Backscatter Ratio Research Articles

Inherent optical properties of suspended particulate matter in the southern Baltic Sea in relation to the concentration, composition and characteristics of the particle size distribution; new forms of multicomponent parameterizations of optical properties

This paper reports on detailed studies of the relationship between the inherent optical properties (IOPs) of suspended particulate matter populations occurring in southern Baltic Sea waters, and the basic biogeochemical and morphological quantities enabling these populations to be generally characterized. Specifically, these IOPs are the spectral absorption coefficients by all particulate matter, phytoplankton pigments and detritus, the spectral scattering and backscattering coefficients by particles, and the backscattering ratio. The biogeochemical properties of suspended matter populations are characterized by measuring concentrations of suspended particulate matter (SPM), particulate organic matter (POM) and particulate inorganic matter (PIM), as well as concentrations of various phytoplankton pigments, including chlorophyll a (Chla). We also take particle size distributions (PSDs) into account. This diverse empirical material is used to describe how the typical relationships between the IOPs of suspended matter and the concentrations of the main water constituents (SPM or Chla) can be influenced by differences in the general composition and size distributions of the suspended matter. We statistically describe the possible impact on these relationships of the variability of composition ratios, such as POM/SPM and Chla/SPM, as well as the impact of the variability of certain characteristics calculated on the basis of PSDs: the average particle diameter weighted by the particle's projected area (DA), the average apparent density of particles (ρa), the average intracellular concentration of chlorophyll a (ci), and the corresponding products of these quantities. We also demonstrate the significant variability of the specific IOPs (SIOPs) by attempting to determine them in the classical way for our southern Baltic data, and we give a possible interpretation of this variability. On the basis of these analyses, we derive new, multicomponent IOP parameterizations that are functions of several variables. These new parameterizations enable the IOPs of suspended matter to be characterized and predicted with better accuracy than with the often-used standard one-component relationships.

Australian Black Summer Smoke Observed by Lidar at the French Antarctic Station Dumont d’Urville

AbstractIn the follow‐up of the Australian ”Black Summer” event that persisted from August 2019 to March 2020, we present the optical properties of the stratospheric aerosols injected into the atmosphere by these wildfires. The outbreak of pyrocumulonimbus (PyroCb) activity triggered between 2019/12/29 and 2020/01/04 has raised the stratospheric aerosol load of the Southern Hemisphere to unprecedented levels. Long‐range transport brought some of the plumes down to the Antarctic region, where general circulation patterns kept them circling around the continent. The 532 nm Rayleigh/Mie/Raman ground‐based lidar of the French Antarctic station Dumont d’Urville (66.6°S–140°E) acquired unprecedented time series of these carbonaceous aerosols starting approximately 20 days after the injection and up to the most recent measurements in October 2019 where local radiosonde reported anomalous ozone depletion as compared to the decadal average. The lidar provides a first and unique time series at high vertical and temporal resolution, complemented by satellite measurements from Ozone Monitoring Instrument, Ozone Mapping and Profiler Suite, and Microwave Limb Sounder. Aerosol backscatter ratio decreases from 1.9 to 1.2 between January and June 2020. Aerosol origin and persistence are characterized, as well as their optical properties and vertical distribution over several months.

Wetlands Delineation with Various Land Covers Using SAR Sentinel-1A Surface Backscatter Ratio

