Downscaling a Reanalysis of Extremely Cold Weather in Southern New Zealand

Abstract

In July 1996 southeastern New Zealand experienced a period of extreme cold, and the resulting pattern of tree damage indicated that cold air ponding had been an important factor. To confirm that hypothesis reanalysis fields (2.5° resolution) have been downscaled dynamically using the Weather Research and Forecasting (WRF) mesoscale model, across nested grids to an inner domain covering an area of about 17 km × 17 km with a horizontal resolution of 222 m. So long as vertical resolution is enhanced near ground relative to the default configuration, WRF resolves small scale drainage flows during strongly stable wintertime conditions over this modest topography (peak elevations above sea level ~400 m, and valleycrest amplitudes ~200 m). A twelve hour spinup resulted in marginally stronger drainage flows than did a six hour spinup, and differences between runs with hydrostatic and non-hydrostatic dynamics also were small. From the evidence reviewed, there seems no reason to regard the meteorology behind the ‘Big Freeze’ as having been categorically unusual.