In August 2018, the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) project released a new level 3 stratospheric aerosol profile data product derived from nearly 12 years of measurements
acquired by the spaceborne Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP). This monthly averaged, gridded level 3 product is based on version 4 of the CALIOP level 1B and level 2 data products, which feature significantly improved calibration that now makes it possible to reliably retrieve profiles of stratospheric aerosol extinction and backscatter coefficients at 532 nm. This paper describes the science algorithm and data handling techniques that were developed to generate the CALIPSO version 1.00 level 3 stratospheric aerosol profile product. Further, we show that the extinction profiles (retrieved using a constant lidar ratio of 50 sr) capture the major stratospheric perturbations in both hemispheres over the last decade resulting from volcanic eruptions, extreme smoke events, and signatures of stratospheric dynamics. Initial assessment of the product by intercomparison with the stratospheric aerosol retrievals from the Stratospheric Aerosol and Gas Experiment III (SAGE III) on the International Space Station (ISS) indicates good agreement in the tropical stratospheric aerosol layer (30
While the bulk of the global distribution of atmospheric aerosols is concentrated within the planetary boundary layer and free troposphere, the persistent aerosol burden in the stratosphere has long been known to have important implications for Earth's climate (Turco et al., 1980). Techniques for the reliable detection of a background aerosol layer in the stratosphere date back to the early 1960s (Junge and Manson, 1961). These aerosols are mostly liquid sulfate particles that are derived from precursor gases like
Most of our current knowledge of the global distribution of stratospheric
aerosols comes from satellite measurements. The earliest of such measurements
were carried out by the Stratospheric Aerosol Measurement II (SAM II) onboard the Nimbus 7 spacecraft, which provided vertical profiles of aerosol extinction at 1
A novel and pioneering technique to retrieve aerosol profiles from space came about with the launch of the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission in April 2006, with a two-wavelength, polarization-sensitive elastic backscatter lidar as the primary payload (Winker et al., 2010). For over 12 years the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) has been providing vertically resolved profiles of aerosol and cloud extinction globally. The primary measurement from a spaceborne elastic backscatter lidar consists of the attenuated backscatter coefficients of the aerosols and clouds in the atmosphere. The strong backscatter from the tropospheric aerosols, combined with CALIOP's relatively strong signal-to-noise ratio (SNR), has been exploited to provide accurate extinction profiles in the troposphere (Young and Vaughan, 2009; Winker et al., 2013; Young et al., 2013, 2016, 2018). In comparison, the aerosol loading in the stratosphere is much lower with correspondingly smaller SNR. As such, retrieving stratospheric aerosol information was not originally a principal target of the CALIPSO mission. However, early results indicated that it might be possible to obtain such information with sufficient averaging of the data (Thomason et al., 2007; Vernier et al., 2009).
One of the issues impacting the retrieval of stratospheric aerosol extinction was the realization that the standard calibration altitude of CALIOP, which was originally fixed at 30–34 km (Powell et al., 2009), was
not completely free of aerosols, and thus applying the molecular
normalization technique at these altitudes would bias the aerosol extinction
profiles (Vernier et al., 2009). This issue has since been addressed with
the release of the version 4 (V4) family of CALIPSO data products in
November 2016. In this version, the calibration altitude for the nighttime
532 nm data, which is the primary calibration for all CALIOP measurements
(all other measurements like the daytime data as well as the 1064 nm
data are calibrated relative to the 532 nm nighttime calibration), was raised
to 36–39 km, where the aerosol loading is expected to be negligible (Kar et
al., 2018a). This largely removed the aerosol contamination issue, making
reliable retrievals of stratospheric aerosols possible. Accordingly, a
stand-alone CALIPSO stratospheric aerosol profile product was developed
that uses the V4 level 1B and level 2 data from the CALIOP measurements.
This is a level 3 monthly averaged product gridded in latitude (5
The CALIPSO level 3 stratospheric aerosol profile product is built primarily
from the V4 level 1B 532 nm attenuated backscatter profiles (
Median values of zonally and vertically (over 30–34 km) averaged 532 nm attenuated scattering ratios for May 2009 nighttime data from V3 and V4. Data over the South Atlantic Anomaly were excluded. A 10-point smoothing of the data has been applied.
