Characteristics of CALIOP attenuated backscatter noise: implication for cloud/aerosol detection

Abstract. To study cloud/aerosol features in the upper troposphere and lower stratosphere (UT/LS) with the NASA's A-Train sensors, a research algorithm is developed for a re-gridded CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Level 1 (L1) backscatter dataset. This paper provides a detailed analysis of the measurement noise of this re-gridded dataset in order to compare the lidar measurements with other collocated measurements (e.g., CloudSat, Microwave Limb Sounder). The re-gridded dataset has a manageable data volume for multi-year analysis. It has a fixed (5 km) horizontal resolution, and the measurement error is derived empirically from the background-corrected backscatter profile on a profile-by-profile basis. The 532-nm and 1064-nm measurement noises, determined from the data at altitudes above 19 km, are analyzed and characterized in terms of the mean (μ), standard deviation (σ), and normalized probability density function (PDF). These noises show a larger variance over landmasses and bright surfaces during day, and in regions with enhanced flux of energetic particles during night, where the instrument's ability for feature detection is slightly degraded. An increasing trend in the nighttime 1064-nm σ appears to be significant, which likely causes the increasing differences in cloud occurrence frequency between the 532-nm and 1064-nm channels. Most of the CALIOP backscatter noise distributions exhibit a Gaussian-like behavior but the nighttime 532-nm perpendicular measurements show multi-Gaussian characteristics. We apply σ – based thresholds to detect cloud/aerosol features in the UT/LS from the subset L1 data. The observed morphology is similar to that from the Level 2 (L2) 05km_CLAY+05km_ALAY product, but the occurrence frequency obtained in this study is slightly lower than the L2 product due to differences in spatial averaging and detection threshold. In the case where the measurement noises of two data sets are different, the normalized PDF has proven useful for quantifying the day-night difference of the CALIOP backscatters, showing higher daytime cloud occurrence frequency in the tropical UT/LS. Other cloud/aerosol properties, such as depolarization ratio and color ratio, can be also evaluated with the PDF method.

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Characteristics of CALIOP attenuated backscatter noise: implication for cloud/aerosol detection

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-2641-2011 ◽

2011 ◽

Vol 11 (6) ◽

pp. 2641-2654 ◽

Cited By ~ 14

Author(s):

D. L. Wu ◽

J. H. Chae ◽

A. Lambert ◽

F. F. Zhang

Keyword(s):

Feature Detection ◽

Atmospheric Model ◽

Horizontal Resolution ◽

Measurement Noise ◽

Orthogonal Polarization ◽

Lidar Measurement ◽

L1 Data ◽

Noise Evaluation ◽

Factor Α ◽

532 Nm

Abstract. A research algorithm is developed for noise evaluation and feature detection of the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Level 1 (L1) backscatter data with an emphasis on cloud/aerosol features in the upper troposphere and lower stratosphere (UT/LS). CALIOP measurement noise of the version v2.01 and v2.02 L1 backscatter data aggregated to (5 km) horizontal resolution is analyzed with two approaches in this study. One is to compare the observed and modeled molecular scatter profiles by scaling the modeled profile (with a fitted scaling factor α) to the observed clear-sky backscatter profiles. This scaling α value is sensitive to errors in the calibrated backscatter and the atmospheric model used. Most of the nighttime 532-nm α values are close to unity, as expected, but an abrupt drop occurred in October 2008 in the daytime 532-nm α, which is likely indicative of a problem in the v2.02 daytime calibrated data. The 1064-nm night α is generally close to 2 while its day α is ~3. The other approach to evaluate the lidar measurement noise is to use the calibrated lidar backscatter data at altitudes above 19 km. With this method, the 532-nm and 1064-nm measurement noises are analyzed and characterized individually for each profile in terms of the mean (μ) and standard deviation (σ), showing larger σ values in general over landmasses or bright surfaces during day and in radiation-hard regions during night. A significant increasing trend is evident in the nighttime 1064-nm σ, which is likely responsible for the increasing difference between the feature occurrence frequencies (532-nm vs. 1064-nm) derived from this study. For feature detection with the research algorithm, we apply a σ–based method to the aggregated L1 data. The derived morphology of feature occurrence frequency is in general agreement with that obtained from the Level 2 (L2) 05 km_CLAY+05 km_ALAY products at 5 km horizontal resolution. Finally, a normalized probability density function (PDF) method is employed to evaluate the day-night backscatter data in which noise levels are largely different. CALIOP observations reveal a higher probability of daytime cloud/aerosol occurrence than nighttime in the tropical UT/LS region for 532-nm total backscatters >0.01 km−1 sr−1.

