On the twilight zone between clouds and aerosols

Koren, Ilan; Remer, L. A.; Kaufman, Yoram J.; Rudich, Yinon; Martins, J. V.

doi:10.1029/2007gl029253

Cited by 316 publications

(390 citation statements)

References 18 publications

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“…This particular combination of changes results in a 16% AOD increase near cloud edge at 532 nm wavelength. Sun-photometer measurements from two recent studies yield comparable AOD increase estimates near cloud edge of $10% [Redemann et al, 2009] and $13% [Koren et al, 2007] at visible wavelengths, bearing in mind that our analysis is at night, while theirs is during the day which may cause AOD underestimates due to 3-D radiative cloud-adjacency effects. A simultaneous increase in R j and decrease in N j and s j is consistent with collision-coalescence with larger aerosols removed by precipitation scavenging, although precipitation scavenging may be minimal given that only $5% of these clouds precipitate [Snodgrass et al, 2009].…”

Section: Resultsmentioning

confidence: 86%

Enhanced aerosol backscatter adjacent to tropical trade wind clouds revealed by satellite‐based lidar

Tackett¹,

Girolamo²

2009

Geophysical Research Letters

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[1] There have been many proposed processes by which clouds modify aerosol properties, suggesting that aerosol properties near clouds are different than far from clouds. We provide the first satellite-based lidar observations of backscatter and color ratio, quantities directly related to aerosol properties, as a function of distance to cloud edge for boundary layer clouds over the western tropical Atlantic. We show backscatter and color ratio are enhanced adjacent to cloud edge, particularly near cloud top and cloud base. Specifically, layer integrated median backscatter increases by 31 ± 3% and 42 ± 2%, at wavelengths of 532 nm, and 1,064 nm, respectively, and layer averaged color ratio increases by 15 ± 5%. Backscatter calculations suggest our observations adjacent to cloud are best explained by an aerosol size distribution with reduced number concentration, increased median radius, and decreased width compared to far from cloud.

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Section: Resultsmentioning

confidence: 86%

Enhanced aerosol backscatter adjacent to tropical trade wind clouds revealed by satellite‐based lidar

Tackett¹,

Girolamo²

2009

Geophysical Research Letters

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“…For example, Koren et al (2007) showed that a belt of forming and evaporating cloud fragments and hydrated aerosols, extending kilometers away from the clouds into cloud-free areas could, have major implications on the estimation of Earth's radiation budget. This phenomenon has been termed the "twilight zone".…”

Section: Introductionmentioning

confidence: 99%

Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties

et al. 2012

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Abstract. One of the major uncertainties in the understanding of Earth's climate system is the interaction between solar radiation and aerosols in the atmosphere. Aerosols exposed to high humidity will change their chemical, physical, and optical properties due to their increased water content. To model hydrated aerosols, atmospheric chemistry and climate models often use the volume weighted mixing rule to predict the complex refractive index (RI) of aerosols when they interact with high relative humidity, and, in general, assume homogeneous mixing. This study explores the validity of these assumptions. A humidified cavity ring down aerosol spectrometer (CRD-AS) and a tandem hygroscopic DMA (differential mobility analyzer) are used to measure the extinction coefficient and hygroscopic growth factors of humidified aerosols, respectively. The measurements are performed at 80 % and 90 %RH at wavelengths of 532 nm and 355 nm using size-selected aerosols with different degrees of absorption; from purely scattering to highly absorbing particles. The ratio of the humidified to the dry extinction coefficients (f RH ext (%RH, Dry)) is measured and compared to theoretical calculations based on Mie theory. Using the measured hygroscopic growth factors and assuming homogeneous mixing, the expected RIs using the volume weighted mixing rule are compared to the RIs derived from the extinction measurements.We found a weak linear dependence or no dependence of f RH(%RH, Dry) with size for hydrated absorbing aerosols in contrast to the non-monotonically decreasing behavior with size for purely scattering aerosols. No discernible difference could be made between the two wavelengths used. Less than 7 % differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20 % difference. However, for substances with growth factor less than 1.15 the volume weighted mixing rule assumption needs to be taken with caution as the imaginary part of the complex RI can be underestimated.

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“…[5] From the ground, Koren et al [2007] showed that AOD retrieved from AERONET sunphotometers [Holben et al, 1998] decreases with time after the passage of clouds, while Angstrom exponent increases. Chiu et al [2009] used the Atmospheric Radiation Measurement (ARM) Shortwave Spectrometer (SWS) measurements to study the transition zone between cloud-free and cloudy areas.…”

Section: Introductionmentioning

confidence: 99%

“…[3] From satellites, many studies found that the brightness of cloud-free areas systematically increases near clouds [Ignatov et al, 2005;Loeb and Manalo-Smith, 2005;Matheson et al, 2005;Zhang et al, 2005;Koren et al, 2007;Loeb and Schuster, 2008]. The enhanced brightness can result from several factors [Twohy et al, 2009;Varnai and Marshak, 2009], including: small-cloud contamination [e.g., Zhang et al, 2005]; cloud-aerosol microphysics like swelling and increase of number concentration of aerosols in the humid environment near clouds [e.g., Su et al, 2008]; and three-dimensional (3D) radiative interactions between clouds and surrounding clear areas [e.g., Wen et al, 2007;Marshak et al, 2008].…”

Section: Introductionmentioning

confidence: 99%

Spectral invariant behavior of zenith radiance around cloud edges observed by ARM SWS

Marshak¹,

Knyazikhin²,

Wiscombe³

2009

Geophysical Research Letters

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[1] The ARM Shortwave Spectrometer (SWS) measures zenith radiance at 418 wavelengths between 350 and 2170 nm. Because of its 1-sec sampling resolution, the SWS provides a unique capability to study the transition zone between cloudy and clear sky areas. A spectral invariant behavior is found between ratios of zenith radiance spectra during the transition from cloudy to cloud-free. This behavior suggests that the spectral signature of the transition zone is a linear mixture between the two extremes (definitely cloudy and definitely clear). The weighting function of the linear mixture is a wavelength-independent characteristic of the transition zone. It is shown that the transition zone spectrum is fully determined by this function and zenith radiance spectra of clear and cloudy regions. An important result of these discoveries is that high temporal resolution radiance measurements in the clear-to-cloud transition zone can be well approximated by lower temporal resolution measurements plus linear interpolation.

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On the twilight zone between clouds and aerosols

Cited by 316 publications

References 18 publications

Enhanced aerosol backscatter adjacent to tropical trade wind clouds revealed by satellite‐based lidar

Enhanced aerosol backscatter adjacent to tropical trade wind clouds revealed by satellite‐based lidar

Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties

Spectral invariant behavior of zenith radiance around cloud edges observed by ARM SWS

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