Relationship between the Indian summer monsoon and the large-scale circulation variability over the Mediterranean

In this study the impact of the Indian summer monsoon on the large scale variability of the atmospheric circulation over the Mediterranean is investigated on an inter-annual time scale. Composite and correlation analysis results are presented, outlining different circulation patterns in the upper and lower troposphere for strong and weak monsoon years respectively. For this purpose ERA-40 Reanalysis monthly mean data at various isobaric levels together with the standardized All India Rainfall Index for boreal summer (June–July–August–September) of a 44- year period were employed. During strong monsoon years many atmospheric circulation systems appear strengthened over Eurasia, resembling a well-organized Rossby wave train over the area. In the upper troposphere a meridional shift of the jet streams over the examined area was also identified during extreme monsoon years. On the other hand, in the lower troposphere enhanced northerlies (Etesians) appear to dominate over Eastern Mediterranean along with intensified subsidence during strong monsoon years

On the link between Indian summer monsoon and the Etesian pattern over the Aegean Sea

The Indian summer monsoon (ISM) is a key factor influencing the eastern Mediterranean climate. During early summer period, the thermal low over Pakistan and northwestern India extends towards the Mediterranean through inner Iraq and Turkey. On the other hand, the Azores high expands eastwards and meets the Balkan high pressure system, forming one common system. Greece lies at the transitional zone between these two pressure systems, where persistent northeasterly wind currents form especially over the Aegean, known as the Etesians. The objective of this study is to investigate the interrelation between the ISM and the Etesian winds over the Aegean Sea during summer for an extended period of 44 years (1958-2001) with the aid of composite and correlation analyses and ERA 40 datasets. More specifically, the study will focus on the ISM activity during and prior to or after ‘Etesian episodes’ in order to determine the extent to which one system may lead another. The present analysis reveals that during enhanced monsoon years two upper level ridges prevail over the greater area of interest, one over western Europe and northern Africa, which is part of the Azores high and a second one over Pakistan region, Persian Gulf and Middle Asia, which extends to the west and connecting to the first one forms a slight trough over Eastern Mediterranean. At lower levels, an intensification of the thermal Asian low as well as of the high pressure system prevailing over western and central Europe is apparent. Concurring with this pattern in upper and lower troposphere, anomalously strong northerlies appear over the Aegean during periods with enhanced monsoon activity, as horizontal surface wind anomalies depict. In agreement with previous studies, mid-level subsidence during excess monsoon rainfall periods is found to be more intense over the Eastern Mediterranean, Iraq, Saudi Arabia and east of the Caspian Sea

