Winter cloudiness variability in the Mediterranean region and its connection to atmospheric circulation features

The temporal and spatial variability of winter total cloud cover in southern Europe and the Mediterranean region and its connection to the synoptic-scale features of the general atmospheric circulation are examined for the period 1950–2005, by using the diagnostic and intrinsic NCEP/NCAR Reanalysis data sets. At first, S-mode factor analysis is applied to the time series of winter cloud cover, revealing five factors that correspond to the main modes of inter-annual variability of cloudiness. The linkage between each of the five factors and the atmospheric circulation is examined by constructing the 500 hPa and 1,000 hPa geopotential height anomaly patterns that correspond to the highest/lowest factor scores. Then, k-means cluster analysis is applied to the factor scores time series, classifying the 56 years into six distinct clusters that describe the main modes of spatial distribution of cloudiness. Eventually, canonical correlation analysis is applied to the factor scores time series of: (1) 500 and 1,000 hPa geopotential heights over Europe and the North Atlantic Ocean and (2) total cloud cover over southern Europe and the Mediterranean, in order to define the main centers of action in the middle and the lower troposphere that control winter cloudiness variability in the various sub-regions of the area under study. Three statistically significant canonical pairs are revealed, defining the main modes of atmospheric circulation forcing on cloudiness variability. North Atlantic oscillation and European blocking activity modulate the highest percentage of cloudiness variability. A statistically significant negative trend of winter cloudiness is found for central and southern Europe and the Mediterranean region. This negative trend is associated with the corresponding positive trends in NAO and European blocking activity.     

Adaptation of the Canadian Fire Weather Index to Mediterranean forests

Natural Hazards - The Canadian Fire Weather Index (FWI) is one of the most used tools to forecast forest fire hazard. In this paper, we propose an adaptation of the FWI to take into account the...     

A 40-year climatology of air temperature extremes in the southern Balkans based on the ERA5 database

Theoretical and Applied Climatology - The air temperature extremes in the southern Balkans and the synoptic conditions that favor their occurrence are examined for the 40-year period...     

Characteristics of the atmospheric circulation associated with cold-season heavy rainfall and flooding over a complex terrain region in Greece

Raingauge data from four meteorological stations in Chalkidiki peninsula (Greece) were used to identify the characteristics of the synoptic circulation patterns associated with cold-season heavy rainfall events and corresponding flooding over the area. Precipitation climatology over the complex topography of Chalkidiki is characterized by limited annual rainfall, but in the occurrence of heavy rainfall episodes daily accumulations are exceptionally high with increased precipitation rates, leading often to severe flooding. Fifty-five cases of high daily accumulations for the period of 1997 to 2010, mostly observed during December and October, were classified into eight clusters by applying S-Mode Factor Analysis and Cluster Analysis to the ERA-Interim grid point reanalysis data. The results revealed that, in most cases, intense rainfall and flooding is produced by synoptic scale disturbances that generate and sustain cyclonic activity over south Italy, the Ionian, and less frequently over the Aegean Sea. In particular, the atmospheric circulation patterns associated with heavy rainfall are characterized by the presence of a southeasterly–easterly low-level humid flow over Chalkidiki in conjunction with the potentially unstable lower troposphere influenced by mid-level cyclonic vorticity advection and enhanced low-level convergence over the complex terrain.     

Sea-level pressure–air temperature teleconnections during northern hemisphere winter

The relationship between sea-level pressure (SLP) and 1,000?hPa air temperature (AT) in winter is investigated over the northern hemisphere, and a statistical forecasting of one of the two parameters from the other is attempted on a monthly basis. Mean monthly SLP and 1,000?hPa level AT values at 563 grid points over the northern hemisphere are utilized for January, February and March, for the period 1949–2002. At first, factor analysis is applied to the data sets as a dimensionality reduction tool. Then, canonical correlation analysis is applied to the resultant factor scores time series for the five SLP–AT pairs: SLP(J)–AT(J), SLP(J)–AT(F), SLP(J)–AT(M), AT(J)–SLP(F) and AT(J)–SLP(M), and the synchronous and time-lag connections between the two parameters are investigated. The areas characterized by a satisfactory monthly or/and bi-monthly forecasting ability are detected. The most satisfactory results refer to the areas affected by the Southern Oscillation. It is found that the SLP teleconnection between the areas of the eastern and the western Pacific in January is related to the regime of AT in the central Pacific Ocean, in both February and March. Also, SLP over the Aleutian and Icelandic lows in January is related to AT over their southwestern and southeastern neighbouring areas in February and March. Finally, it appears that there is also ability for monthly/bi-monthly statistical prediction for some areas affected by the well-known oscillations of North Atlantic Oscillation and Pacific/North American Oscillation. A validation of the statistical prediction methodology is carried out, using real-time series of AT and SLP parameters for some characteristic cases. The results show that the statistical prediction presents a remarkable success. The success rate varies from 67% to 83% for the analysis period 1949–2002 and from 71% to 86% for the recent period 2003–2009.     

Winter convective precipitation variability in southeastern Europe and its connection to middle tropospheric circulation for the 60-year period, 1950–2009

The inter-annual variability of winter convective precipitation rate (CPR) in southeastern Europe and its connection to 500?hPa geopotential height (GH) is examined for the period 1950–2009 by using factor analysis and canonical correlation analysis. Two GH centers of action for CPR are found. The first one is located over Italy and it is associated with the typical winter depression activity regime over the Mediterranean Sea, controlling CPR in southern Italy, the southern Balkans, west Asia Minor, and the adjacent seas. The second one is located over the British Isles and it is associated with blocking activity over western Europe being responsible for a CPR seesaw teleconnection between (1) northern Italy, the Alps and the northwestern Balkans and (2) the south central Mediterranean Sea, south of Sicily. A CPR decrease in most of the areas under study and a CPR increase in the south central Mediterranean Sea are found.     

Intra-annual variation of atmospheric static stability in the Mediterranean region: a 60-year climatology

The seasonal characteristics of atmospheric static stability in the Mediterranean region are examined, for the 60-year period 1948–2007 and for the four 15-year sub-periods 1948–1962, 1963–1977, 1978–1992 and 1993–2007. S-Mode and T-Mode Factor Analysis are applied to the mean 5-day values of K static stability index over the Mediterranean region. Three dominant modes are revealed for both, the intra-annual variation and the spatial distribution of K-index. It is found that these modes are connected to the seasonal characteristics of the main atmospheric circulation systems affecting the region and the thermal properties of the Earth’s surface (land or sea). The differences among the results of the four sub-periods partially reflect the inter-decadal variations of the strength of the above factors.