ABSTRAK Wilayah Kabupaten Ogan Komering Ilir (OKI) yang didominasi lahan basah memerlukan perencanaan tata guna lahan yang tepat untuk mendukung program kesejahteraan dan pelestarian lingkungan. Program tersebut perlu disusun berdasarkan data lahan basah yang tepat dalam penetapan tata guna lahan. Artikel ini mengeksplorasi pemanfaatan data satelit Synthetic Aperture Radar (SAR) Sentinel-1A untuk mengidentifikasi area lahan basah dengan berbagai tutupan lahan untuk menyediakan peta lahan basah skala menengah secara cepat. Peta kelembaban tanah diambil dari platform SEPAL untuk memantau pola kelembaban tanah sepanjang 2015–2017. Profil hamburan balik polarisasi VV (?VV) dan polarisasi VH (?VH) dianalisis untuk data sentinel-1 dari tahun 2015 hingga 2018. Rasio antara nilai VV dan VH (?VV/?VH) serta tekstur VV dan VH digunakan sebagai acuan untuk menentukan lahan basah; ditentukan pada analisis eksperimental profil kelembaban tanah, konstanta dielektrik, dan nilai koefisien hamburan balik dari penelitian lain. Rasio VH/?VV yang rendah (berkisar antara 1,3 dan 1,6) terletak di bagian utara Kabupaten OKI dan sebagian besar ditutupi oleh tanaman akasia. Estimasi ini sejalan dengan peta kesatuan hidrologis gambut dan data validasi berdasarkan ground check lapangan. Secara keseluruhan, rasio ?VH/?VV tahunan bernilai relatif sama, dengan nilai variasi yang kecil. Studi ini menemukan kemampuan Sentinel-1A untuk keperluan klasifikasi tipe vegetasi lahan basah yang akurat, terutama lahan basah bervegetasi herba atau semak, tetapi tidak untuk lahan basah bervegetasi tinggi. Kata kunci : SAR Sentinel-1A, lahan basah, backscatter, program konservasi, keberlanjutan ABSTRACT Ogan Komering Ilir (OKI) Regency area, dominated by wetlands, requires appropriate land use planning to support welfare and environmental conservation programs. This article explores the utilization of Synthetic Aperture Radar (SAR) Sentinel-1A satellite data to identify wetland areas with various land covers to provide a medium scale wetland map quickly. The soil moisture maps were captured from the SEPAL platform to monitor soil moisture patterns throughout 2015–2017. The backscattering profiles of VV (?VV) polarization and VH polarization (?VH) were then analyzed for data sentinel-1A from 2015 to 2018. The ratio between VV and VH (?VV/?VH) values and textures of VV and VH is used as a reference to determine the wetland area; determined on an experimental analysis of soil moisture profile, dielectric constants, and backscattering coefficient values from other studies. The low ?VH/?VV ratio (ranges between 1.3 and 1.6) is located in the northern OKI regency and is mainly covered by acacia plantations. These estimations align with the Peat Hydrology Unit maps and ground check as validation data. Overall, the ?VH/?VV ratio is relatively the same yearly, with a small variation value. This study found the capability of Sentinel-1A for the accurate classification of wetland vegetation types, primarily herbaceous or shrubby vegetated wetlands, but not for high-vegetated wetlands. Keywords : SAR Sentinel-1A, wetland, backscatter, conservation programs, sustainability

Scavenging of Black Carbon Aerosols by Radiation Fog in Urban Central China

Radiation fog episodes are characterized by aerosol radiative properties measured at Hefei in urban central China, which hopefully benefits numerical weather prediction and air quality improvement for local governments. In this study, a high mean aerosol optical depth (AOD) is seen over Hefei during the sampling period, whereas an AOD of ~3.0 at 550 nm is observed during the fog episodes. We redefine the fog scavenging coefficient based on its starting and ending points in time, and a black carbon (BC) scavenging coefficient of 30% is observed. Meanwhile, the fog process cannot reduce aerosol number concentrations at size bins between 0.5 and 0.6 μm, whereas a mean particle scavenging coefficient of 21% at sizes within 0.6–1 μm is seen. Significantly large median aerosol scattering coefficient (2690 Mm−1) and absorption coefficient (446 Mm−1) at 550 nm, and low scattering Angstrom exponent in fog are observed, while distinctive particle size distributions between fog and haze are shown. Particle mean size distribution in fog is lower than that in haze at size bins between 0.7 and 2.1 μm, whereas the reverse is true for sizes within 0.5–0.7 μm and larger than 2.1 μm. Aerosol scattering during fog episodes undergoes a bigger increase than particle absorption, and this increase of scattering in fog is even higher than in haze. Median single scattering albedos of 0.86, 0.82, and 0.76 at 550 nm and aerosol radiative forcing efficiencies of −15.0, −14.0, and −10.0 W/m2 are seen for fog, haze and clear periods, respectively, and more negative radiative forcing efficiency emphasizes the significance of fog episodes on climate forcing. Our study clearly reveals the changes of aerosol radiative properties during radiation fog, particularly a synchronous variation of fog aerosol backscattering ratio with the visibility, indicating that more large particles are formed with fog becoming thicker and are scavenged with the dissipation of fog.

