CALIPSO lidar calibration at 532 nm: version 4 daytime algorithm

Getzewich, Brian; Vaughan, Mark; Hunt, William H.; Avery, Melody; Powell, Kathleen A.; Tackett, Jason L.; Winker, David M.; Kar, Jayanta; Lee, Kam-Pui; Toth, Travis D.

doi:10.5194/amt-11-6309-2018

Cited by 63 publications

(56 citation statements)

References 28 publications

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“…regional mean fractions over middle latitudes in the southern and northern hemisphere are ∼1 % during the daytime and ∼2 % during the nighttime. The sensitivity of CALIPSO is by a factor of ∼2.5 at 18 km, ∼2 at 15 km, and ∼1.5 at 10 km higher at nighttime compared to daytime due to a better signal-to-noise ratio (Winker et al, 2009), which is in line with findings of Hunt et al (2009) and Getzewich et al (2018). As high altitude cirrus clouds show little diurnal cycle and thin cirrus in particular do not show any diurnal pattern (Wylie et al, 1994), we consider the difference in detection sensitivity the leading cause for the difference between CALIPSO night-and daytime measurements.…”

Section: Night-and Daytime Stratospheric Cirrus Cloudssupporting

confidence: 87%

Revisiting global satellite observations of stratospheric cirrus clouds

Zou¹,

Grießbach²,

Hoffmann³

et al. 2020

Preprint

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Abstract. As knowledge about the cirrus clouds in the lower stratosphere is limited, reliable long-term measurements are needed to assess their characteristics, radiative impact and important role in upper troposphere and lower stratosphere (UTLS) chemistry. To investigate the global and seasonal distribution of stratospheric cirrus clouds, we used the latest version (V4.x) of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data. For the identification of stratospheric cirrus clouds, precise information on both, the cloud top height (CTH) and the tropopause height is crucial. Here, we used lapse rate tropopause heights estimated from the ERA-Interim global reanalysis. Considering the uncertainties of the tropopause heights and the vertical sampling grid of the CALIPSO data, we considered cirrus clouds with CTHs more than 0.5 km above the tropopause as being stratospheric. We focused on nighttime CALIPSO measurements, because of their higher detection sensitivity. A six-year mean (2006–2012) global distribution of stratospheric cirrus cloud from CALIPSO showed that higher CTH occurrence frequencies are observed in the tropics than in the extra-tropics. Tropical hotspots of stratospheric cirrus clouds associated with deep convection are located over Equatorial Africa, South and Southeast Asia, the western Pacific and South America. Stratospheric cirrus clouds were more often detected in December–February (15 %) than June–August (8 %) in the tropics (± 20°). At middle (40–60°) and higher latitudes (> 60°), CALIPSO observed on average about 2 % stratospheric cirrus clouds. Observations of stratospheric cirrus cloud with MIPAS are presented here for the first time. Taking into account the MIPAS vertical sampling and broad field of view, we considered cirrus CTHs detected not less than 0.75 km above the tropopause as being stratospheric. Compared to CALIPSO, MIPAS observed twice as many stratospheric cirrus clouds at northern and southern middle latitudes (occurrence frequencies of 4–5 % for MIPAS rather than about 2 % for CALIPSO). We attribute more frequent observations of stratospheric cirrus clouds with MIPAS to higher detection sensitivity of the instrument to optically thin clouds. Sensitivity tests on MIPAS stratospheric cloud detections have been conducted to rule out sampling artefacts. Future work should focus on better understanding the origin of the stratospheric cirrus clouds and their impact on radiative forcing and climate.

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Section: Night-and Daytime Stratospheric Cirrus Cloudssupporting

confidence: 87%

Revisiting global satellite observations of stratospheric cirrus clouds

Zou¹,

Grießbach²,

Hoffmann³

et al. 2020

Preprint

View full text Add to dashboard Cite

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“…Since launch, the CALIPSO project has partnered with the LaRC HSRL team to conduct a mission-long series of validation underflights specifically designed to assess CALIOP's calibration accuracy and monitor long-term trends (Rogers et al, 2011). The airborne HSRL's high SNR, down-looking viewing geometry, and ability to measure the same alongtrack vertical swath as CALIOP yield highly reliable validation measurements in which systematic errors due to aerosol variability are largely eliminated (Gimmestad et al, 2017).…”

Section: Comparisons To Larc Hsrl Measurementsmentioning

confidence: 99%

CALIPSO lidar calibration at 1064 nm: version 4 algorithm

et al. 2019

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Abstract. Radiometric calibration of space-based elastic backscatter lidars is accomplished by comparing the measured backscatter signals to theoretically expected signals computed for some well-characterized calibration target. For any given system and wavelength, the choice of calibration target is dictated by several considerations, including signal-to-noise ratio (SNR) and target availability. This paper describes the newly implemented procedures used to calibrate the 1064 nm measurements acquired by CALIOP (i.e., the Cloud-Aerosol Lidar with Orthogonal Polarization), the two-wavelength (532 and 1064 nm) elastic backscatter lidar currently flying on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission. CALIOP's 532 nm channel is accurately calibrated by normalizing the molecular backscatter from the uppermost aerosol-free altitudes of the CALIOP measurement region to molecular model data obtained from NASA's Global Modeling and Assimilation Office. However, because CALIOP's SNR for molecular backscatter measurements is prohibitively lower at 1064 nm than at 532 nm, the direct high-altitude molecular normalization method is not a viable option at 1064 nm. Instead, CALIOP's 1064 nm channel is calibrated relative to the 532 nm channel using the backscatter from a carefully selected subset of cirrus cloud measurements. In this paper we deliver a full account of the revised 1064 nm calibration algorithms implemented for the version 4.1 (V4) release of the CALIPSO lidar data products, with particular emphases on the physical basis for the selection of “calibration quality” cirrus clouds and on the new averaging scheme required to characterize intra-orbit calibration variability. The V4 procedures introduce latitudinally varying changes in the 1064 nm calibration coefficients of 25 % or more, relative to previous data releases, and are shown to substantially improve the accuracy of the V4 1064 nm attenuated backscatter coefficients. By evaluating calibration coefficients derived using both water clouds and ocean surfaces as alternate calibration targets, and through comparisons to independent, collocated measurements made by airborne high spectral resolution lidar, we conclude that the CALIOP V4 1064 nm calibration coefficients are accurate to within 3 %.

