The CALIPSO Cloud and Aerosol Profile Products report profiles of particle
extinction and backscatter and additional profile information (e.g.,
particulate depolarization ratios) derived from these fundamental products.
Layer optical depths are reported in the Cloud and Aerosol Layer Products. The
layer optical depths are derived from the same retrievals that are used to
compute the extinction and backscatter profiles in the profile products. All of
these extinction products are produced using the same basic algorithm (Young
and Vaughan, 2009).
The cloud and aerosol profile products are reported at a
uniform spatial resolution of 60 m vertically and 5 km horizontally, over a nominal altitude range
from 30 km to -0.5 km. Profile data for tropospheric aerosol and stratospheric aerosol are reported
in the aerosol profile product, 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 resolution of 60 m vertically, and in the
stratospheric region (above 20-km), the aerosol profile products are reported at a resolution
of 180 m vertically. 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. Lidar surface retrieval
information is reported within the Lidar_Surafce_Detection VGroup.
Data Descriptions
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. The data descriptions are grouped into several major
categories, as follows:
Time, expressed in International Atomic Time (TAI). Units are in
seconds, starting from January 1, 1993. For the 5 km profile products,
three values are reported: the time for the first pulse included in the
15 shot average; the time at the temporal midpoint (i.e., at the 8th of
15 consecutive laser shots); and the time for the final pulse.
Profile_UTC
Similar to Profile Time, but for 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.
Day_Night_Flag
Indicates the lighting conditions at an altitude of ~24 km above mean sea
level; 0 = day, 1 = night.
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.
Column Geolocation Information
Latitude
Geodetic latitude, in degrees, of the laser footprint. For the 5 km profile
products, three values are reported: the footprint latitude for the first
pulse included in the 15 shot average; the footprint latitude at the temporal
midpoint (i.e., at the 8th of 15 consecutive laser shots); and the footprint
latitude for the final pulse.
Longitude
Longitude, in degrees, of the laser footprint. For the 5 km profile products,
three values are reported: the footprint longitude for the first pulse
included in the 15 shot average; the footprint longitude at the temporal
midpoint (i.e., at the 8th of 15 consecutive laser shots); and the footprint
longitude for the final pulse.
IGBP_Surface_Type
International Geosphere/Biosphere Programme (IGBP) classification of the
surface type at the lidar footprint. The IGBP surface types reported by
CALIPSO are the same as those used in the
CERES/SARB surface map. The CERES/SARB surface map table
is below.
Optical depth of all clouds, aerosol, or stratospheric layers within a 5
km column. The optical depths are obtained by integrating the 532 nm
cloud/aerosol/stratospheric extinction profile, reported in these profile
products. For aerosols and stratospheric layers the optical depth is provided
at both 532 and 1064 nm wavelengths.
The column optical depths are a column integral product. Any large
uncertainties or poor extinction retrievals from layers within the column
(i.e. clouds, aerosols, or stratospheric features) will propogate downward
and may impact the quality of all (i.e. cloud, aerosol, and
stratospheric) the column optical depths in the column. The following
paragraphs outline notes with regard to specific data products that users
should be aware of when using column optical depth data.
Opacity: We remind data users that the CALIPSO lidar is
only capable of penetrating to the surface if the total column optical depth
is less than ~5. (Note that this value takes into account the contribution of
multiple scattering.) If the column is opaque to the lidar, then the reported
column optical depths are a measure to the apparent base of the lowest
feature observed. Feature opacity can be determined by inspecting the
extinction QC flag for the lowest extinction
coefficient in any 5-km column; if bit 5 (value = 16) is set then the feature
is totally attenuating.
Extinction QC: The extinction QC values in
the column should be examined to determine if any of the extinction
retrievals were bad. Users are reminded that any poor extinction retrievals
in the column may impact the quality of all column optical depths. In
general, solutions where the final lidar ratio is unchanged (extinction QC =
0) or the extinction solution is constrained (extinction QC = 1) yield
physically plausible solutions more often. Conversely, retrievals tend to be
more uncertain in those cases where the lidar ratio for either wavelength
must be reduced.
CAD_Score and
feature sub-type: Features with low absolute CAD_Scores, "special"
CAD_Scores, or uncertain aerosol type classifications may impact the quality
of the column optical depths. For example, if the top-most feature in the
column has a low absolute CAD_Score it is possible that the assigned lidar
ratio may be incorrect; this would impact the extinction retrieval for that
feature which would lead to an incorrect rescaling of all the data below that
feature.
Cloud phase: If there are
clouds in the column that are found to have horizontal oriented ice (HOI)
crystals it is likely that the quality of the column optical depths are low.
The anomalously high backscatter from HOI clouds generally makes the
extinction retrieval more difficult. Because all the data below the HOI cloud
is rescaled by the retrieved optical depth, the extinction data below could
be suspect.
Estimated uncertainty in the column optical depth at each wavelength,
computed according to the formulas give in the
CALIPSO Version 3 Extinction Uncertainty Document (PDF).
