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  • Author or Editor: R. Subramanian x
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V. K. Ponnulakshmi
,
V. Mukund
,
D. K. Singh
,
K. R. Sreenivas
, and
G. Subramanian

Abstract

Broadband flux emissivity schemes are often used to model infrared radiative exchanges in the atmosphere. In particular, such schemes help highlight the interaction of radiation with other transport processes, an aspect that is crucial to an understanding of phenomena relevant to the nocturnal boundary layer (NBL). Although the original schemes were restricted to radiatively black bounding surfaces, an extension of the same to nonblack surfaces has since been frequently used in NBL modeling. Herein, it is shown that the nonblack extension is erroneous and leads to a spurious yet intense near-surface cooling in the opaque bands. This paper presents the correct formulation that eliminates this cooling and discusses in some detail earlier NBL calculations affected by this error.

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V. K. Ponnulakshmi
,
D. K. Singh
,
V. Mukund
,
K. R. Sreenivas
, and
G. Subramanian

Abstract

In an accompanying paper by Ponnulakshmi et al., the prevailing flux emissivity scheme for nonblack surfaces was shown to include an erroneous reflected flux term. The error leads to a spurious cooling within the opaque bands; an expression for the correct broadband-reflected flux was given that eliminated this spurious cooling contribution. Herein, it is shown that the error is generic in nature, and is relevant to any frequency-parameterized radiation scheme applied to nonblack surfaces; such schemes are typically used in longwave radiation budget calculations. The correct reflected flux, previously developed within the framework of a broadband emissivity scheme in Ponnulakshmi et al., is generalized here so as to be applicable to any frequency-parameterized radiation model. The error is illustrated by comparing the bandwise fluxes, obtained using the prevailing and correct narrowband formulations, for a model (tropical) atmosphere. The flux discrepancy is the smallest for opaque bands within which the participating medium emits like a blackbody, and it is largest in frequency intervals where the medium is nearly transparent.

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Vaughan T. J. Phillips
,
Paul J. Demott
,
Constantin Andronache
,
Kerri A. Pratt
,
Kimberly A. Prather
,
R. Subramanian
, and
Cynthia Twohy

Abstract

A framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP’s third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced—namely, soluble organic aerosols.

The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment–Layer Clouds (ICE-L). Selected traverses were confined to temperatures between about −25° and −29°C in layer cloud without homogeneously nucleated ice from aloft. Some of the wave clouds were affected by carbonaceous aerosols from biomass burning and by dust from dry lakebeds and elsewhere. The EP predicts a trend between number concentrations of heterogeneously nucleated ice crystals and apparent black carbon among the five wave clouds, observed by aircraft in ICE-L. It is predicted in terms of IN activity of black carbon.

The EP’s predictions are consistent with laboratory and field observations not used in its construction, for black carbon, dust, primary biological aerosols, and soluble organics. The EP’s prediction of biological ice nucleation is validated using coincident field observations of PBAP IN and PBAPs in Colorado.

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Cynthia H. Twohy
,
Paul J. DeMott
,
Kerri A. Pratt
,
R. Subramanian
,
Gregory L. Kok
,
Shane M. Murphy
,
Traci Lersch
,
Andrew J. Heymsfield
,
Zhien Wang
,
Kim A. Prather
, and
John H. Seinfeld

Abstract

Ice concentrations in orographic wave clouds at temperatures between −24° and −29°C were shown to be related to aerosol characteristics in nearby clear air during five research flights over the Rocky Mountains. When clouds with influence from colder temperatures were excluded from the dataset, mean ice nuclei and cloud ice number concentrations were very low, on the order of 1–5 L−1. In this environment, ice number concentrations were found to be significantly correlated with the number concentration of larger particles, those larger than both 0.1- and 0.5-μm diameter. A variety of complementary techniques was used to measure aerosol size distributions and chemical composition. Strong correlations were also observed between ice concentrations and the number concentrations of soot and biomass-burning aerosols. Ice nuclei concentrations directly measured in biomass-burning plumes were the highest detected during the project. Taken together, this evidence indicates a potential role for biomass-burning aerosols in ice formation, particularly in regions with relatively low concentrations of other ice nucleating aerosols.

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Kerri A. Pratt
,
Andrew J. Heymsfield
,
Cynthia H. Twohy
,
Shane M. Murphy
,
Paul J. DeMott
,
James G. Hudson
,
R. Subramanian
,
Zhien Wang
,
John H. Seinfeld
, and
Kimberly A. Prather

Abstract

During the Ice in Clouds Experiment–Layer Clouds (ICE-L), aged biomass-burning particles were identified within two orographic wave cloud regions over Wyoming using single-particle mass spectrometry and electron microscopy. Using a suite of instrumentation, particle chemistry was characterized in tandem with cloud microphysics. The aged biomass-burning particles comprised ∼30%–40% by number of the 0.1–1.0-μm clear-air particles and were composed of potassium, organic carbon, elemental carbon, and sulfate. Aerosol mass spectrometry measurements suggested these cloud-processed particles were predominantly sulfate by mass. The first cloud region sampled was characterized by primarily homogeneously nucleated ice particles formed at temperatures near −40°C. The second cloud period was characterized by high cloud droplet concentrations (∼150–300 cm−3) and lower heterogeneously nucleated ice concentrations (7–18 L−1) at cloud temperatures of −24° to −25°C. As expected for the observed particle chemistry and dynamics of the observed wave clouds, few significant differences were observed between the clear-air particles and cloud residues. However, suggestive of a possible heterogeneous nucleation mechanism within the first cloud region, ice residues showed enrichments in the number fractions of soot and mass fractions of black carbon, measured by a single-particle mass spectrometer and a single-particle soot photometer, respectively. In addition, enrichment of biomass-burning particles internally mixed with oxalic acid in both the homogeneously nucleated ice and cloud droplets compared to clear air suggests either preferential activation as cloud condensation nuclei or aqueous phase cloud processing.

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T. Eidhammer
,
P. J. DeMott
,
A. J. Prenni
,
M. D. Petters
,
C. H. Twohy
,
D. C. Rogers
,
J. Stith
,
A. Heymsfield
,
Z. Wang
,
K. A. Pratt
,
K. A. Prather
,
S. M. Murphy
,
J. H. Seinfeld
,
R. Subramanian
, and
S. M. Kreidenweis

Abstract

The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well predicted in a 1D parcel model framework. Two different empirical parameterizations were used in the parcel model: a parameterization based on aerosol chemical type and surface area and a parameterization that links ice nuclei number concentrations to the number concentrations of particles with diameters larger than 0.5 μm. This study shows that aerosol size distribution and composition measurements can be used to constrain ice initiation by primary nucleation in models. The data and model results also suggest the likelihood that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous ice nuclei, at least for the lower temperatures examined in this case.

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