As shown in Eq. (1), the attenuated scattering ratios,
The level 3 stratospheric aerosol profile product reports height-resolved
monthly mean profiles of aerosol backscatter and extinction coefficients on
a uniform spatial grid that extends 5
Each level 3 stratospheric aerosol file reports two distinct realizations of the monthly averaged data products. The first of these is the “background” mode, which is designed to represent the long-term background stratospheric aerosol loading. In order to achieve this, we need to remove all readily detectable perturbations within the stratosphere, such as overshooting cirrus clouds, polar stratospheric clouds (PSCs), and strongly scattering injections of smoke, volcanic ash, and other aerosol species that are detected using the layer detection algorithm implemented in the CALIOP level 2 data processing (Vaughan et al., 2009). The second realization is the “all-aerosol” mode, which is designed to represent the time history of aerosol loading in the stratosphere resulting from all possible sources. In this case, the clouds and PSCs are still removed, exactly as is done for the background mode; however, subject to various quality assurance tests, the aerosol layers detected in the level 2 analyses are retained. Details of the averaging algorithms and the various data filtering schemes are provided in the following sections.
The overall design of the level 3 stratospheric aerosol product is shown in Fig. 2. To begin with, three input files are required for each granule under consideration. A CALIOP granule comprises half an orbit of data either from the daytime or the nighttime part of the orbit and divided by the day–night terminator. As noted in Sect. 1, the primary input files used for the present product are the lidar level 1B file, with the corresponding level 2 5 km merged layer and PSC mask files (Pitts et al., 2009) used for filtering. While the levels 1B and 2 merged layer files are based on V4, the currently available level 2 PSC files are based on V3. The latter is only available as a daily file and not for each granule separately. The 5 km merged layer file is a new product in V4 that reports the locations of all aerosol and cloud layers detected at both 5 km (also 20 and 80 km) and single shot (333 m) resolution (Vaughan et al., 2016).
Flowchart illustrating the overall design of the CALIPSO level 3 stratospheric aerosol profile product.
In the background mode, clearing the features detected in the level 2 analyses is done by removing all the level 1B (L1B) attenuated backscatter values (for 15 consecutive L1B profiles) beginning at the top of the uppermost cloud or aerosol layers detected above the local tropopause using the layer heights reported in the 5 km merged layer file. Not only are signals from within the boundary of the layers removed, but the backscatter values at all altitudes below the layers are also removed to avoid issues in correcting for signal attenuation from overlying layers. While the attenuated backscattered signals within and below these layers are removed, this step will retain values that fall below the minimum detectable attenuated backscatter threshold of the CALIPSO layer detection algorithm (McGill et al., 2007). In this sense, the retrieved extinction in this mode will reflect only the aerosol loading below this threshold. Similarly, the signals below the uppermost PSC layers are also removed using the PSC mask file for the PSC-active months in the two hemispheres (December through March in the Arctic and May through October in the Antarctic). The PSC mask files report the occurrence of PSCs in both hemispheres (Pitts et al., 2007, 2009) and are reported for a single day on a 5 km horizontal and 180 m vertical grid for nighttime conditions only.
After clearing all level 2 and PSC layers detected above the local
tropopause, all L1B attenuated backscatter values below the tropopause are
removed. Further, all L1B profiles within the South Atlantic Anomaly (SAA)
region are also removed. In this region, approximately between the Equator
and 50
When creating the all-aerosol mode of the stratospheric aerosol product, it
is necessary to remove any clouds and PSCs, much the same way as for the
background case, but retain the detected layers classified as aerosols by
the CALIPSO cloud–aerosol discrimination (CAD) algorithm (Liu et al., 2009,
2019). It should be mentioned that the CAD algorithm was also modified in V4
in order to be compatible with the new V4 532 nm calibration (Liu et al., 2019). In fact, the CAD algorithm was extended to the stratosphere for the first time in V4. Up until V3, any layer in the stratosphere was simply
classified as a “stratospheric feature” and no distinction was made between clouds and aerosols, which is no longer the case in V4. However, even the V4 CAD algorithm may not perform very well at high altitudes because of low SNR, leading to generally lower absolute values of CAD scores (Liu et al., 2019). In any case, in the stratospheric altitudes above
In the next step, a nominal 5 km resolution profile is constructed by taking the average of these 15 filtered L1B attenuated backscatter profiles. Subsequently, a noise filter is used to screen out strong outliers from these 5 km profiles that might otherwise lead to biases in high-latitude and/or high-altitude regions. The noise filter used for the current version of the product is a reconfigured version of the same filter that is used in the CALIPSO range-dependent automated level 2 layer detection algorithm. Essentially a range-dependent threshold array of attenuated scattering ratios is constructed, which incorporates noise from two types of sources. The first category is the range invariant noise and includes detector dark noise and noise from the solar background light. The second category is the range-dependent noise from single shot measurements and is calculated from the molecular models. Using this range-dependent threshold, outliers are removed (for details see Vaughan et al., 2009, Sect. 2c). After removing the outliers, the 5 km profile is assigned to the appropriate spatial grid. This process is then repeated for all the profiles in the level 1B file. The resulting filtered 5 km profiles are then averaged to create a single mean attenuated backscatter profile for each grid cell.
