I. Pytharoulis

The Low-Level Structure of African Easterly Waves in 1995

Abstract The existence of African easterly waves (AEWs) north of the African easterly jet (AEJ) core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the U.K. Meteorological Office global model analysis from the hurricane season of 1995. At Bamako (12.5°N, 8.0°W) the AEWs were characterized mainly by maximum amplitudes at the level of the AEJ (around 700 mb), whereas at Dakar (14.7°N, 17.5°W) the waves were characterized by maxima between 850 and 950 mb. The low-level waves to the north of the AEJ arise in association with baroclinic interactions between the negative meridional potential vorticity (PV) gradients in the jet core and the positive low-level gradient of potential temperature, θ, enhanced by the presence of low-static-stability air north of the AEJ. These waves follow the positive meridional θ gradients over northern Africa in contrast to the jet-level AEWs that follow the meridional PV gradients at the level of the AEJ. Cross-correlation analysis...

Dynamic processes of mercury over the Mediterranean region: results from the Mediterranean Atmospheric Mercury Cycle System (MAMCS) project

The Mediterranean Atmospheric Mercury Cycle System (MAMCS) project was performed between 1998 and 2000 and involved the collaboration of universities and research institutes from Europe, Israel and Turkey. The main goal of MAMCS was to investigate dynamic processes affecting the cycle of mercury in the Mediterranean atmosphere by combining ad hoc field measurements and modelling tasks. To study the fate of Hg in the Mediterranean Basin an updated emission inventory was compiled for Europe and the countries bordering the Mediterranean Sea. Models were developed to describe the individual atmospheric processes which influence the chemical and physical characteristics of atmospheric Hg, and these were coupled to meteorological models to examine the dispersion and deposition of Hg species in the Mediterranean Basin. One intercomparison and four two-week measurement campaigns were carried out over a three-year period. The work presented here describes the results in general terms but focuses on the areas where definite conclusions were unforthcoming and thus highlights those aspects where, in spite of advances made in the understanding of Hg cycling, further work is necessary in order to be able to predict confidently Hg and Hg compound concentration fields and deposition patterns.

The hurricane‐like Mediterranean cyclone of January 1995

The development of a hurricane-like vortex over the Mediterranean Sea was studied using (mainly) the UK Met. Office Unified Model. The Mediterranean cyclone formed in the morning of 15 January 1995 over the sea between Greece and Sicily. Strong convection was observed prior to its genesis. During the longest part of the cyclones lifetime, strong surface fluxes and, as a result, deep convection existed in its vicinity. Its track was influenced by the surface fluxes and the flow in the wider region. The forecast of the mesoscale and limited-area models reproduced the general characteristics of the actual system as they appeared at the surface and upper-air charts and at the satellite imagery. The investigation of the cyclones characteristics gave strong evidence (including an ‘eye’ and a warm core) to support the initial assertion that it was similar to tropical cyclones and some polar lows. Baroclinic instability does not seem particularly important, although the cyclone formed at the edge of a baroclinic zone. A numerical experiment showed the vortex did not develop in the absence of surface heat and moisture fluxes. Another experiment showed that sensible and latent heat fluxes were equally important in its development. Copyright

A Dynamical Approach to Seasonal Prediction of Atlantic Tropical Cyclone Activity

Abstract Analysis of ECMWF reanalyses and operational analyses covering the period between 1979–98 has confirmed that seasonal Atlantic tropical cyclone activity is strongly and negatively correlated with the observed vertical wind shear present in the main development region (MDR) between July and September. In 1983 and 1995, the least active and most active tropical cyclone years, respectively, anomalous shear was shown to be present in spring and to persist throughout each of the tropical cyclone seasons. While monitoring of MDR shear is recommended for highlighting the risk of such extreme events, the springtime MDR shear is not generally a good indicator of shear in the summer months. Seasonal forecasts of MDR shear made with the U.K. Met Office (UKMO) atmospheric GCM (AGCM) and observed SSTs for the years 1979–97 have been analyzed. The model possesses potential skill for predicting the MDR shear as determined by a consideration of the ensemble mean shear variability and an evaluation of the relativ...

