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Over the last decades, wind energy industry has been growing with an increasing rate. This is highly relevant to the need of new wind farm site selection with certain standards such as high wind potential and accessibility. Even in windy areas, low wind speed persistence can be characterized as an extreme (non-frequent) atmospheric condition for the electricity network as it can lead to low or no energy production. The current work is focused on the estimation of the duration and the frequency of occurrence of low wind speed events using the principles of extreme value theory. The two methods used are the ‘intensity given duration’ and the ‘duration given intensity’ that lead to the same point from different perspectives. The data used is derived from a 10 year, hindcast, high-resolution database developed by the Atmospheric Modeling and Weather Forecasting Group of the University of Athens. The great potential and multinational interest concerning energy applications in the North Sea has led to its selection as a study area. The outcome of the study includes the development of intensity–duration–frequency curves as well as a comparison between the two methodologies adopted. Based on these, the largest period of no energy production for a preselected probability of occurrence is estimated for the area of interest. The results of this work could be potentially supportive for studying the regional climatology. Such information can be included in risk assessment techniques and can be applied among others for energy activities. Copyright © 2017 John Wiley & Sons, Ltd.

Hourly solar and photovoltaic (PV) forecasts for horizons between 0 and 48h ahead were developed using Environment Canada’s Global Environmental Multiscale model. The motivation for this research was to explore PV forecasting in Ontario, Canada, where feed-in tariffs are driving rapid growth in installed PV capacity. The solar and PV forecasts were compared with irradiance data from 10 North-American ground stations and with alternating current power data from three Canadian PV systems. A 1-year period was used to train the forecasts, and the following year was used for testing. Two post-processing methods were applied to the solar forecasts: spatial averaging and bias removal using a Kalman filter. On average, these two methods lead to a 43% reduction in root mean square error (RMSE) over a persistence forecast (skill score=0.67) and to a 15% reduction in RMSE over the Global Environmental Multiscale forecasts without post-processing (skill score=0.28). Bias removal was primarily useful when considering a “regional”forecast for the average irradiance of the 10 ground stations because bias was a more significant fraction of RMSE in this case. PV forecast accuracy was influenced mainly by the underlying (horizontal) solar forecast accuracy, with RMSE ranging from 6.4% to 9.2% of rated power for the individual PV systems. About 76% of the PV forecast errors were within � 5% of the rated power for the individual systems, but the largest errors reached up to 44% to 57% of rated power. © Her Majesty the Queen in Right of Canada 2011. Reproduced with the permission of the Minister of Natural Resources Canada.

A combined modelling system consisting of a meteorological model (Colorado State University/ Regional Atmospheric Modelling System) and a transport package (Hybrid Particle and Concentration Transport Model) is used. The simulations were performed using two types of model initialisation: the first with ECMWF gridded analysis data and the second with one sounding profile. The modelling system has been used to simulate an important air-pollution episode in the Athens urban area. This area is characterised by complex and variable topographic features, and the associated local atmospheric circulations play a significant role in defining the transport and dispersion conditions over the area. The advantages and shortcomings of both types of initialisation, and more specifically the capability of the model to represent and reproduce the local-scale conditions and the dispersion of air pollutants, are discussed.

The ‘force-restore’ approach is commonly used in order to calculate the surface temperature in atmospheric models. A critical point in this method is how to calculate the deep soil temperature which appears in the restore term of the ‘force-restore’ equation. If the prognostic equation for calculating the deep soil temperature is used, some errors in surface temperature calculation and consequently in partitioning the surface energy and land surface water can be introduced. Usually, these errors should appear as a result of incorrect parameterization of surface energy terms in the prognostic equation based on ‘force-restore’ approach. In this paper, the sensitivity of the ‘force-restore’ model for surface temperature to the: (a) changes of soil heat flux; (b) variations of deep soil temperature and (c) changes in soil water evaporation is examined. In addition, the impact of the deep soil temperature variations on partitioning the surface energy and land surface water is discussed. Finally, a new procedure for calculating the deep soil temperature based, on climatological data of soil temperature and its exponential attenuation in the deep soil layers is suggested. All numerical experiments with the LAPS land surface scheme were performed using two data sets, obtained from the micrometeorological measurements over a bare soil at Rimski Sancevi (Yugoslavia), RS, and Caumont (France), HAPEX. Copyright © 1999 Royal Meteorological Society

An extreme rainfall event, associated with a cold front, which occurred on 11-12 January 1997 over Greece is investigated through observational analysis and numerical simulations. the analysed storm system produced large amounts of rainfall, with devastating impacts over central and southern Greece, where at some sites 24-hour accumulated precipitation exceeded 300 mm. the model analysis was performed with the three-dimensional non-hydrostatic Colorado State University Regional Atmospheric Modeling System. the model results caught the larger-scale features present in the observational analyses while developing more realistic finer-scale features. Sensitivity tests are also performed in order to investigate the role of the topography of the Greek peninsula on the enhancement of convection. Indeed, the orientation of the flow was optimal for advection of moist air with a maximized fetch over the sea, and at an angle of impact optimum for orographic lifting ahead of the main topographic axes of continental Greece.

