The impact of an upper tropospheric teleconnection pattern on precipitation extremes over Cyprus

International audienceThe objective of this study is to estimate the duration, frequency and intensity of precipitation extreme episodes in Cyprus, in relation with the two phases of the Eastern Mediterranean teleconnection Pattern (EMP), during winter for the period 1958?2005. A standardised teleconnection index was employed to determine the phases (positive and negative) and the strength of the EMP. The identification of the precipitation extremes was performed with the aid of four climatic indices. It was found that during the positive phase of the pattern, the length of dry periods reduces while that of wet periods increases, being followed by increase of frequency of extreme wet days and precipitation intensity. On the contrary, during the negative phase, the dry spells become longer in accordance with shortening of the wet spells, decrease of the number of extreme wet days and precipitation intensity

Diagnostics of cyclogenesis over the Aegean sea using potential vorticity inversion

In this study an attempt is made to investigate comprehensively the dynamics of a case of cyclogenesis over the Aegean Sea within the context of the potential vorticity. At early stages the cyclogenesis is manifested by a large scale development at the upper levels over Adriatic Sea and Yugoslavia associated with an upper tropospheric potential vorticity anomaly. At later stages a smaller scale development was generated over Aegean Sea associated with a low-level potential vorticity anomaly and a surface warm anomaly. By means of a two-dimensional potential vorticity inversion it is demonstrated that the scale, the position and the strength of the involved anomalies contribute to the surface development, however, the low-level potential vorticity anomaly seems to constitute the most significant feature, more likely to be associated with condensation

Universal thermal climate index (UTCI) and synoptic circulation patterns over the metropolitan city of Athens, Greece

The present study aims to investigate the human thermal stress with respect to the synoptic scale circulation over the metropolitan city of Athens in Greece. The Universal Thermal Climate Index (UTCI) was applied in order to assess the levels of thermal stress employing three-hour meteorological data for four stations of the urban complex for a ten-year period (2006-2015). The patterns of the synoptic atmospheric circulation were classified in eight distinct categories based on a clustering scheme. The obtained results in summer reveal low UTCI values on days that are associated with a cyclonic pattern or a cold advection from the northwest or north. On the contrary, the days with an anticyclonic pattern show high UTCI values. During winter, low UTCI values are observed on days that are associated with a pure cyclonic pattern, a prevailing northern flow and a strong cold air advection from the northwest or north. During nighttime, high UTCI values are noted on days with south-westerly flow. However, this is not observed during daytime and especially around midday. The analysis also shows that the position of the station affects the levels of thermal stress for the same synoptic category. © 2018 Global NEST Printed in Greece

A case study of Saharan cyclogenesis

A case of Saharan cyclogenesis associated with the equatorward intrusion of a trough at the end of the Atlantic storm track is investigated. It is shown that a potential vorticity anomaly resulting from a baroclinic wave life cycle associated with the polar jet interacts with the low-level baroclinicity over subtropical Africa beneath the subtropical jet. It is suggested that low-latitude synoptic-scale cyclogenesis events of this type can be triggered only by upper-level potential vorticity anomalies if they have a sufficient depth scale and that the interaction may be aided by the presence of low static stability associated with a well-mixed boundary layer. A weak cold front also forms during the cyclogenesis event associated with the convergence of the baroclinicity of the polar trough front with the baroclinicity of the subtropical heat low

Hailstorms in Northern Greece: Synoptic patterns and thermodynamic environment

The synoptic scale atmospheric circulation prevailing during hail days in Northern Greece is examined for a period of 26 years (1976-2001) during the warm season. The initial objective of this classification scheme is to serve as a predictive tool for hail forecasting and permit a specification of greater or lesser hail risk, within the forecast activities of the Greek National Hail Suppression Program. The atmospheric circulation was classified into seven synoptic types and their frequency distribution was examined on an annual and monthly basis. Southwesterly flow (SW) and Shortwave trough (SWT) appear to be the most conducive synoptic types for hailstorm activity. Furthermore, the thermodynamic environment favoring the hailstorm development and evolution was examined using selected instability indices. The frequency distribution of the instability indices for a period of 11 years (1991-2001) in association with the synoptic types demonstrated their performance as a forecasting aid for thunderstorm activity to occur in a given synoptic environment

The role of a developing upper diffluent trough in surface cyclogenesis over central Mediterranean

An attempt is made to investigate the role of the jet streaks upstream an upper tropospheric diffluent trough in its evolution. A proof of the relationship that controls the rate of change of the mean absolute vorticity in the region of the diffluent trough is provided. It is demonstrated that for certain value of the rate of the increasing mean regional vorticity, a cut-off low can develop in the upper troposphere, which is likely to trigger the initiation of surface cyclogenesis, under favourable lower tropospheric conditions. Therefore, it seems that this approach offers to the operational meteorologists a simple tool to evaluate the numerical weather prediction of a cyclonic development in southern Europe and to decide for its uncritical accept or rejection when the initial atmospheric conditions in northwest Europe are similar to those described above. This diagnostic tool is then applied to the case of cyclogenesis that occurred on the 29th January 1994 in Greece, in order to demonstrate its practical significance

