Hierarchical Cluster Analysis of a Convection-Allowing Ensemble during the Hazardous Weather Testbed 2009 Spring Experiment. Part I: Development of the Object-Oriented Cluster Analysis Method for Precipitation Fields

Convection-allowingensembleforecastswithperturbationstomodelphysics,dynamics,andinitial(IC)and lateral boundary conditions (LBC) generated by the Center for the Analysis and Prediction of Storms for the NOAA Hazardous Weather Testbed (HWT) Spring Experiments provide a unique opportunity to understandtherelativeimpactofdifferentsourcesofperturbationon convection-allowingensemble diversity.Such impacts are explored in this two-part study through an object-oriented hierarchical cluster analysis (HCA) technique. In this paper, an object-oriented HCA algorithm, where the dissimilarity of precipitation forecasts is quantifiedwithanontraditional object-based threatscore(OTS),is developed. The advantages ofOTS-based HCA relative to HCA using traditional Euclidean distance and neighborhood probability-based Euclidean distance (NED) as dissimilarity measures are illustrated by hourly accumulated precipitation ensemble forecasts during a representative severe weather event. Clusters based on OTS and NED are more consistent with subjective evaluation than clusters based on traditional Euclidean distance because of the sensitivity of Euclidean distance to small spatial displacements. OTS improves the clustering further compared to NED. Only OTS accounts for important features of precipitation areas, such as shape, size, and orientation, and OTS is less sensitive than NED to precise spatial locationandprecipitationamount.OTSisfurtherimprovedbyusingafuzzymatchingmethod.Applicationof OTS-based HCA for regional subdomains is also introduced. Part II uses the HCA method developed in this paper to explore systematic clustering of the convection-allowing ensemble during the full 2009 HWT Spring Experiment period.

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