Lidar Depolarization Ratio Research Articles

Polarization lidar observations of diurnal and seasonal variations in the atmospheric mixing layer above a tropical rural place gadanki, India

This study presents the daily and seasonal variation of the atmospheric mixing layer height (MLH) over Gadanki, India (13.45°N, 79.18°E), a tropical rural location based on polarization lidar observations. The observations spanned the years 2009–2014, encompassing 303 instances, and coinciding with radiosonde and surface weather station measurements. The MLH was determined through the analysis of aerosol profiles and confirmed with the MLH values derived from radiosonde data. The lidar depolarization ratio was employed to characterize aerosol shape. This study aims to establish a connection between aerosol backscatter and its shape through lidar observations, considering diurnal and seasonal variations, while also identifying the influencing factors. This study illustrates four distinct case studies conducted during different seasons to depict aerosol behavior in both convectively active and non-active periods. These case studies unveil the influence of aerosol shape on water intake and subsequent residual layer and cloud formation. The observed fluctuations in MLH and aerosol shape suggest a dynamic relationship between local meteorology and long-range aerosol transport.

Long-term aerosol particle depolarization ratio measurements with HALO Photonics Doppler lidar

Abstract. It has been demonstrated that HALO Photonics Doppler lidars (denoted HALO Doppler lidar hereafter) have the capability for retrieving the aerosol particle depolarization ratio at a wavelength of 1565 nm. For these lidars operating at such a long wavelength, the retrieval quality depends to a large degree on an accurate representation of the instrumental noise floor and the performance of the internal polarizer, whose stability has not yet been assessed for long-term operation. Here, we use 4 years of measurements at four sites in Finland to investigate the long-term performance of HALO Doppler lidars, focusing on aerosol particle depolarization ratio retrieval. The instrumental noise level, represented by noise-only signals in aerosol- and hydrometeor-free regions, shows stable performance for most instruments but clear differences between individual instruments. For all instruments, the polarizer bleed-through evaluated at liquid cloud base remains reasonably constant at approximately 1 % with a standard deviation of less than 1 %. We find these results to be sufficient for long-term aerosol particle depolarization ratio measurements and proceed to analyse the seasonal and diurnal cycles of the aerosol particle depolarization ratio in different environments in Finland, including in the Baltic Sea archipelago, a boreal forest and rural sub-arctic. To do so, we further develop the background correction method and construct an algorithm to distinguish aerosol particles from hydrometeors. The 4-year averaged aerosol particle depolarization ratio ranges from 0.07 in sub-arctic Sodankylä to 0.13 in the boreal forest in Hyytiälä. At all sites, the aerosol particle depolarization ratio is found to peak during spring and early summer, even exceeding 0.20 at the monthly-mean level, which we attribute to a substantial contribution from pollen. Overall, our observations support the long-term usage of HALO Doppler lidar depolarization ratio measurements, including detection of aerosols that may pose a safety risk for aviation.

Global dust optical depth climatology derived from CALIOP and MODIS aerosol retrievals on decadal timescales: regional and interannual variability

