Spaceborne Lidar Cloud-aerosol Lidar With Orthogonal Polarization Research Articles

Evaluation of aerosol number concentrations from CALIPSO with ATom airborne in situ measurements

Abstract. The present study aims to evaluate the available aerosol number concentration (ANC) retrieval algorithms for spaceborne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite with the airborne in situ measurements from the ATom (Atmospheric Tomography Mission) campaign. We used HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory model) to match both the measurements in space and identified 53 cases that were suitable for comparison. Since the ATom data include the dry aerosol extinction coefficient, we used kappa parameterization to adjust the ambient measurements from CALIOP to dry conditions. As both the datasets have a different vertical resolution, we regrid them to uniform height bins of 240 m from the surface to a height of 5 km. On comparing the dry extinction coefficients, we found a reasonable agreement between the CALIOP and ATom measurements with Spearman's correlation coefficient of 0.715. Disagreement was found mostly for retrievals above 3 km altitude. Thus, to compare the ANC, which may vary by orders of magnitude in space and time, we further limit the datasets and only select those height bins for which the CALIOP-derived dry extinction coefficient is within ±50 % of the ATom measurements. This additional filter further increases the probability of comparing the same air parcel. The altitude bins which qualify the extinction coefficient constraint are used to estimate ANC with a dry radius >50 nm (n50,dry) and >250 nm (n250,dry). The POLIPHON (Polarization Lidar Photometer Networking) and OMCAM (Optical Modelling of CALIPSO Aerosol Microphysics) algorithms were used to estimate the n50,dry and n250,dry. The POLIPHON estimates of n50,dry and n250,dry were found to be in good agreement with the in situ measurements, with a correlation coefficient of 0.829 and 0.47, root mean square error (RMSE) of 234 and 13 cm−3, and bias of −97 and 4 cm−3, respectively. The OMCAM estimates of n50,dry and n250,dry were also in reasonable agreement with the in situ measurements, with a correlation coefficient of 0.823 and 0.463, RMSE of 247 and 13 cm−3, and bias of 44 and 4 cm−3, respectively. However, we found that the OMCAM-estimated n50,dry were about an order of magnitude less than the in situ measurements for marine-dominated cases. We propose a modification to the OMCAM algorithm by using an AERONET-based marine model. With the updated OMCAM algorithm, the n50,dry agrees well with the ATom measurements. Such concurrence between the satellite-derived ANC and the independent ATom in situ measurements emboldens the use of CALIOP in studying the aerosol–cloud interactions.

Trans-pacific aerosol vertical structure revealed by spaceborne lidar CALIOP

The 5-year (2011–2015) spaceborne lidar CALIOP (cloud-aerosol lidar with orthogonal polarization) data were aggregated to obtain a seasonal three-dimensional (3-D) distribution of aerosols over the Northern Pacific (NP). Superior to the Moderate-resolution Imaging Spectroradiometer (MODIS) product, which provided only the bulk Aerosol Optical Depth (AOD), the vertical high-resolution Aerosol Extinction Coefficient (AEC) obtained from the CALIOP product enabled us to determine the AOD for the prescribed layers. By integrating the AEC at the prescribed height interval, the AOD beneath and above 2 km was obtained to characterize the aerosol loading in the surface layer (SL) and the free troposphere (FT), respectively. The AOD in the SL and FT present different seasonal variation patterns over the NP. The SL AOD exhibited the highest level in the winter, which was caused by sea salt emissions driven by the strongest winds. However, a much higher FT AOD occurred over the NP in the springtime compared with that of other seasons, indicating that the most significant transpacific transport occurred in the springtime, mainly in the FT. Fine resolution aerosol distributions in cross section along a transpacific direction were extracted from the CALIOP product as well. Along the cross section, aerosol loading was most abundant below 2 km for all the seasons. However, the detectable aerosol layer may extend to 10 km high in the springtime, which is much higher than that in the other seasons, and the aerosol layers below and above 2 km were separated over the remote NP. The CALIOP data demonstrated that dust plays a role in aloft aerosol transport. In terms of aerosol transport in the FT, an 8-fold decrease in the FT AOD occurred during the journey from East Asia to the Eastern Pacific in the summer, which was the sharpest decrease compared with the smallest one, which has 3-fold decrease occurred in the spring. Aerosol transport in the FT contributed 19% of the AOD over western North America (NA) in the spring, which was much higher than that in other seasons, and the aloft aerosol transport between 2 and 4 km high corresponded to a fine PM concentration of 1–2 μg m−3 as a contributor to air quality over western NA.

