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Southerly Changes on the East Coast of New Zealand

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  • 1 Centre for Dynamical Meteorology, Monash University, Clayton, Victoria, Australia
  • | 2 New Zealand Meteorological Service, Wellington, New Zealand
  • | 3 Geography Department, University of Canterbury, Christchurch, New Zealand
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Abstract

A combined observational and climatological study of orographically influenced cold fronts over New Zealand, known locally as “southerly changes,” is presented.

Four southerly changes that occurred along the east coast of New Zealand during the Southerly Change Experiment (SOUCHEX) in January and February of 1988 are analysed in detail using the higher spatial and temporal density of data established for the experimental period. Three of the southerly changes were associated with fronts originating over the Tasman Sea, while the other was not.

A common feature in all four cases was the shallowness of the southerly flow for sonic hours after the surface wind change. The top of the southerly flow layer was less than the typical height of the Southern Alps (2000). Above this there was usually a maximum of the northerly component of the flow at or just above mountain- top levels and in three casts the prefrontal low-level flow was dominated by a warm northwesterly foehn. In the central South Island the northwards motion of the southerly change line at the surface was more rapid on the coast than inland. In this and other respects, the changes had many of the characteristics of “southerly busters” in southeastern Australia, and it sterns likely that the dynamical mechanisms of both kinds of fronts are similar.

Abstract

A combined observational and climatological study of orographically influenced cold fronts over New Zealand, known locally as “southerly changes,” is presented.

Four southerly changes that occurred along the east coast of New Zealand during the Southerly Change Experiment (SOUCHEX) in January and February of 1988 are analysed in detail using the higher spatial and temporal density of data established for the experimental period. Three of the southerly changes were associated with fronts originating over the Tasman Sea, while the other was not.

A common feature in all four cases was the shallowness of the southerly flow for sonic hours after the surface wind change. The top of the southerly flow layer was less than the typical height of the Southern Alps (2000). Above this there was usually a maximum of the northerly component of the flow at or just above mountain- top levels and in three casts the prefrontal low-level flow was dominated by a warm northwesterly foehn. In the central South Island the northwards motion of the southerly change line at the surface was more rapid on the coast than inland. In this and other respects, the changes had many of the characteristics of “southerly busters” in southeastern Australia, and it sterns likely that the dynamical mechanisms of both kinds of fronts are similar.

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