Ignoring multiple scattering concerns for the moment, errors in column
optical depth calculations typically arise from three main sources:
signal-to-noise ratio (SNR) within a layer, calibration accuracy, and the
accuracy of the lidar ratio specified for use in the solution. Except for
constrained solutions, where a lidar ratio estimate can be obtained directly
from the attenuated backscatter data, lidar ratio uncertainties are almost
always the dominant contributor to optical depth uncertainties, and the
relative error in the layer optical depth will always be at least as large as
the relative error in the layer lidar ratio.
Calculation of the layer optical depth uncertainty is an iterative
process. On some occasions when the SNR is poor, or an inappropriate lidar
ratio is being used, the iteration will attempt to converge asymptotically to
positive infinity. Whenever this situation is detected, the iteration is
terminated, and the layer optical depth uncertainty is assigned a fixed value
of 99.99. Any time an uncertainty of 99.99 is reported, the extinction
calculation should be considered to have failed. The associated optical
depths cannot be considered reliable, and should therefore be excluded from
all science studies.
Note: optical depth uncertainties are reported as absolute errors, not
relative errors.
Column Integrated Attenuated Backscatter 532
The integral with respect to altitude of the 532 nm total attenuated
backscatter coefficients. The limits of integration are from the onset of the
backscatter signal at ~40-km, down to the range bin immediately prior to the
surface elevation specified by the
digital elevation map. This quantity
represents the total attenuated backscatter measured within a column.
Physically meaningful values of the column integrated attenuated backscatter
(hereafter, γ′column) range from ~0.01 sr
(completely clear air), to greater than 1.5 sr (e.g., due to anomalous
backscatter from horizontally oriented ice crystals; see
Hu et al. (Optics Express 15, 2007)).
Column_IAB_Cumulative_Probability
The cumulative probability of measuring a total column
integrated attenuated backscatter value equal to the value computed for the
current profile. Values in this field range between 0 and 1. The cumulative
probability distribution function, shown below in Figure 1, was compiled
using all CALIOP total column IAB measurements acquired between 15 June, 2006
and 18 October, 2006.
Mean tropopause height, in kilometers above local mean sea level; derived
from the MERRA-2 data product provided to the CALIPSO project by the
GMAO Data
Assimilation System
Tropopause_Temperature (external)
Mean tropopause temperature, in degrees C; derived from the MERRA-2 data
product provided to the CALIPSO project by the
GMAO Data
Assimilation System
Surface_ Winds (external; aerosol products only)
Provides the mean zonal and meridional component of the surface wind speed
computed over the horizontal distance spanned by the averaged profile; units
are meters per second.
Mean temperature, in degrees C, reported for the midpoint of each range bin
in the profile; derived from the MERRA-2 data product provided to the CALIPSO
project by the GMAO Data Assimilation System.
Pressure (external)
Mean pressure, in hectopascals, reported for the midpoint of each range bin
in the profile; derived from the MERRA-2 data product provided to the CALIPSO
project by the GMAO Data Assimilation System.
Molecular_Number_Density (external)
Mean molecular number density, in molecules per cubic meter, reported for the
midpoint of each range bin in the profile; derived from the MERRA-2 data
product provided to the CALIPSO project by the
GMAO Data
Assimilation System.
Ozone_Number_Density (external)
Mean ozone number density, in molecules per cubic meter, reported for the
midpoint of each range bin in the profile; derived from the MERRA-2 data
product provided to the CALIPSO project by the
GMAO Data
Assimilation System.
Relative_Humidity (external)
Mean relative humidity, reported for the midpoint of each range bin in the
profile; derived from the MERRA-2 data product provided to the CALIPSO project
by the GMAO
Data Assimilation System
Surface Elevation Statistics
Surface_Elevation_Statistics
Provides the minimum, maximum, mean, and standard deviation of the surface
elevation obtained from the
GTOPO30 digital elevation map (DEM) for the horizontal
distance spanned by the averaged profile; units are kilometers.
Surface_Top_Altitude_532 Surface Top Altitude 1064
Top altitude of the surface return in the 532 nm and 1064 nm channels at the
lidar footprint in kilometers above local mean sea level. Contains fill
values when surface is not detected. Surface top altitudes are not guaranteed
to agree in both channels due to the difference in averaging resolutions at
the surface (30 m vs. 60 m at 532 nm and 1064 nm, respectively).
Base altitude of the surface return in the 532 nm and 1064 nm channels at the
lidar footprint in kilometers above local mean sea level. Contains fill values
when surface is not detected. Surface base altitude are not guaranteed to
agree in both channels due to the difference in averaging resolutions at the
surface (30 m vs. 60 m at 532 nm and 1064 nm, respectively) and also due to
the non-ideal detector response in the 532 nm channels which may yield base
altitudes much below that of the 1064 nm channel.
Surface Integrated Attenuated Backscatter 532
Vertically integrated Total Attenuated Backscatter 532 nm of the surface return
from Surface Top Altitude 532 to Surface Base Altitude 532.
Surface_Integrated_Attenuated_Backscatter_1064
Vertically integrated Attenuated Backscatter 1064 nm of the surface return
from Surface Top Altitude 1064 to Surface Base Altitude 1064.