Analysis of the origin of peak aerosol optical depth in springtime over the Gulf of Tonkin

By aggregating MODIS (moderate-resolution imaging spectroradiometer) AOD (aerosol optical depth) and OMI (ozone monitoring instrument) UVAI (ultra violet aerosol index) datasets over 2010–2014, it was found that peak aerosol loading in seasonal variation occurred annually in spring over the Gulf of Tonkin (17–23°N, 105–110°E). The vertical structure of the aerosol extinction coefficient retrieved from the spaceborne lidar CALIOP (cloud-aerosol lidar with orthogonal polarization) showed that the springtime peak AOD could be attributed to an abrupt increase in aerosol loading between altitudes of 2 and 5km. In contrast, aerosol loading in the low atmosphere (below 1km) was only half of that in winter. Wind fields in the low and high atmosphere exhibited opposite transportation patterns in spring over the Gulf of Tonkin, implying different sources for each level. By comparing the emission inventory of anthropogenic sources with biomass burning, and analyzing the seasonal variation of the vertical structure of aerosols over the Northern Indo-China Peninsula (NIC), it was concluded that biomass burning emissions contributed to high aerosol loading in spring. The relatively high topography and the high surface temperature in spring made planetary boundary layer height greater than 3km over NIC. In addition, small-scale cumulus convection frequently occurred, facilitating pollutant rising to over 3km, which was a height favoring long-range transport. Thus, pollutants emitted from biomass burning over NIC in spring were raised to the high atmosphere, then experienced long-range transport, leading to the increase in aerosol loading at high altitudes over the Gulf of Tonkin during spring.

Comparison of Cirrus Cloud Characteristics as Estimated by A Micropulse Ground-Based Lidar and A Spaceborne Lidar CALIOP Datasets Over Lille, France (50.60 °N, 3.14 ° E)

Our goal is to establish a climatology of cirrus cloud properties over Lille, France (50.60°N, 3.14 °E) using a ground-based lidar. A statistical analysis of mid-latitude cirrus clouds from lidar data in Lille over the period 2008-2013 is presented and discussed. The macrophysical properties (cloud base altitude, cloud top altitude, geometrical thickness, mid-cloud temperature) and optical properties (cloud optical thickness and lidar ratio) are evaluated and compared between the ground-based and the spaceborne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) measurements for the period 2008-2013. We found similar results of macrophysical properties derived from both lidars. In addition a difference in the optical properties results is due to the multiple scattering and the heterogeneity of the observed scenes.

Volcanic aerosol layer formed in the tropical upper troposphere by the eruption of Mt. Merapi, Java, in November 2010 observed by the spaceborne lidar CALIOP

Mt. Merapi in Java, Indonesia, erupted in November 2010. The eruption was proved to be the source of the aerosol layer observed by a ground-based lidar at Biak, Indonesia, in January 2011 using data on the global distribution of aerosols observed by the spaceborne cloud-aerosol lidar with orthogonal polarization (CALIOP). These data were used to describe how the volcanic aerosols produced by the volcanic eruption diffused throughout the tropical tropopause layer (TTL). The equivalent maximum total amount of volcanic SO2 estimated from the spatially integrated total amount of aerosols was 0.09Tg, which is one-third to half that of gaseous SO2 after the eruption was observed by the ozone monitoring instrument satellite. The obtained cirrus-cloud-appearance frequency data exhibit a seasonal cycle having its maximum in winter and no detectable variations that are synchronized with the increase in TTL volcanic aerosols.