ON THE STUDY OF THE PHENOMENON OF CYCLOGENESIS OVER THE AEGEAN SEA

AN ATTEMPT IS MADE TO IDENTIFY AND INVESTIGATE THE PHENOMENON OF CYCLOGENESIS OVER THE AEGEAN SEA. THE METHODOLOGY USED FOR THE IDENTIFICATION COMBINES THEEXAMINATION OF SYNOPTIC CHARTS AND THE CALCULATION OF DYNAMIC AND THERMODYNAMIC PARAMETERS. SIX SYNOPTIC CATEGORIES THAT COULD FAVOR THE CYCLOGENESIS OVER THE AEGEAN SEA ARE DISTINGUISHED. THE EXAMINATION OF THE FREQUENCY OF THE IDENTIFIED CASES DEMONSTRATED THAT THE PHENOMENON OF CYCLOGENESIS IS NOT AS RARE AS IT WAS CONSIDERED. IN ADDITION, IT IS OBSERVED THAT THE PHENOMENON IS MOSTLY CHARACTERISED BY RATHER SMALL INTENSITY AND DURATION. IN ORDER TO INVESTIGATE AND DISCUSS THE SYNOPTIC, DYNAMIC AND THERMODYNAMIC CHARACTERISTICS OF THE CYCLOGENESIS, THE ISENTROPIC POTENTIAL VORTICITY ANALYSES ARE STUDIED.THE BAROCLINIC CHARACTER OF THE PHENOMENON IS POSTULATED, DESPITE OF THE RATHER SMALL SCALL AND INTENSITY OF IT. THE CONTRIBUTION OF THE BAROCLINICITY ATTHE LOWER LEVELS OF THE ATMOSPHERE WAS FOUND TO BE IMPORTANT OF THE INITIAL STAGE OF THE CYCLOGENESIS. MOREOVER, THE DEEPENING AND THE SUSTENANCE IN ADDITION REQUIRES THE DYNAMIC FORCING CONDITIONS AT THE UPPER LEVELS.ΣΤΗΝ ΠΑΡΟΥΣΑ ΜΕΛΕΤΗ ΑΝΙΧΝΕΥΕΤΑΙ ΚΑΙ ΕΝΤΟΠΙΖΕΤΑΙ ΤΟ ΦΑΙΝΟΜΕΝΟ ΤΗΣ ΚΥΚΛΟΓΕΝΕΣΗΣ ΣΤΟ ΑΙΓΑΙΟ ΠΕΛΑΓΟΣ, ΜΕ ΤΗ ΒΟΗΘΕΙΑ ΜΙΑΣ ΜΕΘΟΔΟΛΟΓΙΑΣ ΠΟΥ ΣΥΝΔΥΑΖΕΙ ΤΗΝ ΕΞΕΤΑΣΗΣΥΝΟΠΤΙΚΩΝ ΧΑΡΤΩΝ ΚΑΙΡΟΥ ΜΕ ΤΟΝ ΥΠΟΛΟΓΙΣΜΟ ΔΥΝΑΜΙΚΩΝ ΚΑΙ ΘΕΡΜΟΔΥΝΑΜΙΚΩΝ ΠΑΡΑΜΕΤΡΩΝ. ΔΙΑΚΡΙΝΟΝΤΑΙ ΕΞΙ ΣΥΝΟΠΤΙΚΕΣ ΚΑΤΗΓΟΡΙΕΣ ΠΟΥ ΘΑ ΜΠΟΡΟΥΣΑΝ ΝΑ ΔΗΜΙΟΥΡΓΗΣΟΥΝ ΕΥΝΟΙΚΕΣ ΣΥΝΘΗΚΕΣ ΚΥΚΛΟΓΕΝΕΣΗΣ ΣΤΟ ΑΙΓΑΙΟ ΠΕΛΑΓΟΣ. ΑΠΟ ΤΗ ΜΕΛΕΤΗ ΤΗΣ ΣΥΧΝΟΤΗΤΑΣ ΕΜΦΑΝΙΣΗΣ ΠΡΟΚΥΠΤΕΙ ΟΤΙ ΤΟ ΦΑΙΝΟΜΕΝΟ ΔΕΝ ΕΙΝΑΙ ΤΟΣΟ ΣΠΑΝΙΟ, ΟΠΩΣ ΘΕΩΡΟΥΝΤΑΝ ΜΕΧΡΙ ΤΩΡΑ. ΕΠΙΠΛΕΟΝ, ΔΙΑΠΙΣΤΩΝΕΤΑΙ ΟΤΙ Η ΚΥΚΛΟΓΕΝΕΣΗ ΣΤΟ ΑΙΓΑΙΟ ΕΠΙΚΡΑΤΕΙ, ΚΑΤΑ ΚΑΝΟΝΑ, ΣΤΗ ΔΙΑΡΚΕΙΑ ΤΗΣ ΨΥΧΡΗΣ ΠΕΡΙΟΔΟΥ ΤΟΥ ΕΤΟΥΣ ΚΑΙ, ΓΕΝΙΚΑ ΧΑΡΑΚΤΗΡΙΖΕΤΑΙ ΑΠΟ ΣΧΕΤΙΚΑ ΜΙΚΡΟ ΧΡΟΝΟ ΖΩΗΣ ΚΑΙ ΕΝΤΑΣΗ. Η ΙΣΕΝΤΡΟΠΙΚΗ ΑΝΑΛΥΣΗ ΤΟΥ ΔΥΝΑΜΙΚΟΥ ΣΤΡΟΒΙΛΙΣΜΟΥ ΧΡΗΣΙΜΟΠΟΙΕΙΤΑΙ ΓΙΑ ΤΗ ΜΕΛΕΤΗ ΚΑΙ ΕΡΜΗΝΕΙΑ ΤΩΝ ΔΥΝΑΜΙΚΩΝ ΚΑΙ ΘΕΡΜΟΔΥΝΑΜΙΚΩΝ ΧΑΡΑΚΤΗΡΙΣΤΙΚΩΝ ΣΕ ΣΥΓΚΕΚΡΙΜΕΝΕΣ ΠΕΡΙΠΤΩΣΕΙΣ ΤΟΥ ΦΑΙΝΟΜΕΝΟΥ ΤΗΣ ΚΥΚΛΟΓΕΝΕΣΗΣ. ΔΙΑΠΙΣΤΩΝΕΤΑΙ Ο ΒΑΡΟΚΛΙΝΙΚΟΣ ΧΑΡΑΚΤΗΡΑΣ ΤΟΥ ΦΑΙΝΟΜΕΝΟΥ, ΠΑΡΑ ΤΗ ΜΙΚΡΗ ΣΧΕΤΙΚΑ ΚΛΙΜΑΚΑ ΚΑΙ ΕΝΤΑΣΗ ΤΟΥ. ΕΠΙΣΗΣ, ΣΥΜΠΕΡΑΙΝΕΤΑΙ ΟΤΙ ΗΘΕΡΜΟΥΓΡΟΜΕΤΡΙΚΗ ΚΑΤΑΣΤΑΣΗ ΣΤΑ ΚΑΤΩΤΕΡΑ ΣΤΡΩΜΑΤΑ ΤΗΣ ΑΤΜΟΣΦΑΙΡΑΣ ΔΙΑΔΡΑΜΑΤΙΖΕΙ ΣΗΜΑΝΤΙΚΟ ΡΟΛΟ ΣΤΗΝ ΕΚΔΗΛΩΣΗ ΤΗΣ ΚΥΚΛΟΓΕΝΕΣΗΣ, ΕΝΩ Η ΠΕΡΑΙΤΕΡΩ ΒΑΘΥΝΣΗ ΚΑΙ ΔΙΑΤΗΡΗΣΗ ΤΗΣ ΑΠΑΙΤΕΙ ΤΗΝ ΕΠΙΔΡΑΣΗ ΕΠΙΠΛΕΟΝ ΤΩΝ ΔΥΝΑΜΙΚΩΝ ΑΙΤΙΩΝ ΤΗΣ ΑΝΩΤΕΡΗΣ ΑΤΜΟΣΦΑΙΡΑΣ

