Themis Chronis

Publications (27) View all

• Article: Factors regulating the air-sea heat fluxes regime over the Aegean Sea

  Papadopoulos V.P, Bartzokas A, Chronis T, Georgopoulos D, Ferentinos G
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  ABSTRACT: We examine the impact of low frequency atmospheric forcings on the air-sea heat fluxes over the Aegean Sea. The correlation between the air-sea heat flux components and three established (NAO, EAWR and NCP) and two testing climatic indices of potential effect over the Eastern Mediterranean Sea region underlines significant discrepancies between the radiative (shortwave and longwave radiation) and the turbulent (sensible and latent heat) components. The NAO index affects the air-sea heat fluxes over the Aegean Sea region much less than the two innovative indices, the "Mediterranean Index", and the "Eastern Europe Index", which play more effective roles. Moreover, the influence of the sea level atmospheric pressure (SLP) variability over an extended area (Europe and North Africa) on surface fluxes regime is investigated. The SLP anomalies are corroborated as a prominent regulating factor of the air-sea heat fluxes over the Aegean Sea region, especially during the cold season of the year. The analysis of the extreme values in the heat exchange anomalies for the period 1958-2001 highlights the role of SLP field on determining the air-sea heat fluxes regime mainly during winter, when, occasionally, large amounts of heat loss from the sea surface trigger the mechanism of intermediate and deep water formation. It is suggested that wind regime and turbulent components are the modulators of the net air-sea heat flux anomalies throughout the year
  Journal of Climate 01/2012; 25:491-508. · 4.10 Impact Factor
• Article: Correlation between air-sea heat fluxes over the Aegean Sea and the total precipitable water over Europe and North Africa

  Papadopoulos V.P, Bartzokas A, Chronis T, Ruiz S, Zarokanellos N, Ferentinos G
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  ABSTRACT: The relation between the air-sea heat fluxes in the Aegean Sea and the total precipitable water (TPW) over Europe and North Africa is investigated. A one-point linear correlation between the components of surface heat exchange and TPW is employed. During winter and for the shortwave radiation, a dipole of opposite correlation (see-saw teleconnection), is observed between the northeast Europe and the East Mediterranean Sea. This pattern is inverted for the longwave radiation and especially during the summer is limited to a strong positive south pole. Both spatial correlation patterns underline the influence of cloudiness during the winter season and the specific humidity during summer on the radiative fluxes behaviour. Regarding the turbulent air-sea heat flux components - latent and sensible heat - the correlation pattern is enhanced. The winter pattern exhibits positive correlation over Europe with the highest values (r > 0.75) over the Balkan Peninsula. The identified correlation patterns, although they remain dominant, weaken during the summer. The herein findings suggest that the stronger relation between the TPW and the turbulent fluxes is regulated by the wind regime.
  Advances in Science and Research. 01/2011; 6:63-67.
• Article: The Anomalous Terrestrial Gamma-ray Flash of 17 January 2004

  D. M. Smith, B. W. Grefenstette, M. Splitt, S. M. Lazarus, H. K. Rassoul, L. M. Coleman, J. R. Dwyer, E. H. Lay, R. H. Holzworth, M. B. Cohen, R. Said, U. S. Inan, T. G. Chronis, Y. Takahashi
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  ABSTRACT: As far as we know, nearly all Terrestrial Gamma-Ray Flashes (TGFs) are associated with thunderstorms or at least thunderstorm-rich regions, and many have been shown to be associated with individual lightning flashes via observations of radio atmospherics. More than half of all known TGFs have been observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite. Remarkably, the brightest TGF observed by RHESSI -- over twice as bright as its nearest competitor, and an order of magnitude brighter than a typical TGF -- occurred when the satellite was over the Sahara desert, in northeastern Chad, on 17 January 2004. This region sees little lightning and has hosted no other TGFs. We will present the available gamma-ray, meteorological, and radio data corresponding to the time of this event and explore possible explanations, including: propagation of relativistic electrons from a storm at the magnetic conjugate point; TGF production by thunderstorms near the satellite horizon nearly 2500 km away; and a more local relativistic runaway due to an unknown cause of atmospheric electrification not associated with a thunderstorm.
  AGU Fall Meeting Abstracts. 11/2006; -1:1040.
• Article: Evaluation of a long-range lightning detection network with receivers in Europe and Africa

  T.G. Chronis, E.N. Anagnostou
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  ABSTRACT: This study focuses on the performance evaluation of the experimental long-range lightning detection network Zeus, with receivers located in Europe and Africa. The evaluation is carried out in terms of locating error and detection performance. Zeus' receivers record the radio noise (sferics) emitted by cloud-to-ground lightning discharges in the very low frequency (bandwidth between 5 and 15 kHz). The lightning location is retrieved by means of the arrival time difference (ATD) triangulation technique. The presented study includes evaluation of the system's locating error and detection performance over the African continent, equatorial/mid-Atlantic, the Caribbean, and northern South America. This dataset is validated over a period extending from July through October 2004. The Lightning Imaging Sensor onboard NASA's Tropical Rainfall Measuring Mission satellite is used as the validation source. The analysis shows that the number of receivers as well as their relative location (relative position of the flash-event to the network's receivers) plays a key role in the retrieval error magnitude. Median error values within the network's periphery are found to be approximately 20 km, while outside the periphery significantly larger (∼130 km). Simulation of chi-square values, implemented from the ATD algorithm, shows an adequate agreement between theoretical error-related computations and the network's retrievals. In terms of detection performance, the Zeus system is also validated over the same area/period. The chief observation is that the detection performance is likewise dependent on the number of receivers used by the system rather than their relative location.
  IEEE Transactions on Geoscience and Remote Sensing 07/2006; · 2.89 Impact Factor
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  Article: The influence of large dams on surrounding climate and precipitation patterns

  Ahmed Mohamed Degu, Faisal Hossain, Dev Niyogi, Roger Pielke, J Marshall Shepherd, Nathalie Voisin, Themis Chronis
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  ABSTRACT: 1] Understanding the forcings exerted by large dams on local climate is key to establishing if artificial reservoirs inadvertently modify precipitation patterns in impounded river basins. Using a 30 year record of reanalysis data, the spatial gradients of atmospheric variables related to precipi-tation formation are identified around the reservoir shoreline for 92 large dams of North America. Our study reports that large dams influence local climate most in Mediterranean, and semi‐arid climates, while for humid climates the influ-ence is least apparent. Clear spatial gradients of convective available potential energy, specific humidity and surface evaporation are also observed around the fringes between the reservoir shoreline and farther from these dams. Because of the increasing correlation observed between CAPE and extreme precipitation percentiles, our findings point to the possibility of storm intensification in impounded basins of the Mediterranean and arid climates of the United States.
  Geophys. Res. Lett. 01/2011; 38.