Numerical simulations with the WRF model of water vapour vertical profiles: A comparison with LIDAR and radiosounding measurements

Numerical simulations with the Advanced Research-Weather Research and Forecasting model (WRF-ARW) have been performed in order to simulate water vapor mixing ratio (WVMR) vertical profiles over southeastern Italy. An optimal setup of the model has been identified changing the default model land cover database and using a fine model vertical resolution in the lower levels. Among different tests, the selected implementation is the one that best reproduced the experimental WVMR profiles retrieved from the Brindisi radiosounding station and the ground-based Raman lidar measurements at Lecce site.The use of the WRF model to simulate WVMR profiles along the tracks of the spaceborne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) on two case studies has revealed that numerical models are reliable tools, particularly useful when no ground measurements of water vapor are available. In particular, it is shown that the inhomogeneous distribution of moisture in the domain, as emerging from the model simulations, may significantly affect the CALIOP validation procedures. Such procedures are based on the intercomparison of the satellite retrieved aerosol profiles with those measured, nearly at the same time, with a ground-based lidar located few km away from the sites where CALIOP aerosol products are delivered in the selected events.

Estimated desert-dust ice nuclei profiles from polarization lidar: methodology and case studies

Abstract. A lidar method is presented that permits the estimation of height profiles of ice nuclei concentrations (INC) in desert dust layers. The polarization lidar technique is applied to separate dust and non-dust backscatter and extinction coefficients. The desert dust extinction coefficients σd are then converted to aerosol particle number concentrations APC280 which consider particles with radius > 280 nm only. By using profiles of APC280 and ambient temperature T along the laser beam, the profile of INC can be estimated within a factor of 3 by means of APC-T-INC parameterizations from the literature. The observed close relationship between σd at 500 nm and APC280 is of key importance for a successful INC retrieval. We studied this link by means of AERONET (Aerosol Robotic Network) sun/sky photometer observations at Morocco, Cabo Verde, Barbados, and Cyprus during desert dust outbreaks. The new INC retrieval method is applied to lidar observations of dust layers with the spaceborne lidar CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization) during two overpasses over the EARLINET (European Aerosol Research Lidar Network) lidar site of the Cyprus University of Technology (CUT), Limassol (34.7° N, 33° E), Cyprus. The good agreement between the CALIOP and CUT lidar retrievals of σd, APC280, and INC profiles corroborates the potential of CALIOP to provide 3-D global desert dust APC280 and INC data sets.

Depolarization ratio of polar stratospheric clouds in coastal Antarctica: comparison analysis between ground-based Micro Pulse Lidar and space-borne CALIOP observations

Abstract. Polar stratospheric clouds (PSCs) play an important role in polar ozone depletion, since they are involved in diverse ozone destruction processes (chlorine activation, denitrification). The degree of that ozone reduction is depending on the type of PSCs, and hence on their occurrence. Therefore PSC characterization, mainly focused on PSC-type discrimination, is widely demanded. The backscattering (R) and volume linear depolarization (δV) ratios are the parameters usually used in lidar measurements for PSC detection and identification. In this work, an improved version of the standard NASA/Micro Pulse Lidar (MPL-4), which includes a built-in depolarization detection module, has been used for PSC observations above the coastal Antarctic Belgrano II station (Argentina, 77.9° S 34.6° W, 256 m a.s.l.) since 2009. Examination of the MPL-4 δV feature as a suitable index for PSC-type discrimination is based on the analysis of the two-channel data, i.e., the parallel (p-) and perpendicular (s-) polarized MPL signals. This study focuses on the comparison of coincident δV-profiles as obtained from ground-based MPL-4 measurements during three Antarctic winters with those reported from the space-borne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite in the same period (83 simultaneous cases are analysed for 2009–2011 austral winter times). Three different approaches are considered for the comparison analysis between both lidar profile data sets in order to test the degree of agreement: the correlation coefficient (CC), as a measure of the relationship between both PSC vertical structures; the mean differences together with their root mean square (RMS) values found between data sets; and the percentage differences (BIAS), parameter also used in profiling comparisons between CALIOP and other ground-based lidar systems. All of them are examined as a function of the CALIPSO ground-track distance from the Belgrano II station. Results represent a relatively good agreement between both ground-based MPL-4 and space-borne CALIOP profiles of the volume linear depolarization ratio δV for PSC events, once the MPL-4 depolarization calibration parameters are applied. Discrepancies between CALIOP and MPL-4 profiles in vertical layering structure are enhanced from 20 km up, likely due to a decrease of the signal-to-noise ratio (SNR) for both lidar systems at those altitudes. Regarding the results obtained from the mean and the percentage differences found between MPL-4 and CALIOP δV profiles, a predominance of negative values is also observed, indicating a generalized underestimation of the MPL-4 depolarization as compared to that reported by CALIOP. However, absolute differences between those δV-profile data sets are no higher than a 10 ± 11% in average. Moreover, the degree of agreement between both lidar δV data sets is slightly dependent on the CALIPSO ground-track overpass distance from the Belgrano II station. That is, small discrepancies are found when CALIPSO ground-track distance is as close as far from the ground-based station. These results would indicate that MPL-4 depolarization observations would reflect relatively well the PSC field that CALIOP can detect at relatively large distances from the ground-based station. As a consequence, PSC properties can be statistically similar, on average, over large volumes, and hence the present weak disagreement found between both the lidar δV data sets can be likely dominated by small spatial PSC inhomogeneities along the CALIPSO separation from the station. This statement is based on the fact that Belgrano II is a station located well inside the stable Antarctic polar vortex, allowing determined thermodynamic conditions leading to a very low variability in the PSC field, and in their properties.