Abstract. We derived two observation-based global monthly mean dust aerosol optical depth (DAOD) climatological datasets from 2007 to 2019 with a 2∘ (latitude) × 5∘ (longitude) spatial resolution, one based on Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the other on Moderate Resolution Imaging Spectroradiometer (MODIS) observations. In addition, the CALIOP climatological dataset also includes dust vertical extinction profiles. Dust is distinguished from non-dust aerosols based on particle shape information (e.g., lidar depolarization ratio) for CALIOP and on dust size and absorption information (e.g., fine-mode fraction, Ångström exponent, and single-scattering albedo) for MODIS, respectively. The two datasets compare reasonably well with the results reported in previous studies and the collocated Aerosol Robotic Network (AERONET) coarse-mode AOD. Based on these two datasets, we carried out a comprehensive comparative study of the spatial and temporal climatology of dust. On a multi-year average basis, the global (60∘ S–60∘ N) annual mean DAOD is 0.032 and 0.067 according to CALIOP and MODIS retrievals, respectively. In most dust-active regions, CALIOP DAOD generally correlates well (correlation coefficient R>0.6) with the MODIS DAOD, although the CALIOP value is significantly smaller. The CALIOP DAOD is 18 %, 34 %, 54 %, and 31 % smaller than MODIS DAOD over the Sahara, the tropical Atlantic Ocean, the Caribbean Sea, and the Arabian Sea, respectively. Applying a regional specific lidar ratio (LR) of 58 sr instead of the 44 sr used in the CALIOP operational retrieval reduces the difference from 18 % to 8 % over the Sahara and from 34 % to 12 % over the tropical Atlantic Ocean. However, over eastern Asia and the northwestern Pacific Ocean (NWP), the two datasets show weak correlation. Despite these discrepancies, CALIOP and MODIS show similar seasonal and interannual variations in regional DAOD. For dust aerosol over the NWP, both CALIOP and MODIS show a declining trend of DAOD at a rate of about 2 % yr−1. This decreasing trend is consistent with the observed declining trend of DAOD in the southern Gobi Desert at a rate of 3 % yr−1 and 5 % yr−1 according to CALIOP and MODIS, respectively. The decreasing trend of DAOD in the southern Gobi Desert is in turn found to be significantly correlated with increasing vegetation and decreasing surface wind speed in the area.

A case study on the vertical distribution and characteristics of aerosols using ground-based raman lidar, satellite and model over Western India

ABSTRACT Ahmedabad is an urban site in western region of India. Dust loading and its variability are very high over this semi-arid location. A state-of-the-art ground-based Raman lidar has been used for the study of day-to-day variability in vertical distribution of aerosol loading over Ahmedabad (23.02° N, 72.57° E) from 8 to 11 May 2018. We have done a comprehensive study about vertical profiles of aerosols across two wavelengths recorded simultaneously, namely, 355 nm and 532 nm, over this location. An interesting hump feature has been noted on 9 May at around 18 h (Local Time), with an additional peak at 19 h on 10 May. The loading over this region has more fine-mode aerosols than coarse-mode particles. Further, findings from ground-based lidar have been compared with MACC-II model simulations and Cloud-Aerosol Lidar with Orthogonal Polarization on-board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observational. Back-trajectory analysis showed majority of the influx came from middle-east, carrying mineral dust and traces of marine aerosols from Arabian Sea, except for 11 May, where low-level dust carrying trajectories and also coarser dust particles were not seen. This type of manifestation has also been observed while classifying aerosols over Ahmedabad using Lidar Depolarization Ratio and Lidar Ratio obtained from Raman Lidar. Fine mode aerosols have been characterized as industrial, vehicular and marine, while coarser as mineral dust and biomass burning. This study focuses on local variation and classification of aerosols based on size and type, which will further help in better estimation of radiative budget and other impacts of aerosols.

Lidar depolarization ratio of atmospheric pollen at multiple wavelengths

Abstract. Lidar observations during the pollen season 2019 at the European Aerosol Research Lidar Network (EARLINET) station in Kuopio, Finland, were analyzed in order to optically characterize atmospheric pollen. Pollen concentration and type information were obtained by a Hirst-type volumetric air sampler. Previous studies showed the detectability of non-spherical pollen using depolarization ratio measurements. We present lidar depolarization ratio measurements at three wavelengths of atmospheric pollen in ambient conditions. In addition to the depolarization ratio detected with the multiwavelength Raman polarization lidar PollyXT at 355 and 532 nm, depolarization measurements of a co-located Halo Doppler lidar at 1565 nm were utilized. During a 4 d period of high birch (Betula) and spruce (Picea abies) pollen concentrations, unusually high depolarization ratios were observed within the boundary layer. Detected layers were investigated regarding the share of spruce pollen to the total pollen number concentration. Daily mean linear particle depolarization ratios of the pollen layers on the day with the highest spruce pollen share are 0.10 ± 0.02, 0.38 ± 0.23 and 0.29 ± 0.10 at 355, 532 and 1565 nm, respectively, whereas on days with lower spruce pollen share, depolarization ratios are lower with less wavelength dependence. This spectral dependence of the depolarization ratios could be indicative of big, non-spherical spruce pollen. The depolarization ratio of pollen particles was investigated by applying a newly developed method and assuming a backscatter-related Ångström exponent of zero. Depolarization ratios of 0.44 and 0.16 at 532 and 355 nm for the birch and spruce pollen mixture were determined.

