Συνοπτική ανάλυση μιας περίπτωσης ισχυρής χιονόπτωσης στην Αττική

Τον Ιανουάριο του 2017 σημειώθηκε στην Ελλάδα μία περίπτωση έντονης κακοκαιρίας με σημαντικά χαμηλές θερμοκρασίες και χιονοπτώσεις συμπεριλαμβανομένης και της Αττικής. Συγκεκριμένα, από 09 Ιανουαρίου έως 12 Ιανουαρίου στην Αττική επικράτησε ολικός παγετός και παροδικές χιονοπτώσεις κατά τόπους πυκνές στα βόρεια. Σκοπός της εργασίας είναι η μελέτη της ατμοσφαιρικής κυκλοφορίας συνοπτικής κλίμακας καθ’ύψος στην τροπόσφαιρα που οδήγησε στην δημιουργία της χιονόπτωσης στην Αττική. Για το σκοπό αυτό, χρησιμοποιήθηκαν χάρτες επιφανείας, 850hPa, 500hPa, του μοντέλου ECMWF, εικόνες δορυφόρου, εικόνες πραγματικών δεδομένων καθώς και συνοπτικοί χάρτες επιφανείας, 850hPa, 500hPa. Τα δεδομένα αυτά χορηγήθηκαν από την Εθνική Μετεωρολογική Υπηρεσία. Η χιονόπτωση οφείλεται στην σύγκλιση της ψυχρής αέριας μάζας που βρισκόταν βόρεια της Ελλάδας, με τη θερμότερη αέρια μάζα που βρισκόταν στην Αφρική, με ζώνη σύγκλισης πάνω από το Αιγαίο. Ουσιαστικά, η ψυχρή μάζα της Βόρειας Ευρώπης κινήθηκε νότια και η θερμότερη μάζα της Βόρειας Αφρικής, κινήθηκε βορειοανατολικά. Ο σχηματισμός του βαρομετρικού χαμηλού στην περιοχή της Ελλάδας, καθώς και η παραμονή του για μεγάλη διάρκεια, ευνόησε την ψυχρή μεταφορά και το στροβιλισμό της ψυχρής αέριας μάζας, με αποτέλεσμα τον ισχυρό και κατά τόπους ολικό παγετό. Επιπλέον, η σύγκλιση των δύο μαζών, οδήγησε στο σχηματισμό συνεσφιγμένου μετώπου με την Ελλάδα να βρίσκεται στο τριπλό σημείο του, προκαλώντας ισχυρές βροχές και καταιγίδες στην Πελοπόννησο, την Κρήτη τα Δωδεκάνησα και τις Κυκλάδες, αλλά και χιονοπτώσεις στη Θράκη, τη Μακεδονία, τη Θεσσαλία, τις Σποράδες, την Ανατολική και κεντρική Στερεά και την Εύβοια.In this paper, an attempt is made to study the synoptic evolution of surface and middle troposphere barometric systems, which caused snowfall in Athens, during January 2017. Specifically, in 09-12 of January, thunderstorms and low temperatures occurred in Greece, while snowfalls and below zero temperatures occurred in the area of Attica. For this study, surface, 850hPa, 500hPa charts of synoptic conditions and ECMWF model, as well as satellite images, real time weather charts and a tephigram were used. These data were provided by the Meteorological Station of Hellinikon (National Meteorological Service). The synoptic pattern with which the snowfall in Attica is associated, is a surface cyclone that lies (in 09 January) on southern Europe with its center over Sporades. Furthermore, cold arctic air masses coming from northern Europe (Romania) reach the area of Greece, while in the same time, warmer air masses coming from northern Africa approach Greece. The convection of these two types of air masses takes place over the Aegean sea. The snowfalls, thunderstorms and low temperatures in Greece, are due to positive advection of vorticity, the high percentages of humidity and the cold advection in the lower thoposphere

