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CALIPSO HOMECALIPSO User's Guide HOMEData Products Description → Lidar Level 3 Aerosol Profile Monthly Product Version 3.00

CALIPSO: Data User's Guide - Data Quality Statement - Lidar Level 3 Aerosol Profile Monthly Product Version 3.00


Data Quality Statement

Lidar Level 3 Aerosol Profile Monthly Product Information
Half orbit (Night and Day) scene classification data and lidar lighting and land/water indicators.
Release Date Version Data Date Range Maturity Level / Production Strategy
October 2015 3.00 June 2006 to present Standard
April 2013 1.30 March 2013 to September 2015 Beta
December 2011 1.00 June 2006 to February 2013 Beta

Summary Statement for the release of the CALIPSO Lidar Level 3 Aerosol Product Version 3.00, October 2015

The version 3.00 release of the lidar level 3 aerosol profile product contains several improvements to the original version 1 release. New definitions for sky condition now partition the atmosphere into a consistent set of monthly mean aerosol extinction profiles which yield more useful representations of vertical aerosol loading compared to sky conditions in version 1 which were difficult to interpret. Single-species averages for polluted dust and smoke have been added using an improved averaging strategy which generates level 3 aerosol extinction profiles with shapes that more faithfully represent the input level 2 extinction profiles. A revised aerosol optical depth (AOD) calculation removes low-biases that existed in level 3 monthly mean AOD. Mean aerosol extinction near the planetary surface has been improved by (a) mitigating the impact of negative signal anomalies, which cause negative spikes in aerosol extinction at the onset of the surface, and (b) correctly compensating for high biases (i.e., premature base determination) in the level 2 layer base detection algorithm. Two new extinction scale height data sets have been added to identify altitudes below which 63% and 90% of the total mean AOD is found. Several rarely used statistics have also been removed. The major version number for this release (version 3) reflects that version 3 level 2 data products have been used as input for the level 3 product. These improvements are described in greater detail below.

Level 2 Input Data

The following table lists the version numbers of level 2 data used as input for the V3.00 level 3 product. Note that the major version number (3) matches between level 2 and level 3. Minor version number changes to the level 2 products occur at boundaries between months, so a given level 3 file will only contain input for a single version of the level 2 product.

Lidar Level 2 Input Data For Level 3 Aerosol Profile Product, V3.00
Level 2 Version Data Date Range
3.30 March 2013 - present
3.02 November 2011 - February 2013
3.01 June 2006 - October 2011

Level 2 Input Data

Level 3 averages are reported for four different sky conditions. “Sky condition” refers to occurrence and location of clouds within the level 2 profiles which are included in the level 3 average. The definitions for sky conditions have changed in V3.00:

Sky Condition
(V3)
Description of Level 2 Data Usage
All Sky All level 2 columns are averaged, regardless of cloud occurrence
Cloud-Free Only cloud-free level 2 columns are averaged
Cloudy-Sky,
Transparent
Only level 2 columns containing transparent clouds are averaged
Cloudy-Sky,
Opaque
Only level 2 columns containing opaque clouds are averaged

An example of how an individual level 2 granule is parsed into the four sky conditions is shown in Figure 1. Note that it is possible to derive mean extinction for all sky from the cloud-free, cloudy-sky transparent and cloudy-sky opaque sky conditions by weighting the mean extinction values by the “Samples Averaged” statistic for each sky condition.

Figure 1. Level 3 Version 3 sky conditions applied to a single level 2 granule. White indicates columns which are excluded from the particular sky condition.


New single aerosol species averages

Extinction, AOD and sampling statistics are now included for polluted dust-only and smoke-only in addition to dust-only which was reported in version 1.

Revised averaging method for single aerosol species averages

When averaging level 2 extinction for single aerosol species (for example dust-only), the aerosol extinction for all other aerosol species are now assigned extinction = 0.0 km-1 prior to calculating the average. In version 1 of the level 3 product, extinction for those other aerosol species were ignored in the average, which introduced a high bias into the mean extinction at altitudes dominated by other aerosol species (Amiridis et al., 2013).