Active remote sensing of atmospheric dust using relationships between their depolarization ratios and reflectivity.

The backscattered light from agglomerated debris particles shows that an approximate linear correlation exists between the logarithm of the geometric albedo $ \log(A )$ of polydispersions of agglomerated debris particles and their lidar linear or circular depolarization ratios, $ \unicode{x00B5}_L$ and $ \unicode{x00B5}_C$. The nature of the relationship depends on the complex refractive index of the particles in the distribution. This extension of the Umov law can be used for lidar and radar characterizations by placing constraints on the reflectivity of the particles. It suggests that an approximate inverse relationship exists between the lidar ratio and the lidar depolarization ratios whose scaling parameter depends on the refractive index of the aerosol population.

Radar-lidar ratio for ice crystals of cirrus clouds.

Simultaneous measurement of lidar and radar signals returned from the same cirrus clouds is a prospective method for retrieving the cloud microphysics, i.e. size and shape of the ice crystals constituting the clouds. In this study, the ratio of the backscattered signals of lidar and radar called the radar-lidar ratio has been calculated for the first time for typical shapes of ice crystals and wide distribution of the crystals over their sizes. It is shown that it is the lidar-radar ratio that is most sensitive to crystal sizes while the lidar depolarization ratio is most sensitive to crystal shapes.

(UV, VIS) Laboratory evaluation of the lidar depolarization ratio of freshly emitted soot aggregates from pool fire in ambient air at exact backscattering angle

In this paper, a controlled-laboratory experiment is carried out to evaluate the lidar depolarization ratio of freshly emitted soot aggregates in the exact backward scattering direction at 180.0°. The experiment is performed at two wavelengths simultaneously, namely 355 and 532 nm, often used in polarimetric lidar remote sensing. The soot aggregates are generated from a kerosene JET A-1 pool fire in laboratory ambient air and microscopic images confirm the fractal morphology of generated soot aggregates. Then, the Superposition T-Matrix (STM) method is applied to numerically simulate the soot aggregates backscattering properties for different soot particles refractive indices, monomer radii and monomer numbers. The range of these parameters which ensures the lowest discrepancy between the laboratory measurement and the STM-computations is discussed within experimental and numerical error bars. We find that the polydisperse monomers model gives an overall better evaluation of the ratio F22(π)/F11(π). In the polydisperse case, our numerical and laboratory experimental findings agree at both wavelengths for a refractive index m = 2.65 + i1.32 and monomer number Nm > 40 at a mean monomer radius of rp = 30 nm (Nm > 160 at rp = 27.5 nm). We believe this work may be useful for the light scattering and remote sensing communities and may also help future studies aimed at better understanding the impact of soot particle aggregates on the Earth's climate, which still needs to be precisely quantified.

Apparent dust size discrepancy in aerosol reanalysis in north African dust after long-range transport