The SOLA Award in 2021

The Editorial Committee of Scientific Online Letters on the Atmosphere (SOLA) gives The SOLA Award to outstanding paper(s) published each year. We are pleased to announce that The SOLA Award in 2021 will be presented to the paper by Drs. Koichi Shiraishi and Takashi Shibata, entitled “Seasonal variation in high arctic stratospheric aerosols observed by lidar at Ny Ålesund, Svalbard between March 2014 and February 2018” (Shiraishi and Shibata 2021). Stratospheric aerosols play essential roles in radiative and chemical processes in the atmosphere and impact the climate of the Earth. The lidar observations of the temporal and spatial variations of the aerosols will provide essential information to understand the atmospheric processes related to the aerosols. Thus, there have been many studies on lidar observations of aerosols in the low and middle latitude regions. However, the observations in the Arctic are limited. In particular, there are no long-term observations of stratospheric aerosols throughout the year in the high Arctic. This study investigates the seasonal variations of stratospheric aerosols in the high Arctic by using long-term observations for about four years at Ny Ålesund, Svalbard. By carefully removing data affected by polar stratospheric clouds and volcanic eruption, the analysis shows the robust characteristics of the seasonal variations of background aerosols in the stratosphere. It is found that the backscattering ratio takes maximum values at altitudes between 13 and 16 km from December to March and at altitudes between 17 and 20 km from April to November. This observational evidence is highly evaluated in presenting the seasonal variations of stratospheric aerosols in the high Arctic, which will be helpful to understand the dynamical and chemical processes in the stratosphere. The Editorial Committee of SOLA highly evaluates the significance of this study and therefore presents “The SOLA Award.”

Increase in the aerosol backscattering ratio at lower mesosphere in 2019-2021 and its influence on the temperature measurements by the Rayleigh method

Increase in the aerosol backscattering ratio at lower mesosphere in 2019-2021 and its influence on the temperature measurements by the Rayleigh method

Characterization of suspended particulate matter in contrasting coastal marine environments with angle-resolved polarized light scattering measurements.

Optical proxies based on light scattering measurements have potential to improve the study and monitoring of aquatic environments. In this study, we evaluated several optical proxies for characterization of particle mass concentration, composition, and size distribution of suspended particulate matter from two contrasting coastal marine environments. We expanded upon our previous study of Southern California coastal waters, which generally contained high proportions of organic particles, by conducting angle-resolved polarized light scattering measurements in predominantly turbid and inorganic-particle dominated Arctic coastal waters near Prudhoe Bay, Alaska. We observed that the particulate backscattering coefficient bbp was the most effective proxy for the mass concentration of suspended particulate matter (SPM) when compared with particulate scattering and attenuation coefficients bp and cp. Improvements were seen with bbp as a proxy for the concentration of particulate organic carbon (POC), although only if particulate assemblages were previously classified in terms of particle composition. We found that the ratio of polarized-light scattering measurements at 110º and 18º was superior in performance as a proxy for the composition parameter POC/SPM in comparison to the particulate backscattering ratio bbp/bp. The maximum value of the degree of linear polarization DoLPp,max observed within the range of scattering angles 89°-106° was found to provide a reasonably good proxy for a particle size parameter (i.e., 90th percentile of particle volume distribution) which characterizes the proportions of small- and large-sized particles. These findings can inform the development of polarized light scattering sensors to enhance the capabilities of autonomous platforms.

Afterpulse correction for micro-pulse lidar to improve middle and upper tropospheric aerosol measurements

Micro-pulse lidar (MPL) measurements have been widely used in atmospheric research over the past few decades. However, the MPL afterpulse noise has a large impact on the MPL aerosol measurement in the middle and upper troposphere, and an effective correction method is still lacking. Here, a new afterpulse correction approach is presented by using measurements with low-level optically thick clouds to act as the lids blocking atmospheric signals beyond the clouds completely. Examples are provided to illustrate the effectiveness of this correction method. Using one-year 2014 MPL measurements at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) site, the impact of the correction on the aerosol measurements is quantified. The relative error (RE, %) of attenuated backscattering ratio (ABR) without the afterpulse correction is 30% and 190% at the height of 1.00 km and 9.00 km (AGL), and the RE is larger for weaker aerosol signals (ABR<2). The RE of linear depolarization ratio (LDR), which is significantly higher than that of ABR for a given aerosol layer, is highly non-linear above 3.00 km, with a value of 300%-900% for weak aerosols (ABR<2) above 3.00 km. Therefore, the afterpulse correction is critical for the middle and upper tropospheric aerosol observations. We demonstrated that our afterpulse correction can provide reasonable particle depolarization ratio (PDR) to properly identify dust aerosols. The newly developed method can be applied to long-term MPL measurements to support aerosol and mixed-phase/ice cloud interaction study at the NSA site.

