Μελέτη του ατμοσφαιρικού οριακού στρώματος με μεθόδους τηλεπισκόπησης lidar στην Ινδία

Σκοπός της παρούσας εργασίας είναι ο προσδιορισμός του ύψους ΑΟΣ (Ατμοσφαιρικό Οριακό Στρώμα) από μετρήσεις του επίγειου συστήματος lidar PollyXT και η σύγκριση τους με δεδομένα ραδιοβολίσεων, δεδομένα από το δορυφόρο CALIPSO και εκτιμήσεις των ατμοσφαιρικών μοντέλων ECMWF και WRF. Η μελέτη επικεντρώθηκε στην περιοχή του Gual Pahari (28.43ο N, 77.15ο E), 20 km Νότια του Νέου Δελχί για τη χρονική περίοδο από τον Μάρτιο 2008 μέχρι τον Μάρτιο 2009. Ο υπολογισμός του ύψους ΑΟΣ έγινε από 15λεπτους μέσους όρους του οπισθοσκεδαζόμενου σήματος lidar μέσω της τροποποιημένης μεθόδου συνδιακύμανσης. Το ύψος ΑΟΣ από δεδομένα ραδιοβολίσεων προσδιορίστηκε μέσω της μεθόδου Bulk Richardson Number καθώς και από τις κατακόρυφες βαθμίδες σχετικής υγρασίας και δυνητικής θερμοκρασίας. Η σύγκριση ανακτώμενων υψών από το PollyXT και τις ραδιοβολίσεις υποδεικνύει ικανοποιητική συσχέτιση ιδίως για περιπτώσεις ημερήσιου ΑΟΣ. Οι αλγόριθμοι ανίχνευσης στρωμάτωσης του δορυφόρου CALIPSO υπερεκτιμούν το ύψος ΑΟΣ. Η συσχέτιση υψών από το PollyXT και το ECMWF είναι αρκετά ικανοποιητική, κυρίως κατά τη χειμερινή και προ-μουσωνική περίοδο. Η σύγκριση με τα αποτελέσματα του μοντέλου WRF παρουσιάζεται στα πλαίσια χαρακτηριστικών περιπτώσεων μελέτης. Όσον αφορά το εποχικό βάθος ΑΟΣ, αυτό ακολουθεί πολύ καλά τον εποχικό κύκλο θερμοκρασίας και παρουσιάζει τη μεγαλύτερη μεταβλητότητα κατά την προ-μουσωνική περίοδο.The aim of this study is the derivation of PBL (Planetary Boundary Layer) height using measurements from ground-based lidar PollyXT and their comparison with radiosonde data, CALIPSO satellite data and simulations by the atmospheric models ECMWF and WRF. The study focused on Gual Pahari (28.43ο N, 77.15ο E), 20km South of New Delhi between March 2008 and March 2009. The PBL height was derived from the 15min averaged lidar backscatter signal using the modified Wavelet Covariance Transform method. The PBL height was calculated from radiosonde data utilizing the Bulk Richardson Number formula as well as the vertical gradients of relative humidity and potential temperature. The comparison between the heights from PollyXT retrievals and radiosonde data indicates relatively good agreement, especially during daytime. The CALIPSO Layer Detection Algorithms overestimate the PBL height. The correlation between heights from PollyXT and ECMWF is very good, with better agreement during the winter and pre-monsoon period. The comparison with WRF estimations is performed within the context of case studies. Regarding the seasonal PBL depth, it follows the seasonal temperature cycle and exhibits maximum interseasonal variability in the pre-monsoon season

Investigation of aerosol optical properties in the European Arctic using Lidar remote sensing technique

Aerosol strongly affect the radiation balance, especially in the Arctic where climate change is significantly faster compared to lower latitudes. The interaction between aerosol and radiation can be either direct (scattering and absorption) or indirect (aerosol serving as cloud condensation nuclei and ice nucleating particles). Aerosol optical properties can be provided by Lidar (Light Detection and Ranging) systems with high spatial and temporal resolution. In this study, we utilize data from a ground-based Lidar system located in Ny-Ålesund, Spitsbergen and an air-borne system installed onboard the research aircraft Polar5. Our focus is on a rare event of elevated aerosol layers, which persistently appeared over two different parts of the European Arctic during PAMARCMiP (Polar Air-borne Measurements and Arctic Regional Climate Model Simulation Project) campaign in spring 2018. Results show that the detected layers exhibit similar optical properties, namely aerosol backscatter coefficient, which is indicative of aerosol abundance and aerosol depolarization ratio, which is an indicator of the aerosol shape. The main hypothesis is that although the existence of those layers is rare, they impact on the radiation budget of the Arctic. In the next steps of our research, we will investigate the occurrence of similar aerosol layers in the springtime of previous years using long-term measurements from the Lidar system located in Ny-Ålesund. Our goal is to assess the effect of different aerosol layers on the surface radiation budget and gain a better understanding of their role in the amplified Arctic climate change, utilizing radiation measurements from the Ny-Ålesund BSRN (Baseline Surface Radiation Network) station

