Evaluating Precipitation Trends in Alaska

Published studies of precipitation trends in Alaska do not always agree on the direction or magnitude of change. Accurately detecting trends in precipitation can be particularly challenging for a number of reasons. Changes in instrumentation or station location, as well as shifts in major modes of climate variability, can introduce inhomogeneities or step-changes in the observed amount of precipitation. These inhomogeneities, as well as problems such as missing data points, can strongly influence trend analyses performed with standard least-squares regression techniques. To better understand precipitation variability in Alaska, we have implemented a comprehensive trend-analysis plan. We compiled a set of 48 stations throughout the state, using National Climate Data Center (NCDC) records. All stations were statistically evaluated for the presence of inhomogeneities, and detected step-changes were compared to station metadata to determine whether they were related to operational changes. The timing of inhomogeneities was also compared to changes in modes of climate variability with the potential to impact Alaska. After identifying time periods suitable for trend analysis, we performed two trend analyses, the non-parametric Mann-Kendall trend test and linear regression. We compared the results while evaluating whether or not a given station was able to meet each test's assumptions. This suite of analyses allows us to evaluate data quality, trends, and the influence of decadal-scale variability on precipitation in Alaska.