Vertical distributions of aerosol microphysical and optical properties based on aircraft measurements made over the Loess Plateau in China

Aerosol microphysical properties, scattering and absorption characteristics, and in particular, the vertical distributions of these parameters over the eastern Loess Plateau, were analyzed based on aircraft measurements made in 2020 during a summertime aircraft campaign in Shanxi, China. Data from six flights were analyzed. Statistical characteristics and vertical distributions of aerosol concentration, particle size, optical properties, including aerosol scattering coefficient (σsp), backscattering ratio (βsc), Ångström exponent (α), single-scattering albedo (SSA), partially-integrated aerosol optical depth (PAOD), and black carbon concentration (BCc), were obtained and discussed. Mean values of aerosol particle number concentration (Na), particle volume concentration (Va), mass concentration (Ma), surface concentration (Sa), and particle effective diameter (EDa) were 854.92 cm−3, 13.37 μm3 cm−3, 20.06 μg/m3, 170.08 μm3 cm−3, and 0.47 μm, respectively. Mean values of BCc, σsp(450, 525, 635 nm), βsp(525 nm), α(635/450), and SSA were 1791.66 ng m−3, 82.37 Mm−1 at 450 nm, 102.57 Mm−1 at 525 nm, 126.60 Mm−1 at 635 nm, 0.23, 1.47, and 0.92, respectively. Compared with values obtained in 2013, Na decreased by 60% and Ma decreased by 45%, but the scattering coefficients increased in different degrees. In the vertical direction, aerosol concentrations were higher at lower altitudes, decreasing with height. Vertical profiles of σsp, βsp, α(635/450), and BCc measured during the six flights were examined. Two peaks in Na were identified near the top of the boundary layer and between 2000 and 2200 m. Fine particles with EDa smaller than 0.8 μm are dominant in the boundary layer and coarse aerosols existed aloft. Aerosol scattering properties and BCc in the lowest layer of the atmosphere contributed the most to the total aerosol radiative forcing. SSA values were greater than 0.9 below 2500 m, with lower values at higher levels of the atmosphere. On lightly foggy days, SSA values were greater than 0.9, and aerosols played a cooling role in the atmosphere. On hazy days, lower-level SSA values were generally greater than 0.85, with aerosols likely having a warming effect on the atmosphere. 48-hour backward trajectories of air masses during the observation days showed that the majority of aerosol particles in the lower atmosphere originated from local or regional pollution emissions, contributing the most to the total aerosol loading and leading to high values of aerosol concentration and radiative forcing.

Classification and characteristic analysis of the clouds and dust in a dust-carrying precipitation process based on multi-source remote sensing observations

A dust-carrying precipitation event that took place on May 11, 2020 in North China was observed by remote sensing methods including a micropulse lidar (MPL), millimeter-wavelength cloud radar (MMCR) and microwave radiometer from Yanjiaping (YJP) Station, which is the center of the transmission path of this dust storm, and it is the only super station capable of vertical observation of aerosols, clouds and precipitation in North China. The study researched the macro and microphysical characteristics of clouds and sand dust, as well as the possible impact of sand dust on clouds. It is found that dust has a great influence on the CBH measurement of ceilometer but little on the MMCR. The normalized relative backscatter (NRB) and depolarization ratio (DR) of the MPL were comparatively analyzed in terms of the mass concentration of PM10, revealing that the mass concentration of PM10 is linearly positively correlated to the NRB, with the correlation coefficient up to 0.9273. It was also discovered that the 100 μg/m3 is a critical point for the mass concentration of dust particles, and when the PM10 was greater than 100 μg/m3, the DR would fluctuate within a narrow range of 0.32–0.38. A classification method that could be used to distinguish among clear sky, non-dust aerosols, clouds, dust, and cloud-dust mixing zones was proposed. Based on the classification results, the immersion freezing process that might occur in the cloud-dust mixing area was analyzed using Z-LDR and V-SW by reference to the vertical distribution of temperature and humidity.

The Asian tropopause aerosol layer within the 2017 monsoon anticyclone: microphysical properties derived from aircraft-borne in situ measurements

Abstract. The Asian summer monsoon is an effective pathway for aerosol particles and precursors from the planetary boundary layer over Central, South, and East Asia into the upper troposphere and lower stratosphere. An enhancement of aerosol particles within the Asian monsoon anticyclone (AMA), called the Asian tropopause aerosol layer (ATAL), has been observed by satellites. We discuss airborne in situ and remote sensing observations of aerosol microphysical properties conducted during the 2017 StratoClim field campaign within the AMA region. The aerosol particle measurements aboard the high-altitude research aircraft M55 Geophysica (maximum altitude reached of ∼20.5 km) were conducted with a modified ultra-high-sensitivity aerosol spectrometer – airborne (UHSAS-A; particle diameter detection range of 65 nm to 1 µm), the COndensation PArticle counting System (COPAS, detecting total concentrations of submicrometer-sized particles), and the New Ice eXpEriment – Cloud and Aerosol Spectrometer with Detection of POLarization (NIXE-CAS-DPOL). In the COPAS and UHSAS-A vertical particle mixing ratio (PMR) profiles and the size distribution profiles (for number, surface area, and volume concentration), the ATAL is evident as a distinct layer between ∼370 and 420 K potential temperature (Θ). Within the ATAL, the maximum detected PMRs (from the median profiles) were ∼700 mg−1 for particle diameters between 65 nm and 1 µm (UHSAS-A) and higher than 2500 mg−1 for diameters larger than 10 nm (COPAS). These values are up to 2 times higher than those previously found at similar altitudes in other tropical locations. The difference between the PMR profiles measured by the UHSAS-A and the COPAS indicate that the region below the ATAL at Θ levels from 350 to 370 K is influenced by the nucleation of aerosol particles (diameter <65 nm). We provide detailed analyses of the vertical distribution of the aerosol particle size distributions and the PMR and compare these with previous tropical and extratropical measurements. The backscatter ratio (BR) was calculated based on the aerosol particle size distributions measured in situ. The resulting data set was compared with the vertical profiles of the BR detected by the multiwavelength aerosol scatterometer (MAS) and an airborne miniature aerosol lidar (MAL) aboard the M55 Geophysica and by the satellite-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The data of all four methods largely agree with one another, showing enhanced BR values in the altitude range of the ATAL (between ∼15 and 18.5 km) with a maximum at 17.5 km altitude. By means of the AMA-centered equivalent latitude calculated from meteorological reanalysis data, it is shown that such enhanced values of the BR larger than 1.1 could only be observed within the confinement of the AMA.