Optical Properties of Arctic Aerosol during PAMARCMiP 2018

Aerosol strongly affect the radiation balance, especially in the Arctic where climate change is significantly faster compared to lower latitudes, a phenomenon known as Arctic Amplification. The interaction between aerosol and radiation can be either direct (scattering and absorption) or indirect (aerosol serving as cloud condensation nuclei and ice nucleating particles).Aerosol concentration in the accumulation mode exhibits an annual maximum in the Arctic in springtime, forming the Arctic Haze. In this work, elevated layers from the European Arctic are analyzed in terms of their optical and hygroscopic properties

Aerosol investigation during the Arctic Haze season 2018 Optical, Microphysical and Radiative properties

In this work, optical and microphysical properties of Arctic aerosol as well as their radiative impact are investigated. Air-borne Lidar observations along with ground-based measurements are evaluated for the Arctic Haze season of 2018. Aerosol abundance as inferred from particle backscatter was typical for this period of the year, with nearly spherical particles. However, the inversion of microphysical properties yielded high Refractive Index (RI) together with low Single-Scattering Albedo (SSA), suggesting absorbing particles, which are not typical for the Arctic Haze period. A fitted lognormal volume distribution revealed a fine mode with effective radius (reff) of 0.23 μm and a coarse mode with reff=0.75 μm. The total aerosol forcing at ground level was negative according to observations and radiative transfer simulations

Does the Intra-Arctic Modification of Long-Range Transported Aerosol Affect the Local Radiative Budget? (A Case Study)

The impact of aerosol spatio-temporal variability on the Arctic radiative budget is not fully constrained. This case study focuses on the intra-Arctic modification of long-range transported aerosol and its direct aerosol radiative effect (ARE). Different types of air-borne and ground-based remote sensing observations (from Lidar and sun-photometer) revealed a high tropospheric aerosol transport episode over two parts of the European Arctic in April 2018. By incorporating the derived aerosol optical and microphysical properties into a radiative transfer model, we assessed the ARE over the two locations. Our study displayed that even in neighboring Arctic upper tropospheric levels, aged aerosol was transformed due to the interplay of removal processes (nucleation scavenging and dry deposition) and alteration of the aerosol source regions (northeast Asia and north Europe). Along the intra-Arctic transport, the coarse aerosol mode was depleted and the visible wavelength Lidar ratio (LR) increased significantly (from 15 to 64–82 sr). However, the aerosol modifications were not reflected on the ARE. More specifically, the short-wave (SW) atmospheric column ARE amounted to +4.4 - +4.9 W m−2 over the ice-covered Fram Strait and +4.5 W m−2 over the snow-covered Ny-Ålesund. Over both locations, top-of-atmosphere (TOA) warming was accompanied by surface cooling. These similarities can be attributed to the predominant accumulation mode, which drives the SW radiative budget, as well as to the similar layer altitude, solar geometry, and surface albedo conditions over both locations. However, in the context of retreating sea ice, the ARE may change even along individual transport episodes due to the ice albedo feedback

An extended lidar-based cirrus cloud retrieval scheme: first application over an Arctic site

