Relationship between winter orographic precipitation with synoptic and large-scale atmospheric circulation: The case of mount Olympus, Greece

Στην παρούσα εργασία εξετάζονται οι σχέσεις μεταξύ της ατμοσφαιρικής κυκλοφορίας και της χειμερινής (Δεκέμβριος – Μάρτιος) βροχόπτωσης στην περιοχή του Ολύμπου. Απώτερος σκοπός της εργασίας είναι η αξιολόγηση της επίδρασης του ορεινού όγκου του Ολύμπου στη χωρική κατανομή της βροχόπτωσης σε μία μικρής έκτασης περιοχή (100 x 100km2) κατά τη χειμερινή περίοδο. Η εργασία βασίζεται σε βροχομετρικά δεδομένα από 8 σταθμούς, που βρίσκονται περιμετρικά του Ολύμπου, σε υψόμετρα μεταξύ 30 και 1150 μέτρων για την περίοδο μεταξύ 1981 και 2000. Η πολυετής μεταβλητότητα της χειμερινή βροχόπτωσης περιγράφεται από τις συνοπτικές συνθήκες της ατμοσφαιρικής πίεσης στη στάθμη της θάλασσας και από τα γεωδυναμικά ύψη στα επίπεδα των 850 hPa and 500 hPa, αντίστοιχα. Τα συστήματα υψηλής ατμοσφαιρικής πίεσης στην περιοχή της Μεσογείου, μεταξύ των δεκαετιών 1980 και 1990, συνδέονται με τις ελάχιστες τιμές χειμερινής βροχόπτωσης στην περιοχή του Ολύμπου κατά τη διάρκεια των παρατηρήσεων. Το χειμώνα του 1996, η προσθαλάσσια πλευρά του Ολύμπου χαρακτηρίστηκε από πολύ υψηλές τιμές χειμερινής βροχόπτωσης, οι οποίες συνδέονται με μία αυλώνα χαμηλών πιέσεων πάνω από τη Δυτική Μεσόγειο, αλλά και από αυξημένες τροποσφαιρικές υφέσεις στη νότια Αδριατική και το Ιόνιο Πέλαγος. Αυτός ο τύπος της ατμοσφαιρικής κυκλοφορίας δημιουργεί μία ροή αέρα νοτιοανατολικής διεύθυνσης στο Αιγαίο, η οποία επηρεάζει περισσότερο (λιγότερο) τη χειμερινή βροχόπτωση στην προσθαλάσσια (ηπειρωτική) πλευρά του Ολύμπου. Εν αντιθέσει, η επικράτηση υφέσεων στην κεντρική Μεσόγειο και πάνω από τον Κόλπο της Γένοβας, δημιουργεί ατμοσφαιρική κυκλοφορία νότιας / νοτιοδυτικής διεύθυνσης και επηρεάζει περισσότερο τη χειμερινή βροχόπτωση στην ηπειρωτική πλευρά του Ολύμπου (r= -0.80, Σταθμός Ελασσόνας) και λιγότερο την προσθαλάσσια πλευρά του Ολύμπου (r= -0.67; Σταθμός Κατερίνης). Η κατάσταση αυτή τονίζει την επίδραση του ορεινού όγκου του Ολύμπου στην κατανομή της χειμερινής βροχόπτωσης, με μεγάλες παρατηρούμενες διαφορές μεταξύ της ηπειρωτικής και προσθαλάσσιας πλευράς. Μεγάλης κλίμακας φαινόμενα, όπως η ταλάντωση του Βόρειου Ατλαντικού ή του Αρκτικού, επηρεάζουν περισσότερο τη χειμερινή βροχόπτωση στην ηπειρωτική πλευρά του Ολύμπου.The relationship between the winter (DJFM) precipitation and the atmospheric circulation patterns is examined around Mount Olympus, Greece in order to assess the effects of orography and atmospheric dynamics over a small (less than 100 x 100 km) spatial domain. Winter accumulated rainfall datasets from 8 stations spread along the eastern (marine) and western (continental) sides of the Mount Olympus at elevations between 30 m and 1150 m are used during the period 1981 to 2000. Synoptic scale conditions of mean sea-level pressure and geopotential heights at 850 hPa and 500 hPa, were used to explain the multiyear rainfall variability. High pressure systems dominated over the central Mediterranean and most parts of central Europe during the late 1980’s and early 1990’s, are associated with minimum winter rainfall along both sides of Mount Olympus. The winter of 1996 was associated with peak in rainfall along the marine side of the mountain and was characterized by enhancement of upper level trough over the western Mediterranean and increased low tropospheric depressions over the southern Adriatic and the Ionian Seas. This atmospheric circulation pattern facilitated a southeasterly air flow that affected more (less) the marine (continental) sides of the mountain. In contrast, dominance of low pressure systems with cores over the Gulf of Genoa and the Central Mediterranean affect the study area mostly from west/southwest revealing higher correlations with the precipitation in the continental side of the mountain (r= -0.80; Elassona station) and considerably lower correlations with the marine side (r = -0.67; Katerini station). This highlights the orographic barrier of the Mount Olympus revealing large differences between the upward and leeward sides. Large scale atmospheric patterns like the North Atlantic Oscillation and the Arctic Oscillation seem to influence the winter rainfall in the lowlands along the continental side of the mountain