Gravity wave events from mesoscale simulations, compared to polar stratospheric clouds observed from spaceborne lidar over the Antarctic Peninsula

We compare Gravity Waves (GW) and Polar Stratospheric Clouds (PSC) above the Antarctic Peninsula for winters (June to September) between 2006 and 2010. GW activity is inferred from stratospheric temperature and vertical winds from the Weather and Research Forecast mesoscale model (WRF), and documented as a function of time and geography for the studied period. Significant GW activity affects 36% of days and follows the Peninsula orography closely. Volumes of PSC, composed of ice and Nitric Acid Trihydrate (NAT), are retrieved using observations from the spaceborne lidar CALIOP (Cloud‐Aerosol Lidar with Orthogonal Polarization). They are documented against GW activity as a function of time and longitude. Sixty‐three percent of ice PSC are observed during GW events, when the average volume of PSC per profile doubles. Maximum ice PSC volumes are seen directly over the Peninsula (65°W), while maximum NAT PSC volumes appear downstream further East (∼35°W). Effects of GW events on NAT PSC are felt as far East as 40°E. Our results support the importance of gravity waves as a major mechanism driving the evolution of ice PSC in the area, but the effects on NAT PSC are harder to detect. After a GW event ends, volumes of ice PSC get back to their usual levels in less than 24 h, while this process takes more than 48 h for NAT PSC. Daily profiles of H2O and HNO3 mixing ratios, retrieved from MLS observations, are used to derive ice and NAT frost points with altitude and time. Combining these frost points with modeled stratospheric temperatures, the volumes of air able to support ice and NAT crystals are quantified and compared with PSC volumes. Correlation is high for ice crystals, but not for NAT, consistent with their much slower nucleation mechanisms. Observations of ice PSC over the domain are followed by a strong increase (+50–100%) in NAT PSC formation efficiency 2 to 6 h later. This increase is followed by a steep drop (6–10 h later) and a longer period of slow decline (10–24 h later), at the end of which the NAT PSC formation efficiency is less than half its initial value. The fact that these effects tend to cancel each other out, coupled to the important lag in NAT PSC reaction to GW activity, suggest why it is especially difficult to quantify how GW activity impacts NAT PSC cover.

Climatological aspects of seasonal variation of aerosol vertical distribution over central Indo-Gangetic belt (IGB) inferred by the space-borne lidar CALIOP

Abstract Climatological information on the vertical distribution of aerosol in the lower atmosphere is needed to assess the effects of aerosols on climate. Altitude profiles of the backscatter coefficient (at 532 nm), color ratio (CR) and particle depolarization ratio (PDR) derived from space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite were used to investigate the climatological aspects of seasonal variation of aerosol vertical distribution over central Indo-Gangetic belt (IGB). Strong convective activity coupled with capping inversion lead to vertical extension (4–5 km) of aerosol distribution during the pre-monsoon (PrM) season. Trapping of pollution at low altitudes (below 1.5 km) by subsidence have been found during the late post-monsoon (PoM) and winter seasons. Combined results of CR and PDR suggest presence of elevated mineral dust particles during the PrM season, whereas fine mode and spherical (biomass burning/industrial pollution) particles are dominant during the PoM and winter seasons. The observations indicate a seasonal dependency of the aerosol vertical distribution over central IGB.