In the final processing step for each granule, another quality screening is
employed to identify and remove any lingering tenuous cirrus cloud in the
lower stratosphere that might have escaped the layer detection mechanism due
to low backscatter values. For the background mode, we can safely assume
that the background aerosols are uniformly spherical and thus have a near-zero
depolarization ratio. Since ice crystals in even the most tenuous cirrus violate this assumption, we use a threshold of 5 % in the volume depolarization ratio (ratio of the attenuated backscatter measured in the
perpendicular and parallel channels at 532 nm; Hunt et al., 2009) to detect
weakly scattering residual clouds. However, for the all-aerosol mode,
this strategy will not work. This is because volcanic ash is typically
nonspherical, has high volume depolarization values (
Zonally averaged height–latitude cross sections of attenuated
scattering ratio for June 2011:
During this month two strong volcanic eruptions took place, Nabro in the
Northern Hemisphere (13 June; 13
Using a constant threshold to discriminate between different classes of inherently noisy measurements can entail significant risk of misclassification. For example, using a higher attenuated color ratio acceptance threshold to ensure the identification of strong ash plumes (e.g., for Puyehue–Cordón Caulle above) may result in a significant amount of cloud contamination. Similarly, an acceptance threshold set too low will likely exclude all clouds while simultaneously discarding much of the ash signal.
The impact of using the attenuated color ratio and volume depolarization
ratio filters on removing thin cirrus clouds in the all-aerosol and background-only components, respectively, is illustrated in Fig. 4 using the
attenuated scattering ratios measured at 17 km during December 2011. In Fig. 4a, all the aerosol layers are retained, much like the all-aerosol
component, except that neither the volume depolarization ratio filter nor
the attenuated color ratio filter is used. The high scattering ratios
between about 30 and 55
Attenuated scattering ratios at 17 km in December 2011,
Figure 5 shows the profiles of the attenuated scattering ratio for the
background and all-aerosol modes in July 2009 for the grid cell centered at
47.5
Profiles of attenuated scattering ratio at 47.5
After deriving the granule-averaged data, we create monthly averaged gridded profiles of attenuated backscatter by aggregating all profiles during each month of the mission. In addition to the attenuated backscatter coefficient profiles, profiles of molecular and ozone number densities, temperatures, and pressures reported in the L1B files are also averaged and gridded for use in the subsequent retrieval procedures.
Figure 6 depicts the spatial distribution of the number of samples that contributed to the two components at 17 km during July 2009. The grey grid cells over South America and parts of South Atlantic Ocean correspond to the SAA, over which all data samples are rejected. The higher number of samples for the all-aerosol mode over the Asian summer monsoon region reflects the signature of the aerosol in the Asian tropopause aerosol layer (ATAL; Vernier et al., 2011b). A higher number of samples in the all-aerosol mode, albeit to a lesser degree, can also be seen over North America, which is likely related to the Sarychev volcano as mentioned above. Also note the high number of samples over parts of Antarctica, which is partly from oversampling due to orbital configuration and partly related to small particles below the detectability of PSCs by the PSC mask algorithm.
Number of samples contributing to
The monthly mean profiles of the gridded 532 nm attenuated backscatter
coefficient (
Given an appropriate value of the lidar ratio, Eqs. (2) and (3c) can be
solved iteratively to obtain estimates of
In this section we assess the initial performance of the CALIPSO stratospheric aerosol product by first presenting the signatures of various stratospheric aerosol events as captured by the product and then making quantitative comparisons with observations from SAGE III on ISS.