SUMMARY This paper is devoted to the detailed description of an extreme cold surge that occurred 3-13 March 1987, over Greece. This event has been rated as the worst snowfall over the last 100 years and, duc to its severily and persistence, it paralysed the cconornic and communal life of Greece for several days. Emphasis is given to the initiation phase of this event. The structural evolution of the cold surge is analysed using both observations and model results. Model simulations, pcrformed with the Colorado State University-Regional Atmospheric Modelling System, provided the necessary data to diagnose the mesoscale structure of the cold outbreak. Different mechanisms involved in the structural evolution ofthe cold surge are investigated. The gustiness of the observed winds and their significant departure from geostrophy are related to the role of an important isallobaric wind. The progression of this surge presented characteristics of a density current, while near the eastern slopes of the mountain barriers of continental Greece cold-air damming occurred, leading to an accelerated flow parallcl to the mountains. Heavy snowfall events over Greece are often associated with cold-air surges prop- agating from northern and central Europe through the Balkans (Metaxas 1978; Livada- Tselepidaki 1979; Prezerakos and Angouridakis 1984). These cold surges are accompanied with the passage of a fast moving cold front from the north towards the south, very strong winds, especially in the gaps between mountains in northern Greece and over the Aegean Sea, a sharp temperature decrease over the whole country and a sharp pressure rise. The lifetime of these surges is normally from 1 to 3 days, extending up to 10 days in exceptional cases. Their persistence can cause important problems in economic life, since the result- ing snowfall can prohibit road transportation on the main traffic routes. Greek forecasters call these cold fronts 'Balkan fronts' due to their origin. In the frame of this study this terminology will be retained, although the classical term 'cold surge' will also be used interchangeably. The forecast of the evolution of these fronts as well as the persistence of cold surges over Greece is of great importance, not only for the impact of snowfall on economic life but also for the safety of navigation in the Aegean Sea in gale force winds. Cold surges accompanied by severe weather phenomena have also been reported in the Himalayan region (Nakamura and Murakami 1983), over the USA (e.g. Bell and Bosart 1988; Colle and Mass 1995) as well as over Central America (Schultz et al. 1997). All the aforementioned studies underlined the fact that cold surges are accompanied by strong lower tropospheric northerly winds, sharp decreases in temperature and sharp pressure rises. In these studies some interesting features accompanying cold surges were also ad- dressed, mainly cold-air damming near mountain barriers as a result of the interaction of the synoptic-scale flow with topography. Bell and Bosart (1988) concluded that the component of the pressure gradient force parallel to the mountain was the primary cause for the appearance of an accelerated flow nearly parallel to the mountains. Colle and Mass (1995) demonstrated that pressure ridging east of the mountain barrier caused ageostrophic northerly winds, while cold avection of upslope flow led to the damming of cold air. Schultz et al. (1997) showed that the extraordinary equatorial extent of a cold surge was aided by topographic channelling similar to cold-air damming.

A 14-year (1977-1990) record of global solar radiation measurements performed in Athens, Greece, has been utilized to determine the distribution of radiant energy in the various wavelength bands. The monthly mean values of the irradiation ratios in the spectral intervals, blue (0.380-0525 pm), green-orange (0.525-0.630 pm), red (0630-0.710 pm), and photosynthetically active radiation (PAR) (0.380-0.710 pm) and the global irradiation (0.3-24 pm) compare favourably with values reported in the literature for different locations over a wide geographical area. The irradiation ratios exhibit seasonal variations attributable to changes in local air-mass climatology. It is observed that the proportion of the radiant energy in the various spectral bands relative to the global solar irradiation increases as sky conditions change from ‘clear’ to ‘partly cloudy’. Finally, the observed seasonal dependence of the broad-band spectral energy distribution is essentially caused by changes in the composition of the air masses (turbidity, airborne pollutants, clouds) in residence at the measurement site in the course of the year.