Local wind regime in the Zagreb area and mountain circulation

Local wind regime in Zagreb is studied in lower troposphere layer up to the height of 3 km for the spring months. Radiosounding data for Zagreb-Maksimir are used at 00 and 12 GMT for the period 1972 – 1981. It is shown that daytime heating, affecting the slope wind and mountain-valley circulation, is more expressed in April and May throughout the lower troposphere. In early springtime the local circulation is confined to the shallow surface layer, but the influence of mesoscale circulation of the Alpine region is noticed at higher levels. The effects of larger scale circulation on the modification of local winds are examined in a warm period of May 1979. 6-hourly upper air data in Zagreb indicate even larger daily variation of wind and temperature structure in the lower troposphere during this time. It is shown that easterly daytime component in the surface boundary layer weakens at 18 GMT, but the night-time upper level regime with northerly winds is already developed at that time. The onset of upper level daytime regime appears in the early morning (06 GMT) while the downslope winds still persist in the surface boundary layer due to a slow break-down of the surface temperature inversion. The analysis of 6-hourly geostrophic and thermal wind variations based on the surface data in the area to the east of Zagreb give new information on the changes in daily wind regime, which seems to make additional contribution to future studies of local wind regime in Zagreb