Volcanoes are one of the primary sources of stratospheric aerosols (e.g.,
Kremser et al., 2016). Ground-based lidar studies have indicated a positive
trend in stratospheric sulfate aerosol loading since the turn of the
century, which was initially attributed to anthropogenic emissions of
Figure 7 shows the time–altitude cross section of the zonally averaged
extinction in the middle to high northern latitudes (40–60
Time–altitude cross sections of the retrieved extinction coefficients in the all-aerosol mode from January 2007 through December 2017 for
Apart from volcanic material, smoke from strong biomass burning events can also reach the stratosphere during so-called pyrocumulonimbus events (Fromm et al., 2010; Peterson et al., 2018). During the “Black Saturday” event, smoke from strong bushfires in Victoria, Australia, on 7 February 2009 is known to have impacted the stratosphere. Plumes from this blaze eventually reached altitudes of 16–20 km and were readily visible in satellite imagery (de Laat et al., 2012; Glatthor et al., 2013). The signature of this event can also be identified in Fig. 7b, reaching up to nearly 22 km. The signature of another strong pyroCb event can be seen at northern middle to high latitudes (top panel) in August–September, 2017. This event is discussed in detail below. Note the seasonal pattern of high extinction near 12–15 km in the northern middle- to high-latitude summer, seen most clearly between 2012 and 2017. The reason for this is not entirely clear at this time but could again be due to fire events. Cirrus cloud contamination could be another factor. However, the same pattern is also seen in the background mode (not shown) to a slightly lesser degree, which suggests that cloud contamination may not be very significant.
CALIPSO browse images of
The extreme pyroCb event that occurred in August 2017 over British Columbia
in Canada has been extensively studied recently and has been likened to
volcanic perturbations in the stratosphere in terms of intensity and
duration (Khaykin et al., 2018; Ansmann et al., 2018; Haarig et al., 2018;
Peterson et al., 2018). Figure 8 shows an example of the CALIPSO measurements of this pyroCb event. The signature of the smoke plume is seen as extremely high attenuated backscatter (opaque at 532 nm) between 60 and 65
Figure 9 shows the height–latitude cross sections of CALIOP attenuated scattering ratios from the stratospheric aerosol product between August 2017 and November 2017; it captures the evolution of the aforementioned pyroCb event. After the original injection of smoke in August 2017 at midlatitudes, the smoke spreads to lower latitudes as can be seen in these monthly mean spatial distributions from the level 3 stratospheric aerosol product. As in Fig. 3, the feature with a high attenuated scattering ratio near 25–30 km seen in all four panels is the signature of the tropical reservoir of stratospheric aerosols, maintained by a complex interplay of transport from the troposphere and stratospheric dynamics as well as microphysical processes including the Brewer–Dobson circulation, the quasi-biennial oscillation (QBO), evaporation, and sedimentation (Trepte and Hitchman, 1992; Kremser et al., 2016).
Zonally averaged height–latitude cross section of the 532 nm attenuated scattering ratios from August 2017 through November 2017. The white areas in the northern high latitudes in August and the southern high latitudes in November indicate the lack of nighttime data due to continually changing day–night terminator times.
Figure 10 shows the height–latitude cross section of the retrieved 532 nm
extinction coefficients for the all-aerosol mode from March to December
2014, which captures the evolution of the Kelud eruption (February 2014;
7.9
Zonally averaged height–latitude cross sections of 532 nm extinction coefficients (km
Figure 11 shows the spatial distribution of the retrieved extinction
coefficients at 17 km for the month of August 2015 for the all-aerosol mode.
Two strong perturbations of the lower stratosphere can be seen in this plot.
The first is the plume from the Calbuco volcano in Chile, which erupted in
April 2015. The initial plumes would be missed in the level 3
stratospheric aerosol product because data over the SAA region were not
included. However, the plumes quickly spread around the Southern Hemisphere
in a belt between 60 to 30
Retrieved 532 nm extinction coefficients (km
In this section we provide an initial quantitative assessment of the CALIPSO
level 3 stratospheric aerosol product by intercomparison of the retrieved
extinction coefficients with those from the SAGE III instrument aboard the ISS
(SAGE III-ISS). The SAGE III instrument on ISS was launched in February 2017
and has been providing measurements of ozone,
Figure 12a shows zonally averaged mean profiles of extinction coefficients
retrieved from CALIPSO (in blue) and SAGE III (in red) at four latitude
bands using the same 15 months of measurements from the two instruments.
Figure 12b shows the profiles of the fractional differences in the same
latitude bands. The profiles for 0–30
Figure 13 shows the difference in the stratospheric optical depths between
CALIPSO and SAGE III, calculated using the average extinction coefficient
profiles between 20 and 30 km. This region is not likely to be significantly affected
by clouds and is also the region where most of the stratospheric aerosol resides; thus, comparisons here are likely to be
indicative of the overall performance differences between the two sensors.
