Cloud-Aerosol Lidar With Orthogonal Polarization Level Research Articles

New attenuated backscatter profile by removing the CALIOP receiver's transient response

The photomultiplier tubes (PMTs) used to measure altitude-resolved 532 nm backscatter intensity by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) do not recover rapidly following exposure to very high light levels (e.g., from optically dense water clouds). Instead, they exhibit exponentially decaying “noise tails” that can cause substantial misestimates when determining layer base altitudes and deriving cloud extinction coefficients. Here we present a deconvolution correction algorithm to remove these noise tails in the CALIOP level 1 science data products. The corrected 532 nm attenuated backscatter profiles generated by the deconvolution algorithm are evaluated using the corresponding CALIOP observations at 1064 nm and coincident profiles measured at 532 nm by the NASA-Langley airborne high spectral resolution lidar. Results indicate that our deconvolution correction algorithm effectively removes the non-ideal PMT recovery effects on the CALIOP level 1 vertical profiles, showing excellent performance for surface and water cloud lidar returns when the attenuated backscatter coefficients are greater than 0.01 km−1 sr−1.

Long-term (1999–2019) variability of stratospheric aerosol over Mauna Loa, Hawaii, as seen by two co-located lidars and satellite measurements

Abstract. As part of the Network for the Detection of Atmospheric Composition Change (NDACC), ground-based measurements obtained from the Jet Propulsion Laboratory (JPL) stratospheric ozone lidar and the NOAA stratospheric aerosol lidar at Mauna Loa, Hawaii, over the past 2 decades were used to investigate the impact of volcanic eruptions and pyrocumulonimbus (PyroCb) smoke plumes on the stratospheric aerosol load above Hawaii since 1999. Measurements at 355 and 532 nm conducted by these two lidars revealed a color ratio of 0.5 for background aerosols and small volcanic plumes and 0.8 for a PyroCb plume recorded on September 2017. Measurements of the Nabro plume by the JPL lidar in 2011–2012 showed a lidar ratio of (64±12.7) sr at 355 nm around the center of the plume. The new Global Space-based Stratospheric Aerosol Climatology (GloSSAC), Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) Level 3 and Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III-ISS) stratospheric aerosol datasets were compared to the ground-based lidar datasets. The intercomparison revealed a generally good agreement, with vertical profiles of extinction coefficient within 50 % discrepancy between 17 and 23 km above sea level (a.s.l.) and 25 % above 23 km a.s.l. The stratospheric aerosol depth derived from all of these datasets shows good agreement, with the largest discrepancy (20 %) being observed between the new CALIOP Level 3 and the other datasets. All datasets consistently reveal a relatively quiescent period between 1999 and 2006, followed by an active period of multiple eruptions (e.g., Nabro) until early 2012. Another quiescent period, with slightly higher aerosol background, lasted until mid-2017, when a combination of extensive wildfires and multiple volcanic eruptions caused a significant increase in stratospheric aerosol loading. This loading maximized at the very end of the time period considered (fall 2019) as a result of the Raikoke eruption, the plume of which ascended to 26 km altitude in less than 3 months.

CALIPSO level 3 stratospheric aerosol profile product: version 1.00 algorithm description and initial assessment

Abstract. 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∘ N–30∘ S), where the average difference between zonal mean extinction profiles is typically less than 25 % between 20 and 30 km (CALIPSO biased high). However, differences can exceed 100 % in the very low aerosol loading regimes found above 25 km at higher latitudes. Similarly, there are large differences (≥100 %) within 2 to 3 km above the tropopause that might be due to cloud contamination issues.