Recovery of the first ever multi-year lidar dataset of the stratospheric aerosol layer, from Lexington, MA, and Fairbanks, AK, January 1964 to July 1965

Abstract. We report the recovery and processing methodology of the first ever multi-year lidar dataset of the stratospheric aerosol layer. A Q-switched ruby lidar measured 66 vertical profiles of 694 nm attenuated backscatter at Lexington, Massachusetts, between January 1964 and August 1965, with an additional nine profile measurements conducted from College, Alaska, during July and August 1964. We describe the processing of the recovered lidar backscattering ratio profiles to produce mid-visible (532 nm) stratospheric aerosol extinction profiles (sAEP532) and stratospheric aerosol optical depth (sAOD532) measurements, utilizing a number of contemporary measurements of several different atmospheric variables. Stratospheric soundings of temperature and pressure generate an accurate local molecular backscattering profile, with nearby ozone soundings determining the ozone absorption, which are used to correct for two-way ozone transmittance. Two-way aerosol transmittance corrections are also applied based on nearby observations of total aerosol optical depth (across the troposphere and stratosphere) from sun photometer measurements. We show that accounting for these two-way transmittance effects substantially increases the magnitude of the 1964/1965 stratospheric aerosol layer's optical thickness in the Northern Hemisphere mid-latitudes, then ∼ 50 % larger than represented in the Coupled Model Intercomparison Project 6 (CMIP6) volcanic forcing dataset. Compared to the uncorrected dataset, the combined transmittance correction increases the sAOD532 by up to 66 % for Lexington and up to 27 % for Fairbanks, as well as individual sAEP532 adjustments of similar magnitude. Comparisons with the few contemporary measurements available show better agreement with the corrected two-way transmittance values. Within the January 1964 to August 1965 measurement time span, the corrected Lexington sAOD532 time series is substantially above 0.05 in three distinct periods, October 1964, March 1965, and May–June 1965, whereas the 6 nights the lidar measured in December 1964 and January 1965 had sAOD values of at most ∼ 0.03. The comparison with interactive stratospheric aerosol model simulations of the Agung aerosol cloud shows that, although substantial variation in mid-latitude sAOD532 are expected from the seasonal cycle in the stratospheric circulation, the Agung cloud's dispersion from the tropics would have been at its strongest in winter and weakest in summer. The increasing trend in sAOD from January to July 1965, also considering the large variability, suggests that the observed variations are from a different source than Agung, possibly from one or both of the two eruptions that occurred in 1964/1965 with a Volcanic Explosivity Index (VEI) of 3: Trident, Alaska, and Vestmannaeyjar, Heimaey, south of Iceland. A detailed error analysis of the uncertainties in each of the variables involved in the processing chain was conducted. Relative errors for the uncorrected sAEP532 were 54 % for Fairbanks and 44 % Lexington. For the corrected sAEP532 the errors were 61 % and 64 %, respectively. The analysis of the uncertainties identified variables that with additional data recovery and reprocessing could reduce these relative error levels. Data described in this work are available at https://doi.org/10.1594/PANGAEA.922105 (Antuña-Marrero et al., 2020a).