Abstract. North African dust reaches the southeastern United States every summer. Size-resolved dust mass measurements taken in Miami, Florida, indicate that more than one-half of the surface dust mass concentrations reside in particles with geometric diameters less than 2.1 µm, while vertical profiles of micropulse lidar depolarization ratios show dust reaching above 4 km during pronounced events. These observations are compared to the representation of dust in the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) aerosol reanalysis and closely related Goddard Earth Observing System model version 5 (GEOS-5) Forward Processing (FP) aerosol product, both of which assimilate satellite-derived aerosol optical depths using a similar protocol and inputs. These capture the day-to-day variability in aerosol optical depth well, in a comparison to an independent sun-photometer-derived aerosol optical depth dataset. Most of the modeled dust mass resides in diameters between 2 and 6 µm, in contrast to the measurements. Model-specified mass extinction efficiencies equate light extinction with approximately 3 times as much aerosol mass, in this size range, compared to the measured dust sizes. GEOS-5 FP surface-layer sea salt mass concentrations greatly exceed observed values, despite realistic winds and relative humidities. In combination, these observations help explain why, despite realistic total aerosol optical depths, (1) free-tropospheric model volume extinction coefficients are lower than those retrieved from the micro-pulse lidar, suggesting too-low model dust loadings in the free troposphere, and (2) model dust mass concentrations near the surface can be higher than those measured. The modeled vertical distribution of dust, when captured, is reasonable. Large, aspherical particles exceeding the modeled dust sizes are also occasionally present, but dust particles with diameters exceeding 10 µm contribute little to the measured total dust mass concentrations after such long-range transport. Remaining uncertainties warrant a further integrated assessment to confirm this study's interpretations.

Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom.

Oceanographic lidar can provide remote estimates of the vertical distribution of suspended particles in natural waters, potentially revolutionizing our ability to characterize marine ecosystems and properly represent them in models of upper ocean biogeochemistry. However, lidar signals exhibit complex dependencies on water column inherent optical properties (IOPs) and instrument characteristics, which complicate efforts to derive meaningful biogeochemical properties from lidar return signals. In this study, we used a ship-based system to measure the lidar attenuation coefficient (α) and linear depolarization ratio (δ) across a variety of optically and biogeochemically distinct water masses, including turbid coastal waters, clear oligotrophic waters, and calcite rich waters associated with a mesoscale coccolithophore bloom. Sea surface IOPs were measured continuously while underway to characterize the response of α and δ to changes in particle abundance and composition. The magnitude of α was consistent with the diffuse attenuation coefficient (Kd), though the α versus Kd relationship was nonlinear. δ was positively related to the scattering optical depth and the calcite fraction of backscattering. A statistical fit to these data suggests that the polarized scattering properties of calcified particles are distinct and contribute to measurable differences in the lidar depolarization ratio. A better understanding of the polarized scattering properties of coccolithophores and other marine particles will further our ability to interpret polarized oceanographic lidar measurements and may lead to new techniques for measuring the material properties of marine particles remotely.

Modeling study of scattering and absorption properties of tar-ball aggregates.

Atmospheric tar balls (TBs) form an important class of atmospheric brown carbon (BrC) particulates. The morphology of the individual TBs is typically described as amorphous and nearly spherical. However, several studies reported observations of TBs aggregated with other aerosols or agglomerations consisting of up to tens of individual TBs. We use the superposition $T$T-matrix method to compute the scattering matrix elements and optical cross sections for a variety of TB aggregates, each of which is composed of a number of monomers whose sizes follow a lognormal distribution. The results for a TB aggregate can differ fundamentally from those calculated for two simplified models commonly used in climate modeling; viz., the external mixture of TBs and the respective volume-equivalent sphere model. Clustering of individual TBs into an aggregate can either enhance or weaken absorption depending on the wavelength, the monomer size, and how absorptive the BrC material is. In the case of strongly absorptive BrC, aggregation results in enhanced absorption only at 1064 nm, while at 355 and 532 nm TB aggregates become less effective absorbers relative to the corresponding external mixtures. The effect of aggregation is always to increase the single-scattering albedo and asymmetry parameter, sometimes more than tenfold. The significant scattering-matrix differences between a TB aggregate, the "equivalent" external mixture, and the volume-equivalent sphere model demonstrate the failure of the conventional Lorenz-Mie theory to represent the scattering properties of morphologically complex BrC aerosols. We show that TB aggregates can help explain exceptionally strong and spectrally dependent lidar depolarization ratios reported in several recent studies.