Accurate and precise characterization of cirrus cloud geometrical and optical properties is essential for better constraining their radiative footprint. A lidar-based retrieval scheme is proposed here, with its performance assessed on fine spatio-temporal observations over the Arctic site of Ny-Ålesund, Svalbard. Two contributions related to cirrus geometrical (dynamic Wavelet Covariance Transform (WCT)) and optical properties (constrained Klett) are reported. The dynamic WCT rendered cirrus detection more robust, especially for thin cirrus layers that frequently remained undetected by the classical WCT method. Regarding optical characterization, we developed an iterative scheme for determining the cirrus lidar ratio (LRci) that is a crucial parameter for aerosol - cloud discrimination. Building upon the Klett-Fernald method, the LRci was constrained by an additional reference value. In established methods, such as the double-ended Klett, an aerosol-free reference value is applied. In the proposed constrained Klett, however, the reference value was approximated from cloud-free or low cloud optical depth (COD up to 0.2) profiles and proved to agree with independent Raman estimates. For optically thin cirrus, the constrained Klett inherent uncertainties reached 50% (60-74%) in terms of COD (LRci). However, for opaque cirrus COD (LRci) uncertainties were lower than 10% (15%). The detection method discrepancies (dynamic versus static WCT) had a higher impact on the optical properties of low COD layers (up to 90%) compared to optically thicker ones (less than 10%). The constrained Klett presented high agreement with two established retrievals. For an exemplary cirrus cloud, the constrained Klett estimated the COD355 (LRci355) at 0.28 ± 0.17 (29 ± 4 sr), the double-ended Klett at 0.27 ± 0.15 (32 ± 4 sr) and the Raman retrievals at 0.22 ± 0.12 (26 ± 11 sr). Our approach to determine the necessary reference value can also be applied in established methods and increase their accuracy. In contrast, the classical aerosol-free assumption led to 44  sr LRci overestimation in optically thin layers and 2-8 sr in thicker ones. The multiple scattering effect was corrected using Eloranta (1998) and accounted for 50-60% extinction underestimation near the cloud base and 20-30% within the cirrus layers

Cirrus cloud properties over Ny-Alesund as retrieved from lidar measurements (2011-2020)

The present dataset reports the properties of cirrus clouds over the research site of Ny-Alesund, Svalbard, as retrieved from lidar measurements from 2011 to 2020. Several essential geometrical (cloud thickness (GT), cloud base and top heights) and optical properties (Cloud Optical Depth (COD), lidar ratio (LR), linear particle depolarization ratio (LPDR) and color ratio (CR)) were derived. Additionally, meteorological parameters (temperature, wind speed and wind direction) were extracted from AWIPEV radiosonde ascents. The meteorological parameters were subsetted within the altitude of cirrus clouds and only measurements up to 2 hours apart from the cirrus observations were used. In this way, the meteorological parameters can be considered representative of the prevailing meteorological conditions in the presence of cirrus clouds. The original radiosonde datasets are already available at doi:10.1594/PANGAEA.914973

Aerosol Investigation During the Arctic Haze Season of 2018: Optical and Hygroscopic Properties

In the beginning of March 2018, Lidar measurements were performed on Svalbard, Arctic Ocean, in order to analyse the optical and hygroscopic properties of Arctic aerosol. In this study, aerosol backscatter showed significant higher values in lower altitudes. The analysis of the Colour Ratio (CR) revealed smaller particles in lower altitudes, with larger particles appearing only above Investigation of the hygroscopic character was done by applying the growth parameter introduced by Gassó et al. (2000). It was found that the method of Zieger et.al. (2010) can be successfully extended to backscatter and CR data from Lidar measurements. Ice nucleation was examined in ice supersaturation conditions, with no ice cloud formation observed. This indicated that the role of Arctic aerosol as ice nuclei is still a poorly understood issue

Properties of Cirrus Clouds over the European Arctic (Ny-Ã lesund, Svalbard)

Cirrus is the only cloud type capable of inducing daytime cooling or heating at the top of the atmosphere (TOA) and the sign of its radiative effect highly depends on its optical depth. However, the investigation of its geometrical and optical properties over the Arctic is limited. In this work the long-term properties of cirrus clouds are explored for the first time over an Arctic site (Ny-Ã lesund, Svalbard) using lidar and radiosonde measurements from 2011 to 2020. The optical properties were quality assured, taking into account the effects of specular reflections and multiple-scattering. Cirrus clouds were generally associated with colder and calmer wind conditions compared to the 2011-2020 climatology. However, the dependence of cirrus properties on temperature and wind speed was not strong. Even though the seasonal cycle was not pronounced, the winter-time cirrus appeared under lower temperatures and stronger wind conditions. Moreover, in winter, geometrically- and optically-thicker cirrus were found and their ice particles tended to be more spherical. The majority of cirrus was associated with westerly flow and westerly cirrus tended to be geometrically-thicker. Overall, optically-thinner layers tended to comprise smaller and less spherical ice crystals, most likely due to reduced water vapor deposition on the particle surface. Compared to lower latitudes, the cirrus layers over Ny-Ã lesund were more absorbing in the visible spectral region and they consisted of more spherical ice particles