Observations of foehn interaction with lake breeze and valley wind

circulations, Lake Tekapo, New Zealand

Hamish McGowan

School of Geography, Planning and Architecture, University of Queensland, Brisbane,

Australia

Research collaborators• Prof. Andy Sturman

Centre for Atmospheric Research, University of Canterbury, Christchurch, NZ

• Dr. Meinolf KossmannGerman Meteorological Service (DWD), Offenbach a. M., Germany

• Dr. Peyman Zawar-RezaCentre for Atmospheric Research, University of Canterbury, Christchurch, NZ

Aim Of The Research• To develop a clearer understanding of the

interaction of thermal and dynamic forcing of airflow in complex terrain.

• Improve knowledge of the way in which various atmospheric processes interact in particularly complex environments to develop equally complex local wind regimes.

Background• 6 years of complex terrain windfield research –

foehn triggered dust storms.

• Frequent observation of foehn events that were not forecast (potential wind hazard).

• Increased development of New Zealand’s alpine region (tourism, agriculture) requiring improved local and meso-scale forecasting capabilities

South Islandof

New Zealand

Source NASA

Lake Tekapostudy area

(April 1998)

Source NASA

Looking north over Lake Tekapo

View south over Lake Tekapo

Godley Valley

The upper Godley Valley and Main Divide

MeteorologicalMonitoring Network

1997 - 1999

‘Typical’ foehn

• Moderate to strong foehn nor’wester dominates regional windfield

• Wind speeds in alpine valleys may > 40 ms-1

• Significant orographic rainfall and spillover precipitation• Barrier jet formation upwind of barrier with well developed lee

trough formation downwind

14 February 1999• Alpine foehn not forecast!

• Rawinsonde soundings identified ridge-top (2500 m agl.) light W/NW gradient flow at 5ms-1

• Thermally driven circulations recorded at most coastal and lowland locations

• Foehn onset recoded in the Godley Valley at 1000NZST – linked to local surface heating

Foehn advance and retreat, 14 Feb 1999

Vertical wind speed and direction profiles – Godley Valley

1600

Foehn – Lake Breeze Interaction• Typical light foehn – lake breeze interaction (a). Cold air

ramping (an important mechanism for pollution dispersion).

• 14 February 1999 situation (b). May result in fumigation over lake and associated shoreline

Foehn Foehn

Valley wind

(a) (b)

Valley wind

Conclusions • The wind field at Lake Tekapo represents a complex superposition of multi-

scale wind systems: lake/land breezes, up-/down-slope winds, mountain/valley winds, plain-

to-basin circulation and foehn

• Foehn onset ‘initially’ governed by airflow at 500hPa, surface sensible heat flux, thermally induced local and regional scale circulations.

• During light/moderate foehn conditions the topographically channelled foehn regularly decouples from the surface and flows over the lake breeze (cold air ramping).

• The foehn may ground over the downwind shoreline in response to increased surface mixing (fumigation possible)

• A climatology of alpine foehn events is to be developed (ie to improve forecasting of alpine foehn, role of foehn in mean annual temps)