Improved cloud-phase determination of low-level liquid and mixed-phase clouds by enhanced polarimetric lidar

Abstract. The unambiguous retrieval of cloud phase from polarimetric lidar observations is dependent on the assumption that only cloud scattering processes affect polarization measurements. A systematic bias of the traditional lidar depolarization ratio can occur due to a lidar system's inability to accurately measure the entire backscattered signal dynamic range, and these biases are not always identifiable in traditional polarimetric lidar systems. This results in a misidentification of liquid water in clouds as ice, which has broad implications on evaluating surface energy budgets. The Clouds Aerosol Polarization and Backscatter Lidar at Summit, Greenland employs multiple planes of linear polarization, and photon counting and analog detection schemes, to self evaluate, correct, and optimize signal combinations to improve cloud classification. Using novel measurements of diattenuation that are sensitive to both horizontally oriented ice crystals and counting system nonlinear effects, unambiguous measurements are possible by over constraining polarization measurements. This overdetermined capability for cloud-phase determination allows for system errors to be identified and quantified in terms of their impact on cloud properties. It is shown that lidar system dynamic range effects can cause errors in cloud-phase fractional occurrence estimates on the order of 30 % causing errors in attribution of cloud radiative effects on the order of 10–30 %. This paper presents a method to identify and remove lidar system effects from atmospheric polarization measurements and uses co-located sensors at Summit to evaluate this method. Enhanced measurements are achieved in this work with non-orthogonal polarization retrievals as well as analog and photon counting detection facilitating a more complete attribution of radiative effects linked to cloud properties.

Upper air thermal inversion and their impact on the summer monsoon rainfall over Goa – A case study

Profiles of periodic GPS Radiosonde ascends collected from a station at the west coast of India (Goa) during summer monsoon months (June to September) of 2009 and 2013 have been used to analyze the thermal inversion statistics at various heights and their repercussions on the regional weather is studied. The interaction of contrasting air masses over the northern Arabian Sea often produces a two layer structure in the lower 5000 m close to the coastal station with warm and dusty air (Summer Shamal) occupying the space above the cool and moist Low Level Jet (LLJ) by virtue of their density differences. The warm air intrusion creates low lapse rate pockets above LLJ and modifies the gravitational stability strong enough to inhibit convection. It is observed that the inversion occurring in the lower 3000 m layer with an optimum layer thickness of 100–200 m has profound influence on the weather beneath it. We demonstrated the validity of the proposed hypothesis by analyzing the collocated data from radiosonde, lidar and the rain gauge during 16th July 2013 as a case study. The lidar depolarization ratio provides evidence to support the two layer structure in the lidar backscatter image. The presence of dust noticed in the two layer interface hints the intrusion of warm air that makes the atmosphere stable enough to suppress convection. The daily rainfall record of 2013 surprisingly coincides with the patterns of a regional break like situation centered at 16th July 2013 in Goa.

Study of African Dust with Multi-Wavelength Raman Lidar During “Shadow” Campaign in Senegal

West Africa and the adjacent oceanic regions are very important locations for studying dust properties and their influence on weather and climate. The SHADOW (Study of SaHAran Dust Over West Africa) campaign is performing a multi-scale and multi-laboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instruments at an observation site located at IRD (Institute for Research and Development) Center, Mbour, Senegal (14°N, 17°W). In this paper, we present the results of lidar measurements performed during the first phase of SHADOW which occurred in March-April, 2015. The multiwavelength Mie-Raman lidar acquired 3β+2α+1δ measurements during this period. This set of measurements has permitted particle intensive properties such as extinction and backscattering Angstrom exponents (BAE) for 355/532 nm wavelengths corresponding lidar ratios and depolarization ratio at 532 nm to be determined. The backscattering Angstrom exponent during the dust episodes decreased to ~-0.7, while the extinction Angstrom exponent though being negative, was greater than -0.2. Low values of BAE can likely be explained by an increase in the imaginary part of the dust refractive index at 355 nm compared to 532 nm.

