheric Measurements Using Gliding UASs

Atmospheric aerosols and ozone (O3) have lifetimes of days to weeks and continuously evolve chemically and physically. Frequent and globally spaced vertical profiles of O3, aerosol optical density, particle size distribution, hygroscopic growth, and light absorption coefficients are highly desired in order to understand their controlling processes and subsequent effects on air quality and climate. High costs and logistical restrictions prohibit frequent profiling on a global scale using current technologies. We propose a new approach using state-of-the-art technologies including 3D printing and an unpowered small Unmanned Aircraft System to make the desired measurements at a fraction of the cost of current conventional methods.

The Radiative Properties of Small Clouds: Multi-Scale Observations and Modeling

Warm, liquid clouds and their representation in climate models continue to represent one of the most significant unknowns in climate sensitivity and climate change. Our project combines ARM observations, LES modeling, and satellite imagery to characterize shallow clouds and the role of aerosol in modifying their radiative effects.