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Global Ocean Studies from CALIOP/CALIPSO by Removing Polarization Crosstalk Effects

Remote Sensing ◽

10.3390/rs13142769 ◽

2021 ◽

Vol 13 (14) ◽

pp. 2769

Author(s):

Xiaomei Lu ◽

Yongxiang Hu ◽

Ali Omar ◽

Rosemary Baize ◽

Mark Vaughan ◽

...

Keyword(s):

Polarization Plane ◽

Optical Power ◽

Satellite Observations ◽

Global Ocean ◽

Reference Plane ◽

Orthogonal Polarization ◽

True Value ◽

Before And After ◽

Oriented Parallel ◽

532 Nm

Recent studies indicate that the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite provides valuable information about ocean phytoplankton distributions. CALIOP’s attenuated backscatter coefficients, measured at 532 nm in receiver channels oriented parallel and perpendicular to the laser’s linear polarization plane, are significantly improved in the Version 4 data product. However, due to non-ideal instrument effects, a small fraction of the backscattered optical power polarized parallel to the receiver polarization reference plane is misdirected into the perpendicular channel, and vice versa. This effect, known as polarization crosstalk, typically causes the measured perpendicular signal to be higher than its true value and the measured parallel signal to be lower than its true value. Therefore, the ocean optical properties derived directly from CALIOP’s measured signals will be biased if the polarization crosstalk effect is not taken into account. This paper presents methods that can be used to estimate the CALIOP crosstalk effects from on-orbit measurements. The global ocean depolarization ratios calculated both before and after removing the crosstalk effects are compared. Using CALIOP crosstalk-corrected signals is highly recommended for all ocean subsurface studies.

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Retrieval of cirrus cloud optical thickness and top altitude from geostationary remote sensing

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-4123-2014 ◽

2014 ◽

Vol 7 (4) ◽

pp. 4123-4161 ◽

Cited By ~ 1

Author(s):

S. Kox ◽

L. Bugliaro ◽

A. Ostler

Keyword(s):

Optical Thickness ◽

High Spectral Resolution ◽

Orthogonal Polarization ◽

Cirrus Cloud ◽

Backpropagation Neural Network ◽

Novel Approach ◽

Infrared Imager ◽

Cloud Optical Thickness ◽

High Spectral Resolution Lidar ◽

532 Nm

Abstract. A novel approach for the detection of cirrus clouds and the retrieval of optical thickness and top altitude based on the measurements of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the geostationary Meteosat Second Generation (MSG) satellite is presented. Trained with 8 000 000 co-incident measurements of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission the new "cirrus optical properties derived from CALIOP and SEVIRI algorithm during day and night" (COCS) algorithm utilizes a backpropagation neural network to provide accurate measurements of cirrus optical depth τ at λ =532 nm and top altitude z every 15 min covering almost one third of Earth's atmosphere. The retrieved values are validated with independent measurements of CALIOP and the optical thickness derived by an airborne high spectral resolution lidar.