Surface_532_Integrated_Depolarization_Ratio
Depolarization ratio of surface return at 532 nm from Surface Top Altitude
532 to Surface Base Altitude 532, computed as the ratio of vertically integrated
Perpendicular Attenuated Backscatter 532 to vertically integrated parallel
attenuated backscatter 532 nm.
Surface_1064_Integrated_Depolarization_Ratio
Depolarization ratio of surface return at 532 nm from Surface Top Altitude
1064 to Surface Base Altitude 1064, computed as the ratio of vertically
integrated Perpendicular Attenuated Backscatter 532 to vertically integrated
parallel attenuated backscatter 532 nm.
Surface_532_Integrated_Attenuated_Color_Ratio
Attenuated color ratio of surface return from Surface Top Altitude 532 to
Surface Base Altitude 532, computed as the ratio of vertically integrated
Attenuated Backscatter 1064 to vertically integrated Total Attenuated
Backscatter 532.
Surface_1064_Integrated_Attenuated_Color_Ratio
Attenuated color ratio of surface return from Surface Top Altitude 1064 to
Surface Base Altitude 1064, computed as the ratio of vertically integrated
Attenuated Backscatter 1064 to vertically integrated Total Attenuated
Backscatter 532.
Bit-mapped 16-bit integers describing the success of surface detection within the
indicated channel, the surface detection method, information about
saturated surfaces
or surfaces affected by the
negative signal anomaly,
and diagnostic failure information. Bits are interpreted as follows:
Bit(s)
Interpretation
1
Surface detected (0 = no, 1 = yes)
2-3
Surface detection method; values interpreted as follows
0 = derivative test
1 = multi-shot averaged data test
2 = single shot surface detection fraction test
3 = unused
4-6
Is saturated; 532 parallel, 532 perpendicular and 1064, respectively
7-9
Has negative signal anomaly; 532 parallel, 532 perpendicular and 1064, respectively
Background noise estimate computed from
RMS baseline noise measurements
between 65 and 80 km AMSL, rescaled to create a pseudo attenuated backscatter
coefficient with units of per kilometer per steradian. The
version 4 surface detection algorithm
requires the surface signal to exceed the scaled RMS
background noise estimate by a multiplicative constant.
Surface_Peak_Signal_532 Surface_Peak_Signal_1064
Maximum attenuated backscatter value of the surface signal within the
indicated channel.
Number of 1 km resolution profiles within each 5 km resolution profile where
surface was detected in the indicated channel.
Feature Spatial Information Within Column
Column_Feature_Fraction
The fraction of the 5-km horizontally averaged profile, between 30-km and the
DEM surface elevation, which has been
identified as containing a feature (i.e., either a cloud, an aerosol, or a
stratospheric layer).
Samples_Averaged
Specifies the number of full resolution samples averaged for each profile
range bin; for the purposes of this computation, 'full resolution' is taken
to mean 30 meters vertically, and a single shot (~1/3-km) horizontally. Thus
a single range bin below an altitude of ~20.2 km (resolution = 60-m vertical,
5-km horizontal) will have at most 480 samples averaged (i.e., for those
layers that required 80-km averaging for detection, 240 shots horizontally by
two 30-m range bins vertically).
Aerosol_Layer_Fraction Cloud_Layer_Fraction
Reports the fraction (by area) within each 5 km horizontal * 60 m vertical
range bin containing aerosols or clouds in the profile products. The Aerosol
and Cloud Layer Fractions are conceptually identical and the same procedure
is used to calculate both quantities.
Since the array elements of the profile products can be larger than the
native resolution of the extinction retrieval (5 km * 30 m vs. 5 km * 60 m in
the lower troposphere), and because atmospheric features can be identified at
horizontal resolutions of 1 km and 1/3 km, the atmospheric composition within
each profile range bin is not guaranteed to be homogeneous. Thus, the Aerosol
and Cloud Layer Fractions report the fraction (by area) of each 5 km * 60 m
profile range bin identified as containing aerosols or clouds by the Scene
Classification Algorithms. By referencing Cloud Layer Fraction, the
fractional amount of cloud clearing performed within each profile range bin
of aerosol backscatter and extinction can be determined.
Figure 4: Cloud clearing scenarios for strongly scattering clouds
detected at single shot resolution. Red indicates clouds detected at 1/3
km resolution, blue indicates clouds found at 1 km or coarser resolution,
yellow indicates an aerosol layer found at 5 km resolution, and white
indicates clear air. Scenarios: Clouds embedded in aerosol (upper panel),
clouds embedded in clear air (middle panel), and dense clouds embedded in
within a weakly scattering cloud layer (lower panel). Each row extends 5
km horizontally and 30 m vertically. Each column extends 1/3 km
horizontally.