Particulate matter estimation from photochemistry: a modelling approach using neural networks and synoptic clustering

We report on the development and validation of a neural network (NN) model of PM10 concentrations in terms of photochemical measurements of NO, NO_2 and O_3 and temporal parameters that include the day of the week and the day of the year with its sinusoidal variation. A long-term record (≈10 yr) from 2001–2012 (inclusive) assembled from measurements taken at 10 station nodes in the air quality monitoring network of the Greater Athens Area in Greece has been used. Eight synoptic categorizations of the circulation at 850 hPa were used to partition the data record, and to train individual NNs with Bayesian regularization using 90% of available data for different atmospheric conditions. The time series of PM10 estimates was then reconstructed from the partitioned output. As a control, a NN without synoptic clustering was trained on the same data. The remaining 10% of the data was used for testing the simulation performance. NN models with synoptic clustering achieved an average root mean square error (RMSE) ≈ 16 µg/m^3 across the station nodes with an average index of agreement (IA) of 0.71 (somewhat better than the control network whose performance statistics were RMSE ≈ 17 µg/m^3 and IA = 0.61, respectively). For routine measurements below the EU Air Quality Directive limit value of 50 µg/m^3, the average error is as low as RMSE ≈ 11 µg/m^3 across the station nodes. NN models were found to strongly outperform analogous MLR models over all station nodes