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Airborne measurements of aerosol optical properties related to early spring transport of mid-latitude sources into the Arctic

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-5011-2010 ◽

2010 ◽

Vol 10 (11) ◽

pp. 5011-5030 ◽

Cited By ~ 48

Author(s):

R. A. de Villiers ◽

G. Ancellet ◽

J. Pelon ◽

B. Quennehen ◽

A. Schwarzenboeck ◽

...

Keyword(s):

Optical Properties ◽

The Arctic ◽

Depolarization Ratio ◽

Aerosol Optical Properties ◽

Accumulation Mode ◽

Color Ratio ◽

Aerosol Sources ◽

Aerosol Backscatter ◽

532 Nm

Abstract. Airborne lidar and in-situ measurements of the aerosol properties were conducted between Svalbard Island and Scandinavia in April 2008. Evidence of aerosol transport from Europe and Asia is given. The analysis of the aerosol optical properties based on a multiwavelength lidar (355, 532, 1064 nm) including volume depolarization at 355 nm aims at distinguishing the role of the different aerosol sources (Siberian wild fires, Eastern Asia and European anthropogenic emissions). Combining, first aircraft measurements, second FLEXPART simulations with a calculation of the PBL air fraction originating from the three different mid-latitude source regions, and third level-2 CALIPSO data products (i.e. backscatter coefficient 532 nm,volume depolarization and color ratio between 1064 and 532 nm in aerosol layers) along the transport pathways, appears a valuable approach to identify the role of the different aerosol sources even after a transport time larger than 4 days. Optical depth of the aerosol layers are always rather small (<4%) while transported over the Arctic and ratio of the total attenuated backscatter (i.e. including molecular contribution) provide more stable result than conventional aerosol backscatter ratio. Above Asia, CALIPSO data indicate more depolarization (up to 15%) and largest color ratio (>0.5) for the northeastern Asia emissions (i.e. an expected mixture of Asian pollution and dust), while low depolarization together with smaller and quasi constant color ratio (≈0.3) are observed for the Siberian biomass burning emissions. A similar difference is visible between two layers observed by the aircraft above Scandinavia. The analysis of the time evolution of the aerosol optical properties revealed by CALIPSO between Asia and Scandinavia shows a gradual decrease of the aerosol backscatter, depolarization ratio and color ratio which suggests the removal of the largest particles in the accumulation mode. A similar study conducted for a European plume has shown aerosol optical properties intermediate between the two Asian sources with color ratio never exceeding 0.4 and moderate depolarization ratio being always less than 8%, i.e. less aerosol from the accumulation mode.

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Measuring and Modeling the Feature Detection Threshold Functions of Colormaps

IEEE Transactions on Visualization and Computer Graphics ◽

10.1109/tvcg.2018.2855742 ◽

2019 ◽

Vol 25 (9) ◽

pp. 2777-2790 ◽

Cited By ~ 10

Author(s):

Colin Ware ◽

Terece L. Turton ◽

Roxana Bujack ◽

Francesca Samsel ◽

Piyush Shrivastava ◽

...

Keyword(s):

Feature Detection ◽

Detection Threshold ◽

Threshold Functions

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CALIPSO lidar calibration at 1064 nm: version 4 algorithm

Atmospheric Measurement Techniques ◽

10.5194/amt-12-51-2019 ◽

2019 ◽

Vol 12 (1) ◽

pp. 51-82 ◽

Cited By ~ 19

Author(s):

Mark Vaughan ◽

Anne Garnier ◽

Damien Josset ◽

Melody Avery ◽

Kam-Pui Lee ◽

...

Keyword(s):

Molecular Model ◽

Signal To Noise Ratio ◽

High Spectral Resolution ◽

Orthogonal Polarization ◽

Radiometric Calibration ◽

Calibration Target ◽

Full Account ◽

High Spectral Resolution Lidar ◽

532 Nm ◽

Calibration Coefficients

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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Aerosol classification using airborne High Spectral Resolution Lidar measurements – methodology and examples

Atmospheric Measurement Techniques ◽

10.5194/amt-5-73-2012 ◽

2012 ◽

Vol 5 (1) ◽

pp. 73-98 ◽

Cited By ~ 252

Author(s):

S. P. Burton ◽

R. A. Ferrare ◽

C. A. Hostetler ◽

J. W. Hair ◽

R. R. Rogers ◽

...

