Contributing factors to Koshava wind characteristics

Abstract

Koshava is a local wind usually observed in the cold part of the year over the large part of Serbia, parts of Romania, parts of Hungary, and east Croatia. The main characteristics of the Koshava wind are its high wind speed, southeasterly direction, persistence, and gustiness. This paper analyses the synoptic [Mediterranean cyclones (MCs) and Eurasian anticyclones (EAs)] as well as the mesoscale contributors to the Koshava wind. Time and spatial features of the pressure systems have been analysed using an automatic cyclone tracking scheme. The results show that the MCs and EAs that generate Koshava are approximately 1000 km away from the Koshava region. It is demonstrated that the strong anticyclones are the main trigger for the Koshava wind. Less than 3% of the Koshava winds occurred without the occurrence of either an anticyclone or a cyclone. The Synoptic Koshava Index (SKI), developed in this study and defined as the difference between the area-averaged mean sea level pressures in the anticyclone and cyclone regions, has been shown to be a good indicator of the Koshava occurrence. Koshava has also been investigated from the perspective of the gap flow windstorm. The across-mountain pressure difference, ΔP, is the most important mesoscale contributor to the Koshava characteristics. The across-mountain potential temperature difference, Δ , together with ΔP, can be successfully used to predict Koshava's occurrence. A simple (linear) probabilistic model for forecasting the Koshava mean hourly wind speed has been constructed. The results demonstrate that the occurrence of the Koshava wind can be predicted with the significantly higher accuracy than the Koshava mean hourly speed. The correlation between Koshava and winds at higher levels is small.

Investigation of an extreme Koshava wind episode of 30 January–4 February 2014

“…The central pressure in the anticyclone that caused the EKE was 1055 mb, which according to Romanić et al () has an exceedance probability of occurrence of 0.1%. The Mediterranean cyclone, situated in the Gulf of Genoa, was not particularly strong as its central pressure was 3 mb below the average central pressure of the cyclones which typically generate Koshava (Romanić et al , ). Therefore, the strong anticyclone was the main trigger for the EKE.…”

“…The Synoptic Koshava Index (SKI; Romanić et al ()), defined as the normalized area‐averaged MSLP difference between the anticyclone and cyclone regions is presented in Figure a. The SKI values above 25 are noticeable during EKE while the values above 35 can be observed for the days with the strongest Koshava wind.…”

“…The high values of SKI demonstrate a strong pressure difference between the anticyclone and cyclone regions, which indubitably generated the strong Koshava velocities. Romanić et al 's () results show that the SKI values above 35 have a probability of occurrence below 1% (less than 0.3% chance of the SKIs above 40). This case study suggests that the SKI is a reliable prognostic Koshava parameter.…”

“…The major Koshava contributors on the mesoscales are the across‐mountain MSLP and potential temperature differences between west parts of the Wallachia Valley and the Koshava region (Romanić et al , ). The across‐mountain MSLP difference between Drobeta‐Turnu Severin and VG reached 10 mb during the EKE, whereas the across‐mountain potential temperature difference between these two stations was below −7 °C.…”

“…The southeasterly winds need to last for at least 2 days and possess the mean hourly wind speeds above 2 m s −1 to be considered as Koshava (Romanić et al , ). Romanić et al () demonstrated that the Eurasian anticyclones and Mediterranean cyclones, together with the orography of the Balkan region are the main Koshava drivers. The pressure gradients created by these two pressure systems move air masses from Ukraine and Moldavia towards the Adriatic Sea.…”