Simulating the thermal behaviour of a building during summer period in the urban environment

In this study an attempt is made to investigate the impact of the urban heat island (UHI) effect on the energy consumption of a building using both, an accurate, extensively validated, transient simulation model and a neural network one. The energy consumption is calculated for a typical building during the summer period at 20 different sites of the Greater Athens area with the aid of the TRNSYS model. It was found that the UHI affects significantly the thermal behaviour of the building, implying much lower cooling load cost and energy cost in the suburban areas. A neural network model was then appropriately designed and tested for the estimation of the energy consumption, employing as an input, the UHI intensity. The results of both methods were tested and compared and it was found that there is a good agreement between the cooling rate values calculated by the TRNSYS programme and those derived by the neural model. Moreover, the employment of the neural model helped in quantifying the contribution of each input parameter in the calculation of the cooling rate, and it was demonstrated that the UHI effect is a predominant parameter, affecting considerably the energy consumption of a building in the Mediterranean region. © 2006 Elsevier Ltd. All rights reserved

An experimental study of aerosol distribution over a Mediterranean urban area

In this study an attempt is made to investigate the aerosol spatial and size distributions at different heights over the Greater Athens Area (GAA), Greece, under sea breeze conditions and clear sky and to further discuss possible implications for aerosol characteristics. The data used are airborne measurements of aerosol collected during two flights that were performed within the context of the 1997 STAAARTE experimental campaign. The aerosol measurements cover particle diameters from 0.1 to 45.5 μm. The horizontal and vertical distribution revealed that higher concentrations exist within or just above the atmospheric boundary layer, while greater concentrations are observed over the sea compared to land at high altitudes. At all altitudes the number size distributions show dominant diameter ranges between 0.1 and 0.3 μm at all altitudes. The volume distributions are characterised by two modes, one in the accumulation and the other in the coarse particle regime. At lower altitudes, fresh combustion emissions more likely cause the predominance of the size range 0.1-0.3 μm while enhanced physical and chemical processes that favour the growth of smaller particles to larger sizes could also act. The relative humidity does not seem to affect the observed number size distributions at low altitudes, where relative humidity is below 70% while at 4000 m the distributions seem to change over the sea where the humidity increases. © 2006 Elsevier B.V. All rights reserved

An experimental study of aerosol distribution over a Mediterranean urban area

In this study an attempt is made to investigate the aerosol spatial and size distributions at different heights over the Greater Athens Area (GAA), Greece, under sea breeze conditions and clear sky and to further discuss possible implications for aerosol characteristics. The data used are airborne measurements of aerosol collected during two flights that were performed within the context of the 1997 STAAARTE experimental campaign. The aerosol measurements cover particle diameters from 0.1 to 45.5 μm. The horizontal and vertical distribution revealed that higher concentrations exist within or just above the atmospheric boundary layer, while greater concentrations are observed over the sea compared to land at high altitudes. At all altitudes the number size distributions show dominant diameter ranges between 0.1 and 0.3 μm at all altitudes. The volume distributions are characterised by two modes, one in the accumulation and the other in the coarse particle regime. At lower altitudes, fresh combustion emissions more likely cause the predominance of the size range 0.1-0.3 μm while enhanced physical and chemical processes that favour the growth of smaller particles to larger sizes could also act. The relative humidity does not seem to affect the observed number size distributions at low altitudes, where relative humidity is below 70% while at 4000 m the distributions seem to change over the sea where the humidity increases. © 2006 Elsevier B.V. All rights reserved

The role of the interaction between polar and subtropical jet in a case of depression rejuvenation over the Eastern Mediterranean

In this paper, an attempt is made to investigate the synoptic-scale atmospheric conditions and dynamic processes leading to the rejuvenation of depressions or cyclogenesis over the Eastern Mediterranean during the cold period of the year. A case study analysis is carried out when the Polar Front Jet (PFJ) is positioned to the south of its normal seasonal position and interacts with the Subtropical Jet Stream (SJS), which is positioned to the north of its normal seasonal position. It was found that the vigorous rejuvenation of a northeastwards moving Atlas mountains depression on 15 March 1998 over Cyprus region is associated with an increase of the hydrodynamic instability due to the juxtaposition of a polar front jet streak to the subtropical jet stream. Furthermore, the rejuvenation is related to the combined effect of the direct and indirect cross vertical circulations appearing on the right side of the entrance of a polar jet streak and the left side of a subtropical jet streak exit, respectively. © Springer-Verlag 2005