Study of the Hurricane-like Mediterranean cyclone of January 1995

Abstract The development of a hurricane-like cyclone over the Mediterranean sea has been studied using observational data and the UKMO Unified Model. The formation of the Mediterranean cyclone took place in the morning of the 15th of January 1995 over the sea between Greece and Sicily. Deep convection existed in the vicinity of the cyclone during its lifetime. Its track was influenced by surface fluxes and the flow in the wider region. The forecast of the mesoscale model reproduced the characteristics of the actual system. The investigation of the cyclones characteristics gave strong evidence to support the initial assertion that it was similar to tropical cyclones and some polar lows (including an ‘eye’ and a warm core). A numerical experiment showed that the vortex did not develop in the absence of surface heat and moisture fluxes. Another experiment showed that sensible and latent heat fluxes were equally important in its development. Baroclinic instability did not seem particularly important, although the formation took place at the edge of a baroclinic zone.

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.

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.

Ten-year operational dust forecasting – Recent model development and future plans

The Sahara desert is one of the major sources of mineral dust on Earth, producing up to 2x108 t yr-1. A combined effort has been devoted during the last ten years at the University of Athens (UOA) from the Atmospheric Modeling and Weather Forecasting Group (AM&WFG) to the development of an analysis and forecasting tool that will provide early warning of Saharan dust outbreaks. The developed tool is the SKIRON limited-area forecasting system, based on the Eta limited area modeling system with embedded algorithms describing the dust cycle. A new version of the model is currently available, with extra features like eight-size particle bins, radiative transfer corrections, new dust source identification and utilization of rocky soil characterization and incorporation of more accurate deposition schemes. The new version of SKIRON modeling system is coupled with the photochemical model CAMx in order to study processes like the shading effect of dust particles on photochemical processes and the production of second and third generation of aerosols. Moreover, another new development in the AM&WFG is based on the RAMS model, with the incorporation of processes like dust and sea-salt production, gas and aqueous phase chemistry and particle formation. In this study, the major characteristics of the developed (and under development) modeling systems are presented, as well as the spatiotemporal distribution of the transported dust amounts, the interaction with anthropogenically-produced particles and the potential implications on radiative transfer.

Modelling Framework for Atmospheric Mercury over the Mediterranean Region: Model Development and Applications

The atmospheric part of the mercury cycle is considered as very complicated because of the various physicochemical processes involved. The temporal and spatial scales of various processes are varying according to mercury species. While Hg 0 is considered as long-range transport pollutant, Hg II is fast reacting and deposits quickly (wet and dry). Hg P has behaviour similar to the other particulate in the atmosphere. There is enough evidence now about the various disturbances in what are considered as background quantities. The most important reasons are (i) the increase of emissions from sources like coal burning, waste incinerators, cement production, mining etc, (ii) the lack of understanding of important physicochemical processes like fluxes, transport, transformation and deposition. Because of these verified disturbances, during the last years, a considerable effort has been devoted to reduce the mercury emissions. At the framework of the EU/DG-XII project MAMCS a significant effort has been devoted at the development of appropriate models for studying the mercury cycle in the atmosphere. The model development is performed within the atmospheric models RAMS and SKIRON/Eta. In this development we tried to transfer and utilize the modeling techniques applied in conventional air pollution modelling studies. In addition, we had to develop new methodologies for processes like re-emissions from soil and water bodies and gas to particle formation. The developed modeling systems have been applied in the Mediterranean Region where the multi-scale atmospheric processes (thermal and mechanical circulations at regional and mesoscale) are considered as important, according to a number of past air pollution studies. Seasonal-type of simulation has been performed and annual deposition patterns have been estimated. As it was found, the regional-scale pattern and the trade wind systems (from North to South) and the photochemistry are the key factors for controlling the mercury deposition, especially the Hg P .

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.