1 Trends in Snowfall versus Rainfall in the Western United States Noah Knowles [email protected] U.S. Geological Survey 345 Middlefield Rd., MS 496 Menlo Park, CA 94025 Michael D. Dettinger U.S. Geological Survey and Scripps Institution of Oceanography, University of California, San Diego La Jolla, CA 92093-0224 Daniel R. Cayan Scripps Institution of Oceanography, University of California, San Diego and U.S. Geological Survey La Jolla, CA 92093-0224 Originally submitted to Journal of Climate, April 29, 2005. Accepted requiring revisions July 13, 2005. Revised version submitted September 26, 2005. Accepted December 19, 20052 Trends in Snowfall versus Rainfall in the Western United States Noah Knowles Michael D. Dettinger Daniel R. Cayan ABSTRACT The water resources of the western U.S. depend heavily on snowpack to store part of wintertime precipitation into the drier summer months. A well-documented shift towards earlier runoff in recent decades has been attributed to 1) more precipitation falling as rain instead of snow, and 2) earlier snowmelt. The present study addresses the former, documenting a regional trend toward smaller ratios of winter-total snowfall water equivalent (SFE) to winter-total precipitation (P) during the period 1949-2004. The trends toward reduced SFE are a response to warming across the region, with the most significant reductions occurring where winter wet-day minimum temperatures, averaged over the study period, were warmer than -5ºC. Most SFE reductions were associated with winter wet-day temperature increases between 0 and +3°C over the study period. Warmings larger than this occurred mainly at sites where the mean temperatures were cool enough that precipitation form was less susceptible to warming trends. The trends toward reduced SFE/P were most pronounced in March regionwide and in January near the West Coast, corresponding to widespread warming in these months. While mean temperatures in March were sufficiently high to allow the warming trend to produce SFE/P declines across the study region, mean January temperatures were cooler, with the result that January SFE/P impacts were restricted to the lower elevations near the West Coast. Extending the analysis back to 1920 shows that although the trends presented here may be partially attributable to interdecadal climate variability (PDO), they also appear to result from still longer-term climate shifts.3 1. Introduction One of the most common, and common-sense, projections of the impact of global warming on the western United States is that warming will reduce the volumes and persistence of snowpacks across the region (e.g., Gleick 1987, Lettenmeier and Gat 1990, Dettinger et al. 2004, Knowles and Cayan 2004, Stewart et al. 2004). Warming in the western states is expected to reduce the fraction of precipitation that falls as snow rather than rain and hasten the onset of snowmelt once snowpacks have formed. In this context, recent observations in many rivers of the mountainous western United States and Canada indicate an alarming tendency for streamflow from snow-dominated basins to arrive progressively earlier in recent decades in response to large-scale warming (Roos 1991, Dettinger and Cayan 1995, Cayan et al. 2001). Widespread trends towards less winter’s-end (April) snowpack water content have also been reported (Mote 2003, Mote et al. 2005). Trends in the dates of onset of rapid snowmelt runoff in spring (Cayan et al. 2001, Stewart et al. 2005) indicate that an important part of the changes in runoff timing has been earlier onset of springtime snowmelt across the region, but the possible contribution of shifts towards more rainfall and less snowfall has received less attention to date. In the northeastern states, trends toward increases in the fraction of precipitation as rainfall have already been documented (Huntington et al. 2004). To better understand the nature of observed changes in snowpack and streamflow timing in the West, historical changes in the relative contributions of rainfall and snowfall are assessed here. Western warming trends historically have been (and presumably will continue to be) marked by strong seasonal and geographic patterns (e.g., Diaz and Quayle 1980, Dettinger et al. 1995, Cayan et al. 2001). Because of the general wintertime maximum of snowfall and precipitation in the region, contributions of snow to western precipitation are likely to be most affected by wintertime (November-March) temperatures, whereas changes in onset of snowmelt (once snow is on the ground) are more likely to be sensitive to springtime temperatures. Thus snow deposition and snowmelt are expected to be differently sensitive to warming trends in different seasons, and the warming trends associated with snowfall and snowmelt changes may be distinguishable by differences in their geographic patterns and rates of change. Much work has been accomplished to map trends in snowmelt response; this study documents a parallel set of trends that has changed the relative contributions of snowfall to western precipitation. In Section 2, the data used and the methods applied are discussed, and the robustness of the approach is addressed. In Section 3a, trends in winter precipitation form are presented, and in Section 3b, the influence of temperature on these trends is examined. In Section 3c, the monthly patterns underlying the seasonal trends are presented. In Section 3d, the role of climate variability in generating trends in precipitation form is investigated. Finally, the main results are summarized in Section 4, and their implications discussed.4 2. Data and Methods The measure of snowfall that will be used in this study is the snowfall liquid-water equivalent (SFE), defined as the precipitation totals on days for which newly fallen snow was recorded. These data and the temperature data used in this study were derived from the historical Summary of the Day (SOD) observations from cooperative weather stations in the 11 westernmost states of the conterminous US (Fig. 1), obtained from the National Climatic Data Center (NCDC). The observations used here comprise daily snowfall depth (S, actual depth as opposed to liquid equivalent), precipitation (P, regardless