engleski

Numerical simulations of the severe rainfall in Pula on 25th September 2010

A case with the severe precipitation in the night and early morning on 25th September 2010 in Istra, Croatia, is used as a testbed for the numerical weather prediction model ALADIN (Aire Limiteé Adaptation Dynamique dévelopement InterNational). The operational ALADIN forecast (OPER) at CMHS is run with 8 km horizontal resolution, it uses ARPEGE initial and boundary conditions and digital filter initialization (DFI). The parallel suite of ALADIN at DHMZ has similar characteristics but it uses initial conditions obtained from data assimilation (DA) cycle. Forecasts obtained from operational and parallel suites show that the precipitation event west of Istria peninsula was forecasted by both suites (OPER and DA) even several days in advance, but high precipitation maximum at southern part of the peninsula was not captured. Possible reason for that is that observed severe precipitation was caused by convective activity supported by synoptic conditions and/or local conditions that were not represented correctly in the initial conditions or the model was not able to represent its development. In order to investigate this, different sets of experiments have been performed. These tests include the sensitivity to different initial and lateral boundary conditions, model resolution and convective paratmeterization scheme. Different options for obtaining the initial and boundary conditions that are operationally available in the Croatian Meteorological and Hydrological Service (CMHS) are tested. The first set for LBC comes from ARPEGE (Action de Recherche Petite Echelle Grande Echelle) and the second is obtained from the IFS (Integrated Forecast System) model that is run operationally at ECMWF (European Center for Medium-Range Weather Forecast). Also possibility of improving the initial conditions via the data assimilation using these sets of LBC s is investigated. Several experiments using high- resolution (2 km) non-hydrostatic ALADIN model runs have been performed. The experiment where the prognostic parametrization of convection has been used have indicated the existence of the secondary maximum over the Istrian land. Most of the precipitation was given by the convection scheme. This result suggests the importance of using the convective parametrization even in the resolutions in which it is assumed that the convection is resolved. Further experiments, with a diagnostic convection scheme and without any deep convection scheme will be performed in order to investigate this problem further.

flash flood; heavy rain

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