A Coherent Approach to Evaluating Precipitation Forecasts over Complex Terrain

Precipitation forecasts provided by high-resolution NWP models have a degree of realism that is very appealing to most users of meteorological data. However, it is a challenge to demonstrate whether or not such forecasts contain more skillful information than their lower resolution counterparts. A verification procedure must be based on equally detailed observations that are also realistic in areas where ground observations are not available and remote sensing data can only increase the accuracy of the location of rain events at the cost of decreased accuracy in estimating the amount of rain that has actually reached the ground. Traditional verification methods based on station or grid point comparison yield poor results for high-resolution fields due to the double penalty error that is attributed to finite space and time displacement that such methods do not account for. A complete approach to evaluating precipitation forecasts over complex terrain is suggested. The method is based on realistic gridded precipitation observations generated by an interpolation method that uses long climate data series to determine the geographical characteristics that this parameter is best correlated with as well as remote sensing estimates as background information to cover the areas where observations are insufficient. Spatial verification methodologies are subsequently applied to a convective event that accentuate the relative skill of high-resolution COSMOGR forecasts in revealing characteristics in the precipitation patterns such as structure and intensity

Developing Gridded Climate Data Sets of Precipitation for Greece Based on Homogenized Time Series

The creation of realistic gridded precipitation fields improves our understanding of the observed climate and is necessary for validating climate model output for a wide range of applications. The challenge in trying to represent the highly variable nature of precipitation is to overcome the lack of density of observations in both time and space. Data sets of mean monthly and annual precipitations were developed for Greece in gridded format with an analysis of 30 arcsec (∼800 m) based on data from 1971 to 2000. One hundred and fifty-seven surface stations from two different observation networks were used to cover a satisfactory range of elevations. Station data were homogenized and subjected to quality control to represent changes in meteorological conditions rather than changes in the conditions under which the observations were made. The Meteorological Interpolation based on Surface Homogenized Data Basis (MISH) interpolation method was used to develop data sets that reproduce, as closely as possible, the spatial climate patterns over the region of interest. The main geophysical factors considered for the interpolation of mean monthly precipitation fields were elevation, latitude, incoming solar irradiance, Euclidian distance from the coastline, and land-to-sea percentage. Low precipitation interpolation uncertainties estimated with the cross-validation method provided confidence in the interpolation method. The resulting high-resolution maps give an overall realistic representation of precipitation, especially in fall and winter, with a clear longitudinal dependence on precipitation decreasing from western to eastern continental Greece

A Statistical Investigation of the Impact of the Indian Monsoon on the Eastern Mediterranean Circulation

The Indian summer monsoon (ISM) is a prominent feature of the summer circulation in the Northern Hemisphere (NH) and has been found to modulate the weather and climate conditions in many remote regions. This study investigates the most recurrent patterns of summertime midlatitude circulation, over the eastern Mediterranean (EM) and also globally, that are most associated with the ISM. Monthly data of 44 summers from the ERA40 dataset are used and two multidimensional statistical methods, the Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA), are implemented. The ISM is found to be related to subsidence anomalies in the middle and more extendedly in the upper troposphere over the central and eastern Mediterranean and with an Etesian-like pattern regarding the field of the lower troposphere winds. An equatorial Rossby wave pattern, extending westward from an ISM heat source up to EM and N. Africa, was identified to be associated with the variability of ISM. The observed relationship between the ISM and the EM circulation features can be attributed to this equatorial Rossby wave response to the monsoon forcing. CCA implementation revealed the interconnection of the aforementioned PCA results with an ISM action center over the northern Arabian Sea and the monsoon trough region

