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CALIPSO HOMEUser’s Guide HOMEData Products Description → Lidar Level 2 Vertical Feature Version 4.20 Product

CALIPSO: Data User’s Guide - Data Product Descriptions - Lidar Level 2 5 km Vertical Feature Mask (VFM) Version 4.20 Product


Version 4.20 Level 2 Vertical Feature Mask Product Description

The CALIPSO Level 2 lidar vertical feature mask data product describes the vertical and horizontal distribution of cloud and aerosol layers observed by the CALIPSO lidar. Each range bin in the Lidar Level 0 data is characterized by a single 16-bit integer, with the various bits in the integer representing flags that describe some aspect of the data measured within the bin. Instructions on how to decode these integer data are given in the sections below. Also provided are quality summaries for each classification reported. The data are recorded in nominal increments of 15 consecutive laser pulses, which is nominally equivalent to a distance of 5 km along the laser ground-track. A Single_Shot_Detection VGroup is reported in the vertical feature mask product which provides information at single shot (1/3 km) resolution for a subset of science datasets. The names of science data sets within this VGroup are prefixed with "ss".


Data Descriptions

In the text below we provide brief descriptions of individual data fields reported in the CALIPSO vertical feature mask product. Where appropriate, we also provide an assessment of the quality and accuracy of the data in the current release. The data descriptions are grouped into several major categories, as follows:

Additionally all the science data sets (SDSs) are listed in the table to the right, click on the SDS name to go directly to the description.

Science Data Set (SDS)
Profile_ID
Latitude
Longitude
Profile_Time
Profile_UTC_Time
Day_Night_Flag
Land_Water_Mask
Minimum_Laser_Energy_532
Spacecraft_Position
Feature_Classification_Flags
ssProfile_ID
ssLatitude
ssLongitude
ssProfile_Time
ssProfile_UTC_Time
ssLand_Water_Mask
ssLaser_Energy_532


Spatial and Temporal Parameters

Profile_ID
A unique identifier for each profile. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution.

Latitude
Latitude, in degrees, of the laser footprint. One value is reported at the temporal midpoint of each 15 shot data segment. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution.

Longitude
Longitude, in degrees, of the laser footprint. One value is reported at the temporal midpoint of each 15 shot data segment. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution.

Profile_Time (external)
Time expressed in International Atomic Time (TAI). Units are in seconds, starting from January 1, 1993. One value is reported at the temporal midpoint of each 15 shot data segment. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution.

Profile_UTC_Time (external)
Time expressed in Coordinated Universal Time (UTC), and formatted as 'yymmdd.ffffffff', where 'yy' represents the last two digits of year, 'mm' and 'dd' represent month and day, respectively, and 'ffffffff' is the fractional part of the day. One value is reported at the temporal midpoint of each 15 shot data segment. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution.

Day_Night_Flag (external)
Indicates the lighting conditions at an altitude of ~24 km above mean sea level: 0 = day, 1 = night.

Land_Water_Mask (external)
This is a 15 arc second resolution land/water mask provided by the CloudSat Science Team. The data is stored as an 8-bit integer. The values indicate the surface type at the lidar footprint, and are interpreted as follows:

  • 0 = shallow ocean
  • 1 = land
  • 2 = coastlines
  • 3 = shallow inland water
  • 4 = intermittent water
  • 5 = deep inland water
  • 6 = continental ocean
  • 7 = deep ocean

Please see section 4.5 in PC-SCI-503: CALIPSO Data Products Catalog (Release 4.30) (PDF) for more information. Science data sets within the Single Shot Detection VGroup, also prefixed with "ss", report properties of features detected at single shot (1/3 km) resolution.

Minimum_Laser_Energy_532
This field reports the minimum 532 nm laser pulse energy, in Joules, within each 80 km along-track data segment (80 km = 240 laser pulses). The 80 km distance matches the largest horizontal extent considered in CALIOP’s standard level 2 data analyses. Since layers can be detected at horizontal resolutions as large as 80 km, anomalously low laser energies in coarse resolution upper layers can potentially introduce biases in the spatial and optical property retrievals of underlying layers detected at finer spatial resolutions. The Minimum_Laser_Energy_532 SDS enables ready identification of these problematic situations. See the Data Advisory web page for information on the occurrence of anomalously low laser energy shots, their impact on data quality, and for specific guidance on how to use Minimum_Laser_Energy_532 to identify affected profiles.