Figure 2 below demonstrates this effect. Saharan dust transporting westward during Boreal summer initially remains elevated and then descends gradually into the marine boundary layer (MBL) towards the Caribbean (Liu et al, 2008). Dust-only extinction should therefore peak above the MBL just west of the North Africa coast. In level 3 version 1, the dust-only extinction profile in this region shows two peaks - one at 4 km and a second, larger peak in the MBL. The lower peak occurs because the fraction of dust samples decreases rapidly near the ocean surface where marine aerosol is the dominant species (Figure 1, right panel). Since the marine aerosol extinction was ignored during the averaging process, the mean dust-only extinction near the surface is magnified because numerous “dust-free” extinction values of 0.0 km-1 (i.e., where the aerosol is marine, not dust) are not included in the averages. Crucially, the profile shape for dust-only extinction becomes badly deformed solely because a small amount of dust existed at the same altitude as marine aerosol. This deformation is not an accurate portrayal of the profile shapes reported by the CALIOP level 2 aerosol profile product, but is instead an artifact of the single aerosol species averaging procedure used in the version 1 analyses. In the version 3 product, all non-dust aerosol extinction is set to 0.0 km-1 during the averaging process which yields a dust-only extinction profile with a single peak near 4 km that is more representative of dust-only extinction in this region. A similar methodology is implemented for polluted dust-only and smoke-only averages in level 3 version 3.

Figure 2: Level 3 mean dust-only extinction averaged according to level 3 version 1 and version 3 methodologies (left panel) and the fraction of dust samples out of all possible aerosol subtypes (right panel) for July 2008 at night. Inset map indicates region examined (red box).


References

Amiridis, V., U. Wandinger, E. Marinou, E. Giannakaki, A. Tsekeri, S. Basart, S. Kazadzis, A. Gkikas, M. Taylor, J. Baldasano and A. Ansmann, 2013: “Optimizing Saharan dust CALIPSO retrievals”, Atmos. Chem. Phys., 13, 12089-12106, doi:10.5194/acp-13-12089-2013.

Liu, Z., et al. (2008), “CALIPSO lidar observations of the optical properties of Saharan dust: A case study of long-range transport”, J. Geophys. Res., 113, D07207.

Revised AOD mean computation

Mean aerosol optical depth (AOD) reported in version 3 of the level 3 product is computed as the vertical integral of the mean level 3 extinction profile. In contrast, version 1 mean aerosol optical depth was computed as the average of level 2 column AODs. The version 1 method caused a low-biased mean AOD because the columns being averaged together have unequal geometric depths due to cloud cover, opacity or data quality filtering (Figure 3). A simple example demonstrates this. Imagine two level 2 aerosol extinction profiles - one profile having valid aerosol extinction all the way down to the surface and the other profile having an opaque cloud with a small amount of aerosol extinction above. Computing the vertical integral for these two profiles would yield one relatively large AOD and one small AOD since there is less aerosol above cloud. Averaging these AODs together would give a rather small AOD. Conversely, averaging these two extinction profiles together and then integrating the mean extinction would yield a larger AOD that is representative of the aerosol loading at all altitudes. This is the version 3 method. It also has the benefit that the mean AOD is consistent with the mean extinction profiles reported.

Figure 3: Version 1 Level 3 AOD for one month calculated according to the version 1 method (average of column AODs) and the version 3 method (integral of mean extinction).


New extinction scale heights

New science data sets, “AOD 63% Below” and “AOD 90% Below” indicate the altitude which contains 63% and 90% of the total mean aerosol optical depth below.