Shown in Figure 4 are 3 possible scenarios illustrating how the Cloud
Layer Fraction would be reported for each 5 km x 60 m cloud profile range
bin. There are at most 30 single shot (1/3 km x 30 m) cloud layers in each 5
km x 60 m cloud profile range bin - fifteen 1/3 km horizontally and two 30 m
vertically. In the top panel, red indicates clouds found at 1/3 km resolution
and the yellow indicates an aerosol layer found at a 5 km horizontal
resolution after the 1/3 km clouds had been removed. In this case, the cloud
fraction for the top row would be 11/30 = 0.36. In the middle panel, no
features were detected at any coarser spatial resolution after the 1/3 km
features were removed. The Atmospheric Volume Description
for the 5 km average would report the cell as being "clear air",
but the cloud fraction for the top row would still be 11/30 = 0.36. In the
lower panel, a cloud was detected in the data remaining after all 1/3 km
features had been removed. In this case the cloud fraction would be 1 for all
rows shown.
The Aerosol and Cloud Layer Fractions must be values between zero and one,
yet both layer fractions are reported as integers between 0 and 30. For
example, a value for the Aerosol_Layer_Fraction reported as 11 would indicate
a fraction of 11/30 = 0.367.
Atmospheric_Volume_Description
Atmospheric Volume Description is a profile descriptive flag containing
the Feature Classification Flags
associated with each 5 km x 60 m (or 5 km x 180 m) range bin in the
Profile Products. The Feature Classification Flags provide assessments of (a)
feature type (e.g., cloud vs. aerosol vs. stratospheric layer); (b) feature
subtype; (c) layer ice-water phase (clouds only); and (d) the amount of
horizontal averaging required for layer detection. Note that the
interpretation of final three bits in the atmospheric volume description
(i.e., the averaging required for detection) is slightly different from the
interpretation that would be used for the feature classification flags. These
differences are summarized in the table below.
Value
Atmospheric volume description
Feature Classification flag
0
not applicable
not applicable
1
5 km
1/3 km
2
20 km
1 km
3
80 km
5 km
4
5 km w/ subgrid feature detected at 1/3 km
20 km
5
20 km w/ subgrid feature detected at 1/3 km
80 km
6
80 km w/ subgrid feature detected at 1/3 km
not used
7, 8
not used
not used
How profile descriptive flags are stored
Atmospheric Volume Description, CAD Score
and Extinction QC [532|1064] are all profile
descriptive flags that are stored in the Level 2 Profile Products in the same
manner explained here.
Ideally, each profile descriptive flag would be an array of the size [#
altitude bins, # profiles] with each array element providing a complete
description of the range-resolved atmospheric state. However, because the
range resolution of the Level 1 profile data below ~8.3-km is 30 m, and
because the feature-finder, scene classification, and extinction algorithms
all operate at this finer spatial resolution, providing a genuinely complete
description of the atmospheric state for each 60 m Level 2 range bin requires
that the profile descriptive flags be stored as 3-D arrays of the size
[#profiles, # altitude bins, 2]. The first dimension, [ : , : , 1],
corresponds to the standard altitude array of the Profile Products. Thus,
below 8.3 km, the first dimension contains the descriptive flags of the
higher of the two full resolution (30 m) bins that comprise the single 60 m
bin reported in the Profile Products. Meanwhile, below 8.3 km, the second
dimension [: , : , 2] contains the descriptive flags for the lower of the two
30 m range bins. Above 8.3 km, where the range resolution of the Level 1 data
is 60 m or greater, the descriptive flags for each single 60 m (or 180 m)
range bin are replicated in both array elements.
Figure 5: Wholly fictitious but heuristically useful schematic of
layer detection results for a data segment extending 80-km
horizontally and 480-m vertically. Yellow/orange/brown indicates an
aerosol layer detected at horizontal averaging resolutions of,
respectively, 80, 20 or 5 km. Shades of blue likewise represent
clouds at 80, 20, and 5 km. Red represents a surface detected layer
at 5 km, and the white regions are (presumably) clear air, where no
features were found. The right-hand side of the figure shows the
atmospheric volume descriptor for columns 1 and 16.
The extinction QC flags are bit-mapped 16-bit integers, reported for each
layer and for each wavelength for which an extinction retrieval was
attempted. Aerosol extinction is computed for both wavelengths; cloud
extinction is only reported at 532 nm. The information content of each bit is
as follows
Bit
Value
Interpretation
0
0
unconstrained retrieval; initial lidar ratio unchanged during solution process
0
1
constrained retrieval
1
2
Initial lidar ratio reduced to achieve successful full-column solutions
2
4
Suspicious retrieval due to layer or overlying integrated attenuated backscatter being too high or excessive lidar ratio reductions
3
8
Lidar ratio has been reduced and has converged, but backscatter uncertainty solution does not exist
4
16
Layer being analyzed has been identified by the feature finder as
being totally attenuating (i.e., opaque)
5
32
Estimated optical depth error exceeds the maximum allowable value
6
64
Negative signal anomaly detected
7
128
Retrieval terminated at maximum iterations for a constrained retrieval
8
256
No solution possible within allowable lidar ratio bounds
9
512
Two-way particulate transmittance has converged but constrained retrieval still not achieved
10
1024
Backscatter coefficients not converging and maximum lidar ratio correction iterations reached
11
2048
Uncertainties not converging and maximum lidar ratio correction iterations achieved
12
4096
Lidar ratio converged but retrieval still not converging
The bit assignments are additive, so that (for example) an extinction QC
value of 18 represents an unconstrained retrieval (bit 1 is NOT set) for
which the lidar ratio was reduced to prevent divergence (+2; bit 2 is set),
and for which the feature finder has indicated that the layer is opaque (+16;
bit 5 is set). For the version 2.01 release, bits 10-15 are not used.