Seasonal Variability of Atmospheric Aerosol Parameters over Greater Noida Using Ground Sunphotometer Observations

Atmospheric aerosols over northern India are subject of significant temporal and spatial variability and many studies have been carried out to investigate their physico-chemical and optical properties. The present work emphasizes on examining the aerosol optical properties and types over Greater Noida, Delhi region, using ground-based sun photometer data during the period 2010–2012. The analysis reveals a relatively high mean aerosol optical depth at 500 nm (AOD 500 = 0.82 ± 0.39), associated with a moderate Angstrom exponent α 440–870 of 0.95 ± 0.37. Both parameters, exhibit significant daily, monthly and seasonal variability with higher values of AOD 500 during post-monsoon (0.98 ± 0.50) and winter (0.87 ± 0.35) seasons associated with high α values (\u3e 1.1) suggesting significant urban and biomass-burning contribution. On monthly basis, the highest AOD is found during July and November and the lowest one in the transition months of March and September. The aero ol-type discrimination vi a the relationship AOD vs. α shows a clear dominance of urban/industrial and biomass-burning aerosols during post-monsoon and winter in fractions of 74.5% and 72%, respectively, while aerosols of desert-dust characteristics were most frequent in pre-monsoon (41.7%) and monsoon (21%) seasons. In general, the analysis shows a rather well-mixed aerosol type under very turbid atmosphere, which is associated with the long range transport of pollutants through the westerly winds from the Thar desert and biomass burning in the western parts of India

Editorial: Impact of the COVID-19 lockdown on the atmosphere

[no abstract available

Aerosol climatology: on the discrimination of aerosol types over four AERONET sites

International audienceAerosols have a significant regional and global effect on climate, which is about equal in magnitude but opposite in sign to that of greenhouse gases. Nevertheless, the aerosol climatic effect changes strongly with space and time because of the large variability of aerosol physical and optical properties, which is due to the variety of their sources, which are natural, and anthropogenic, and their dependence on the prevailing meteorological and atmospheric conditions. Characterization of aerosol properties is of major importance for the assessment of their role for climate. In the present study, 3-year AErosol RObotic NETwork (AERONET) data from ground-based sunphotometer measurements are used to establish climatologies of aerosol optical depth (AOD) and Ångström exponent ? in several key locations of the world, characteristic of different atmospheric environments. Using daily mean values of AOD at 500 nm (AOD500) and Ångström exponent at the pair of wavelengths 440 and 870 nm (? 440?870), a discrimination of the different aerosol types occurring in each location is achieved. For this discrimination, appropriate thresholds for AOD500 and ? 440?870 are applied. The discrimination of aerosol types in each location is made on an annual and seasonal basis. It is shown that a single aerosol type in a given location can exist only under specific conditions (e.g. intense forest fires or dust outbreaks), while the presence of well-mixed aerosols is the accustomed situation. Background clean aerosol conditions (AOD500<0.06) are mostly found over remote oceanic surfaces occurring on average in ~56.7% of total cases, while this situation is quite rare over land (occurrence of 3.8?13.7%). Our analysis indicates that these percentages change significantly from season to season. The spectral dependence of AOD exhibits large differences between the examined locations, while it exhibits a strong annual cycle

Variability and Trends of Aerosol Properties over Kanpur, Northern India using AERONET Data (2001-10)