Keyword(s):

Spectral Resolution ◽

High Spectral Resolution ◽

Color Ratio ◽

Lidar Measurements ◽

Aerosol Classification ◽

Langley Research Center ◽

High Spectral Resolution Lidar ◽

Aerosol Types ◽

532 Nm ◽

Aerosol Type

Abstract. The NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) on the NASA B200 aircraft has acquired extensive datasets of aerosol extinction (532 nm), aerosol optical depth (AOD) (532 nm), backscatter (532 and 1064 nm), and depolarization (532 and 1064 nm) profiles during 18 field missions that have been conducted over North America since 2006. The lidar measurements of aerosol intensive parameters (lidar ratio, depolarization, backscatter color ratio, and spectral depolarization ratio) are shown to vary with location and aerosol type. A methodology based on observations of known aerosol types is used to qualitatively classify the extensive set of HSRL aerosol measurements into eight separate types. Several examples are presented showing how the aerosol intensive parameters vary with aerosol type and how these aerosols are classified according to this new methodology. The HSRL-based classification reveals vertical variability of aerosol types during the NASA ARCTAS field experiment conducted over Alaska and northwest Canada during 2008. In two examples derived from flights conducted during ARCTAS, the HSRL classification of biomass burning smoke is shown to be consistent with aerosol types derived from coincident airborne in situ measurements of particle size and composition. The HSRL retrievals of AOD and inferences of aerosol types are used to apportion AOD to aerosol type; results of this analysis are shown for several experiments.

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Retrieval of cirrus cloud optical thickness and top altitude from geostationary remote sensing

Atmospheric Measurement Techniques ◽

10.5194/amt-7-3233-2014 ◽

2014 ◽

Vol 7 (10) ◽

pp. 3233-3246 ◽

Cited By ~ 24

Author(s):

S. Kox ◽

L. Bugliaro ◽

A. Ostler

Keyword(s):

Optical Thickness ◽

High Spectral Resolution ◽

Orthogonal Polarization ◽

Cirrus Cloud ◽

Backpropagation Neural Network ◽

Novel Approach ◽

Infrared Imager ◽

Cloud Optical Thickness ◽

High Spectral Resolution Lidar ◽

532 Nm

Abstract. A novel approach for the detection of cirrus clouds and the retrieval of optical thickness and top altitude based on the measurements of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the geostationary Meteosat Second Generation (MSG) satellite is presented. Trained with 8 000 000 co-incident measurements of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission the new "cirrus optical properties derived from CALIOP and SEVIRI algorithm during day and night" (COCS) algorithm utilizes a backpropagation neural network to provide accurate measurements of cirrus optical depth τ at λ = 532 nm and top altitude z every 15 min covering almost one-third of the Earth's atmosphere. The retrieved values are validated with independent measurements of CALIOP and the optical thickness derived by an airborne high spectral resolution lidar.

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Transport of aerosol to the Arctic: analysis of CALIOP and French aircraft data during the spring 2008 POLARCAT campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-8235-2014 ◽

2014 ◽

Vol 14 (16) ◽

pp. 8235-8254 ◽

Cited By ~ 24

Author(s):

G. Ancellet ◽

J. Pelon ◽

Y. Blanchard ◽

B. Quennehen ◽

A. Bazureau ◽

...

