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Detailed Data Quality Summary for the CALIPSO Version 1.00 Lidar Level 3 Stratospheric Aerosol Profile Data Product



Data Version: 1.00
Data Release Date: August 2018
Data Date Range: June 13, 2006 to December 31, 2016

The lidar level 3 (L3) stratospheric aerosol profile product reports monthly mean profiles of aerosol extinction, particulate backscatter, attenuated scattering ratio and stratospheric aerosol optical depth on a spatial grid of 5° in latitude, 20° in longitude and 900 m in altitude. Profiles are derived from level 1B attenuated backscatter (β′). The product has two components. For the “Background” component, all layers detected by CALIOP level 2 are removed prior to averaging β′, including clouds, aerosols, and polar stratospheric clouds (PSC). For the “All aerosol” component, only cloud and PSC layers are removed, thereby retaining β′ for detected aerosol layers in addition to “Background” β′.

Input Data

The following table lists the dates and version number of CALIOP data used to build the level 3 stratospheric aerosol profile product.

CALIOP Product Version Data Date Range
Level 1B V4.10 June 13, 2006 to present
Level 2 - 5 km MLay V4.10 June 13, 2006 to present
Level 2 PSC Mask V1.00
V1.10
June 13, 2006 to November 30, 2016
December 1, 2016 to present

A Brief Description of the Algorithm

There are five fundamental steps in generating output for the level 3 stratospheric aerosol product.

1. Features are removed from the level 1B attenuated backscatter (β′) depending on the component. The “All aerosol” component removes β′ for the following layers reported by the CALIOP level 2 products: clouds, stratospheric aerosol with the “PSC aerosol” subtype, and PSCs identified by the PSC mask product, while retaining stratospheric aerosol layers with all other subtypes. The “Background” component removes β′ for all features reported by the CALIOP level 2, including clouds, aerosols, and PSCs.

2. The β′ profiles are then averaged to 5 km horizontal resolution.

3. Quality filters are applied next to remove untrustworthy β′ measurements. Specifically, all data over the South Atlantic Anomaly region is discarded due to high noise. Outliers are eliminated using a spike filter that uses a thresholding scheme similar to the one used in the automatic layer detection scheme in the CALIOP level 2 algorithm (Vaughan et al., 2009). The quality-filtered β′ profiles from 1 km below the MERRA-2 tropopause height to 36 km are then averaged onto the 5° latitude x 20° longitude x 900 m altitude grid; each gridded-average value is hereafter termed as a “sample”.

4. The gridded data from the single granule are further filtered to remove undetected thin clouds in different ways, depending on the component. For the “Background” component, samples having volume depolarization ratios > 0.05 are rejected. For the “All aerosol” component, samples having attenuated color ratio > 0.5 are rejected to eliminate thin clouds while simultaneously preserving possible ash layers (Vernier et al., 2013). In addition, the “All aerosol” component rejects samples containing detected aerosol layers having | CAD score | < 20 because there is no confidence in cloud-aerosol discrimination. Monthly averaged gridded datasets for attenuated backscatter coefficients, meteorological data, and ancillary data are then created which serve as the inputs to the extinction module in the next step.

5. Aerosol extinction ( σp ), particulate backscatter ( βp ), attenuated scattering ratio ( R′ ), and stratospheric aerosol optical depth are then retrieved/calculated from the monthly mean gridded attenuated backscatter profiles. Particulate backscatter is solved from equations (1) and (2):

βp(r) = β′ (r) / T2m · T2O3 · T2p - βm(r) (1)
   
T2p = exp(-2 · S · βp(r′)dr′)
(2)
Where, T2m, T2O3 and T2p refer to the two way transmittance from the molecules, ozone and particulates respectively and βm is the modeled molecular backscatter coefficient. T2m and T2O3 are computed from the meteorological model (MERRA-2) used for CALIOP V4.10 data products.

In order to compute the aerosol extinction coefficient, a lidar ratio is required for stratospheric aerosol. The stratospheric aerosol lidar ratio (S) is given a uniform value of 50 sr, globally for this version. The extinction coefficient is computed subsequently using the equation:

σp = S · βp (3)

The extinction retrievals are carried out from 36 km in altitude down to 1 km below the tropopause. The stratospheric aerosol optical depth is obtained by integrating the monthly mean aerosol extinction coefficients. Attenuated scattering ratios are computed using equation (4):

R′ = β′ / (βm · T2m · T2O3 ) (4)

Preliminary Quality Assessment

Figure 1. Spatial distribution of the extinction coefficients retrieved in June 2011 in a) background component, and b) all aerosol component.

Figure 1 shows height-latitude cross sections of the aerosol extinction coefficient from the two components for the month of June 2011. During this month two strong volcanic eruptions took place, Nabro in the northern hemisphere (June 13th, 13°N, 41°E) and Puyehue-Cordon Caulle (June 4th, 40°S, 72°W). The composition of Nabro plume was mostly sulfate while the composition of Puyehue-Cordon Caulle was mostly ash (De Vries et al.,2014, Vernier et al., 2013). The background component (Fig. 1a) shows no signature of Nabro and some signature of Cordon Caulle (possibly from aerosol below the layer detection threshold). However, in the all aerosol component (Fig. 1b), signatures of both the volcanoes are captured, with the ash-laden Cordon plumes appearing particularly strong.

Uncertainty estimates are provided for all the retrieved quantities, e.g., particulate backscatter and extinction coefficients. These are the primary quality assurance metrics for this product. Data users are advised to use them for their applications appropriately. The current version uses a constant value of the lidar ratio at all locations and altitudes. This is likely to introduce errors in places where the lidar ratio may vary with altitude. Similarly, the use of constant depolarization ratio and color ratio thresholds for filtering out the thin clouds may not always ensure complete cloud removal.

References:

De Vries, M.J.M. Penning et al., “Characterisation of a stratospheric sulfate plume from the Nabro volcano using a combination of passive satellite measurements in nadir and limb geometry”, Atmos. Chem. Phys., 14, 8149-8163, doi:10.5194/acp-14-8149-2014, 2014.

Vaughan, M. et al., “Fully automated detection of cloud and aerosol layers in the CALIPSO lidar measurements”, J. Atmos. Ocean. Tech., 26, 2034-2050, doi:10.1175/2009JTECHA1221.1, 2009.

Vernier, J.-P., et al., “An advanced system to monitor the 3D structure of diffuse volcanic ash clouds”, J. Appl. Meteorol. Climatol., 52, 2125-2138, doi:10.1175/JAMC-D-12-0279.1, 2013.



NASA
Last Updated: August 10, 2018
Curator: Charles R. Trepte
NASA Official: Charles R. Trepte

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