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Csirmaz K.,Hungarian Meteorological Service Storm Warning Observatory
Idojaras | Year: 2015

A new forecasting method for maximum size of hail stones is presented in this paper by using the outputs of a high-resolution, non-hydrostatic numerical weather prediction model (NWP). The method was tested applying simulations of a real case supercell storm producing damaging hail with the Weather, Research and Forecasting (WRF) model. Numerical simulations were made with two distinct horizontal resolutions, 2 km and 100 m, applying nesting technique to assess the effect of the resolution on the storm microphysical properties and the maximum size of the hail stones on the surface. The WRF was able to simulate the main observed characteristics of the supercell on both resolutions. However, the numerical simulation with finer resolution gave better agreement with the radar observation and the observed maximum hail size on the surface. It was found that the horizontal resolution has significant influence on the magnitude and evolution of the microphysical processes in the storm. The numerical simulation with finer resolution produced not only significantly larger maximum mixing ratios of graupel/hail than the 2 km one did, but the volume integrated content of graupel/hail particles in the storm was also larger in the case of finer resolution. This difference can be attributed to the stronger updraft in the case of 100 m resolution. The analysis of various production shows that graupel/hail particles were mainly formed by the heterogeneous freezing of supercooled rain drops and by the freezing of rain drops due to the collision with cloud ice in the midlevels (between 3 and 8 km). Subsequent accretion of graupel/hail particles occurred by the collision with cloud and rain water. Larger graupel/hail content aloft in the case of finer simulation resulted in larger surface mixing ratios which directly led to larger maximum hail sizes on the ground. © 2015, Hungarian Meteorological Service. All rights reserved. Source

Horvath A.,Hungarian Meteorological Service Storm Warning Observatory | Nagy A.,Hungarian Meteorological Service Storm Warning Observatory | Simon A.,Hungarian Meteorological Service | Nemeth P.,Hungarian Meteorological Service
Idojaras | Year: 2015

In this paper, an overview of the complex nowcasting system of the Hungarian Meteorological Service is presented. The system named MEANDER started to work as a linear extrapolation process to provide warnings on convective storms. The role of the numerical weather prediction (NWP) models have been increased by involving the Weather Research and Forecasting (WRF) Model into nowcasting procedures. In the current state, MEANDER system consists of 2 main segments: NWP and linear parts. In the NWP segment, WRF model is used in two steps: in the first step, WRF (WRFALPHA) is run at a 2.5×2.5 km horizontal resolution grid, using non-hydrostatic dynamics and ECMWF model data as initial and boundary conditions. The second step is a higher resolution (1.2×1.2 km) WRF model run – named WRF-BETA –, that uses lateral conditions and first guess data coming from WRF-ALPHA outputs and assimilates radar reflectivity, satellite and surface observation data. The domain of WRF-BETA is included in the domain of WRF-ALPHA. The applied nowcasting-specific assimilation helps the model to develop significant precipitating weather systems on the right location in the right time. WRF-BETA outputs provides such background information for the nowcasting system that makes the forecast of the linear segment more exact. In the linear part, the actual objective analysis is considered at the beginning and the NWP prediction at the end of the nowcasting period. In the meantime, linear interpolation is applied. Radar data has key role in the nowcasting procedures in the linear segment, too. There are several derived parameters that are used for calculating the SYNOP-type present weather parameter for all grid points in the analysis and for the entire forecast time. The MEANDER system has a warning process that is able to create weather warnings for all districts of Hungary, helping decisions of forecasters. © 2015, Hungarian Meteorological Service. All rights reserved. Source

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