Observation and simulation study of atmospheric aerosol nonsphericity over the Loess Plateau in northwest China

Aerosol nonsphericity, which is not well depicted in model calculations, seriously affects aerosol optical properties and subsequently alters the radiative forcing of the earth–atmosphere system. Based on aerosol backscattering linear depolarization ratio data observed by a polarization lidar at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from September 2009 to August 2012 and numerical computations, the spatial and temporal distribution of the aerosol depolarization ratio, parameterization of the derived aspect ratio and influence of water vapor on aerosol nonsphericity were investigated. Aerosol nonsphericity varied considerably by season, with a pronounced maximum in the spring, when more nonspherical aerosols were transported upward to the free troposphere; moreover, the column-averaged lidar depolarization ratios were 0.13, 0.09, 0.08 and 0.10 for the spring, summer, autumn and winter, respectively. The derived aerosol aspect ratio, a simplified parameter that describes the particle nonsphericity, ranged from 1.00 to 1.30 and peaked at approximately 1.06. A modified log-normal function, which was fitted to the frequency distribution of the derived aspect ratios, yielded a log-normal distribution parameterization for this parameter and provided a better shape input for the aerosol optical modeling. The monthly averaged aspect ratios reached a maximum of 1.13 during the spring and a minimum of 1.04 in autumn. The depolarization ratios decreased significantly with column-integrated increasing precipitable water in summer when there was sufficient precipitable water.

Wind vector retrieval algorithm from spaceborne lidar data

The principal purpose of this paper is to extract entire sea surface wind’s information from spaceborne lidar, and particularly to utilize a appropriate algorithm for removing the interference information due to whitecaps and subsurface water. Wind speeds are obtained through empirical relationship with sea surface mean square slopes. Wind directions are derived from relationship between wind speeds and wind directions implied in CMOD5n geophysical models function (GMF). Whitecaps backscattering signals were distinguished with the help of lidar depolarization ratio measurements and rectified by whitecaps coverage equation. Subsurface water backscattering signals were corrected by means of inverse distance weighted (IDW) from neighborhood non-singular data with optimal subsurface water backscattering calibration parameters. To verify the algorithm reliably, it selected NDBC’s TAO buoy-laying area as survey region in camparison with buoys’ wind field data and METOP satellite ASCAT of 25 km single orbit wind field data after temporal-spatial matching. Validation results showed that the retrieval algorithm works well in terms of root mean square error (RMSE) less than 2m/s and wind direction’s RMSE less than 21 degree.

Influence of biogenic pollen on optical properties of atmospheric aerosols observed by lidar over Gwangju, South Korea

For the first time, optical properties of biogenic pollen, i.e., backscatter coefficients and depolarization ratios at 532 nm were retrieved by lidar observations. The extinction coefficient was derived with the assumption of possible values of the extinction-to-backscatter (lidar) ratio. We investigate the effect of the pollen on the optical properties of the observed atmospheric aerosols by comparing lidar and sun/sky radiometer measurements carried out at the lidar site. The observations were made with a depolarization lidar at the Gwangju Institute of Science & Technology (GIST) in Gwangju, Korea (35.13°N, 126.50°E) during an intensive observational period that lasted from 5 to 7 May 2009. The pollen concentration was measured with a Burkard trap sampler at the roof top of the Gwangju Bohoon hospital which is located 1 km away from the lidar site. During the observation period, high pollen concentrations of 1360, 2696, and 1952 m−3 day−1 were measured on 5, 6, and 7 May, respectively. A high lidar depolarization ratio caused by biogenic pollen was only detected during daytime within the planetary boundary layer which was at 1.5–2.0 km height above ground during the observational period. The contribution of biogenic pollen to the total backscatter coefficient was estimated from the particle depolarization ratio. Average hourly values of pollen optical depth were retrieved by integrating the pollen extinction coefficients. We find average values of 0.062 ± 0.037, 0.041 ± 0.028 and 0.067 ± 0.036 at 532 nm on 5, 6, and 7 May, respectively. The contribution of pollen optical depth to total aerosol optical depth was 2–34%. The sun/sky radiometer data show that biogenic pollen can affect optical properties of atmospheric aerosol by increasing aerosol optical depth and decreasing the Ångström exponent during daytime during the season of high pollen emission.