Feature_Classification_Flags
For each layer detected in the CALIPSO backscatter data, we derive a set of feature classification flags that report (a) feature type (e.g., cloud vs. tropospheric aerosol vs. stratospheric aerosol); (b) feature subtype; (c) layer ice-water phase (clouds only); and (d) the amount of horizontal averaging required for layer detection. Quality assessments are also provided for all layer classifications. The complete set of flags is stored as a single 16-bit integer. The following table is reproduced from the CALIPSO Data Products Catalog.

Reproduced from PC-SCI-503: CALIPSO Data Products Catalog (Release 4.70) (PDF)
Table 83: Feature Classification Flag Definition
Bits Field Description Bit Interpretation
1-3 Feature Type 0 = invalid (bad or missing data)
1 = "clear air"
2 = cloud
3 = tropospheric aerosol
4 = stratospheric aerosol
5 = surface
6 = subsurface
7 = no signal (totally attenuated)
4-5 Feature Type QA 0 = none
1 = low
2 = medium
3 = high
6-7 Ice/Water Phase 0 = unknown / not determined
1 = ice
2 = water
3 = oriented ice crystals
8-9 Ice/Water Phase QA 0 = none
1 = low
2 = medium
3 = high
10-12 Feature Sub-type  
If feature type = tropospheric aerosol, bits 10-12 will specify the aerosol type 0 = not determined
1 = clean marine
2 = dust
3 = polluted continental/smoke
4 = clean continental
5 = polluted dust
6 = elevated smoke
7 = dusty marine
If feature type = cloud, bits 10-12 will specify the cloud type. 0 = low overcast, transparent
1 = low overcast, opaque
2 = transition stratocumulus
3 = low, broken cumulus
4 = altocumulus (transparent)
5 = altostratus (opaque)
6 = cirrus (transparent)
7 = deep convective (opaque)
If feature type = stratospheric aerosol, bits 10-12
will specify stratospheric aerosol type.
0 = invalid
1 = PSC aerosol
2 = volcanic ash
3 = sulfate/other
4 = elevated smoke
5 = spare
6 = spare
7 = spare
13 Cloud / Troposheric Aerosol / Stratospheric Aerosol QA 0 = not confident
1 = confident
14-16 Horizontal averaging required for detection (provides a
coarse measure of feature backscatter intensity)
0 = not applicable
1 = 1/3 km
2 = 1 km
3 = 5 km
4 = 20 km
5 = 80 km

User notes for the feature classification flags.

  • Bits 1-3, Feature Type
    Invalid (bad or missing data)

    Invalid features are defined as those layers for which the 532 nm integrated attenuated backscatter, γ′532, is less than zero. In version 3 of the CALIPSO Level 2 data products, invalid layers will only occur in those very rare cases when (a) a layer is detected at one of the coarser averaging resolutions (i.e., 20 km or 80 km) and (b) above this layer there are also overlying layers detected at finer resolutions (typically 5 km) that have large and widely varying optical depths. The non-uniform attenuation corrections applied to the backscatter signal within the lower layer can result in an unfavorable redistribution of signal magnitudes; e.g., large corrections applied to profiles with a substantial fraction of negative attenuated backscatter values, and smaller corrections applied to profiles with all positive values. In especially perverse cases, γ′532 for the rescaled and reaveraged data can be negative, even though prior to rescaling γ′532 was comfortably above zero.


    "Clear Air"

    Regions are where the signal has not been totally attenuated by overlying layers and in which no feature is detected are classified as being clear air. Because the determination of clear air depends not only on the cloud/aerosol content of the atmosphere, but also on the minimum detectable backscatter of the CALIOP layer detection scheme, regions containing especially weak layers can, on occasion, be misclassified as clear air. Detection threshold issues are addressed in detail in the CALIPSO layer detection ATBD (PDF).