Estimations of global shortwave direct aerosol radiative effects above opaque water clouds using a combination of A-Train satellite sensors

Abstract. All-sky direct aerosol radiative effects (DARE) play a significant yet still uncertain role in climate. This is partly due to poorly quantified radiative properties of aerosol above clouds (AAC). We compute global estimates of shortwave top-of-atmosphere DARE over opaque water clouds (OWCs), DAREOWC, using observation-based aerosol and cloud radiative properties from a combination of A-Train satellite sensors and a radiative transfer model. There are three major differences between our DAREOWC calculations and previous studies: (1) we use the depolarization ratio method (DR) on CALIOP (Cloud–Aerosol Lidar with Orthogonal Polarization) Level 1 measurements to compute the AAC frequencies of occurrence and the AAC aerosol optical depths (AODs), thus introducing fewer uncertainties compared to using the CALIOP standard product; (2) we apply our calculations globally, instead of focusing exclusively on regional AAC “hotspots” such as the southeast Atlantic; and (3) instead of the traditional look-up table approach, we use a combination of satellite-based sensors to obtain AAC intensive radiative properties. Our results agree with previous findings on the dominant locations of AAC (south and northeast Pacific, tropical and southeast Atlantic, northern Indian Ocean and northwest Pacific), the season of maximum occurrence and aerosol optical depths (a majority in the 0.01–0.02 range and that can exceed 0.2 at 532 nm) across the globe. We find positive averages of global seasonal DAREOWC between 0.13 and 0.26 W m−2 (i.e., a warming effect on climate). Regional seasonal DAREOWC values range from −0.06 W m−2 in the Indian Ocean offshore from western Australia (in March–April–May) to 2.87 W m−2 in the southeast Atlantic (in September–October–November). High positive values are usually paired with high aerosol optical depths (>0.1) and low single scattering albedos (<0.94), representative of, for example, biomass burning aerosols. Because we use different spatial domains, temporal periods, satellite sensors, detection methods and/or associated uncertainties, the DAREOWC estimates in this study are not directly comparable to previous peer-reviewed results. Despite these differences, we emphasize that the DAREOWC estimates derived in this study are generally higher than previously reported. The primary reasons for our higher estimates are (i) the possible underestimate of the number of dust-dominated AAC cases in our study; (ii) our use of Level 1 CALIOP products (instead of CALIOP Level 2 products in previous studies) for the detection and quantification of AAC aerosol optical depths, which leads to larger estimates of AOD above OWC; and (iii) our use of gridded 4∘×5∘ seasonal means of aerosol and cloud properties in our DAREOWC calculations instead of simultaneously derived aerosol and cloud properties from a combination of A-Train satellite sensors. Each of these areas is explored in depth with detailed discussions that explain both the rationale for our specific approach and the subsequent ramifications for our DARE calculations.

On the Limits of CALIOP for Constraining Modeled Free Tropospheric Aerosol

AbstractThe spaceborne Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument provides valuable information on the vertical distribution of global aerosol and is often used to evaluate vertical aerosol distributions in general circulation models (GCMs). Here we show, however, that the detection limit of the CALIOP retrievals mean background aerosol is not detected, leading to substantially skewed statistics that moreover differ significantly by product. In the CALIOP Level 2 product this missing low‐backscatter aerosol results in the retrieved aerosol distribution significantly overrepresenting aerosol backscatter and extinction in the middle and upper troposphere if taken to be representative of the undetected aerosol. The CALIOP Level 3 product assumes no aerosol where none is detected, which then leads to an underestimation in the aerosol extinction profile in the upper troposphere. Using the ECHAM‐HAM GCM, we estimate that the mean fraction of aerosol undetected by CALIOP daytime (nighttime) retrievals is 41% (44%) globally.

Community Intercomparison Suite (CIS) v1.4.0: a tool for intercomparing models and observations

Abstract. The Community Intercomparison Suite (CIS) is an easy-to-use command-line tool which has been developed to allow the straightforward intercomparison of remote sensing, in situ and model data. While there are a number of tools available for working with climate model data, the large diversity of sources (and formats) of remote sensing and in situ measurements necessitated a novel software solution. Developed by a professional software company, CIS supports a large number of gridded and ungridded data sources "out-of-the-box", including climate model output in NetCDF or the UK Met Office pp file format, CloudSat, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization), MODIS (MODerate resolution Imaging Spectroradiometer), Cloud and Aerosol CCI (Climate Change Initiative) level 2 satellite data and a number of in situ aircraft and ground station data sets. The open-source architecture also supports user-defined plugins to allow many other sources to be easily added. Many of the key operations required when comparing heterogenous data sets are provided by CIS, including subsetting, aggregating, collocating and plotting the data. Output data are written to CF-compliant NetCDF files to ensure interoperability with other tools and systems. The latest documentation, including a user manual and installation instructions, can be found on our website (http://cistools.net). Here, we describe the need which this tool fulfils, followed by descriptions of its main functionality (as at version 1.4.0) and plugin architecture which make it unique in the field.