Revised low-bias filter for undetected surface-attached aerosol

When the CALIPSO feature detection algorithm does not detect the entire vertical extent of an aerosol layer and/or the aerosol base extension algorithm is invoked, a gap of “clear-air” can exist beneath the layer base and the surface. If left as “clear-air”, level 3 mean extinction would exhibit a low bias in range bins near the surface. To guard against this in version 1 of the level 3 product, “clear-air” was ignored below the lowest layer in a column whose base was below 2.5 km. However, setting this threshold altitude at 2.5 km proved to be overly aggressive, and would occasionally introduce high biases into the level 3 profiles. With version 3, the lowest layer base altitude threshold is lowered to 250 meters, which is much more consistent with the “clear-air” gap of 90-120 meters from the surface that is seen in the level 2 products. This revision focuses the filter application more precisely where it is needed rather than at higher altitudes which, as mentioned, can otherwise yield a high bias in level 3 mean extinction.

New negative signal anomaly mitigation strategy

A recently discovered phenomenon, dubbed a “negative signal anomaly” sometimes occurs when the level 1B attenuated backscatter becomes anomalously negative at the abrupt onset of a strongly scattering target such as the planetary surface or a dense cloud. Aerosol extinction can also become anomalously negative when attenuated backscatter affected by the anomaly is included in the extinction retrieval. Subsequent level 3 mean extinction would therefore exhibit a low bias near the surface where the negative signal anomaly is present. To mitigate the impact of this anomaly, all level 2 aerosol extinction coefficients within 60 meters of the planetary surface are excluded from level 3 calculations. Globally, this yields a 2-5% decrease in AOD, but increases AOD in regions strongly affected by the negative signal anomaly. Crucially, anomalously low spikes in level 3 extinction in lowest altitude range bins have been removed.

Removed statistics

The following statistics have been removed from the level 3 aerosol profile product compared to V1.x.

  • Extinction 532 median, skew, RMS
  • Extinction 532 dust median, skew, RMS, percentiles
  • AOD median, skew, RMS, percentiles
  • AOD dust median, skew, RMS, percentiles
  • AOD and AOD dust statistics were previously reported for all four sky conditions in every level 3 file. In V3.00, AOD is only reported for the sky condition of the level 3 file.

Known Issues

The following sections describe issues with the level 3 product known at the time of its release in October 2015.

Undetected aerosol influence on level 3 aerosol extinction statistics

Regions classified as “clear air” by the CALIPSO feature detection algorithms are assumed to have aerosol extinction = 0.0 km-1 when calculating level 3 averages of aerosol extinction. CALIOP has a limited detection sensitivity however, and weakly scattering layers go undetected, resulting in a low bias. This occurs most often in the upper troposphere where aerosol layers tend to be optically weak, in the daytime since the SNR is smaller than at nighttime resulting in more layer misdetection, and beneath optically thick clouds due to significant signal attenuation. More details of why some aerosol features may go undetected can be found in the level 2 aerosol layer data quality summary. Biases due to missed detections are believed to be small at night in most regions, though potential biases have not yet been comprehensively quantified by validation intercomparisons. Biases can be more pronounced in the daytime when strongly scattering layers are not detected due to enhanced solar noise over bright surfaces. The impact of this effect has not been quantified at this time.

Meteorological context statistics not calculated for sky condition

Currently, each meteorological context parameter is generated for every grid cell by averaging together all profiles of the parameter that exist within the grid cell together, regardless of cloud cover or aerosol quality screening. As a consequence, there is not a one-to-one correspondence between the region of sky used to generate aerosol extinction statistics and the region of sky used to generate the meteorological context statistics. For example, the aerosol extinction reported for the “Cloud-Free” sky condition will not include extinction samples in cloudy columns, but the Temperature statistics (for example), and all other meteorological parameters would still contain those samples in cloudy columns.

Samples Searched statistic does not include cloudy samples

The “Samples Searched” statistic is intended to represent the number of atmospheric samples searched by the level 3 algorithm regardless of whether they were accepted, rejected or ignored in the averaging process. This statistic should include cloudy samples, but it does not in the V3.00 release. This will be corrected in a forthcoming release.



NASA
Last Updated: November 28, 2017
Curator: Charles R. Trepte
NASA Official: Charles R. Trepte

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