CALIPSO Quality Statements:
Lidar Level 2 Cloud and Aerosol Profile Products
Version Releases: 2.01, 2.02
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CALIPSO Quality Statements: |
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This document provides a high-level quality assessment of the cloud and aerosol profile products derived from the CALIPSO lidar measurements, as described in Sections 2.5 and 2.6 of the CALIPSO Data Products Catalog (Version 2.4) (PDF). As such, it represents the minimum information needed by scientists and researchers for appropriate and successful use of these data products. We strongly suggest that all authors, researchers, and reviewers of research papers review this document periodically, and familiarize themselves with the latest status before publishing any scientific papers using these data products.
These data quality summaries are published specifically to inform users of the accuracy of CALIOP data products as determined by the CALIPSO Science Team and Lidar Science Working Group (LSWG). This document is intended to briefly summarize key validation results; provide cautions in those areas where users might easily misinterpret the data; supply links to further information about the data products and the algorithms used to generate them; and offer information about planned algorithm revisions and data improvements.
The primary new parameters included in the version 2.0 release are aerosol and cloud extinction and backscatter profiles (Aerosol Profile Product and Cloud Profile Product), layer optical depth, aerosol type, and cloud ice/water phase (Aerosol and Cloud Layer Product), and aerosol type and cloud ice/water phase in the Vertical Feature Mask (VFM). Although extinction or optical depth appear in several different products, all extinction retrievals are produced by the same algorithm. Therefore, the Data Quality Summaries include a section which discusses general characteristics of the extinction and optical depth data applicable to all products.
| WARNING! |
| The backscatter and extinction coefficients reported in the the version 2.01 release of the CALIPSO profile products are unvalidated, beta-quality data products. As such, they still contain a number of errors and/or inconsistencies. Furthermore, the current products contain no data quality information, and hence cannot be used as standalone products. Users are cautioned against the indiscriminate use of these data products; using them as the basis for research findings, journal publications, and/or presentations at scientific conferences is strongly discouraged. |
The CALIOP Version 2 data release includes extinction products for the first time. The Cloud Profile Product and Aerosol Profile Product report profiles of particle extinction and backscatter. The Cloud and Aerosol Layer Product includes layer optical depths. The layer optical depths are derived from the same retrievals which are used to compute the extinction and backscatter profiles in the Profile Products, although the horizontal averaging of the final products may be different. All of these extinction products are produced using the same basic algorithm.
There are layers for which the optical depth can be reliably measured directly from the CALIOP backscatter signal. For these layers, the measured optical depths are reported in the layer products, and the layer two-way transmittance is used to constrain the extinction solution, so that an optimal estimate of the layer lidar ratio is retrieved. These constrained solutions are the most reliable retrievals. The implementation in Version 2 is fairly conservative though, and only a small fraction of the retrievals are constrained. Uncertainties of the other, unconstrained, retrievals are larger, and primarily depend on how closely the lidar ratio initially assumed by the algorithm agrees with the true lidar ratio, and on how well the attenuation of overlying layers has been estimated. Errors in the initial lidar ratio and attenuation correction propagate non-linearly into subsequent retrievals, so that extinction errors do not average out but instead produce biases.
In the case of opaque layers - i.e., where the lidar signal does not penetrate to the base of the layer - the reported optical depth refers only to the upper portion of the layer where there is measurable lidar signal. About 20% of cloud layers are identified as being opaque, as are a few very dense aerosol layers. In these cases CALIOP does not give the true layer base height and underestimates the true layer optical depth. The Opacity Flag in the Cloud and Aerosol Layer Products identifies layers which are opaque. There is no corresponding flag in the profile products.
About 90% of cloud retrievals and 99% of aerosol retrievals conclude successfully, in the sense that a physically possible solution is found (retrieved extinction is non-negative and finite). The results of the remaining retrievals can also be found in the data products, but are non-physical. For these cases, the extinction QC flags reported in the layer products will enumerate the condition(s) that caused the failure. There are no corresponding flags in the profile products.
The extinction retrieval will, if necessary to avoid a non-physical solution, adjust the initial lidar ratio as required to produce a physical solution. The most common problem is that the extinction solution diverges and the retrieved values tend toward infinity if the assumed lidar ratio is too large. For weakly scattering layers the lidar ratio is most often left unchanged, as a physical solution is usually obtained on the first iteration. However, if the initial lidar ratio is much larger than the true value, divergence can occur even for optically thin (optical depth < 0.5) layers. When divergence occurs the lidar ratio is automatically decreased and the retrieval is repeated until a convergent solution is obtained.
In the current implementation of the extinction retrieval algorithm, the lidar ratio adjustment scheme does not result in realistic retrievals in aerosol layers in cases where the initial divergence is due to a misclassification of aerosol type. When divergence occurs, it can be because the true lidar ratio is much less than the lidar ratio selected by the algorithm, or that the attenuation of overlying layers was overcorrected, or both. Aerosol retrievals where the final lidar ratio is not equal to the initial lidar ratio should be regarded as invalid and ignored. The lidar ratio is adjusted in only about 5% of the aerosol retrievals.