Analysis of Desert Dust Outbreaks Over Southern Europe Using CALIOP Data and Ground-Based Measurements

Saharan dust outbreaks are one of the major aerosol sources in Southern Europe. Dust affects the radiative balance of the Earth–atmosphere system and impacts health. The assessment of Saharan outbreaks requires information about the vertical distribution of dust in the atmosphere, usually obtained from lidar measurements, such as the ones performed by the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) sensor. In this paper, we have used CALIOP level-1 daytime data in combination with aerosol optical depth (AOD) measurements from Aerosol Robotic Network (AERONET) ground-based photometers to obtain aerosol profiles during 23 dust outbreaks over Southern Europe. These profiles are used for two main purposes: analyzing the lidar ratio values and vertical distribution of dust and evaluating the performance of version 3 of CALIOP algorithms. The mean lidar ratio obtained from the synergic method ranged between 47.7 and 49.0 sr, depending on the criterion used to average CALIOP signal. The dust layers traveled at a wide range of altitudes, from surface level up to 7.6 km. We found that the AOD values retrieved from CALIOP level 2 (version 3) and AERONET were not correlated $({r} . In addition, we observed that the mean AOD obtained by satellite retrievals, between 0.16 and 0.18, was lower than the mean AERONET value, i.e., 0.33, by 50%. These differences in the AOD were found to be caused by two main reasons: misclassification of dense dust layers as clouds by CALIOP algorithms, mainly over 4 km, and limitations of CALIOP algorithms to detect weak aerosol layers, particularly between 1.5 and 2.5 km.

Spaceborne observations of the lidar ratio of marine aerosols

Abstract. Retrievals of aerosol optical depth (AOD) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor require the assumption of the extinction-to-backscatter ratio, also known as the lidar ratio. This paper evaluates a new method to calculate the lidar ratio of marine aerosols using two independent sources: the AOD from the Synergized Optical Depth of Aerosols (SODA) project and the integrated attenuated backscatter from CALIOP. With this method, the particulate lidar ratio can be derived for individual CALIOP retrievals in single aerosol layer, cloud-free columns over the ocean. Global analyses are carried out using CALIOP level 2, 5 km marine aerosol layer products and the collocated SODA nighttime data from December 2007 to November 2010. The global mean lidar ratio for marine aerosols was found to be 26 sr, roughly 30% higher than the current value prescribed by the CALIOP standard retrieval algorithm. Data analysis also showed considerable spatiotemporal variability in the calculated lidar ratio over the remote oceans. The calculated marine aerosol lidar ratio is found to vary with the mean ocean surface wind speed (U10). An increase in U10 reduces the mean lidar ratio for marine regions from 32 ± 17 sr (for 0 < U10 < 4 m s−1) to 22 ± 7 sr (for U10 > 15 m s−1). Such changes in the lidar ratio are expected to have a corresponding effect on the marine AOD from CALIOP. The outcomes of this study are relevant for future improvements of the SODA and CALIOP operational product and could lead to more accurate retrievals of marine AOD.

