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David L. Portep and H. H. Shih


The National Oceanic and Atmospheric Administration collects tide and water-level data by using an acoustic tide gauge in its Next Generation Water Level Measurement System (NGWLMS). The elevation of the water is calculated from the round-trip travel time of an acoustic wave generated from a source mounted above the water. At some, sites, solar radiation on the tide well can set up a nonuniform temperature structure in the well. This temperature effect can modify the travel time of the sound pulse, thereby introducing an offset into the estimate of the water level and hence into the computation of datums, such as mean sea level, which is a significant global change index. This diurnal temperature effect was quantified by computing day and night datums for tide stations located at La Jolla, California, and Baltimore, Maryland. By employing the difference in these datums, a method of delineating and removing this temperature effect was developed. This analysis resulted in 1) a quantifiable method for determining the need for maintaining temperature sensors at NGWLMS locations, 2) an optimized temperature correction formula, and 3) an important finding that in most cams the temperature effect will have little impact on significant global change indices such as the, yearly mean sea level.

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