Theodore S. Karacostas

Climatological aspects of extreme precipitation in Europe, related to mid-latitude cyclonic systems

Climatic aspects of extreme European precipitation are studied. Daily pluviometric data from 280 stations across Europe, covering the period from 1958 to 2000, are used. First, the criteria for extreme precipitation cases and episodes are communicated using threshold and spatial definitions. The cases and episodes meeting these criteria are grouped according to their area of appearance. Most of them are located in three major areas: Greece, the Alps, and the Iberian Peninsula. The existence of trends in the annual and seasonal time series of these extreme events is examined. Decreasing trends are found in most of the cases, for Greece, the Iberian Peninsula, and Europe, as a whole. The Alps present a different behavior, with no trend at all in the southern part, and a possible increasing trend in the northern part. Finally, the positive impact of altitude in the frequency of occurrence of extreme precipitation episodes in Europe is discussed.

Drought assessment for the potential of precipitation enhancement in northern Greece

SummaryThe paper is part of a series earlier studies in which the precipitation enhancement potential in central and northern Greece is assessed. A drought analysis is performed in this study. In particular, the Palmer Drought Severity Index (PDSI) is used for the quantitative estimation of droughts on a monthly basis. Emphasis is placed on drought identification and estimation as well as the severity and areal extent of the phenomenon. The results indicate that there are several drought periods which are common to all stations in central and northern Greece, characterized by significant severity and time duration. There are also a large number of drought periods during the summer months as well as during the winter months.

A Study of Fog Characteristics Using a Coupled WRF–COBEL Model Over Thessaloniki Airport, Greece

An attempt is made to couple the one dimensional COBEL-ISBA (Code de Brouillard à l’Échelle Locale-Interactions Soil Biosphere Atmosphere) model with the WRF (Weather Research and Forecasting)–ARW (Advanced Research WRF) numerical weather prediction model to study a fog event that formed on 20 January 2008 over Thessaloniki Airport, Greece. It is the first time that the coupling of COBEL and WRF models is achieved and applied to a fog event over an airport. At first, the performance of the integrated WRF–COBEL system is investigated, by validating it against the available surface observations. The temperature and humidity vertical profiles were used for initializing the model. The performance of WRF–COBEL is considered successful, since it realistically simulated the fog onset and dissipation better than the WRF alone. The COBEL’s sensitivity to initial conditions such as temperature and specific humidity perturbations was also tested. It is found that a small increase of temperature (~1°C) counteracts fog development and results in less fog density. On the other hand, a small decrease of temperature results in much denser fog formation. It is concluded that the integrated model approach for aviation applications can be useful to study fog impact on local traffic and aviation.

The Rain Enhancement Experiment in Puglia, Italy: Statistical Evaluation

A randomized rain enhancement experiment was carried out during 1988‐94 in the area of Bari and Canosa, Italy, on the Adriatic coast. It was commissioned by the Italian Department of Agriculture and Forestry and the region of Puglia, with TECNAGRO, a nonprofit Italian company, as overall manager, and with EMS, an Israeli company, as field operator. The original purpose was to study rain-producing weather systems in southern Italy, establish similarities with Israel, and transfer Israeli technology. The experiment was a cross-over design with two alternating target areas, a buffer in between, and two additional control areas. Seeding was by injection of silver iodide into clouds by aircraft flying near the bases of clouds along predetermined tracks upwind of the target area. The experimental units were rainy days. Based on historical rain gauge data, it was estimated that 303 rainy days were required to establish a 15% rain increase at a significance level of 0.05 and 90% power. In 1995, TECNAGRO asked the Scientific Committee for a statistical evaluation to investigate if a seeding effect could be established before the original goal of 303 seeding days was reached. The results of the analysis of the 260 available rainy days were that no discernable seeding effect could be found. This was evident from the root double ratio (RDR) and root regression ratio (RRR), which yielded RDR 2 1 52 0.083 6 0.089 and RRR 2 1 52 0.004 6 0.057, respectively (the 6 sign represents the standard error of the estimate). Based on that result, it was decided to terminate the Puglia seeding experiment. Preliminary exploratory studies suggest that the two target areas might have been affected differently by seeding and that an apparent substantial seeding effect occurred in the Bari area under conditions of moderate precipitable water between 700 and 850 mb. If these findings are confirmed by the recommended meteorological analyses and airflow studies, a new experiment with an appropriate design might be justified.

Hailstorm in the region of Central Macedonia, Greece : A kinematic study

The purpose of this study is the analysis of radar data, digitally recorded, during an operational hail suppression program in the region of Central Macedonia, Greece, for the warm period of the years 1997-2001. Data have been quality controlled, so that only those storms unaffected by external factors be examined. Kinematic characteristics, such as lifetime and distance traveled by hailstorms, as well as direction of motion and speed, have been related to the type of storms, the synoptic environment and the season. It has been found that singlecells are short-lived and travel short distances, while multicells are long-lived and travel long distances. On the contrary, their corresponding speed distributions are similar. The deviation of the direction of motion from mean wind is smaller for singlecells than for multicells. Two classes of synoptic circulation types can be identified: one which favours long-lived storms traveling long distances and another which favours short-lived storms traveling short distances. September and July exhibit the maximum and minimum average storm speeds as a direct implication of synoptic disturbances passage and convection, prevailing respectively. Finally, storms overcoming orographic barriers decelerate in general on the windward side and accelerate on the lee side of mountains.