Improvement of CALIOP Cloud Masking Algorithms for Better Estimation of Dust Extinction Profiles

Mineral dust suspended in the atmosphere affects the Earth’s radiation budget. To accurately predict the effect of dust on the climate system, information regarding its extinction profiles is needed. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite has enabled the global observation of the vertical distributions of aerosols and clouds since June 2006. To correctly retrieve extinction coefficients from CALIOP signals, the lidar-observed layers must be classified into aerosols or clouds. The cloud masking algorithms of CALIOP should be improved since the cloud mask products occasionally misclassify dense dust as clouds. This study attempts to discriminate misclassified clouds from the CALIOP cloud mask with a discriminant analysis. The training data are collected by tests with the CloudSat cloud mask, the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask, and relative humidity. Discrimination of dust from clouds is successful in cases over land and water surfaces during the daytime and nighttime. In contrast, the discrimination model of previous studies was inadequate during the nighttime since training data were not collected during the nighttime. The accuracy rate of the linear discriminant function classification is 91.7 % for misclassified clouds. The cloud mask is most frequently misclassified in the Taklimakan Desert. The proportion of misclassified clouds to the observed dust is ∼34.6 % (below 2 km) in the desert. Comparison of our results with CALIOP level 3 products indicates that the extinction profile using the improved cloud mask is at most twice larger than that of CALIOP level 3 products. This study suggests that the smaller extinction coefficients of CALIOP level 3 products are mainly caused by misclassification of dust as clouds in the vertical feature mask.

Multi-instrumental observation of an exceptionally strong Saharan dust outbreak over Portugal

[1] An exceptionally strong outbreak of Saharan dust was observed in southern Portugal from 4 to 9 April 2011. The event was monitored with a multiwavelength Raman lidar, a CIMEL Sun photometer, the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) and two Moderate Resolution Imaging Spectroradiometer (MODIS). Those instruments provided vertically resolved and columnar information about the optical properties of the mineral dust plume. Maximum aerosol optical depths in the free troposphere of 1.8 and 1.9 were observed with Raman lidar at 355 and 532 nm, respectively. The mean extinction-related Ångström exponents (355, 532 nm) from Raman lidar averaged over the whole period were 0.0 ± 0.2. Backscatter-related Ångström exponents (355, 532 nm and 532, 1064 nm) were 0.4 on average. Mean lidar ratios calculated from Raman lidar measurements were 45 ± 8 sr at 355 nm and 53 ± 7 sr at 532 nm. The mean linear particle depolarization ratio at 532 nm was 0.28 ± 0.04. Furthermore, intrusion of Saharan dust into the planetary boundary layer could be observed with different ground-based in-situ instruments. Maximum particle mass concentration values (PM10) of 162 μg m−3 were detected at ground. The daily threshold of 50 μg m−3 for PM10 was exceeded on three subsequent days during the considered period. The ratio of coarse to fine mode particle number concentration reached maximum values larger than 1, coinciding with the maximum in PM10mass concentration. A comparison of ground-based Raman lidar measurements and CALIOP Level 2 data was done for one CALIPSO overpass. A good agreement was found for backscatter coefficients.

An accuracy assessment of the CALIOP/CALIPSO version 2/version 3 daytime aerosol extinction product based on a detailed multi-sensor, multi-platform case study

Abstract. The Cloud Aerosol LIdar with Orthogonal Polarization (CALIOP), on board the CALIPSO platform, has measured profiles of total attenuated backscatter coefficient (level 1 products) since June 2006. CALIOP's level 2 products, such as the aerosol backscatter and extinction coefficient profiles, are retrieved using a complex succession of automated algorithms. The goal of this study is to help identify potential shortcomings in the CALIOP version 2 level 2 aerosol extinction product and to illustrate some of the motivation for the changes that have been introduced in the next version of CALIOP data (version 3, released in June 2010). To help illustrate the potential factors contributing to the uncertainty of the CALIOP aerosol extinction retrieval, we focus on a one-day, multi-instrument, multiplatform comparison study during the CALIPSO and Twilight Zone (CATZ) validation campaign on 4 August 2007. On that day, we observe a consistency in the Aerosol Optical Depth (AOD) values recorded by four different instruments (i.e. space-borne MODerate Imaging Spectroradiometer, MODIS: 0.67 and POLarization and Directionality of Earth's Reflectances, POLDER: 0.58, airborne High Spectral Resolution Lidar, HSRL: 0.52 and ground-based AErosol RObotic NETwork, AERONET: 0.48 to 0.73) while CALIOP AOD is a factor of two lower (0.32 at 532 nm). This case study illustrates the following potential sources of uncertainty in the CALIOP AOD: (i) CALIOP's low signal-to-noise ratio (SNR) leading to the misclassification and/or lack of aerosol layer identification, especially close to the Earth's surface; (ii) the cloud contamination of CALIOP version 2 aerosol backscatter and extinction profiles; (iii) potentially erroneous assumptions of the aerosol extinction-to-backscatter ratio (Sa) used in CALIOP's extinction retrievals; and (iv) calibration coefficient biases in the CALIOP daytime attenuated backscatter coefficient profiles. The use of version 3 CALIOP extinction retrieval for our case study seems to partially fix factor (i) although the aerosol retrieved by CALIOP is still somewhat lower than the profile measured by HSRL; the cloud contamination (ii) appears to be corrected; no particular change is apparent in the observation-based CALIOP Sa value (iii). Our case study also showed very little difference in version 2 and version 3 CALIOP attenuated backscatter coefficient profiles, illustrating a minor change in the calibration scheme (iv).