Complete information about the conditions under which each extinction QC bit
is toggled can be found in the
CALIPSO Extinction Retrieval ATBD (PDF) and in
Young et al, 2018.
The cloud-aerosol discrimination (CAD) score provides a numerical confidence level for the
classification of layers by the CALIOP cloud-aerosol discrimination
algorithm. The CAD algorithm separates clouds and aerosols based on multi-dimensional
histograms of scattering properties (e.g., intensity and spectral dependence) as
a function of geophysical location. In areas where there is no overlap or
intersection between these histograms, features can be classified with complete
confidence (i.e., |CAD score| = 100). In the current release (version 4) the
CAD algorithm uses five-dimensional (5D) probability density functions (PDFs),i.e.
layer mean attenuated backscatter at 532 nm, layer-integrated attenuated backscatter color ratio, altitude, latitude and layer-integrated volume depolarization ratio.
These PDFs were newly developed to take into account the significantly improved
calibration of CALIOP version 4 level 1 data and using a latitude resolution of
5° (as opposed to 10° in earlier versions) leading to an overall improvement
in CAD reliability. Detailed descriptions of the CAD algorithm can be found
in Sections 4 and 5 of the CALIPSO Scene Classification ATBD (PDF). Further
information on the CAD algorithm architecture may be found in
Liu et al., 2010, 2009, 2004.
For the profile product, the CAD score for a layer is replicated over the
samples spanning the vertical extent of the layer. For a more complete description
of CAD score for layers in version 4, the reader is referred to the layer data
description.
The standard CAD scores reported in the CALIPSO layer products range
between -100 and 100. The sign of the CAD score indicates the feature type:
positive values signify clouds, whereas negative values signify aerosols. The
absolute value of the CAD score provides a confidence level for the
classification. The larger the magnitude of the CAD score, the higher our
confidence that the classification is correct. An absolute value of 100
therefore indicates complete confidence. Absolute values less that 100
indicate some ambiguity in the classification; that is, the scattering
properties of the feature are represented to some degree in both the cloud
PDF and in the aerosol PDF. In this case, a definitive classification cannot
be made; that is, although we can provide a "best guess"
classification, this guess could be wrong, with a probability of error
related to the absolute value of the CAD score. A value of 0 indicates that a
feature has an equal likelihood of being a cloud and an aerosol. Users are
encouraged to refer to the CAD score when the cloud and aerosol
classification results are used and interpreted.
Beginning with the version 2.01 release, several "special" CAD
score values have been added. These are listed in the table below. Each of
these new values represents a classification result that is based on
additional information beyond that normally considered in the standard CAD
algorithm.
CAD score
Interpretation
-101
Negative mean attenuated backscatter encountered; layer is most likely
an artifact, and its spatial and optical properties should be excluded from
all science analyses.
101
Initially classified as aerosol, but layer integrated depolarization
mandates classifying layer as cloud
(version 2 only; obsolete
in version 3 and later versions)
102
Layer exhibits very high integrated backscatter and very low
depolarization characteristic of oriented ice crystals
(version 2 only; obsolete
in version 3 and later versions)
103
Layer integrated attenuated backscatter at 532 nm is suspiciously high;
feature authenticity and classification are both highly uncertain
104
Boundary layer clouds that were found
to be opaque at the initial 5 km horizontal averaging resolution used by
the layer detection algorithm; however, these layers are not uniformly
filled with high-resolution clouds (i.e., layers detected at a 1/3 km
horizontal resolution), and the 532 nm mean attenuated backscatter
coefficient of the data that remains after cloud clearing is negative.
Studies examining the spatial properties and distributions of clouds can
safely include the spatial properties of these layers; however, the
associated measured and derived optical properties should be excluded from
all science studies.
105
Layer detected at one of the coarser
averaging resolutions (20 km or 80 km) for which the initial estimates of
measured properties have been negatively impacted by either (a) the
attenuation corrections applied to account for the optical depths of
overlying layers, or (b) the extension of the layer base altitude are
boundary layer clouds that were found to be opaque at the initial 5-km
horizontal averaging resolution used by the layer detection algorithm;
however, these layers are not uniformly filled with high-resolution clouds
(i.e., layers detected at a 1/3 km horizontal resolution), and the 532 nm
mean attenuated backscatter coefficient of the data that remains after
cloud clearing is negative. Studies examining the spatial properties and
distributions of clouds can safely include the spatial properties of these
layers; however, the associated measured and derived optical properties
should be excluded from all science studies.
