Cloud discrimination in probability density functions of limb-scattered sunlight measurements

Normand, Elise; Wiensz, J. T.; Bourassa, Adam; Degenstein, D. A.

doi:10.5194/amt-6-3359-2013

Cited by 4 publications

(2 citation statements)

References 24 publications

(50 reference statements)

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“…Sulfate aerosols in the UTLS can be an important component of the total aerosol optical depth, particularly in middle-to-high latitudes after moderate volcanic eruptions (Ridley et al, 2014), making this an important region for accurate measurements. However, the radiance signal from clouds and cloud/aerosol mixtures can appear similar to volcanically enhanced aerosols, so distinguishing them has proved challenging and many methods have been developed to screen clouds from limb aerosol records (Thomason & Vernier, 2013;Normand et al, 2013;Eichmann et al, 2016;Liebing, 2016;Chen et al, 2016). Section 3.2 implements an updated cloud detection algorithm applied to the OSIRIS data set.…”

Section: Utls Sensitivitymentioning

confidence: 99%

A Multiwavelength Retrieval Approach for Improved OSIRIS Aerosol Extinction Retrievals

et al. 2019

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The Optical Spectrograph and InfraRed Imaging System (OSIRIS) on board the Odin satellite has been used to provide vertically resolved aerosol extinction since 2001. The OSIRIS version 5.07 aerosol product has been used in numerous studies and now provides a 17‐year record of global stratospheric aerosol. This work presents the new version 7 OSIRIS aerosol extinction retrieval. A multiwavelength aerosol extinction algorithm has been developed to reduce measurement geometry biases and improve extinction retrieval in the upper troposphere and lower stratosphere. The Chen et al. (2016, https://doi.org/10.5194/amt-9-1239-2016) cloud detection algorithm has been adapted for the OSIRIS wavelength range for improved cloud screening and polar stratospheric cloud detection, and comparisons after volcanic eruptions and with the CALIPSO‐GOCCP product show promising results. The version 7 product shows comparable agreement with version 5.07 when compared to coincident SAGE II and SAGE III measurements, and improved agreement with CALIPSO time series. The algorithm has been applied to the complete set of OSIRIS measurements, and the new data set is now publicly available.

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Section: Utls Sensitivitymentioning

confidence: 99%

A Multiwavelength Retrieval Approach for Improved OSIRIS Aerosol Extinction Retrievals

et al. 2019

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“…The lowest tangent height was not constant along the orbit. It varied from 5 km at the poles to 12 km at the Equator as a function of latitude φ with z th (φ) = 12 − 7 * cos(90− abs(φ)) (Raspollini and Ceccherini, 2011). Thus only cloud top heights above 8.5 km were retrieved in the tropics and extratropics (30 • N/S).…”

Section: Mipasmentioning

confidence: 99%

Global cloud top height retrieval using SCIAMACHY limb spectra: model studies and first results

et al. 2016

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Abstract. Cloud top heights (CTHs) are retrieved for the period 1 January 2003 to 7 April 2012 using height-resolved limb spectra measured with the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) on board ENVISAT (ENVIronmental SATellite). In this study, we present the retrieval code SCODA (SCIAMACHY cloud detection algorithm) based on a colour index method and test the accuracy of the retrieved CTHs in comparison to other methods. Sensitivity studies using the radiative transfer model SCIATRAN show that the method is capable of detecting cloud tops down to about 5 km and very thin cirrus clouds up to the tropopause. Volcanic particles can be detected that occasionally reach the lower stratosphere. Upper tropospheric ice clouds are observable for a nadir cloud optical thickness (COT)  ≥  0.01, which is in the subvisual range. This detection sensitivity decreases towards the lowermost troposphere. The COT detection limit for a water cloud top height of 5 km is roughly 0.1. This value is much lower than thresholds reported for passive cloud detection methods in nadir-viewing direction. Low clouds at 2 to 3 km can only be retrieved under very clean atmospheric conditions, as light scattering of aerosol particles interferes with the cloud particle scattering. We compare co-located SCIAMACHY limb and nadir cloud parameters that are retrieved with the Semi-Analytical CloUd Retrieval Algorithm (SACURA). Only opaque clouds (τN,c > 5) are detected with the nadir passive retrieval technique in the UV–visible and infrared wavelength ranges. Thus, due to the frequent occurrence of thin clouds and subvisual cirrus clouds in the tropics, larger CTH deviations are detected between both viewing geometries. Zonal mean CTH differences can be as high as 4 km in the tropics. The agreement in global cloud fields is sufficiently good. However, the land–sea contrast, as seen in nadir cloud occurrence frequency distributions, is not observed in limb geometry. Co-located cloud top height measurements of the limb-viewing Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT are compared for the period from January 2008 to March 2012. The global CTH agreement of about 1 km is observed, which is smaller than the vertical field of view of both instruments. Lower stratospheric aerosols from volcanic eruptions occasionally interfere with the cloud retrieval and inhibit the detection of tropospheric clouds. The aerosol impact on cloud retrievals was studied for the volcanoes Kasatochi (August 2008), Sarychev Peak (June 2009), and Nabro (June 2011). Long-lasting aerosol scattering is detected after these events in the Northern Hemisphere for heights above 12.5 km in tropical and polar latitudes. Aerosol top heights up to about 22 km are found in 2009 and the enhanced lower stratospheric aerosol layer persisted for about 7 months. In August 2009 about 82 % of the lower stratosphere between 30 and 70° N was filled with scattering particles and nearly 50 % in October 2008.

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Pluto’s Haze Abundance and Size Distribution from Limb Scatter Observations by MVIC

et al. 2021

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The New Horizons spacecraft observed Pluto and Charon at solar-phase angles between 16° and 169°. In this work, we use the Multispectral Visible Imaging Camera (MVIC) observations to construct multiwavelength phase curves of Pluto’s atmosphere, using the limb scatter technique. Observational artifacts and biases were removed using Charon as a representative airless body. The size and distribution of the haze particles were constrained using a Titan fractal aggregate phase function. We find that monodispersed and log-normal populations cannot simultaneously describe the observed steep forward scattering, indicative of wavelength-scale particles, and the non-negligible backscattering indicative of particles much smaller than the wavelength. Instead, we find it necessary to use bimodal or power-law distributions, especially below ∼200 km, to properly describe the MVIC observations. Above 200 km, where the atmosphere is isotropically scattering, a monodisperse, log-normal, or a bimodal/power law approximating a monodispersed population is able to fit the phase curves well. As compared to the results of previously published articles, we find that Pluto’s atmosphere must contain haze particle number densities an order of magnitude greater for small (∼10 nm) and large (∼1 μm) radii, and relatively fewer intermediate sizes (∼100 nm). These conclusions support a lower aggregate aerosol growth rate than that found by Gao et al., indicating a higher charge-to-radius ratio, upwards of 60e − μm−1. In order to generate large particles with a lower growth rate, the atmosphere must also have a lower sedimentation velocity (<∼0.01 m s−1 at 200 km), which is possible with a fractal dimension of less than 2.

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Cloud discrimination in probability density functions of limb-scattered sunlight measurements

Cited by 4 publications

References 24 publications

A Multiwavelength Retrieval Approach for Improved OSIRIS Aerosol Extinction Retrievals

A Multiwavelength Retrieval Approach for Improved OSIRIS Aerosol Extinction Retrievals

Global cloud top height retrieval using SCIAMACHY limb spectra: model studies and first results

Pluto’s Haze Abundance and Size Distribution from Limb Scatter Observations by MVIC

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