DEFINITION OF CLIMATE REGIONS IN THE NORTHERN PLAINS USING AN OBJECTIVE CLUSTER MODIFICATION TECHNIQUE

Spatially homogeneous climate regions were developed from long-term mo nthly temperature and precipitation data for a subset of the U.S. Nort hern Plains. Climate regions were initially defined using the ''best'' of three agglomerative and hierarchical clustering methodologies; the n the clusters were objectively modified using a ''pseudohierarchical' ' iterative improvement technique. Under the premise of hierarchical c luster analysis, once an object has been assigned to a cluster, it can not later be reassigned to a different cluster, even if it is statisti cally desirable. The objective modification technique used herein is e mployed to compensate for this problem. Principal component analysis ( PCA) was used to reduce a 147-station dataset, consisting of 24 climat ic variables averaged over the 1931-1990 period, to three orthogonal c omponents. The new standardized scores, which explain 93% of the origi nal dataset variance, were then subjected to the Ward's, average linka ge, and complete linkage clustering methods. The average linkage metho d produced the most representative statistical results in identifying the climate regions. An iterative improvement technique was then utili zed to test ''border station'' membership and to modify the climate re gion boundaries. Fifteen climate regions resulted from the clustering (with two single-station clusters in the Black Hills alone), although they are just one possible partitioning of the data. The within-cluste r variability is generally the same for the 15 climate regions and the corresponding 21 National Climatic Data Center (NCDC) climate divisio ns. However, since the within-cluster variability tends to decrease wi th increasing cluster number, this result favors the new climate regio ns. Additionally, the new climate regions are shown to be superior to the NCDC climate divisions in terms of between-cluster variability. Th ese results suggest that the NCDC climate divisions could be redefined , improving their climatic homogeneity.