Validation of the TROPOMI/S5P aerosol layer height using EARLINET lidars

The purpose of this study is to investigate the ability of the Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) to derive accurate geometrical features of lofted aerosol layers, selecting the Mediterranean Basin as the study area. Comparisons with ground-based correlative measurements constitute a key component in the validation of passive and active satellite aerosol products. For this purpose, we use ground-based observations from quality-controlled lidar stations reporting to the European Aerosol Research Lidar Network (EARLINET). An optimal methodology for validation purposes has been developed and applied using the EARLINET optical profiles and TROPOMI aerosol products, aiming at the in-depth evaluation of the TROPOMI aerosol layer height (ALH) product for the period 2018 to 2022 over the Mediterranean Basin. Seven EARLINET stations were chosen, taking into consideration their proximity to the sea, which provided 63 coincident aerosol cases for the satellite retrievals. In the following, we present the first validation results for the TROPOMI/S5P ALH using the optimized EARLINET lidar products employing the automated validation chain designed for this purpose. The quantitative validation at pixels over the selected EARLINET stations illustrates that the TROPOMI ALH product is consistent with the EARLINET lidar products, with a high correlation coefficient R=0.82 (R=0.51) and a mean bias of -0.51±0.77 km and -2.27±1.17 km over ocean and land, respectively. Overall, it appears that aerosol layer altitudes retrieved from TROPOMI are systematically lower than altitudes from the lidar retrievals. High-albedo scenes, as well as low-aerosol-load scenes, are the most challenging for the TROPOMI retrieval algorithm, and these results testify to the need to further investigate the underlying cause. This work provides a clear indication that the TROPOMI ALH product can under certain conditions achieve the required threshold accuracy and precision requirements of 1 km, especially when only ocean pixels are included in the comparison analysis. Furthermore, we describe and analyse three case studies in detail, one dust and two smoke episodes, in order to illustrate the strengths and limitations of the TROPOMI ALH product and demonstrate the presented validation methodology. The present analysis provides important additions to the existing validation studies that have been performed so far for the TROPOMI S5P ALH product, which were based only on satellite-to-satellite comparisons.</p

Δυναμική μελέτη μιας περίπτωσης medicane

Η παρούσα διπλωματική εργασία αποσκοπεί στη δυναμική μελέτη μιας περίπτωσης ισχυρής κυκλογένεσης που έλαβε χώρα στη Μεσόγειο, την περίοδο 26-30 Σεπτεμβρίου 2018, και για ορισμένο χρονικό διάστημα παρουσίασε δομή μεσογειακού κυκλώνα με τροπικά χαρακτηριστικά (medicane). Για τη μελέτη του συνοπτικού αλλά και δυναμικού περιβάλλοντος του συγκεκριμένου χαμηλού, χρησιμοποιήθηκαν δορυφορικές εικόνες και δεδομένα ανάλυσης του Ευρωπαϊκού Κέντρου Μεσοπρόθεσμης Πρόγνωσης Καιρού (ECMWF) από την Εθνική Μετεωρολογική Υπηρεσία. Ταχεία βάθυνση της επιφανειακής ύφεσης παρουσιάστηκε κατά το χρονικό διάστημα 27 Σεπτεμβρίου 00UTC –28 Σεπτεμβρίου 06 UTC με εκρηκτικό ρυθμό πτώσης της πίεσης, ενώ το σύστημα απέκτησε μορφή μεσογειακού κυκλώνα με τροπικά χαρακτηριστικά κατά το διάστημα 27 Σεπτεμβρίου 18UTC – 29 Σεπτεμβρίου 00UTC, σύμφωνα με μαθηματικό υπολογισμό που λαμβάνει υπόψη δυναμικά κριτήρια. Τα ανώτερα στρώματα συνέβαλαν δυναμικά στη δημιουργία του επιφανειακού χαμηλού, όπως συμβαίνει σε μια συνηθισμένη κυκλογένεση των μέσων γεωγραφικών πλατών. Στα κατώτερα στρώματα μια προϋπάρχουσα βαροκλινική ζώνη πάνω από τις ακτές της Λιβύης συνδυάστηκε με θετική ανωμαλία της επιφανειακής θερμοκρασίας θάλασσας. Κατά τη διάρκεια που ο κυκλώνας παρουσίασε τροπικά χαρακτηριστικά, οι διαβατικές διεργασίες στα κατώτερα στρώματα, λειτούργησαν σαν κινητήριος δύναμη στην εξέλιξη της επιφανειακής κυκλογένεσης και κυριάρχησαν των αντίστοιχων δυναμικών διεργασιών των ανωτέρων στρωμάτων της τροπόσφαιρας. Στις διαβατικές διεργασίες συγκαταλέγονται οι ροές λανθάνουσας κι αισθητής θερμότητας στην κατώτερη τροπόσφαιρα.In this study, a case of explosive cyclogenesis that took place over the central Mediterranean sea during 26-30 September 2018, and obtained a Tropical-Like Cyclone structure (medicane), is being examined. For this study, satellite images and data-analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), employing a regular 0.1250.125 latitude – longitude grid of the ERA – Interim dataset, are being used. Data are provided by the Hellenic National Meteorological Service (HNMS). During 27 September 00UTC – 28 September 06UTC explosive deepening took place with central mean sea level pressure of about 998,91hPa. The formation of the Mediterranean cyclone with hurricane-like structure, took place in 27 September 18UTC – 29 September 00UTC, according to mathematic criteria, considering dynamical processes. The cyclogenesis, occurred as a result of upper-level dynamics, as an ordinary case of mid-latitudes areas. Thus, the baroclinic instability of lower-levels over Libya, was combined with a Sea Surface Temperature anomaly. During the medicane activity, diabatic processes of lower levels played a leading role in the cyclone’s evolution, in contrast with upper-troposphere dynamics, that did not contribute. Diabatic processes include surface sensible and latent heat fluxes in lower troposphere