Figure 1 (below) shows a comparison of column aerosol optical depth (AOD) from CALIOP with MODIS AOD. The comparison is for all CALIOP nighttime aerosol measurements in cloud-free columns during the month of January 2007 vs. all daytime MODIS AOD during the same month. In computing the distribution of CALIOP column AOD in Figure 1, retrievals where the initial lidar ratio was adjusted during the retrieval were screened out, as were any aerosol layers with integrated attenuated backscatter values greater than 0.01 /km/sr. While the two distributions agree fairly well, with MODIS appearing to slightly over-estimate values at the lower end of the optical depth range, global maps of monthly-average AOD show significant regional differences. A similar comparison of the distribution of MODIS AOD with daytime CALIOP shows CALIOP has a significant low bias with respect to MODIS. The difference is probably due to a combination of differences in day and night calibration and somewhat lower detection sensitivity during daytime.
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Figure 1: Comparison of CALIPSO aerosol optical depths to those derived from MODIS
(Preliminary - January 2007, nighttime data, final lidar ratio = initial lidar ratio only) |
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A histogram of cirrus optical depths from the Cloud Layer Product shows a bimodal distribution (see Figure 2, left panel). The peak on the right, at an optical depth of ~2.5, appears to be an artifact due to the behavior of the retrieval algorithm when the initial retrieval diverges and the lidar ratio is reduced to produce a convergent solution. This happens most often in totally attenuating (i.e., opaque) clouds, and when the true cirrus lidar ratio is significantly smaller than the initial value assumed by the algorithm. The second peak disappears when we consider only those solutions for which the lidar ratio was unchanged (i.e., the extinction QC flag = 0; see Figure 2, right panel). In any case, errors grow rapidly when the true cirrus lidar ratio is different from the assumed value. Errors in optical depth for thin cirrus (optical depth < 0.5) are on the order of 50% for unconstrained retrievals.
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Figure 2: CALIPSO cirrus cloud optical depths
(Preliminary - August 2006, nighttime only) |
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Although the production code applies the extinction retrieval algorithm to all layers detected, the CALIOP extinction retrieval algorithm was developed for retrievals of aerosol and ice clouds, not water clouds. Results from water clouds are unreliable and should be ignored, except for constrained retrievals. One of the new parameters in the Version 2 Cloud and Aerosol Layer Products and the VFM is cloud ice/water phase. There is no corresponding flag in the profile products.
The CALIPSO data processing system generates separate sets of profile products for clouds and aerosols. The cloud profile products are reported at a uniform spatial resolution of 60-m vertically and 5-km horizontally, over a nominal altitude range from 20-km to -0.5-km. The aerosol profile products are reported over a broader altitude range of 30-km to -0.5-km. Profile data for all features detected in the stratosphere are reported in the aerosol profile product, thus users seeking to obtain CALIPSO retrievals of backscatter and extinction profiles of polar stratospheric clouds are directed to the aerosol profile products. Due to constraints imposed by CALIPSO's on-board data averaging scheme, the vertical resolution of the aerosol profile data varies as a function of altitude. In the tropospheric region between 20-km to -0.5-km, the aerosol profile products are reported at a spatial resolution of 120-m vertically and 40-km horizontally. In the stratospheric region (above 20-km), the aerosol profile products are reported at a spatial resolution of 360-m vertically and 40-km horizontally. In the text below we provide brief descriptions of individual data fields reported in the CALIPSO cloud and aerosol profile products. Where appropriate, we also provide an assessment of the quality and accuracy of the data in the current release.
WARNING! The backscatter coefficients reported in the the version 2.01 release of the CALIPSO profile products are unvalidated, beta-quality data products. As such, they still contain a number of errors and/or inconsistencies. Furthermore, the current products contain no data quality information, and hence cannot be used as standalone products. Users are cautioned against the indiscriminate use of these data products; using them as the basis for research findings, journal publications, and/or presentations at scientific conferences is strongly discouraged.
WARNING! The extinction coefficients reported in the the version 2.01 release of the CALIPSO profile products are unvalidated, beta-quality data products. As such, they still contain a number of errors and/or inconsistencies. Furthermore, the current products contain no data quality information, and hence cannot be used as standalone products. Users are cautioned against the indiscriminate use of these data products; using them as the basis for research findings, journal publications, and/or presentations at scientific conferences is strongly discouraged.
| Lidar Level 2 Cloud and Aerosol Profile Information Half orbit (Night and Day) averaged cloud and aerosol profile data and ancillary data |
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| Release Date | Version | Data Date Range | Maturity Level |
| October 2008 | 2.02 | September 14, 2008 to present | Beta |
| January 25, 2008 | 2.01 | June 13, 2006 to September 13, 2008 | Beta |
Version 2.02 of the Level 2 data products is a maintenance release that implements the following changes.