Regional Climatic Features of the Arabian Peninsula

The climate of the Arabian Peninsula is characterized by significant spatial and temporal variations, due to its complex topography and the large-scale atmospheric circulation. Furthermore, the role of dust in the formation of regional climate is considered to be crucial. In this work, the regional climatology for the Arabian Peninsula has been studied by employing a high resolution state of the art atmospheric model that included sophisticated physical parameterization schemes and online treatment of natural aerosol particles. The simulations covered a 30-year period (1986–2015) with a temporal resolution of 3 h and a spatial distance of 9 km. The main focus was given to the spatial and temporal variations of mean temperature and temperature extremes, wind speed and direction, and relative humidity. The results were evaluated using in situ measurements indicating a good agreement. An examination of possible climatic changes during the present climate was also performed through a comprehensive analysis of the trends of mean temperature and temperature extremes. The statistical significant trend values were overall positive and increased over the northwestern parts of the examined area. Similar spatial distributions were found for the daily minimum and maximum temperatures. Higher positive values emerged for the daily maxima

Precipitation Climatology for the Arid Region of the Arabian Peninsula—Variability, Trends and Extremes

The Arabian Peninsula is a region characterized by diverse climatic conditions due to its location and geomorphological characteristics. Its precipitation patterns are characterized by very low annual amounts with great seasonal and spatial variability. Moreover, extreme events often lead to flooding and pose threat to human life and activities. Towards a better understanding of the spatiotemporal features of precipitation in the region, a thirty-year (1986-2015) climatic analysis has been prepared with the aid of the state-of-the-art numerical modeling system RAMS/ICLAMS. Its two-way interactive nesting capabilities, explicit cloud microphysical schemes with seven categories of hydrometeors and the ability to handle dust aerosols as predictive quantities are significant advantages over an area where dust is a dominant factor. An extended evaluation based on in situ measurements and satellite records revealed a good model behavior. The analysis was performed in three main components; the mean climatic characteristics, the rainfall trends and the extreme cases. The extremes are analyzed under the principles of the extreme value theory, focusing not only on the duration but also on the intensity of the events. The annual and monthly rainfall patterns are investigated and discussed. The spatial distribution of the precipitation trends revealed insignificant percentage differences in the examined period. Furthermore, it was demonstrated that the eastern part and the top half of the western Arabian Peninsula presented the lowest risk associated with extreme events. Apart from the pure scientific interest, the present study provides useful information for different sectors of society and economy, such as civil protection, constructions and reinsurance

Precipitation Climatology for the Arid Region of the Arabian Peninsula-Variability, Trends and Extremes

The Arabian Peninsula is a region characterized by diverse climatic conditions due to its location and geomorphological characteristics. Its precipitation patterns are characterized by very low annual amounts with great seasonal and spatial variability. Moreover, extreme events often lead to flooding and pose threat to human life and activities. Towards a better understanding of the spatiotemporal features of precipitation in the region, a thirty-year (1986-2015) climatic analysis has been prepared with the aid of the state-of-the-art numerical modeling system RAMS/ICLAMS. Its two-way interactive nesting capabilities, explicit cloud microphysical schemes with seven categories of hydrometeors and the ability to handle dust aerosols as predictive quantities are significant advantages over an area where dust is a dominant factor. An extended evaluation based on in situ measurements and satellite records revealed a good model behavior. The analysis was performed in three main components; the mean climatic characteristics, the rainfall trends and the extreme cases. The extremes are analyzed under the principles of the extreme value theory, focusing not only on the duration but also on the intensity of the events. The annual and monthly rainfall patterns are investigated and discussed. The spatial distribution of the precipitation trends revealed insignificant percentage differences in the examined period. Furthermore, it was demonstrated that the eastern part and the top half of the western Arabian Peninsula presented the lowest risk associated with extreme events. Apart from the pure scientific interest, the present study provides useful information for different sectors of society and economy, such as civil protection, constructions and reinsurance