PBL Height Retrievals during ASKOS Campaign

This study analyzes the structure of the Planetary Boundary Layer (PBL) at Mindelo, Cabo Verde, where the ASKOS Campaign took place from 2021 to 2022. Datasets from ground-based remote sensing instruments and radiosondes are used to derive the PBL height, by applying the Wavelet Covariance Transform (WCT), Threshold (TM), and Gradient Method (GM). Two case studies are described in detail, one with a significant dust load (23 September 2022) and one with relatively less dust load (12 September 2022). In the first case, the PBL top is found lower, and the methods used for the retrievals are characterized by larger uncertainties. In the second case, a higher and more convective PBL is observed. Additionally, results are compared with ECMWF outputs, establishing good agreement

Geometrical and Microphysical Properties of Clouds Formed in the Presence of Dust above the Eastern Mediterranean

In this work, collocated lidar–radar observations are used to retrieve the vertical profiles of cloud properties above the Eastern Mediterranean. Measurements were performed in the framework of the PRE-TECT experiment during April 2017 at the Greek atmospheric observatory of Finokalia, Crete. Cloud geometrical and microphysical properties at different altitudes were derived using the Cloudnet target classification algorithm. We found that the variable atmospheric conditions that prevailed above the region during April 2017 resulted in complex cloud structures. Mid-level clouds were observed in 38% of the cases, high or convective clouds in 58% of the cases, and low-level clouds in 2% of the cases. From the observations of cloudy profiles, pure ice phase occurred in 94% of the cases, mixed-phase clouds were observed in 27% of the cases, and liquid clouds were observed in 8.7% of the cases, while Drizzle or rain occurred in 12% of the cases. The significant presence of Mixed-Phase Clouds was observed in all the clouds formed at the top of a dust layer, with three times higher abundance than the mean conditions (26% abundance at −15 °C). The low-level clouds were formed in the presence of sea salt and continental particles with ice abundance below 30%. The derived statistics on clouds’ high-resolution vertical distributions and thermodynamic phase can be combined with Cloudnet cloud products and lidar-retrieved aerosol properties to study aerosol-cloud interactions in this understudied region and evaluate microphysics parameterizations in numerical weather prediction and global climate models

PBL Height Retrievals at a Coastal Site Using Multi-Instrument Profiling Methods

The objective of this study was the estimation of the dynamic evolution of the Planetary Boundary Layer (PBL) height, using advanced remote sensing measurements from Finokalia Station, where the Pre-TECT Campaign took place during 1&ndash;26 April 2017. PollyXT Raman Lidar and Halo Wind Doppler Lidar profiles were used to study the daily vertical evolution of the PBL. Wavelet Covariance Transform (WCT) and Threshold Method (TM) were performed on different products acquired from Lidars. According to the analysis, all methods and products are able to provide reasonable boundary-layer height estimates, each of them showing assets and barriers under certain conditions. Two cases are presented in detail, indicating the limited daytime evolution of a coastal area, the decisive role of wind speed-direction in the formation of a shallow or high boundary layer and the differences when using aerosols or turbulence as tracers for the PBL height retrieval. Comparison between the observed PBL and ECMWF model results was made, establishing the importance of actual PBL measurements, in coastal regions with complex topography

