THE UTILITY OF SURFACE AND UPPER AIR DATA IN SYNOPTIC CLIMATOLOGICAL SPECIFICATION OF SURFACE CLIMATIC VARIABLES

The relative merits of applying 1000 and 500 hPa map classifications t o specifying departures in daily and monthly climatic elements have be en examined for a network of 82 stations over New Zealand The map clas sification technique was based on eigenanalysis of twice daily geopote ntial height fields at 2.5 degrees resolution, followed by cluster ana lysis. Five significant EOFs were obtained at each level and subsequen t cluster analysis resulted in stable patterns of seven and ten synopt ic classes at the 1000 and 500 hPa levels respectively. The applicatio n of EOF analysis and cluster analysis to the combined data from both levels led in each case to equivalent-barotropic patterns, so that it was not possible to distinguish between varying vertical structures as sociated with the same low-level circulation. On a daily basis, the te n 500 hPa classes were better able to distinguish between maximum and minimum temperatures and daily precipitation, but comparable results w ith the seven 1000 hPa map types were obtained for the duration of bri ght sunshine and daily wind run. The difference in skill was at least partly related to the number of synoptic classes at each level. Monthl y mean departures for all variables over 1980-1993 were estimated by r egression from the relative frequencies of each synoptic class, from t he weighted daily mean departure patterns for each class, and from the mean EOF values for the month. In general the first two methods showe d similar skill, with estimates based on the 500 hPa data slightly sup erior to those based on 1000 hPa analyses, but combinations of the pre dictors at both levels gave the best results. Direct estimates from th e monthly mean EOFs proved better overall than those based on daily ma p classifications, limiting the value of the stochastic weather genera tor approach to the down-scaling of New Zealand's regional climate. (C ) 1997 by the Royal Meteorological Society.