    Cloud / Tropospheric Aerosol / Stratospheric Aerosol

    The V4 cloud aerosol discrimination (CAD) algorithm uses the altitude-and-latitude-dependent feature integrated color ratio, χ′, the layer-integrated volume depolarization ratio, δv, and the feature mean attenuated backscatter coefficient, <β′532>, to compute the CAD score. These parameters depend on the quality of the 532 nm and 1064 nm channel calibrations. Significant errors in the calibration of either channel may result in the misclassification of a particular feature.

    The probability distribution functions (PDFs) that are used by the version 4 CAD algorithm were developed following substantial revisions in the calibration of CALIOP Level 1 data for both 532 nm and 1064 nm channels. These PDFs are binned by altitude (1 km increments between 0 km and 30 km) and latitude (between 90°S and 90°N in 5° increments). The version 4 CAD algorithm is applied to all layers in the troposphere and stratosphere including the single shot layers (333m), although the PDFs did not include the optical properties of the 333m layers. With the new PDFs, the discrimination between clouds and high-altitude dust and smoke layers should improve.

    The application of version 4 CAD algorithm in the stratosphere (unlike in all earlier versions) now allows classification of stratospheric aerosols as distinct from clouds. All layers with attenuated backscatter centroids above the local tropopause as determined from the MERRA 2 meteorology are classified as stratospheric aerosol layers, whereas those with centroids below the tropopause are classified as tropospheric aerosol layers. The correctness of this classification thus depends critically on the accuracy of the tropopause heights obtained from the MERRA 2 reanalysis. Note that some extended aerosol plumes may straddle the tropopause, causing the feature type to switch between tropospheric aerosol and stratospheric aerosol.

    Surface/Subsurface

    Altitude regions lying below the surface elevation detected by the CALIOP surface detection algorithm are classified as subsurface regions.


    No signal

    The backscatter signal in those regions lying directly beneath opaque clouds, aerosols, and/or stratospheric layers is totally attenuated by the overlying features, and thus these regions are classified as having "no signal".


  • Bits 4-5, Feature Type QA
    Invalid
    Not applicable. Always 0.

    "Clear Air"
    Not applicable. Always 0.

    Cloud / Tropospheric Aerosol / Stratospheric Aerosol
    For cloud and aerosol layers, feature type QA is directly related to the cloud-aerosol discrimination (CAD) score, as follows:

    high confidence  =  |CAD score| ≥ 70
    medium confidence  =  50 ≤ |CAD score| < 70
    low confidence  =  20 ≤ |CAD score| < 50
    no confidence  =  |CAD score| < 20

    Surface / Subsurface
    Always high.

    No signal
    Not applicable; always 0.

  • Bits 6-7, Ice/Water Phase

    Cloud phase is determined from relations between depolarization ratio, backscatter intensity, temperature, and attenuated backscatter color ratio. The cloud phase algorithm used in Version 3 is new and completely different from that used in Version 2. The Version 3 algorithm classifies cloud layers as water, randomly-oriented ice, or horizontally-oriented ice. In those cases where the classification is ambiguous, the phase is reported as "unknown/not determined". The classification of "mixed phase cloud" has been eliminated; the version 3 algorithm does not attempt to determine if more than one phase is present within a layer. The version 3 algorithm distinguishes between two separate classes of ice clouds: those dominated by randomly oriented particles, and those containing a substantial fraction of horizontally oriented crystals.


  • Bits 8-9, Ice/Water Phase Quality Assessment

    Changes in the interpretation of the ice-water phase QA flags between version 2 and version 3 are described in the table below.

    Value Version 4Interpretation
    0 no confidence
    1 low confidence
    2 medium confidence
    3 high confidence

  • Bits 10-12, Feature Sub-type

    Aerosols

    The tropospheric aerosol selection scheme shown in Figure 1 uses the observed backscatter strength and depolarization to identify aerosol type, to the extent possible, from among one of the seven types. The volume depolarization is directly related to the hydration state of the aerosol. The backscatter and volume depolarization are not sufficient to fully constrain the model selection, however. Therefore, the selection algorithm uses surface type to aid in the type identification. The input parameters - the magnitude of attenuated backscatter, altitude, location, surface type, depolarization ratio, and mean attenuated backscatter coefficient measurements - are used to identify the type following one of several pathways. The volume depolarization ratio is used to identify aerosol types that have a substantial mass fraction of non-spherical particles, e.g., a mixture of smoke and dust. The integrated attenuated backscatter (γ′) is used to discern instances of transient high aerosol loading over surfaces where this is not usually expected, e.g., a smoke or dust layer over the ocean. For aerosols in polar regions, the algorithm takes into consideration the high aerosol loading events caused by arctic haze. Once the type is identified, the aerosol lidar ratio, Sa is chosen from a lookup table that currently consists of six pairs of 532 nm and 1064 nm values.