Modeling the Dynamics of Dense Pyroclastic Flows on Venus: Insights Into Pyroclastic Eruptions

AbstractOn Venus, relatively young deposits near volcanic and coronal summits with unique radar characteristics have been proposed to be emplaced by pyroclastic density currents (PDCs). The proposed units are laterally extensive, long‐runout deposits showing moderate to high radar backscatter and circular polarization ratio in 12.6 cm wavelength synthetic aperture radar data. Previous studies have hypothesized that a recent resumption of volcanic activity in the form of PDC‐forming eruptions could have emplaced these deposits. We model the dynamics of dense PDCs using a 2D, depth‐averaged framework focusing on regions where stereo‐derived topography coverage is available; this includes the flanks of Irnini Mons, Anala Mons, Didilia Corona, and Pavlova Corona. Two different mechanisms of initiation which includes impulsive collapse of an eruption column and sustained pyroclastic fountaining are considered. The results emphasize the importance of pyroclastic flow fluidization via high pore pressure in emplacing long‐runout deposits along gently sloping (<2°) volcanic flanks. We also show that collapse of columns >1.2–1.4 km tall as well as pyroclastic fountains lasting >400 s with fountain heights of 50 m are capable of generating pyroclastic flows that could emplace some of the smaller deposits studied. For the large deposits at Irnini Mons, more energetic flows resulting from taller column heights would be necessary; the dynamics of such flows under Venus's conditions are not well understood. Distinguishing between the two initiation styles, that is, column collapse and sustained fountaining is not feasible with currently available datasets and would require higher resolution imagery and topography data.

Potential of C-band Synthetic Aperture Radar Sentinel-1 time-series for the monitoring of phenological cycles in a deciduous forest

Annual time-series of the two satellites C-band SAR (Synthetic Aperture Radar) Sentinel-1A and 1B data over five years were used to characterize the phenological cycle of a temperate deciduous forest. Six phenological metrics of the start (SOS), middle (MOS) and end (EOS) of budburst and leaf expansion stage in spring, and the start (SOF), middle (MOF) and end (EOF) of leaf senescence in autumn were extracted using an asymmetric double sigmoid function (ADS) fitted to the time-series of the ratio (VV/VH) of backscattering at co-polarization VV (vertical–vertical) and at cross polarization VH (vertical-horizontal). Phenological metrics were also derived from other four vegetation proxies (Normalized Difference Vegetation Index NDVI time-series from Sentinel-2A and 2B images, and in situ measurements of NDVI measurements, Leaf Area Index LAI and litterfall temporal dynamics). These estimated phenological metrics were compared to phenological observations obtained by visual observations from the ground, achieved using binoculars by three inter-calibrated observers, on a bi-weekly basis during the budburst and weekly during the senescence. We observe a decrease in the backscattering coefficient (σ0) at VH cross polarization during the leaf development and the expansion phase in spring and an increase during the senescence phase, contrary to what is usually observed on various types of crops. In vertical polarization, σ0VV shows very little variation throughout the year. S-1 time-series of VV/VH ratio provide a good description of the seasonal vegetation cycle allowing the estimation of spring and autumn phenological metrics. Estimates provided by VV/VH of budburst dates using MOS criterion differ by approximately 8 days on average (mean average deviation) from phenological observations. During senescence phase, estimates using MOF criterion are later and deviate by about 20 days from phenological observations of leaf senescence while the differences are of the order of 2 to 4 days between the phenological observations and estimates based on in situ NDVI and LAI time-series, respectively. A deviation of about 7 days, comparable to that observed during budburst, is obtained between the estimates of senescence (MOF) from S-1 and those determined from the in situ monitoring of litterfall. While in spring, leaf emergence and expansion described by LAI or NDVI explain the increase of VV/VH (or the decrease of σ0VH), during senescence, S-1 VV/VH is decorrelated from LAI or NDVI and is better explained by litterfall temporal dynamics. This behavior resulted in a hysteresis phenomenon observed on the relationships between VV/VH and NDVI or LAI. For the same LAI or NDVI, the response of VV/VH is different depending on the phenological phase considered. This study shows the high potential offered by Sentinel-1 SAR C-band time-series for the detection of forest phenology, thus overcoming the limitations caused by cloud cover in optical remote sensing of vegetation phenology.

Seasonal variation of the sound-scattering zooplankton vertical distribution in the oxygen-deficient waters of the NE Black Sea