Natural and anthropogenic aerosols over northern India play an important role in influencing the regional radiation budget, causing climate implications to the overall hydrological cycle of South Asia. In the context of regional climate change and air quality, we discuss aerosol loading variability and trends at Kanpur AERONET station located in the central part of the Indo-Gangetic plains (IGP), during the last decade (2001-10). Ground-based radiometric measurements show an overall increase in column-integrated aerosol optical depth (AOD) on a yearly basis. This upward trend is mainly due to a sustained increase in the seasonal/monthly averaged AOD during the winter (Dec-Feb) and post-monsoon (Oct-Nov) seasons (dominated by anthropogenic emissions). In contrast, a neutral to weak declining trend is observed during late pre-monsoon (Mar-May) and monsoon (Jun-Sep) months, mainly influenced by inter-annual variations of dust outbreaks. A general decrease in coarse-mode aerosols associated with variable dust activity is observed, whereas the statistically significant increasing post-monsoon/winter AOD is reflected in a shift of the columnar size distribution towards relatively larger particles in the accumulation mode. Overall, the present study provides an insight into the pronounced seasonal behavior in aerosol loading trends and, in general, is in agreement with that associating the findings with those recently reported by satellite observations (MODIS and MISR) over northern India. Our results further suggest that anthropogenic emissions (due mainly to fossil-fuel and biomass combustion) over the IGP have continued to increase in the last decade

Influence of continental advection on aerosol characteristics over Bay of Bengal (BoB) in winter: results from W-ICARB cruise experiment

The transport of aerosols and pollutants from continental India to the adjoining oceanic areas is a major topic of concern and several experimental campaigns have been conducted over the region focusing on aerosol characteristics and their climate implications. The present study analyzes the spectral aerosol optical depth (AOD) variations over Bay of Bengal (BoB) during Winter-Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) from 27 December 2008 to 30 January 2009 and investigates the influence of the adjoining landmass to the marine aerosol field. High AOD₅₀₀ values (>0.7) occurred over northern BoB due to outflow of aerosols and pollutants from the densely populated Indo-Gangetic Plains (IGP); low AOD₅₀₀ (0.1–0.2) was observed in central and southern BoB, far away from the mainland. The Angstrom exponent "α" was observed to be high (>1.2) near coastal waters, indicating relative abundance of accumulation-mode continental aerosols. On the other hand, over southern BoB its values dropped below ~0.7. National Center for Environmental Prediction (NCEP) reanalysis data on winds at 850 and 700 hPa, along with air-mass trajectories calculated using Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, suggested transport of continental aerosols from central and northern India over the BoB. On the other hand, when the ship was crossing the eastern BoB, the aerosol loading was strongly affected by air-masses originating from Southeast Asia, causing an increase in AOD and α. Biomass-burning episodes over the region played an important role in the observed aerosol properties. Terra/Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) AOD₅₅₀ and cruise measured AOD₅₅₀ showed good agreement (<I>R</I>² = 0.86 and 0.77, respectively) over BoB, exhibiting similar AOD and α spatio-temporal variation

Extremely large anthropogenic-aerosol contribution to total aerosol load over the Bay of Bengal during winter season

Ship-borne observations of spectral aerosol optical depth (AOD) have been carried out over the entire Bay of Bengal (BoB) as part of the W-ICARB cruise campaign during the period 27 December 2008–30 January 2009. The results reveal a pronounced temporal and spatial variability in the optical characteristics of aerosols mainly due to anthropogenic emissions and their dispersion controlled by local meteorology. The highest aerosol amount, with mean AOD&lt;sub&gt;500&lt;/sub&gt;&gt;0.4, being even above 1.0 on specific days, is found close to the coastal regions in the western and northern parts of BoB. In these regions the Ångström exponent is also found to be high (~1.2–1.25) indicating transport of strong anthropogenic emissions from continental regions, while very high AOD&lt;sub&gt;500&lt;/sub&gt; (0.39&amp;plusmn;0.07) and &amp;alpha;&lt;sub&gt;380–870&lt;/sub&gt; values (1.27&amp;plusmn;0.09) are found over the eastern BoB. Except from the large &amp;alpha;&lt;sub&gt;380–870&lt;/sub&gt; values, an indication of strong fine-mode dominance is also observed from the AOD curvature, which is negative in the vast majority of the cases, suggesting dominance of an anthropogenic-pollution aerosol type. On the other hand, clean maritime conditions are rather rare over the region, while the aerosol types are further examined through a classification scheme based on the relationship between α and &lt;i&gt;d&lt;/i&gt;&amp;alpha;. It was found that even for the same α values the fine-mode dominance is larger for higher AODs showing the strong continental influence over the marine environment of BoB. Furthermore, there is also an evidence of aerosol-size growth under more turbid conditions indicative of coagulation and/or humidification over specific BoB regions. The results obtained using OPAC model show significant fraction of soot aerosols (~6 %–8 %) over the eastern and northwestern BoB, while coarse-mode sea salt particles are found to dominate in the southern parts of BoB

