The Automated Surface Observing System (ASOS) Program Office of the United States (US) National Weather Service (NWS) has established a Planned Product Improvement (PPI) Program to enhance sensor performance in the ASOS. The current ASOS anemometer, which is a conventional cup and vane design, was determined to be deficient under severe icing conditions and is a candidate for replacement under the PPI Program.
A sensor technology was not specified in the requirements for a replacement ASOS anemometer. However, the technical proposal evaluation resulted in the selection of four candidate anemometers all of which were based on sonic technology.
Sonic anemometers have direction and speed errors as a function of both azimuth orientation and wind speed, due to the shadow effects of the transducers and the flow distortion caused by the array assembly. It is desirable to have a standard test method to assess both speed and direction performance characteristics simultaneously and present the data to the user in a concise format.
There is no existing National Weather Service test method for evaluating sonic anemometers. The American Society for Testing and Materials (ASTM) has a standard test method for determining sonic anemometer characteristics, but that test method is more appropriate for design and development verification than for an end-user production validation test.
This paper will describe a test method which has been accepted as the basis for a committee working draft standard by the International Organization for Standardization (ISO). This paper will also describe the NWS wind tunnel test experience with sonic anemometry. This is an update of the paper presented at the WMO TECO-98 in Casablanca, Morocco