Evaluation of the AWAP daily precipitation spatial analysis with an independent gauge network in the Snowy Mountains

Abstract

The Bureau of Meteorology's Australian Water Availability Project (AWAP) daily precipitation analysis provides high resolution rainfall data by interpolating rainfall gauge data, but when evaluated against a spatially dense independent gauge network in the Snowy Mountains large systematic biases are identified. Direct comparisons with the gauge data in May–September between 2007 and 2014 reveal average root mean square errors of about 4.5 mm, which is slightly greater than the average daily precipitation amount, and the errors are larger for higher elevation gauges. A standard Barnes objective analysis is per-formed on the combined set of independent gauges and Bureau of Meteorology gauges in the region to examine the spatial characteristics of the differences. The largest differences are found on the western (windward) slopes, where the Barnes analysis is up to double the value of the AWAP analysis. These differences are attributed to a) the lack of Bureau of Meteorology gauges in the area to empirically represent the precipitation climatology, and b) the inability of the AWAP analysis to account for the steep topography exposed to the prevailing winds. At high elevation (>1400 m) the Barnes analysis suggests that the precipitation amount is about fifteen percent greater than that of the AWAP analysis, where the difficulties of measuring frozen precipitation likely have a large impact.