Abstract. At the northeastern Black Sea research site, observations from 2010–2020 allowed us to study the dynamics and evolution of the vertical distribution of mesozooplankton in oxygen-deficient conditions via analysis of sound-scattering layers associated with dominant zooplankton aggregations. The data were obtained with profiler mooring and zooplankton net sampling. The profiler was equipped with an acoustic Doppler current meter, a conductivity–temperature–depth probe, and fast sensors for the concentration of dissolved oxygen [O2]. The acoustic instrument conducted ultrasound (2 MHz) backscatter measurements at three angles while being carried by the profiler through the oxic zone. For the lower part of the oxycline and the hypoxic zone, the normalized data of three acoustic beams (directional acoustic backscatter ratios, R) indicated sound-scattering mesozooplankton aggregations, which were defined by zooplankton taxonomic and quantitative characteristics based on stratified net sampling at the mooring site. The time series of ∼ 14 000 R profiles as a function of [O2] at depths where [O2] < 200 µm were analyzed to determine month-to-month variations of the sound-scattering layers. From spring to early autumn, there were two sound-scattering maxima corresponding to (1) daytime aggregations, mainly formed by diel-vertical-migrating copepods Calanus euxinus and Pseudocalanus elongatus and chaetognaths Parasagitta setosa, usually at [O2] = 15–100 µm, and (2) a persistent monospecific layer of the diapausing fifth copepodite stages of C. euxinus in the suboxic zone at 3 µm < [O2] < 10 µm. From late autumn to early winter, no persistent deep sound-scattering layer was observed. At the end of winter, the acoustic backscatter was basically uniform in the lower part of the oxycline and the hypoxic zone. The assessment of the seasonal variability of the sound-scattering mesozooplankton layers is important for understanding biogeochemical processes in oxygen-deficient waters.

The subsurface biological structure of Southern Ocean eddies revealed by BGC-Argo floats

Mesoscale eddies play several key roles in the Southern Ocean by redistributing momentum, potential vorticity, heat and salt. However, the role of eddies in Southern Ocean biogeochemistry has received less attention. Previous studies have been based on satellites or models. In this study, Southern Ocean eddies were matched with BGC-Argo float profiles to investigate the subsurface biological structure of Southern Ocean eddies. We focused on four frontal regions in the Indian sector of the Southern Ocean where data density is greater than in the other basins. The seasonality and regional variability of subsurface chlorophyll and backscatter structures were revealed by calculating anomalies in eddies relative to outside of eddies, in each season and frontal region. Chlorophyll anomalies in the mixed layer were mostly positive for both cyclones and anticyclones in all regions in spring, summer and autumn. Anomalies in winter were close to zero. Eddy pumping was the most likely mechanism sustaining high chlorophyll in cyclones. Eddy-induced Ekman pumping, in combination with deep vertical mixing was likely the driver of enhanced chlorophyll in anticyclones. Backscatter anomalies were mostly consistent with chlorophyll anomalies. Chlorophyll to backscatter ratios (chlorophyll:bbp) were also calculated to help understand the role of light acclimation in the phytoplankton. There were strong subsurface maxima for in-eddy chlorophyll:bbp in spring and summer. Such structures were weak in autumn and winter, but chlorophyll:bbp in the upper 30 m in autumn and winter was mostly higher than spring and summer in the same region. Photo-acclimation to low light at depth was evident for both cyclones and anticyclones in spring and summer in the euphotic zone, while in autumn and winter it was observed throughout the upper water column. The majority of positive chlorophyll anomalies were due to both an increase of biomass and photo-acclimation while few cases were due to photo-acclimation only.

Optical properties and seasonal distribution of aerosol layers observed by lidar over Jinhua, southeast China

To explore the vertical distribution of the aerosol optical parameters over Jinhua City, southeast China, a Dual-wavelength Mie Polarization Raman Lidar (DMPRL) system was operated to obtain the atmospheric backscattering signals from June 2013 to May 2014. Profiles of aerosol occurrence frequency and aerosol optical parameters, including backscattering ratio, particle depolarization ratio, and color ratio, were retrieved from the lidar signals. The cases with a cloud base below 6 km and profiles on rainy days were eliminated. The multi-layer vertical aerosol structure was found in Jinhua area and the altitude-resolved optical properties varies with seasons. In autumn, winter and spring, the aerosol layer occurrence frequencies were higher than summer at 3–6 km, especially in spring. The mean particle depolarization ratio at 4–6 km exceeded 0.1 in autumn, winter, and spring, whereas it was <0.05 in summer. Aerosol with high color ratios appeared at 0–2 km in autumn. The analysis of one-year backward trajectories exhibits a dust dominating aerosol layer at 4–6 km in autumn, winter, and spring, a partially sea salt dominating aerosol layer at 0–2 km in autumn and a partially dust aerosol layer at 0–2 km in winter. Six cases were used to study the influence of aerosol advection and local sources vertical aerosol properties distribution. Through our analysis, aerosols at different altitudes are influenced by different sources, which are important for understanding the spatiotemporal characteristics of aerosol layers over Jinhua. The vertical distribution of aerosol properties in Jinhua area is affected by locally emitted aerosol , transported dust and ocean aerosol.

