Predictability of intense weather events over northern Greece

In the framework of WaveForUs project (http://wave4us.web.auth.gr), the Laboratory of Meteorology and Climatology (http://meteo.geo.auth.gr) of the Aristotle University of Thessaloniki in Greece, produces four-day operational numerical weather predictions (NWPs) for Europe, Greece and northern Greece - Thermaikos Gulf, which constitutes the target area. The NWP system is based on the nonhydrostatic Weather Research and Forecasting model with the Advanced Research dynamic solver (WRF-ARW) and resolves the latter region with a horizontal grid spacing of 1.667 km. The WRF-ARW forecasts are employed (offline) by the partners in wave and coastal circulation models in order to produce high-resolution operational sea-state forecasts for Thermaikos Gulf. The meteorological and sea-state forecasts are disseminated to end users and the public.

The goal of this research is to investigate the predictability of intense weather events which affected the area of interest and understand the sensitivity of model performance to the surface conditions, using the abovementioned WRF-ARW system. Selected cases of high impact weather in northern Greece, associated with intense convective activity and/or strong synoptic scale forcing, are employed in the numerical experiments. These events were characterized either by large amounts and spatial variability of precipitation or gale force winds. The precipitation locally exceeded 100 mm/day, the 10m wind speed reached 24 m/s with gusts up to 31m/s, while significant damages were reported in large areas.

The WRF-ARW model was integrated in three 2-way telescoping nests, which covered: i) Europe, the Mediterranean sea and northern Africa (D01), ii) the central and a large part of eastern Mediterranean sea, including Greece and the Aegean sea (D02) and iii) the wider region of northern Greece - Thermaikos Gulf (D03) at horizontal grid-spacings of 15km, 5km and 1.667km respectively. Fine-resolution USGS data (30'' x 30'') were used in the control runs to define topography and land use. Operational NCEP/GFS analyses and 3-hourly forecasts (0.5x0.5 deg. lat.-long.) of the 1200 UTC cycle were employed as initial and boundary conditions of the coarse domain. The Ferrier scheme, the Betts-Miller-Janjic scheme, the RRTMG, the Monin-Obukhov (Eta), the Mellor-Yamada-Yanjic and the NOAH Unified model represented microphysical processes, sub-grid scale convection, longwave and shortwave radiation, surface layer, boundary layer and soil physics, respectively.

Sensitivity experiments were performed in order to investigate the sensitivity of model performance to: (a) the horizontal resolution of topography, (b) the land use dataset and (c) the source/resolution of sea-surface temperatures (SSTs; NCEP, JPL/NASA) as well as their temporal evolution. The control runs utilized USGS topography (30'' x 30'') at all domains while very high resolution (3'' x 3'') elevation data from Shuttle Radar Topography Mission (SRTM) were employed in the inner domain of the experiments.. The impact of USGS land use (30'' x 30'') was compared against the one of Corine Land Cover 2000 (100m x 100m) in the inner domain. The NCEP and JPL/NASA SSTs which were employed in the 3rd set of experiments are available daily at a horizontal increment of 1/12x1/12 deg. and 0.01x0.01 deg. lat.-long., respectively. The model results were statistically analyzed and compared to the available surface observations and radar data. The results appeared to be sensitive to the surface conditions especially during convective events. Finally, an effort was made to identify and analyze the main factors that are responsible for the differences in model performance.

Acknowledgments This research work of WaveForUs project (contract number: 11SYN_9_975) is co-funded by the European Union (European Regional Development Fund) and Greek national funds, through the action "COOPERATION 2011: Partnerships of Production and Research Institutions in Focused Research and Technology Sectors" in the framework of the Operational Programme "Competitiveness and Entrepreneurship" and Regions in Transition (OPC II, NSRF 2007-2013).