    In summary, the algorithm classifies aerosol layers that have volume depolarization ratio (δv) greater than 0.2 as desert dust and 0.075 < δv < 0.2 as polluted dust. Note that polluted dust could be a component of urban pollution, i.e., it is not confined to desert regions but is any type of aerosol composed of some dust-like particles. Likewise, layers classified as dusty marine could be mixtures of dust and urban pollution if they reside over ocean.Of the non-depolarizing aerosols, layers with tops above 2.5 km are assumed to be smoke, and layers with tops less than 2.5.km above the surface are either clean continental if the layer IAB is small or polluted continental/smoke if the layer IAB is large. Note that layers classified as "polluted continental/smoke" can contain smoke, urban pollution or both types since it is not possible to unambiguously discriminate smoke from urban pollution based on CALIOP measurables.

    VFM Figure 1 image.

    Figure 1. Tropospheric aerosol subtyping flowchart implemented in version 4.


    Clouds
    Cloud layers are classified into the standard meteorological cloud types defined by the ISCCP). The classification decisions are based on cloud top pressure/altitude, cloud opacity, and cloud fraction within an 80 km segment.

    Stratospheric Aerosols
    The stratospheric aerosol selection scheme, shown in Figure 2, first identifies PSC related aerosols having layer centroid temperature less than -70C and existing during PSC season, assumed to be December to February in the Arctic and May to October in the Antarctic. Non-PSC related stratospheric aerosol layers are identified as one of three stratospheric aerosol species based on depolarization, color ratio and layer integrated attenuated backscatter (γ′). Thresholds for these three subtypes were developed empirically from CALIOP observations of six volcanic plume events and several smoke events which exceeded the tropopause. Weakly scattering layers are immediately classified as sulfate/other since depolarization and color ratio fidelity is compromised for layers with low signal. Layers exceeding the γ′ threshold having high depolarization are classified as volcanic ash while layers with moderate to low depolarization and enhanced color ratio are classified as smoke. Layers exceeding the γ′ threshold which having low depolarization and low color ratio are classified as sulfate/other.

    VFM Figure 2 image.

    Figure 2. Stratospheric aerosol subtype flowchart implemented in version 4.

  • Bit 13, Cloud / Aerosol / PSC Subtype Quality Assessment

  • Bits 14-16, Horizontal Averaging
    Specifies the amount of horizontal averaging required for a feature to be detected. For all data versions up to and including 3.01 release, the values decoded from the bits in this field will be either 1/3 km, 1 km, 5 km, 20 km, or 80 km.

ssLaser_Energy_532 (external)
This field reports the minimum 532 nm laser energy, in Joules, contained within each horizontal 80 km segment (the coarsest resolution, see the layer detection ATBD) used by the feature detection algorithm.

Layout of the Feature Classification Flag data block

The Feature_Classification_Flag values are stored as a sequence of 5515 element arrays (i.e., as an N x 5515 matrix, where N is the number of separate records in the file). Each array represents a 5 km "chunk" of data, and each array element contains the feature classification information for a single range resolution element in the Level 0 lidar data downlinked from the satellite. As shown in the table below, the vertical and horizontal resolution of the CALIPSO data varies as a function of altitude above mean sea level (see Hunt et al., 2009). The image above provides a pictorial representation of the mapping of the one-dimensional array of Feature_Classification_Flag values into a two-dimensional array of range-resolved lidar data samples. The numbers in each block of the image indicate the 1-D array indices associated with each spatial averaging regime in the 2-D lidar backscatter data. Only the starting and ending indices are shown.