Californian Wildfire Smoke Over Europe: A First Example of the Aerosol Observing Capabilities of Aeolus Compared to Ground‐Based Lidar

AbstractIn September 2020, extremely strong wildfires in the western United States of America (i.e., mainly in California) produced large amounts of smoke, which was lifted into the free troposphere. These biomass‐burning‐aerosol (BBA) layers were transported from the US west coast toward central Europe within 3–4 days turning the sky milky and receiving high media attention. The present study characterizes this pronounced smoke plume above Leipzig, Germany, using a ground‐based multiwavelength‐Raman‐polarization lidar and the aerosol/cloud product of ESA’s wind lidar mission Aeolus. An exceptional high smoke‐AOT &gt;0.4 was measured, yielding to a mean mass concentration of 8 μg m−3. The 355 nm lidar ratio was moderate at around 40–50 sr. The Aeolus‐derived backscatter, extinction and lidar ratio profiles agree well with the observations of the ground‐based lidar PollyXT considering the fact that Aeolus’ aerosol and cloud products are still preliminary and subject to ongoing algorithm improvements.

Measurement Report: Lidar measurements of stratospheric aerosol following the 2019 Raikoke and Ulawun volcanic eruptions

Abstract. At 18:00 UTC on 21 June 2019 the Raikoke volcano in the Kuril islands began a large-magnitude explosive eruption, sending a plume of ash and sulfur dioxide into the stratosphere. A Raman lidar system at Capel Dewi Atmospheric Observatory, UK, was deployed to measure the vertical extent and optical depth of the volcanic aerosol cloud following the eruption. The elastic channel at 355 nm allowed measurements up to 25 km, but the Raman channel was only sensitive to the troposphere. Therefore, retrievals of backscatter ratio profiles from the raw backscatter measurements required aerosol-free profiles derived from nearby radiosondes and allowance for aerosol extinction using a lidar ratio of 40–50 sr. Small amounts of aerosol were measured prior to the arrival of the volcanic cloud (27 June–5 July 2019), from pyroconvection over Canada. Model simulations by de Leeuw et al. (2020) and Kloss et al. (2020) show that volcanic ash may have reached Europe from 1 July onwards and was certainly present over the UK after 10 July. The lidar detected a thin layer at an altitude of 14 km late on 3 July, with the first detection of the main aerosol cloud on 13 July. In this initial period the aerosol was confined below 16 km, but eventually the cloud extended to 20.5 km. A sustained period of clearly enhanced stratospheric aerosol optical depths began in early August, with a maximum value (at 355 nm) around 0.05 in mid-August and remaining above 0.02 until early November. Thereafter, optical depths decayed to around 0.01 by the end of 2019 and remained around that level until May 2020. The altitude of peak backscatter varied considerably (between 14 and 18 km) but was generally around 15 km. However, on one notable occasion on 25 August 2019, a layer around 300 m thick with peak lidar backscatter ratio around 1.5 was observed as high as 21 km.

Crop Monitoring and Classification Using Polarimetric RADARSAT-2 Time-Series Data Across Growing Season: A Case Study in Southwestern Ontario, Canada

Multitemporal polarimetric synthetic aperture radar (PolSAR) has proven as a very effective technique in agricultural monitoring and crop classification. This study presents a comprehensive evaluation of crop monitoring and classification over an agricultural area in southwestern Ontario, Canada. The time-series RADARSAT-2 C-Band PolSAR images throughout the entire growing season were exploited. A set of 27 representative polarimetric observables categorized into ten groups was selected and analyzed in this research. First, responses and temporal evolutions of each of the polarimetric observables over different crop types were quantitatively analyzed. The results reveal that the backscattering coefficients in cross-pol and Pauli second channel, the backscattering ratio between HV and VV channels (HV/VV), the polarimetric decomposition outputs, the correlation coefficient between HH and VV channelρ ρHHVV, and the radar vegetation index (RVI) show the highest sensitivity to crop growth. Then, the capability of PolSAR time-series data of the same beam mode was also explored for crop classification using the Random Forest (RF) algorithm. The results using single groups of polarimetric observables show that polarimetric decompositions, backscattering coefficients in Pauli and linear polarimetric channels, and correlation coefficients produced the best classification accuracies, with overall accuracies (OAs) higher than 87%. A forward selection procedure to pursue optimal classification accuracy was expanded to different perspectives, enabling an optimal combination of polarimetric observables and/or multitemporal SAR images. The results of optimal classifications show that a few polarimetric observables or a few images on certain critical dates may produce better accuracies than the whole dataset. The best result was achieved using an optimal combination of eight groups of polarimetric observables and six SAR images, with an OA of 94.04%. This suggests that an optimal combination considering both perspectives may be valuable for crop classification, which could serve as a guideline and is transferable for future research.