[1] Enhanced sea level oscillations with devastating effects occurred on 26/27 June 2003 in the east Adriatic. The phenomenon was recognized as a resonantly coupled air-sea interaction, where the sea wave is forced by the propagating atmospheric pressure disturbance. This study examines the dynamics and predictability of the atmospheric component of the coupled system. First, the initiation, amplification and maintenance of the system are examined. The source of the perturbation is traced back to the Alpine region, where the potentially unstable air in the westerly current lifts over the Alps resulting in convection and wave development. The wave and convective activity apparently couple in a wave-CISK manner and propagate southeastward down the eastern Adriatic coast. Due to the potentially unstable air above the Adriatic, the system additionally amplifies along its track. The dissipation of the system occurs after the landfall. Second, the performance of the numerical model at the resolution that is similar to operational NWP model resolution is examined. This severe event is highly susceptible to the details of the model configuration, presumably due to the sensitivity in coupling of the wave and MCS. Thus although the model is able to reproduce and forecast this kind of event at a fairly coarse resolution, the details of the spatial structure, as well as the time of onset, are questionable. In this case, the greatest sensitivity is primarily in the treatment of moist and convective processes.

The dynamics, structure and temporal evolution of a severe Adriatic bora, which occurred during 14 and 15 November 2004 was inspected. Numerical simulation of the investigated episode was performed by the mesoscale model MM5. The model was validated against radiosonde data and wind data from one automatic meteorological station and three ultrasonic anemometers. Two anemometers where located in the region extremely favorable for the bora occurrence (Senj and Vratnik Pass), while the third one was placed in the mainland (Zagreb-Horvatovac). The model reproduced well the onset and the strength of the investigated bora, as well as the establishment of bora-induced potential vorticity (PV) banners. On the other hand, surface turbulent kinetic energy (T KE) was poorly predicted. Inspection of gap wind characteristics indicated the absence of strong dissipation in the flow through a mountain pass, which gives rise to a horizontally elongated jet. Appearance of wave breaking in the lee of a mountain peak leads to the creation of a mountain wake. Shear lines between individual jets and wakes created in this way are then responsible for the generation of PV banners. Also, the ability of the model to predict hourly wind gusts was validated using a recently developed method. Zusammenfassung Dynamik, Struktur und zeitlicher Verlauf der schweren adriatischen Bora vom 14. und 15. November 2004 wurden untersucht. Zur numerischen Simulation wurde das Modell MM5 herangezogen. Die Ergebnisse der Berechnungen wurden mit Windmessungen von einer automatischen Wetterstation und drei Ultraschall-Anemometern verglichen. Zwei der Anemometer befanden sich an bevorzugt von der Bora heimgesuchten küstennahen Lokalitäten (Senj, Vratnik-Pass), das dritte war im Hinterland (Zagreb, Horvatovac). Das Modell gab das Einsetzen und die Stärke der untersuchten Bora und die damit zusammenhängende potentielle Vorticity zufriedenstellend wieder. Die kinetische Turbulenzenergie am Boden wurde hingegen unbefriedigend vorausgesagt. Eine Untersuchung der Passwinde ergab keine Hinweise auf nennenswerte Dissipation in der Strömung durch einen Gebirgspass, so dass ein horizontal verlängerter Strahl entstand. Sich am Gebirgskamm überschlagende Strömungswellen führten zur Entstehung von Wirbelschleppen auf der Leeseite. Überlagerung der auf diese Weise entstandenen strahlartigen Strömungen und Wirbelschleppen definiert die potentielle Vorticity. Die Eignung des hier vorgestellten Modells zur Vorhersage von Windböen wurde mit einer jüngst entwickelten Methode bestätigt.

No abstract

No abstract

No abstract

A performance of the wind gust estimate (WGE) method on the bora wind has been examined. Numerical simulations of several bora episodes have been performed using a non‐hydrostatic mesoscale model MEMO6. The model captured well the onset and cessation of the bora while the agreement between simulated and observed wind speeds differed from episode to episode. In cases with accurately simulated wind speeds, the WGE results were very good, thus indicating that the method could be used in forecasting bora gusts. The performance of the WGE method for the examined bora cases also suggested a possibility of further simplification of the method for the bora applications. Inspection of the bora flow and its thermodynamical structure revealed that after the bora onset the shear instabilities completely overwhelm the buoyant forces. This means that the parcels with the maximum wind speed in the boundary layer will always be able to reach the surface and result in wind gusts. Therefore, it is enough to have only a vertical profile of the wind speed. Although completely derived from the physical considerations, this represents a very simple way of determining bora wind gusts and can thus be easily implemented in the operational bora wind forecasting models.

The aim of this study was to investigate possible effects of two hypothetical scenarios of the urbanization of Zagreb's surroundings on the local winds, which are established under summertime anticyclonic conditions. For this purpose, the nonhydrostatic mesoscale meteorological model MEMO was applied to the greater Zagreb area. Three simulations were performed. One employed the current land-use distribution, while the other two corresponded to an increase of the densely urbanized area by 12.5% (test 1) and 37.5% (test 2), respectively. Apart from the hypothetically urbanized areas, where average surface wind speed reductions of 8% and 18% were obtained for test 1 and test 2, respectively, the rest of the domain was not signi®cantly affected by hypothetical urbanization. The differences between the wind vectors for the predicted current state and the hypothetical state were more pronounced and found at higher altitudes during the night compared to daytime values. For all three simulations the same diurnal variation of the depth of anabatic=katabatic wind¯ow generated on south-facing slopes of 1 km high mountain Medvednica was obtained. During the night the depth of well-developed katabatic¯ow was about 370 m, while during the day the depth of anabatic ow grew from about 550 m in the late morning up to about 1140 m in the late afternoon.

No abstract