Example code for transforming a 1-D array of feature classification flags into a rectangular, altitude-registered matrix is available for both IDL and MatLab.

VFM Data Packing.
Profile Spatial Resolution
Altitude Region Vertical Resolution
(meters)
Horizontal Resolution
(meters)
Profiles
per 5 km
Samples
per Profile
Base (km) Top (km)
-0.5 8.2 30 333 15 290
8.2 20.2 60 1000 5 200
20.2 30.1 180 1667 3 55
Total 545

Metadata Parameters

Product_ID
an 80-byte (max) character string specifying the data product name. For all CALIPSO Level 2 lidar data products, the value of this string will be "L2_Lidar".

Date_Time_at_Granule_Start
a 27-byte character string that reports the date and time at the start of the file orbit segment (i.e., granule). The format is yyyy-mm-ddThh:mm:ss.ffffffZ.

Date_Time_at_Granule_End
a 27-byte character string that reports the date and time at the end of the file orbit segment (i.e., granule). The format is yyyy-mm-ddThh:mm:ss.ffffffZ.

Date_Time_at_Granule_Production
This is a 27-byte character string that defines the date at granule production. The format is yyyy-mm-ddThh:mm:ss.ffffffZ.

Lidar_L1_Production_Date_Time
For each CALIOP Lidar Level 2 data product, the Lidar Level 1 Production Date Time field reports the file creation time and date for the CALIOP Level 1 lidar data file that provided the source data used in the Level 2 analyses.

Number_of_Good_Profiles
This is a 32-bit integer specifying the number of good attenuated backscatter profiles contained in the granule.

Number_of_Bad_Profiles
This is a 32-bit integer specifying the number of bad attenuated backscatter profiles contained in the granule.

Initial_Subsatellite_Latitude
This field reports the first subsatellite latitude of the granule.

Initial_Subsatellite_Longitude
This field reports the first subsatellite longitude of the granule.

Final_Subsatellite_Latitude
This field reports the last subsatellite latitude of the granule.

Final_Subsatellite_Longitude
This field reports the last subsatellite longitude of the granule.

Orbit_Number_at_Granule_Start
This field reports the orbit number at the granule start time.

Orbit_Number_at_Granule_End
This field reports the orbit number at the granule stop time.

Orbit_Number_Change_Time
This field reports the time at which the orbit number changes in the granule.

Path_Number_at_Granule_Start
This field reports the path number at the granule start time.

Path_Number_at_Granule_End
This field reports the path number at the granule stop time.

Path_Number_Change_Time
This field reports the time at which the path number changes in the granule.

Lidar_Data_Altitude
This field defines the lidar data altitudes (583 range bins) to which lidar Level 1 profile products are registered.

GEOS_Version
This is a 64-bit character that reports the version of the MERRA-2 data product provided by the GMAO.

Classifier_Coefficients_Version_Number
Version number of the classifier coefficients file that stores the five-dimensional probability distribution functions used by the cloud-aerosol discrimination (CAD) algorithm

Classifier_Coefficients_Version_Date
Creation date of the classifier coefficients file that stores the five-dimensional probability distribution functions used by the cloud-aerosol discrimination (CAD) algorithm

Production_Script
Provides the configuration information and command sequences that were executed during the processing of the CALIOP Lidar Level 2 data products. Documentation for many of the control constants found within this field is contained in the CALIPSO Lidar Level 2 Algorithm Theoretical Basis Documents


Data Quality Statements

Lidar Level 2 Vertical Feature Mask (VFM) Information
Half orbit (Night and Day) scene classification data and lidar lighting and land/water indicators.
Release Date Version Data Date Range Production Strategy
October 10, 2018 4.20 June 13, 2006 to present Standard
November 8, 2016 4.10 June 13, 2006 to May 31, 2018 Standard

Summary Statement for the release of the CALIPSO Lidar Level 2 Version 4.20 Products, October 10, 2018

View Version 4.20 Detailed Quality Statement.

Summary Statement for the release of the CALIPSO Lidar Level 2 Version 4.10 Products, November 08, 2016

View Version 4.10 Detailed Quality Statement.


NASA
Last Updated: October 01, 2021
Curator: Charles R. Trepte
NASA Official: Charles R. Trepte

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