Characteristics of CALIOP attenuated backscatter noise: implication for cloud/aerosol detection

Abstract. A research algorithm is developed for noise evaluation and feature detection of the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Level 1 (L1) backscatter data with an emphasis on cloud/aerosol features in the upper troposphere and lower stratosphere (UT/LS). CALIOP measurement noise of the version v2.01 and v2.02 L1 backscatter data aggregated to (5 km) horizontal resolution is analyzed with two approaches in this study. One is to compare the observed and modeled molecular scatter profiles by scaling the modeled profile (with a fitted scaling factor α) to the observed clear-sky backscatter profiles. This scaling α value is sensitive to errors in the calibrated backscatter and the atmospheric model used. Most of the nighttime 532-nm α values are close to unity, as expected, but an abrupt drop occurred in October 2008 in the daytime 532-nm α, which is likely indicative of a problem in the v2.02 daytime calibrated data. The 1064-nm night α is generally close to 2 while its day α is ~3. The other approach to evaluate the lidar measurement noise is to use the calibrated lidar backscatter data at altitudes above 19 km. With this method, the 532-nm and 1064-nm measurement noises are analyzed and characterized individually for each profile in terms of the mean (μ) and standard deviation (σ), showing larger σ values in general over landmasses or bright surfaces during day and in radiation-hard regions during night. A significant increasing trend is evident in the nighttime 1064-nm σ, which is likely responsible for the increasing difference between the feature occurrence frequencies (532-nm vs. 1064-nm) derived from this study. For feature detection with the research algorithm, we apply a σ–based method to the aggregated L1 data. The derived morphology of feature occurrence frequency is in general agreement with that obtained from the Level 2 (L2) 05 km_CLAY+05 km_ALAY products at 5 km horizontal resolution. Finally, a normalized probability density function (PDF) method is employed to evaluate the day-night backscatter data in which noise levels are largely different. CALIOP observations reveal a higher probability of daytime cloud/aerosol occurrence than nighttime in the tropical UT/LS region for 532-nm total backscatters >0.01 km−1 sr−1.

CALIOP Aerosol Subset Processing for Global Aerosol Transport Model Data Assimilation

A system for processing Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite-based 0.532 and 1.064 elastic and polarization lidar datasets for global aerosol transport model assimilation is described. A method for constructing one-degree along-track and cloud-free signal composite averages, consistent with Navy Aerosol Analysis and Prediction System (NAAPS) model gridding, using CALIOP Level 1B attenuated backscatter and Level 2 cloud boundary-height products is outlined. Optimal vertical resolutions and relative signal uncertainties for the composite signal averages are described for both day and nighttime measurement scenarios. Depolarization profiles are described for the 0.532 channel as well as attenuated color ratio profiles using 0.532 and 1.064 attenuated backscatter measurements. Constrained by NAAPS model aerosol optical depths, processed attenuated backscatter profiles are inverted to solve for extinction and backscatter coefficients, their ratio, and extinction coefficient profiles which serve as the basis for data assimilation.