The impacts of these changes on the Level 2 data products are as follows:
Layer detection: As a result of the first two changes, the 532 nm and 1064 nm calibration constants are larger, on average, by ~1%, resulting in corresponding decreases in the magnitudes of the attenuated backscatter coefficients at both wavelengths. These changes in the level 1 data result only small changes to the layer detection statistics. For example, the difference in the total number of layers detected by the two different versions on August 12, 2006 was 4: 9680 layers were detected by the version 2.01 code, versus 9676 layers by the version 2.02 code.
Cloud-aerosol discrimination: with one exception, there were only minimal changes in cloud-aerosol discrimination results. The exception occurs in the polar regions when PSCs are present. For the August 12, 1006 test case, corrections to the interpolation algorithms applied to the GMAO data result in a slight upward shift in the tropopause heights, and as a consequence, more clouds and fewer stratospheric layers are identified in the version 2.02 results.
Ice-water phase determination: because this classification is based on depolarization ratio and temperature no substantial changes, there were no substantial changes in the assessments of cloud thermodynamic state.
Aerosol subtype identification: Correcting the level 2 runtime script error will reduce the number of layers identified as smoke, and increase the number of layers identified as sea salt.
Cloud and aerosol extinction profiles and optical properties: changes in backscatter and extinction coefficients at the tops of layers are small, and proportional to the changes in the calibration coefficients ... however, due to the cumulative nature of error propagation in the extinction retrieval, differences increase with increasing penetration depths, and can grow large when the optical depths of the clouds are large (i.e., > 3).
PLEASE NOTE: users of the CALIOP extinction and backscatter profile data should read and thoroughly understand the information provide in the Profile Products Data Quality Summary. This summary contains an expanded description of the extinction retrieval process from which the layer optical depths are derived, and provides essential guidance in the apppropriate use of all CALIOP extinction-related data products. Validation and improvements to the profile products QA are ongoing efforts, and additional data quality information will be included with future releases.
Dense aerosol layers (primarily very dense dust and smoke over and close to the source regions) are sometimes labeled as cloud. Because the CAD algorithm operates on individual layers, without a contextual awareness of any surrounding features, it can happen that small but strongly scattering regions within an extended aerosol layer can occasionally be labeled as cloud. This occurs because the optical properties (backscatter and color ratio) within the region are similar to what would be expected for the relatively faint clouds that fall within the PDF overlap region. These misclassifications are often apparent from studying the Level 1 browse images. Based on the initial analysis of the CALIOP measurements, the cloud and aerosol distributions show variabilities that depend on season and on geophysical location. The globally averaged PDFs used in the current release will have a larger overlap between the cloud and aerosol than would occur for more regionally specific statistics. For future versions of the CAD algorithm, we expect to develop and deploy PDFs that will correctly reflect both seasonal and latitudinal variations.
Many optically thin clouds, both ice and water, are encountered in the polar regions. The current CAD PDFs do not work as well in the polar regions as at lower latitudes and misclassifications of clouds as aerosol are more common. In particular, thin ice clouds which can extend from the surface to several kilometers in altitude, are sometimes misclassified as aerosol.
Correct classification of heterogeneous layers is always difficult, and the process can easily go awry. An example of a heterogeneous layer would be an aerosol layer that is vertically adjacent to a cloud or contains an embedded cloud, but which is nonetheless detected by the feature finder as a single entity. By convention, heterogeneous layers should be classified as clouds. However, depending on the relative strengths of the components, these layers are sometimes erroneously identified as aerosol.
Some so-called features identified by the layer detection scheme are not legitimate layers, but instead are artifacts due to the noise in the signal, multiple scattering effects, or to artificial signal enhancements caused by non-ideal detector transient response or an over estimate of the attenuation due to overlying layers. These erroneous "pseudo-features" are neither cloud nor aerosol; however, because they are not properly interdicted in the processing stream, the CAD algorithm nonetheless attempts to assign them to one class or the other. Very frequently these layers can be identified by their very low CAD scores (typically less than 20).
The extinction products are produced by first identifying an aerosol type and then using the appropriate values of Sa and the multiple scattering factor, η(z). Note that multiple scattering corrections have not yet been implemented for the current data release, so that η(z) = 1 for all aerosol types. The accuracy of the Sa value used in the lidar inversions depends on the correct identification of the type of aerosol. In turn, the accuracy of the subsequent optical depth estimate depends on the accuracy of Sa.
The underlying paradigm of the type classification is that a variety of emission sources and atmospheric processes will act to produce air masses with a typical, identifiable aerosol 'type'. This is an idealization, but one that allows us to classify aerosols based on observations and location in a way to gain insight into the geographic distribution of aerosol types and constrain the possible values of Sa for use in aerosol extinction retrievals.
The aerosol subtype product is generated downstream of the cloud-aerosol discrimination (CAD) scheme and, therefore, depends on the cloud-aerosol classification scheme in a very fundamental way. If a cloud feature is misclassified as aerosol, the aerosol subtype algorithm will identify this 'aerosol' as one of the aerosol subtypes. The user must exercise caution where the aerosol subtype looks suspicious or unreasonable. Such situations can occur with some frequency in the southern oceans and the polar regions.
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