106
Suspected fringe of cirrus initially classified as aerosol by the CAD
algorithm and subsequently reclassified as no-confidence horizontally oriented
ice cloud. These layers are in contact with cirrus (medium or high confidence,
randomly or horizontally oriented ice clouds) and have optical properties which
make it difficult to distinguish between cloud and aerosol. Due to their proximity
to cirrus and low SNR, these "aerosol" layers are more often misclassified
cirrus fringes and are thusly reclassified as cloud. Only aerosol layers detected
at 20 km and 80 km resolution above 4 km in altitude and having layer centroid
temperatures below 0°C are reclassified by the "cirrus fringe amelioration" algorithm.
Particulate total backscatter coefficients reported for each profile range
bin in which the appropriate particulates (i.e., clouds or aerosols) were
detected; those range bins in which no particulates were detected contain
fill values (-9999). Range bins where particulates were detected but the extinction
retrieval failed are assigned a fill value of -333.Units are kilometers-1
steradians-1. For the 532 nm data, the particulate total
backscatter coefficients are derived from the sum of the parallel and
perpendicular backscatter measurements recorded aboard the CALIPSO satellite
(i.e., β532 total = β532 parallel +
β532 perp).
Uncertainty in the particulate total backscatter coefficients reported for
each profile range bin in which the appropriate particulates were detected;
these are absolute uncertainties, not relative, thus the units are identical
to the units of the total backscatter coefficients (i.e.,
kilometers-1 steradians-1); those range bins in which
no particulates were detected contain fill values (-9999). Range bins where
particulates were detected but the extinction retrieval failed are assigned a
fill value of -333.
Opaque water cloud uncertainties are assigned fill values of -29.
Uncertainties are computed according to the procedures described in the
CALIPSO Version 3 Extinction Uncertainty Document (PDF).
Perpendicular_Backscatter_Coefficient_532
Particulate backscatter coefficients derived from the 532 nm perpendicular
channel measurements, reported for each profile range bin in which the
appropriate particulates (i.e., clouds or aerosols) were detected; those
range bins in which no particulates were detected contain fill values
(-9999). Range bins where particulates were detected but the extinction retrieval
failed are assigned a fill value of -333.Units are kilometers-1 steradians-1.
Uncertainty in the perpendicular channel backscatter coefficients reported
for each profile range bin in which the appropriate particulates were
detected; these are absolute uncertainties, not relative, thus the units are
identical to the units of the total backscatter coefficients (i.e.,
kilometers-1 steradians-1); those range bins in which
no particulates were detected contain fill values (-9999). Uncertainties are
computed according to the procedures described in the
CALIPSO Version 3 Extinction Uncertainty Document (PDF).
Particulate extinction coefficients reported for each profile range bin in
which the appropriate particulates (i.e., clouds or aerosols) were detected;
those range bins in which no particulates were detected contain fill values
(-9999). Range bins where particulates were detected but the extinction retrieval
failed are assigned a fill value of -333. Units are kilometers-1.
Uncertainty in the particulate extinction coefficients reported for each
profile range bin in which the appropriate particulates were detected; these
are absolute uncertainties, not relative, thus the units are identical to the
units of the particulate extinction coefficients (i.e.,
kilometers-1); those range bins in which no particulates were
detected contain fill values (-9999). Range bins where
particulates were detected but the extinction retrieval failed are assigned a
fill value of -333.
Opaque water cloud uncertainties are assigned fill values of -29. Uncertainties are computed according to
the procedures described in the
CALIPSO Version 3 Extinction Uncertainty Document (PDF)
Particulate Depolarization Profiles
Particulate_Depolarization_Ratio_Profile_532
The particulate depolarization ratio, δp(z), is a
post-extinction quantity, calculated from ratio of the layer integrated
perpendicular and parallel polarization components of particulate backscatter
coefficient at a given altitude z, using
Here β⊥,P and
β||,P perpendicular and parallel components of
particulate backscatter coefficient at 532 nm, respectively. The quality of
the estimate for δp is determined not only by the
SNR of the backscatter measurements in parallel and perpendicular channels,
but also the accuracy of the range-resolved two-way transmittance estimates
within the layer. The two-way transmittances due to molecules and ozone can
be well characterized via the model data obtained from the
GMAO.
The two-way transmittances due to particulates, however, are only as accurate
as the CALIOP extinction retrieval. Opaque cirrus cloud layers can be
particularly prone to errors in the particulate depolarization ratio, as very
large attenuation corrections are applied to the weak signals at the base of
the layers, and on those occasions where one channel or the other becomes
totally attenuated, this situation can generate very large, negative
particulate depolarization ratio estimates. For layers that are not opaque,
δp is generally reliable. However, in weakly
scattering layers, the quality of the daytime estimate can be degraded by a
factor of 2-4 due to the larger background noise compared with the nighttime
estimate. Range bins where particulates were detected but the extinction retrieval
failed are assigned a fill value of -333.
