No abstract

Abstract. The HYdrological cycle in the Mediterranean EXperiment (HyMeX) is intended to improve the capabilities of predicting high-impact weather events. Within its framework, the aim of the first special observation period (SOP1), 5 September to 6 November 2012, was to study heavy precipitation events and flash floods. Here, we present high-impact weather events over Croatia that occurred during SOP1. Particular attention is given to eight intense observation periods (IOPs), during which high precipitation occurred over the eastern Adriatic and Dinaric Alps. During the entire SOP1, the operational model forecasts generally well represented medium intensity precipitation, but heavy precipitation was frequently underestimated by the ALADIN model at an 8 km grid spacing and was overestimated at a higher resolution (2 km grid spacing). During IOP2, intensive rainfall occurred over a wider area around the city of Rijeka in the northern Adriatic. The short-range maximum rainfall totals were the largest ever recorded at the Rijeka station since the beginning of measurements in 1958. The rainfall amounts measured in intervals of 20, 30 and 40 min were exceptional, with return periods that exceeded a thousand, a few hundred and one hundred years, respectively. The operational precipitation forecast using the ALADIN model at an 8 km grid spacing provided guidance regarding the event but underestimated the rainfall intensity. An evaluation of numerical sensitivity experiments suggested that the forecast was slightly enhanced by improving the initial conditions through variational data assimilation. The operational non-hydrostatic run at a 2 km grid spacing using a configuration with the ALARO physics package further improved the forecast. This article highlights the need for an intensive observation period in the future over the Adriatic region to validate the simulated mechanisms and improve numerical weather predictions via data assimilation and model improvements in descriptions of microphysics and air-sea interactions.

The evaluation of several climatological background-error covariance matrix (defined as the B matrix) estimation methods was performed using the ALADIN limited-area modeling data-assimilation system at a 4 km horizontal grid spacing. The B matrices compared were derived using the standard National Meteorological Center (NMC) and ensemble-based estimation methods. To test the influence of lateral boundary condition (LBC) perturbations on the characteristics of ensemble-based B matrix, two ensemble prediction systems were established: one used unperturbed lateral boundary conditions (ENS) and another used perturbed lateral boundary conditions (ENSLBC). The characteristics of the three B matrices were compared through a diagnostic comparison, while the influence of the different B matrices on the analysis and quality of the forecast were evaluated for the ENSLBC and NMC matrices. The results showed that the lateral boundary condition perturbations affected all the control variables, while the smallest influence was found for the specific humidity. The diagnostic comparison showed that the ensemble-based estimation method shifted the correlations toward the smaller spatial scales, while the LBC perturbations gave rise to larger spatial scales. The influence on the analysis showed a smaller spatial correlation for the ensemble B matrix compared to that of the NMC, with the most pronounced differences for the specific humidity. The verification of the forecast showed modest improvement for the experiment with the ensemble B matrix. Among the methods tested, the results suggest that the ensemble-based data-assimilation method is the favorable approach for background-error covariance calculation in high-resolution limited-area data assimilation systems.

No abstract

<p>An experiment has been carried out in the Adriatic Sea, in the framework of the Middle Adriatic Upwelling and Downwelling (MAUD) project. The CTD and ADCP data were collected by the yo-yo and shipborne measurements performed during the cruises whereas the temperature, pressure and dissolved oxygen time series were recorded by the probes deployed at the sea bottom. Additionally, the SST satellite data and the meteorological time series originating from permanent coastal stations were considered. Moreover, the high-resolution, 2-km meteorological (ALADIN) and 2.5-km oceanographic (ROMS) models were used to reproduce and interpret the experimental findings. Analysis of the data has concentrated on the end of May 2017, when a dense water dome was documented by the CTD measurements in the area between the island of Blitvenica (close to the east coast) and the island of Jabuka (in the open sea). Its center was observed at a distance of about 20 km from the coast. The dome left its mark on the sea surface, with the temperature above its center being slightly lower than in the surrounding areas as documented by both in situ and remotely-sensed data. The vmADCP measurements suggested that the surface circulation around the dome was cyclonic. At the time, a decrease of temperature close to the east coast was documented by the bottom probes and satellite images. The meteorological data and modeling results showed that the northern winds prevailed during the May 2017 experiment, suggesting that the open-sea and coastal upwelling occurred at the same time. In order to verify the interpretation, several schematized numerical experiments were conducted. The modelled wind fields were first decomposed into the curl and curl-free components, using the Helmholtz-Hodge decomposition. The components were then used to impose the forcing on the Adriatic model, assuming flat bottom and realistic bathymetry. Schematized simulations revealed that the wind curl was responsible for the offshore rising of pycnocline through Ekman pumping and therefore for the open-sea upwelling. On the other hand, in simulations with the curl-free wind component the pycnocline rose only close to the east coast and thus the coastal upwelling was reproduced.</p>