Validation of the TROPOMI/S5P aerosol layer height using EARLINET lidars

The purpose of this study is to investigate the ability of the Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) to derive accurate geometrical features of lofted aerosol layers, selecting the Mediterranean Basin as the study area. Comparisons with ground-based correlative measurements constitute a key component in the validation of passive and active satellite aerosol products. For this purpose, we use ground-based observations from quality-controlled lidar stations reporting to the European Aerosol Research Lidar Network (EARLINET). An optimal methodology for validation purposes has been developed and applied using the EARLINET optical profiles and TROPOMI aerosol products, aiming at the in-depth evaluation of the TROPOMI aerosol layer height (ALH) product for the period 2018 to 2022 over the Mediterranean Basin. Seven EARLINET stations were chosen, taking into consideration their proximity to the sea, which provided 63 coincident aerosol cases for the satellite retrievals. In the following, we present the first validation results for the TROPOMI/S5P ALH using the optimized EARLINET lidar products employing the automated validation chain designed for this purpose. The quantitative validation at pixels over the selected EARLINET stations illustrates that the TROPOMI ALH product is consistent with the EARLINET lidar products, with a high correlation coefficient RCombining double low line0.82 (RCombining double low line0.51) and a mean bias of -0.51±0.77 km and -2.27±1.17 km over ocean and land, respectively. Overall, it appears that aerosol layer altitudes retrieved from TROPOMI are systematically lower than altitudes from the lidar retrievals. High-albedo scenes, as well as low-aerosol-load scenes, are the most challenging for the TROPOMI retrieval algorithm, and these results testify to the need to further investigate the underlying cause. This work provides a clear indication that the TROPOMI ALH product can under certain conditions achieve the required threshold accuracy and precision requirements of 1 km, especially when only ocean pixels are included in the comparison analysis. Furthermore, we describe and analyse three case studies in detail, one dust and two smoke episodes, in order to illustrate the strengths and limitations of the TROPOMI ALH product and demonstrate the presented validation methodology. The present analysis provides important additions to the existing validation studies that have been performed so far for the TROPOMI S5P ALH product, which were based only on satellite-to-satellite comparisons.Atmospheric Remote Sensin

Is the near-spherical shape the "new black" for smoke?

We examine the capability of near-sphericalshaped particles to reproduce the triple-wavelength particle linear depolarization ratio (PLDR) and lidar ratio (LR) values measured over Europe for stratospheric smoke originating from Canadian wildfires. The smoke layers were detected both in the troposphere and the stratosphere, though in the latter case the particles presented PLDR values of almost 18% at 532 nm as well as a strong spectral dependence from the UV to the near-IR wavelength. Although recent simulation studies of rather complicated smoke particle morphologies have shown that heavily coated smoke aggregates can produce large PLDR, herein we propose a much simpler model of compact near-spherical smoke particles. This assumption allows for the reproduction of the observed intensive optical properties of stratospheric smoke, as well as their spectral dependence. We further examine whether an extension of the current Aerosol Robotic Network (AERONET) scattering model to include the near-spherical shapes could be of benefit to the AERONET retrieval for stratospheric smoke cases associated with enhanced PLDR. Results of our study illustrate the fact that triple-wavelength PLDR and LR lidar measurements can provide us with additional insight when it comes to particle characterization. © 2020 Author(s)

La forme quasi sphérique est-elle le « nouveau noir » de la fumée ?

International audienceWe examine the capability of near-spherical-shaped particles to reproduce the triple-wavelength particle linear depolarization ratio (PLDR) and lidar ratio (LR) values measured over Europe for stratospheric smoke originating from Canadian wildfires. The smoke layers were detected both in the troposphere and the stratosphere, though in the latter case the particles presented PLDR values of almost 18 % at 532 nm as well as a strong spectral dependence from the UV to the near-IR wavelength. Although recent simulation studies of rather complicated smoke particle morphologies have shown that heavily coated smoke aggregates can produce large PLDR, herein we propose a much simpler model of compact near-spherical smoke particles. This assumption allows for the reproduction of the observed intensive optical properties of stratospheric smoke, as well as their spectral dependence. We further examine whether an extension of the current Aerosol Robotic Network (AERONET) scattering model to include the near-spherical shapes could be of benefit to the AERONET retrieval for stratospheric smoke cases associated with enhanced PLDR. Results of our study illustrate the fact that triple-wavelength PLDR and LR lidar measurements can provide us with additional insight when it comes to particle characterization.Nous examinons la capacité des particules de forme quasi-sphérique à reproduire le PLDR (Particle Linear Depolarization Ratio) mesurées en Europe pour la fumée stratosphérique provenant des incendies de forêt au Canada. Des couches de fumée ont été détectées à la fois dans la troposphère et la stratosphère, bien que dans ce dernier cas, les particules présentaient des valeurs PLDR de près de 18% à 532 nm ainsi qu'une forte dépendance spectrale de l'UV à proche infrarouge. L'hypothèse selon laquelle les particules de fumée ont une forme presque sphérique permet la reproduction du PLDR et du rapport Lidar (LR) observés, alors que 20% n'était pas possible lors de l'utilisation de formes plus compliquées. Les résultats présentés ici sont étayés par des découvertes récentes dans la littérature, montrant que jusqu'à présent la forme quasi sphérique (ou des formes étroitement similaires) est la seule morphologie trouvée capable de reproduire les propriétés optiques intensives observées de la fumée stratosphérique, ainsi que leur dépendance spectrale