Effects of Crop Residue Burning on Aerosol Properties, Plume Characteristics, and Long-Range Transport over Northern India

Aerosol emissions from biomass burning are of specific interest over the globe due to their strong radiative impacts and climate implications. The present study examines the impact of paddy crop residue burning over northern India during the postmonsoon (October-November) season of 2012 on modification of aerosol properties, as well as the long-range transport of smoke plumes, altitude characteristics, and affected areas via the synergy of ground-based measurements and satellite observations. During this period, Moderate Resolution Imaging Spectroradiometer (MODIS) images show a thick smoke/hazy aerosol layer below 2-2.5 km in the atmosphere covering nearly the whole Indo-Gangetic Plains (IGP). The air mass trajectories originating from the biomass-burning source region over Punjab at 500 m reveal a potential aerosol transport pathway along the Ganges valley from west to east, resulting in a strong aerosol optical depth (AOD) gradient. Sometimes, depending upon the wind direction and meteorological conditions, the plumes also influence central India, the Arabian Sea, and the Bay of Bengal, thus contributing to Asian pollution outflow. The increased number of fire counts (Terra and Aqua MODIS data) is associated with severe aerosol-laden atmospheres (AOD(500 nm) \u3e 1.0) over six IGP locations, high values of Angstrom exponent (\u3e1.2), high particulate mass 2.5 (PM2.5) concentrations (\u3e100-150 mu gm(-3)), and enhanced Ozone Monitoring Instrument Aerosol Index gradient (similar to 2.5) and NO2 concentrations (similar to 6 x 10(15) mol/cm(2)), indicating the dominance of smoke aerosols from agricultural crop residue burning. The aerosol size distribution is shifted toward the fine-mode fraction, also exhibiting an increase in the radius of fine aerosols due to coagulation processes in a highly turbid environment. The spectral variation of the single-scattering albedo reveals enhanced dominance of moderately absorbing aerosols, while the aerosol properties, modification, and mixing atmospheric processes differentiate along the IGP sites depending on the distance from the aerosol source, urban influence, and local characteristics

Heterogeneity in pre-monsoon aerosol types over the Arabian Sea deduced from ship-borne measurements of spectral AODs

Ship-borne sunphotometer measurements obtained in the Arabian Sea (AS) in the pre-monsoon season (18 April–10 May 2006) during a cruise campaign (ICARB) have been used to retrieve the Aerosol Optical Depth (AOD; &amp;tau;) and the Ångström wavelength exponent (α). The continents surrounding the AS produce natural and anthropogenic aerosols that have distinctive influences on α and its spectral distribution. The α values were estimated by means of the least-squares method over the spectral bands 340–1020 nm and 340–870 nm. The spectral distribution of AOD in logarithmic co-ordinates could be fit using a 2nd order polynomial with higher accuracy in the wavelength band 340–1020 nm than in the 340–870 nm band. A polynomial fit analytically parameterizes the observed wavelength dependencies of AOD with least errors in spectral variation of α and yields accurate estimates of the coefficients (&lt;i&gt;a&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt; and &lt;i&gt;a&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt;). The coarse-mode (positive curvature in the ln&amp;tau;&lt;sub&gt;&amp;lambda;&lt;/sub&gt; vs. ln&amp;lambda;) aerosols are mainly depicted in the Northern part of the AS closely associated with the nearby arid areas while fine-mode aerosols are mainly observed over the far and coastal AS regions. In the study period the mean AOD at 500 nm is 0.25&amp;plusmn;0.11 and the α&lt;sub&gt;340-1020&lt;/sub&gt; is 0.90&amp;plusmn;0.19. The α&lt;sub&gt;340-870&lt;/sub&gt; exhibits similar values (0.92&amp;plusmn;0.18), while significant differences revealed for the constant terms of the polynomial fit (&lt;i&gt;a&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt; and &lt;i&gt;a&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt;) proportionally to the wavelength band used for their determination. Observed day-to-day variability in the aerosol load and optical properties are direct consequence of the local winds and air-mass trajectories along with the position of the ship