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“…Changes made between V3 and V4 extend all the way back to improving both the 532 nm nighttime and daytime calibration, as well as re-calibration of the 1064 nm channel. These V4 calibration changes are documented extensively in a series of papers by Kar et al, 2018;Getzewich et al, 2018;and Vaughan et al, 2019a. Since the phase algorithm makes use of all three CALIOP 5 channels to make thermodynamic cloud phase decisions, changes to these calibrations in V4 can impact the distribution of assigned thermodynamic cloud phases by creating subtle changes in the γ′532-δp,eff relationships.…”

Section: Impact Of Caliop Cloud Algorithm Changesmentioning

confidence: 90%

“…In the CALIOP Level 1 data processing stream, the lidar 532 nm parallel and perpendicular polarization signal profiles are first calibrated to provide attenuated backscatter in each channel , Getzewich et al, 2018. Then as a first step in Level 2 processing, a feature detection algorithm isolates locations of elevated backscatter in the signal return profiles 30 from 30 km to the surface, at horizontal along-track averaging resolutions including 333m single laser shots from the surface to 8.2 km, and otherwise ranging from 1 to 80 km ).…”

Section: Cloud Phase Determination By Caliopmentioning

confidence: 99%

CALIOP V4 Cloud Thermodynamic Phase Assignment and the Impact of Near-Nadir Viewing Angles

Avery¹,

Ryan²,

Getzewich³

et al. 2020

Preprint

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Abstract. Accurate determination of thermodynamic cloud phase is critical for establishing the radiative impact of clouds on climate and weather. Depolarization of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) 532 nm signal provides an independent piece of information for determining cloud phase, a critical addition to other methods of thermodynamic phase discrimination that rely on temperature, cloud top altitude or a temperature-based cloud phase climatology. The CALIOP phase algorithm primarily uses layer-integrated depolarization and attenuated backscatter to determine the dominant thermodynamic phase of hydrometeors present in a vertical cloud layer segment, at horizontal resolutions varying between 333 m and 80 km. Ice cloud backscatter observations taken with a 0.3° near-nadir view include a significant amount of specular reflection from hexagonal smooth crystal faces that are oriented perpendicularly to the incident lidar beam. These specular reflections are often caused by horizontally oriented ice crystals (HOI), and are shown to occur between 0 and −40° C, with a peak in the distribution globally at −15° C. To avoid these reflections, the viewing angle was changed from 0.3 to 3° in November 2007. Since then the instrument has been observing clouds almost continuously for almost 12 more years. Recent viewing angle testing occurring during 2017 at 1, 1.5 and 2° quantifies the impact of changing the viewing angle to CALIOP global observations of attenuated backscatter and depolarization. These CALIOP results verify earlier observations by POLDER, showing that at the peak of the HOI distribution the mean backscatter from ice clouds decreases by 50 % and depolarization increases by a factor of 5 as the viewing angle increases from 0.3 to 3°. This has provided more data for a thorough evaluation of phase determination at the 3° viewing angle and suggested changes to the CALIOP cloud phase algorithm for Version 4 (V4). Combined with extensive calibration changes that impact the 532 nm and 1064 nm backscatter observations, V4 represents the first major Level 2 phase algorithm adjustment since 2009. For V4 the algorithm has been simplified to exclude over-identification of HOI at 3°, particularly in cold clouds. The V4 algorithm also considers temperature at the 532 nm cloud layer centroid, as opposed to cloud top or mid-cloud temperature as a secondary means of determining cloud phase and has been streamlined for more consistent identification of water and ice clouds. In this paper we summarize the major Version 3 (V3) to V4 cloud phase algorithm changes. We also characterize the impacts of applying the V4 phase algorithm globally and describe changes that can be expected when comparing V4 with V3. In V4 there are more cloud layers detected in V4, with edges that may extend further than in V3, for a combined increase in total atmospheric cloud volume of 6–9 % for high confidence cloud phases and 1–2 % for all cloudy bins. Some cloud layer boundaries have changed because 532 nm layer-integrated attenuated backscatter in V4 has increased due to improved calibration and extended layer boundaries, while the corresponding depolarization has stayed about the same. Collocated CALIPSO Imaging Infrared Radiometer (IIR) observations of ice and water cloud particle microphysical indices complement the CALIOP ice and water cloud phase determinations.

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CALIPSO lidar calibration at 532 nm: version 4 daytime algorithm

Cited by 63 publications

References 28 publications

Revisiting global satellite observations of stratospheric cirrus clouds

Revisiting global satellite observations of stratospheric cirrus clouds

CALIPSO lidar calibration at 1064 nm: version 4 algorithm

CALIOP V4 Cloud Thermodynamic Phase Assignment and the Impact of Near-Nadir Viewing Angles

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