A study of orographic blocking and barrier wind development upstream of the Southern Alps, New Zealand

Upper level and surface wind data for 1994 are used to provide an initial i dentification of the orographic effect on regional airflow patterns upwind of the mountain barrier. A case study of the development of upstream blocki ng and barrier jets is also provided. The predominance of gradient airflow from between northwest and southwest t hrough this region results in frequent trans-mountain winds. The mountains are seen to have a major effect on airflow in the lowest 2000 m above sea l evel, with clear evidence of orographic blocking and barrier wind developme nt. Some variability in the extent of this blocking was noted during 1994, which appeared to be associated with changes in the synoptic circulation an d air mass characteristics. The frequent occurrence of southwesterly winds between 300m and 2000m indicates significant defection of the predominant w inds to follow the southwest-northeast orientation of the mountains. These southwesterly barrier winds occur in opposition to the apparent pressure gr adient. Northeasterly barrier winds occur mainly below 300 m, and represent a down-gradient, localised flow that is frequently separated from overlyin g northwesterly gradient winds by a transitional layer, within which the wi nd backs with height. The controls of the extent of orographic blocking are only assessed superficially, due to the lack of good thermodynamic data up stream of the mountains, although a combination of wind speed and atmospher ic stability is obviously important. These initial results provide a useful insight into the extent of orographic effects on regional windfields, whic h will serve as the basis for future observational and modelling studies.