Between 30
Fractional difference in 532 nm optical depth between CALIPSO and
SAGE III calculated using extinction coefficients from 20 to 30 km as a
function of latitude. The dashed red lines demarcate the
For an initial assessment of the CALIPSO stratospheric aerosol product, we
have used the aerosol retrievals from SAGE III acquired between June
2017 and August 2018. The solar occultation technique used for SAGE III
retrievals does not rely on any assumptions on aerosol species or size
distribution. Further, the retrieval wavelengths from SAGE III (521 nm)
and CALIPSO (532 nm) are quite close, and thus the comparison of the
extinction retrievals will not be significantly impacted by errors in the
Ångström exponent. The previous section demonstrated that the retrieved
aerosol extinction coefficients reported by the CALIPSO level 3
stratospheric aerosol product agree well with those reported by SAGE III
between 20 and 30 km within tropical latitudes (30
The adopted lidar ratio for the CALIOP stratospheric aerosol retrievals can
be assessed by using the independent extinction retrievals from SAGE III
and the attenuated backscatter measurements from CALIOP. For this we rewrite
Eq. (3c) as
Figure 14 shows the height–latitude cross section of the estimated lidar
ratios from the SAGE III–ISS and CALIOP measurements. For this figure, we
have used data from both the instruments from June 2017 through March 2018,
excluding data from August 2017 through November 2017 to avoid the effect of
smoke from the strong pyroCb event of August–September 2017 as discussed
above. The data from both instruments beyond March 2018 were not used to
avoid the impact of the Ambae volcano, which erupted in April 2018. As for
the comparisons presented in Sect. 3, we have averaged the SAGE III–ISS
data over each month, interpolated to the CALIPSO altitude grid, and
computed the lidar ratios, which were then averaged to obtain the climatological distribution shown in Fig. 14. As before, we have cloud-cleared the SAGE III–ISS data below 20 km by using only those 521 nm
extinction coefficients for which the 521 to 1022 nm extinction ratio exceeded 2. The Ångström exponent obtained from these two wavelengths was used to scale the SAGE III extinction at 521 to 532 nm. As can be seen, the lidar ratio values in the bulk of the stratosphere with significant aerosol loading are in the range 45–50 sr, quite similar to the canonical range in the stratosphere (Kremser et al., 2016), with the mean value between 18 and 30 km and between 40
Spatial distribution of the stratospheric aerosol 532 nm lidar ratio obtained from the extinction retrievals from SAGE III and backscatter measurements from CALIOP.
Is there any evidence of low lidar ratios in the high-latitude stratosphere
as seen in Fig. 14? O'Neill et al. (2012) studied aerosol plumes from the
Sarychev volcano from high Arctic observations from the Polar Environmental
Atmospheric Research Laboratory (PEARL) station (80.05
In the presence of large ash and even sulfate injections from volcanoes, lidar
ratios can be significantly different and can evolve with time. From the
so-called “constrained” retrievals, when the lidar ratios of layers can be
obtained directly from the attenuation measurements from CALIOP (Young and
Vaughan, 2009), the median lidar ratio of sulfate as well as ash-dominant
layers from several volcanoes has been found to be
In this paper we have provided a detailed account of the algorithm used to construct the recently released CALIPSO level 3 stratospheric aerosol profile product version 1.00. Further, we have given a qualitative as well as an initial quantitative assessment of the aerosol extinction retrievals. We have shown that the product captures significant stratospheric aerosol injections (e.g., from volcanic eruptions and wildfires) and clearly illustrates perturbations from stratospheric dynamics over the lifetime of the mission. Comparisons with extinction retrievals obtained from SAGE III show quite good agreement to within about 25 % in the mean between 20 and 30 km and between about 30
CALIPSO lidar level 1B, level 2, and level 3 stratospheric aerosol data products (Vaughan et al., 2016) are available from the Atmospheric Science Data Center at NASA LaRC (
All authors contributed to the development of the algorithm for the CALIPSO level 3 stratospheric aerosol product, which is described in this paper. JK wrote the initial draft of the paper, and KPL, MAV, JLT, and BJG contributed through data analysis, additional figures, and augmenting the text. CRT, DMW, and PLL provided overall guidance on the structure of the paper.
The authors declare that they have no conflict of interest.
Jayanta Kar would like to thank Jean-Paul Vernier, Mike Pitts, Larry Thomason, and Stuart Young for useful discussions during the development of the CALIPSO stratospheric product. SAGE III–ISS aerosol data were obtained from the Langley Atmospheric Science Data Center. The referees are thanked for useful comments and suggestions that significantly improved the paper. Jayanta Kar would like to acknowledge illuminating discussions with Mike Pitts on PSCs and Larry Thomason on SAGE III–ISS aerosol data.
This paper was edited by Alexander Kokhanovsky and reviewed by Andrew Prata, Michael Fromm, Sergey Khaykin, and one anonymous referee.