Stjepan Ivatek-Šahdan

Use of high-resolution dynamical adaptation in operational suite and research impact studies

The article presents a very high-resolution dynamical adaptation of the wind field with the hydrostatic version of the ALADIN model used in the operational suite, including a sensitivity study of a recent case of bura and a MAP IOP 15 case study. Bura is a strong wind with severe gusts ; it occurs suddenly and affects traffic on the eastern Adriatic. It has a large spatial variability and depends on an upstream terrain configuration. The described approach makes it possible to predict the occurrence, strength and spatial variability of the wind field in the mountainous area of the eastern Adriatic coast. Although this area is often affected by severe wind events (bura), measured wind data do not provide adequate coverage and are usually not representative enough. Good performance of the operational wind prediction encouraged its use for estimating the expected extreme wind speeds on the route of a new highway. Two sensitivity tests were carried out to explore the impact of the neglected processes. It was found that the impact is negligible in the case of severe bura event.

Evolution and Structure of Two Severe Cyclonic Bora Events: Contrast between the Northern and Southern Adriatic

Abstract While statistical analyses and observations show that severe bora with maximum gusts exceeding 40 m s−1 can occur in all parts of the Adriatic, the bora research to date has been mainly focused on the dynamics and structure of severe bora in the northern Adriatic. Examined to a significantly lesser degree is a less predictable counterpart in the southern Adriatic, where the Dinaric Alps are higher, broader, and steeper, and where the upwind bora layer is generally less well defined. Identification of the main differences in the sequence of mesoscale and macroscale events leading to the onset of bora in the northern and southern parts of the eastern Adriatic is of fundamental importance for its forecasting. To this end, presented here is a comparative analysis of the evolution and structure of two typical severe cyclonic bora events—one “northern” (7–8 November 1999) and one “southern” (6–7 May 2005) event. The analysis utilizes airborne, radiosonde, and ground-based observations, as well as the h...

The case study of bura of 1st and 3rd February 2007

Two cases when the operational forecast seriously underestimated the wind speed maxima are analysed. The first one in the night between 1 st and 2 nd February 2007 and the second one in the evening of 3 rd February 2007. The two cases are analyzed using measured data from Split and Makarska automatic stations as well as vertical soundings from Zagreb and Zadar and ALADIN model simulations. For the purpose of this study, ALADIN 72 hour forecast was run on 2 km resolution using the complete set of physics parametrizations, hydrostatic and nonhydrostatic dynamics. Results show the potential benefit of nonhydrostatic dynamics for operational forecast does not lie in improvement of the 10 m wind forecast as much as in forecasting clear air turbulence associated with the lee waves.

Forecasting Weather in Croatia Using ALADIN Numerical Weather Prediction Model

This chapter gives an overview on weather forecasting using the set-up of the NWP model ALADIN that is used for operational weather forecast in CMHS as an example for operation weather forecasting. ALADIN is a state-of-the-art modern NWP model. Using ALADIN we exemplarily discuss short-comings and challenges in modern operational weather forecasting. A high-resolution LAM is intended to predict the sub-synoptic weather features forced by topography or other local characteristics that can be absent in the main synoptic pattern. Successful prediction of these small-scale features enables usage of the LAM forecast in predicting the conditions important for the flight safety, vehicle road safety or navigation at sea. The operational suite has to be tuned in order to predict the high-impact weather events of local character that could be missing in the large scale forecast. The domain properties as well as the forecast model complexity are formed according to the needs of the forecast users and the computing capabilities.

Effects of different initial and boundary conditions in ALADIN/HR simulations during MAP IOPs

In order to better understand the influence of initial and boundary conditions on the numerical model results for MAP IOP5 (heavy precipitation) and IOP15 (severe Bora wind), numerical experiments were performed using the operational ALADIN/HR (hydrostatic) limited area model. Three experiments were performed. For the reference experiment (ARPE), ARPEGE operational analyses from 1999 were used. The other two experiments (ECAR and ECMW) were initialized from the 4D-Var ECMWF MAP reanalysis. In ECAR, the surface analysis was used from ARPEGE. The results presented here emphasise the huge importance of initial low-level moisture for the convective precipitation during IOP5. The humidity distribution in the ECMW experiment shows predominantly more humid conditions over the Dinaric Alps than in ARPE and ECAR. As a result, the simulation based upon the ECMWF MAP reanalysis produces fairly realistic results. The intensive surface convergence in the wind field and the orographic lifting of warm and humid air from the Adriatic Sea resulted in heavy precipitation in western Croatia. On the other hand, the different initial conditions during the MAP IOP15, with respect to the wind fields only, do not have a significant influence.

Operational Wave Modelling in the Adriatic Sea with the Wind Wave Model

AbstractThe accurate modelling of sea surface gravity waves is essential for accurate oceanic forecasting with high sea waves being a major concern for navigation and coastal activities. It is also very important for oceanic modelling, with the wave input being key to the accurate modelling of oceanic surface stress, sediment resuspension, and also to oceanic current modelling. In the Croatian Meteorological Institute, we have implemented the Wind Wave Model III as an operational model. The wind forcing used is based on the numerical weather prediction model ALADIN/HR. The model uses near-surface winds dynamically adapted to 2 km grid spacing over the 3-day forecast range. The boundary condition at the Otranto Strait is obtained from the WAM model forecasts computed at ECMWF. The model setup uses an unstructured grid to make the forecasts. The numerical modellization uses an implicit scheme that we describe. We found an underestimate of significant wave height by 8 cm, an absolute error of 21 cm and a correlation of 91% on comparing with the altimeter of the SARAL satellite. Comparison with wave radar and buoys show no underestimate and smaller absolute errors.