Keyword(s):

Calibration Procedure ◽

Airborne Lidar ◽

The Arctic ◽

Orthogonal Polarization ◽

Transport Pathways ◽

Airborne Measurements ◽

Surface Measurements ◽

Regional Analyses ◽

532 Nm

Abstract. Lidar and in situ observations performed during the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, Climate, Chemistry, Aerosols and Transport (POLARCAT) campaign are reported here in terms of statistics to characterize aerosol properties over northern Europe using daily airborne measurements conducted between Svalbard and Scandinavia from 30 March to 11 April 2008. It is shown that during this period a rather large number of aerosol layers was observed in the troposphere, with a backscatter ratio at 532 nm of 1.2 (1.5 below 2 km, 1.2 between 5 and 7 km and a minimum in between). Their sources were identified using multispectral backscatter and depolarization airborne lidar measurements after careful calibration analysis. Transport analysis and comparisons between in situ and airborne lidar observations are also provided to assess the quality of this identification. Comparison with level 1 backscatter observations of the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) were carried out to adjust CALIOP multispectral observations to airborne observations on a statistical basis. Recalibration for CALIOP daytime 1064 nm signals leads to a decrease of their values by about 30%, possibly related to the use of the version 3.0 calibration procedure. No recalibration is made at 532 nm even though 532 nm scattering ratios appear to be biased low (−8%) because there are also significant differences in air mass sampling between airborne and CALIOP observations. Recalibration of the 1064 nm signal or correction of −5% negative bias in the 532 nm signal both could improve the CALIOP aerosol colour ratio expected for this campaign. The first hypothesis was retained in this work. Regional analyses in the European Arctic performed as a test emphasize the potential of the CALIOP spaceborne lidar for further monitoring in-depth properties of the aerosol layers over Arctic using infrared and depolarization observations. The CALIOP April 2008 global distribution of the aerosol backscatter reveal two regions with large backscatter below 2 km: the northern Atlantic between Greenland and Norway, and northern Siberia. The aerosol colour ratio increases between the source regions and the observations at latitudes above 70° N are consistent with a growth of the aerosol size once transported to the Arctic. The distribution of the aerosol optical properties in the mid-troposphere supports the known main transport pathways between the mid-latitudes and the Arctic.

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CALIPSO Lidar Calibration at 532 nm: Version 4 Nighttime Algorithm

10.5194/amt-2017-365 ◽

2017 ◽

Author(s):

Jayanta Kar ◽

Mark A. Vaughan ◽

Kam-Pui Lee ◽

Jason L. Tackett ◽

Melody A. Avery ◽

...

Keyword(s):

Signal To Noise Ratio ◽

Calibration Procedure ◽

Stratospheric Aerosol ◽

Airborne Lidar ◽

High Spectral Resolution ◽

Orthogonal Polarization ◽

Radiometric Calibration ◽

Data Set ◽

Langley Research Center ◽

532 Nm

Abstract. Data products from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) were recently updated following the implementation of new (version 4.1) calibration algorithms for all of the level 1 attenuated backscatter measurements. In this work we present the motivation for and the implementation of the version 4.1 nighttime 532 nm parallel channel calibration. The nighttime 532 nm calibration is the most fundamental calibration of CALIOP data, since all of CALIOP’s other radiometric calibration procedures – i.e., the 532 nm daytime calibration and the 1064 nm calibrations during both nighttime and daytime – depend either directly or indirectly on the 532 nm nighttime calibration. The accuracy of the 532 nm nighttime calibration is significantly improved by raising the molecular normalization altitude from 30–34 km to 36–39 km to substantially reduce stratospheric aerosol contamination. Due to the greatly reduced molecular number density and consequently reduced signal-to-noise ratio at the higher altitudes used to avoid aerosols, the signal is averaged over a larger number of samples. The new calibration procedure is shown to eliminate biases introduced in earlier versions and consequently leads to an improved representation of stratospheric aerosols. Validation results using airborne lidar measurements are also presented. Biases relative to collocated measurements acquired by the Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) are reduced from 3.6 % ± 2.2 % in the version 3 data set to 1.6 % ± 2.4 % in the version 4.1 release.

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