EXPLORATORY ANALYSIS OF THE DIFFERENCE BETWEEN TEMPERATURE OBSERVATIONS RECORDED BY ASOS AND CONVENTIONAL METHODS

The Automated Surface Observing System is currently replacing conventi onal observations at the National Weather Service, the Federal Aviatio n Administration, and other stations that report hourly observations. From a climatological viewpoint, it is necessary to compare the data f rom the old and new measuring systems in order to gain an understandin g of their differences. These differences may become important when us ing time series for applications such as the computation of climatic n ormals, the development of homogeneous datasets for long periods of re cord for the investigation of climatic change, the placing of events i nto historical perspective, or the analysis of extreme weather events. This exploratory study of temperature data was undertaken to determin e first whether there is a data continuity problem between the two obs erving systems and second, if there is a problem, to identify the magn itude of the problem. The most important conclusion from this study is that differences in site characteristics, even at the same airport, p lay as much, if not more, of a role in assessing the comparability of measurements from the two observing systems as does the instrument sys tem bias. The instrument bias at most stations is on the order of a fe w tenths of a degree Fahrenheit, but the siting differences can lead t o biases on the order of a couple of degrees. Not only is there a diff erence in the magnitude of the biases, but there is also a difference in the direction; the instrument bias is usually negative, but the sit ing biases can be either positive or negative.