Particulate_Depolarization_Ratio_Uncertainty_532
The uncertainties reported for the particulate depolarization ratios provide
an estimate for random error in the particulate depolarization ratio for each
range bin (i.e., the ratio of perpendicular and parallel components of
retrieved particulate backscatter coefficient within the feature). Based on
an assessment of several days of test data (January 1-3, 2007), the
uncertainty for aerosol profile products is typically (a median value) ~0.18
and ~0.7 for nighttime and daytime measurements, respectively. For cloud
profile products, it is typically ~0.33 and ~0.58 during night and day,
respectively. Range bins where particulates were detected but the extinction
retrieval failed are assigned a fill value of -333.
The multiple scattering profiles, η532(z) and
η1064(z), are specified at each wavelength
according to layer type and subtype. Values range between 0 and 1; 1
corresponds to the limit of single scattering only, with smaller values
indicating increasing contributions to the backscatter signal from multiple
scattering. Multiple scattering effects are different in aerosols, ice
clouds, and water clouds. A discussion of multiple scattering factors for
ice clouds and several aerosol types can be found in
Winker, 2003 (PDF). Multiple scattering in water clouds
is discussed in
Winker and Poole (1995).
Ice clouds: In Version 3 and earlier, ice clouds were assigned a
range-independent multiple scattering factor of η532 = 0.6.
In Version 4, the multiple scattering factor is implemented as a sigmoid
approximation function of the layer
attenuated backscatter centroid temperature,
with η532 increasing from
0.46 at 270 K to 0.76 at 190 K.
This approximation function was derived from extensive analysis of collocated
measurements acquired by the CALIPSO lidar and the CALIPSO IIR,
which reconciled observed and theoretical ratios
of 532 nm optical depths derived from Version 3 CALIOP measured two-way
transmittances to the absorption optical depth retrieved from IIR measurements
at 12.05 μm
(Garnier et al., 2015).
The theoretical ratios are computed assuming
severely roughened aggregated columns.
Water clouds: In Version 3 and later versions, ice clouds were assigned a
range-independent multiple scattering factor of
η532 = 0.6 is used. Based on Monte Carlo
simulations of multiple scattering, this value appears to be appropriate for
semitransparent water clouds (τ < 1). (It is purely
coincidental this is the same value used for ice clouds.) For denser water
clouds (τ > 1) the multiply-scattered component of the
signal becomes much larger than the single-scattered component,
η532 becomes dependent on both cloud extinction
and range into the cloud, and the retrieval becomes very sensitive to errors
in the multiple scattering factor used. In these cases the multiple
scattering cannot be properly accounted for in the current retrieval
algorithm and retrieval results are unreliable.
Aerosols: simulations of multiple scattering effects on retrievals
of aerosol layer optical depth indicate the effects are small in most cases.
There is uncertainty in these estimates, however, due to poor knowledge of
aerosol scattering phase functions. Validation comparisons conducted to date
do not indicate significant multiple scattering effects on aerosol extinction
profile retrievals. Multiple scattering effects may become significant in
dense aerosol layers (σ > 1 /km), but in these cases
retrieval errors are usually dominated by uncertainties in the lidar ratio or
failure to fully penetrate the layer. In version 2 and later, multiple
scattering factors for both wavelengths are set to unity.
IWC (Ice Water Content) Profiles
Ice_Water_Content_Profile (cloud products only)
Ice water content (IWC) is reported for all ice clouds with valid extinction
retrievals detectedby CALIOP. IWC values of 0.00001.54 gm,-3 account for 99.5%
of the values measured by CALIOP, although values up to 10 g/m3 are considered
to be physically possible.
Cloud ice water content is a provisional data product that is
calculated as a parameterized function of the CALIOP retrieved and interpolated
MERRA-2 temperature. This parametrization comes from an emipical relationship
derived from aircraft in situ observations, as described in Heymsfield et al., 2014:
(1)
IWC(z) = (0.917/3) σ(z) Deff, with Deff = a exp(bT(z))
Here Deff is the effective particle diameter, with
empirically derived coefficients a and b are that are derived for three
different temperature ranges: For 0 > T > -56 C, a = 308.4 and
b = 0.0152; for -56 > T > 71 C, a = 9.1744 x 104 and b = 0.117; and
for -71 > T > -85 C, a = 83.3 and b = 0.0184.
Here, σ is the 532 nm volume extinction coefficient in
km-1, and c0 = 119 gm-3 and c1 =
1.22 are coefficients derived from an observed empirical relationship between
lidar extinction and an extensive set of in situ measurements of cloud
particle properties from numerous field campaigns [1]. The relationship
between 532 nm extinction and IWC was developed using IWC data between 0-1.0
gm-3 with temperatures between -70 and 0 °C. Cloud ice amount
has been shown to vary with temperature, cloud particle size distribution,
and by location inside a cloud. A temperature-dependent parameterization is
being considered and tested for the next CALIOP data release. The effect of
particle size distribution on IWC as seen by CALIOP is also currently being
evaluated by comparison with in situ cloud data. Preliminary results show
that CALIOP IWC has sufficient spatial resolution and precision to
realistically resolve cloud morphology. A more detailed preliminary
evaluation of the CALIOP version 3 IWC is available as an ILRC extended
abstract [2], which includes CALIOP IWC probability distributions and example
browse images. For a brief discussion containing critical information needed
to intelligently use CALIOP IWC, please see the "data screening"
section, below. Range bins where particulates were detected but the extinction
retrieval failed are assigned a fill value of -333.