A systematic hailpad calibration procedure for operational hail suppression in Greece

SummaryHailpads are used to provide quantitative hailfall measurements in several hail experiments and hail suppression operations around the world. The dented hailpads record the time-integrated size distribution and concentration of hailfall. In the five-year Greek National Hail Suppression Program (GNHSP) hailpad data have been used to estimate the global (impact) energy of hailswaths for the evaluation of the GNHSP.In this paper a systematic hailpad calibration procedure is developed applicable to operational programs. To meet this objective a calibration experiment has been conducted consisting of several tests to: consider differences between pad types; to examine the effects of ultra violet-light on hailpads for varying periods of time; to investigate the effect of painting and inking of the hailpad surfaces; to consider the effect of analysts variability, loose hailpad stands, and bouneing; and to develop calibration eqqations. The concluded results seem to justify the design and performance of the hailpad calibration procedure.

Turbulent Kinetic Energy Budgets over Mountainous Terrain

Abstract The objective of this study is to describe the characteristics of the airflow and turbulence structure over mountainous terrain. Turbulent characteristics of the airflow were measured using well-instrumented aircraft. The shear, buoyancy, transport of energy and eddy dissipation rate terms were obtained from direct measurements. The turbulent kinetic energy budgets were determined with respect to height and horizontal distance upwind and downwind of the mountain. The change of turbulence intensity was also demonstrated by comparing power spectra as a function of height, as well as a function of distance upwind and downwind of the mountain. The results show that all measurable terms were significant. The shear production and the eddy dissipation rate were the dominant terms. The buoyancy and vertical transport terms were smaller but still important. The imbalance term was estimated to be relatively small.

Numerical simulation of airborne cloud seeding over Greece, using a convective cloud model

An extensive work has been done by the Department of Meteorology and Climatology at Aristotle University of Thessaloniki and others using a three-dimensional cloud resolving model to simulate AgI seeding by aircraft of three distinct hailstorm cases occurred over Greece in period 2007-2009. The seeding criterion for silver iodide glaciogenic seeding from air is based on the beneficial competition mechanism. According to thermodynamic analysis and classification proposed by Marwitz (1972a, b, and c) and based on their structural and evolutionary properties we classified them in three groups as singlecell, multicell and supercell hailstorms. The seeding optimization for each selected case is conducted by analysis of the thermodynamic characteristics of the meteorological environment as well as radar reflectivity fields observed by the state of the art Thunderstorm Identification, Tracking, Analysis and Nowcasting (TITAN) software applied in the Greek National Hail Suppression Program (GNHSP). Results of this comprehensive study have shown positive effects with respect to hailfall decrease after successful seeding as our primarily objective. All three cases have illustrated 15-20% decrease in accumulated hailfall at the ground Seeded clouds have exhibited earlier development of precipitation and slight dynamical enhancement of the updraft and rainfall increase of ~10- 12.5%. The results have emphasized a strong interaction between cloud dynamics and microphysics, especially the subgrid scale processes that have impact on agent transport and diffusion in a complex environment. Comparisons between modelled and observed radar reflectivity also show a relatively good agreement. Simulated cloud seeding follows the operational aircraft seeding for hail suppression. The ability of silver-iodide particles to act as ice nuclei has been used to perform airborne cloud seeding, under controlled conditions of temperature and humidity. The seeding effects depend upon applying the seeding methodology in proper seeding time, right placement and agent dose rate.

Numerical Study of the Medicane of November 2014

A hurricane-like cyclone with an ‘eye’, eyewall convection and strong winds affected central Mediterranean basin on 7–8 November 2014. The maximum observed sustained wind speed was 22 m/s (tropical storm strength) at Lampedusa. Significant damages were reported from this island and the coastal regions of eastern Sicily. Thus, it is essential to study medicanes and calibrate the numerical weather prediction models in order to simulate them adequately. Operational ECMWF analyses are used together with the non-hydrostatic Weather Research and Forecasting numerical model with the Advanced Research dynamic solver (WRF-ARW ver 3.7.1). The aims of this study are to simulate the system and investigate the sensitivity of the model on the microphysical scheme, the number of vertical levels and the global input dataset. The main characteristics of the medicane are represented in good agreement with observations and analyses, but, no single setup is able to provide the best reproduction of all its features.

Watershed Size, an Alternative or a Misguided Parameter for River’s Waterpower? Implementation in Macedonia, Greece

This paper examines the effectiveness of using the watershed size, as an alternative stand-alone parameter that substitutes the river’s waterpower. The waterpower of each river is the main factor shaping the surface waters and is directly related with the water cycle balance. Watershed size was used by the (WFD) Water Framework Directive 2000/60 for Community action in the field of water policy as an alternative for the river’s waterpower. In this study, 49 watersheds of Macedonia, Greece were examined and classified by their watershed size, according to the WFD. The mean annual temperature and precipitation was estimated at each one of these catchments based on data from 64 different meteorological stations, which belong to the Ministry of Rural Development and Food and the examined time period was thirty years (1974–2004). The geology of Macedonia was classified in 2 classes at scale 1:500,000 and in 6 classes at scale 1:50,000. The corresponding mean annual surface runoff was estimated respectively to each geological classification. The results were compared with the suggested modified WFD watershed size classification and all were evaluated respectively.