Interpretation of Accurate UV Polarization Lidar Measurements: Application to Volcanic Ash Number Concentration Retrieval

Abstract In this paper, accurate UV polarization measurements are performed on a volcanic ash cloud after long-range transport at Lyon, France (45.76°N, 4.83°E). The volcanic particles are released from the mid-April 2010 eruption of the Eyjafjallajökull Icelandic volcano (63.63°N, 19.62°W). The aerosol UV depolarization, which arises from nonspherical volcanic ash particles, serves as an independent means to discriminate ash from nonash particles in the volcanic cloud. This discrimination is only feasible if the intrinsic ash particle depolarization ration δash is accurately determined. In this paper, the δash value [δash = (40.5 ± 2.0)%] is derived from literature laboratory measurements on a scattering matrix to ensure ash particle specificity. It is shown that traditional approaches, based on direct lidar depolarization ratio δa measurements, are only valid very close to the source region, because δa may be very different from δash, when nonash particles are present. For the first time, observed lidar depolarization ratios, in the range from a few percent to 40%, are hence interpreted in comparison with δash taken as a reference. It is then shown how to use the sensitive and accurate UV polarization measurements to access to the size-averaged number concentration vertical profile of volcanic ash particles in the troposphere. Vertical profiles of the backscattering coefficient specific to volcanic ash particles, providing altitude-resolved ash particle number concentrations, are presented in the troposphere after optical scattering computation. This new methodology can be applied to other aerosols events and for other optical remote sensing experiments.

A new method for retrieval of the extinction coefficient of water clouds by using the tail of the CALIOP signal

Abstract. A method is developed based on Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) level 1 attenuated backscatter profile data for deriving the mean extinction coefficient of water droplets close to cloud top. The method is applicable to low level (cloud top <2 km), opaque water clouds in which the lidar signal is completely attenuated beyond about 100 m of penetration into the cloud. The photo multiplier tubes (PMTs) of the 532 nm detectors (parallel and perpendicular polarizations) of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) both exhibit a non-ideal recovery of the lidar signal after striking a strongly backscattering target (such as water cloud or surface). Therefore, the effects of any transient responses of CALIOP on the attenuated backscatter profile of the water cloud must first be removed in order to obtain a reliable (validated) attenuated backscatter profile. Then, the slope of the exponential decay of the validated water cloud attenuated backscatter profile, and the multiple scattering factor are used for deriving the mean extinction coefficient of low-level water cloud droplets close to cloud top. This novel method was evaluated and compared with the previous method which combined the cloud effective radius (3.7-μm) reported by MODIS with the lidar depolarization ratios measured by CALIPSO to estimate the mean extinction coefficient. Statistical results show that the extinction coefficients derived by the new method based on CALIOP alone agree reasonbably well with those obtained in the previous study using combined CALIOP and MODIS data. The mean absolute relative difference in extinction coefficient is about 13.4%. An important advantage of the new method is that it can be used to derive the extinction coefficient also during night time, and it is also applicable when multi-layered clouds are present. Overall, the stratocumulus dominated regions experience larger day-night differences which are all negative and seasonal. However, a contrary tendency consisted in the global mean values. The global mean cloud water extinction coefficients during different seasons range from 26 to 30 km−1, and the differences between day and night time are all positive and small (about 1–2 km−1). In addition, the global mean layer-integrated depolarization ratios of liquid water clouds during different seasons range from 0.2 to 0.23, and the differences between day and night also are small, about 0.01.

Depolarization ratio measurement using single photomultiplier tube in micropulse lidar

The conventional dual polarization micropulse lidar uses two separate photomultiplier tubes (PMT) to detect both the copolarized and cross-polarized beam. The prominent sources of error in the depolarization ratio measurement are mismatch in PMT, improper selection of discriminator threshold and unequal PMT high voltage. In the present work a technique for the measurement of lidar depolarization ratio using only one PMT sensor has been developed. The same PMT detects both copolarized and cross-polarized lidar backscatter. A stepper motor is used along with the mirrors to bring both the received polarization signals over the PMT window. Application of the same PMT minimizes the error caused in the depolarization ratio measurement due to error in photon counting of an individual channel. The design description of this technique along with the preliminary results depicting its functionality has been mentioned in this article.