Resolution
IWC is reported at 60 m vertically, with a horizontal spatial resolution
of 5 km along-track, and effectively the width of the laser beam
across-track.
Precision
The precision of IWC is directly linked to the precision of the
associated extinction retrieval. The precision of the extinction retrieval is
ultimately limited by signal-to-noise ratio, and this varies between night
and day and according to the overhead two-way 532 nm transmission. Therefore,
the precision of CALIOP IWC has to be evaluated for each individual case. The
team is currently developing a best-case precision estimate for nighttime
high altitude Cirrus clouds.
Accuracy
Because this is a provisional data product, assessment of IWC accuracy is
ongoing. This assessment can be approached in two different ways;
(1) by establishing the accuracy of the 532 nm extinction and Ice particle
area to mass conversion empirical relationship on which it is
based, or (2) by assessing the IWC product directly. Direct comparison of
CALIOP IWC with other measured IWC values includes evaluates a combination of
both the extinction retrieval and the IWC parameterization. The uncertainty
reported in the data products is related to the uncertainty in the CALIOP
extinction retrieval. If the total IWC uncertainty is needed the user is referred
to Heymsfield et al., 2014 for the aircraft data-based uncertainty in the empirical
extinction to IWC relationship, and to Molod et al., 2015, for uncertainty in the
MERRA-2 interpolated temperature.
Data screening
Users that do not wish to dig more deeply are recommended to use data of
type=2 (cloud) and phase=1 (randomly-oriented ice) or 3 (horizontally-oriented ice)
with a valid extinction retrieval with extinction QCFlag of 0, 1,
2, 16 or 18, as described above. Valid IWC values are considered to be 0.00001 - 10 g/m3.
There is a more detailed discussion of the various factors impacting IWC accuracy, below.
CALIOP IWC is a highly derived data product. Besides the parameterization, it relies on these activities:
Cloud determination (CAD)
Bits 6 and 7 in the atmospheric volume descriptor
indicates a feature of type 2=cloud, determined using 5-dimensional
probability distribution functions as described above. A CAD "score"
higher than 70 indicates "high confidence" in the cloud/aerosol
discrimination, although the relationship between "CAD score" and
phase confidence is currently under evaluation.
Cloud phase determination
The atmospheric volume descriptor indicates that the cloud phase is
1=randomly oriented ice (ROI) or 3=horizontally oriented ice (HOI) as determined
for cloud particles (type=2) primarily using the integrated layer attenuated
backscatter coefficient and depolarization. Although extinction is available,
IWC is not calculated for cloud particle phase 2=water. Users should use
caution with HOI data because the preferred horizontal orientation of ice
particles in the 0.3 degree, nadir-viewing data (before November 28, 2007)
causes anomalously large backscatter that makes the extinction retrievals
more difficult, which in turn may affect the accuracy of the IWC. This problem
was much alleviated by tilting the viewing angle to 3 degrees after
November, 2007, and also by improvement in the Version 4 extinciton retrievals,
so it is not considered to be a problem for the Version 4 data after November 28, 2007.
Extinction retrievals
Users that wish to understand the nuances and details of CALIOP extinction
retrievals are referred to the V4 lidar ratio, multiple scattering and extinction
discussions, above. The extinction quality flag provides information about
the type and reliability of extinction retrievals. Because IWC parameterization
relies on accurate extinction retrieval, it is recommended to use IWC only
where the extinction retrieval is valid. For this reason, data screening
for accurate IWC should follow that for extinction coefficients.
IWC uncertainty has a range of 0-99.99 gm-3, and is derived
directly from the extinction retrieval uncertainty. This is the estimated
CALIOP measurement uncertainty, and does not characterize uncertainty in the
IWC parameterizationavailable in Heymsfield et al., 2014, or in the
interpolated MERRA-2 temperature profile (Molod et al., 2015).
Comparison of CALIOP IWC with direct in situ measurements from various
aircraft field campaigns, and is ongoing. A recent evaluation from the ATTREX
field campaign is available in Thornberry et al., 2016 (JGR, submitted).
Range bins where particulates were detected but the extinction retrieval
failed are assigned a fill value of -333.
Future IWC parameterizations may include temperature dependency, based on
further in situ data comparisons. Currently suggested temperature-dependant
parameterizations do not produce good results for high-altitude tropical
clouds [6] and therefore are not used.
File 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.
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.
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_Level_1_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_Single_Shot_Records_in_File
for internal use only
Number_of_Average_Records_in_File
for internal use only
Number_of_Features_Found
for internal use only
Number_of_Cloud_Features_Found
for internal use only
Number_of_Aerosol_Features_Found
for internal use only
Number_of_Indeterminate_Features_Found
for internal use only
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-byte character that reports the version of the GEOS data
product provided by the GMAO.
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 Cloud and Aerosol Profile Information Half orbit (Night and Day) lidar cloud and aerosol profile data and ancillary data