Atmospheric Rivers, Floods and the Water Resources of California

Abstract

California's highly variable climate and growing water demands combine to pose both water-supply and flood-hazard challenges to resource managers. Recently important efforts to more fully integrate the management of floods and water resources have begun, with the aim of benefitting both sectors. California is shown here to experience unusually large variations in annual precipitation and streamflow totals relative to the rest of the US, variations which mostly reflect the unusually small average number of wet days per year needed to accumulate most of its annual precipitation totals (ranging from 5 to 15 days in California). Thus whether just a few large storms arrive or fail to arrive in California can be the difference between a banner year and a drought. Furthermore California receives some of the largest 3-day storm totals in the country, rivaling in this regard the hurricane belt of the southeastern US. California's largest storms are generally fueled by landfalling atmospheric rivers (ARs). The fractions of precipitation and streamflow totals at stations across the US that are associated with ARs are documented here and, in California, contribute 20-50% of the state's precipitation and streamflow. Prospects for long-lead forecasts of these fractions are presented. From a meteorological perspective, California's water resources and floods are shown to derive from the same storms to an extent that makes integrated flood and water resources management all the more important. OPEN ACCESSWater 2011, 3 446

How will orographic precipitation respond to surface warming? An idealized thermodynamic perspective

“…Our model applies a warm-rain microphysical parameterization to dynamical fields derived from linear mountain wave theory (see Appendix A for details). Simulations are performed at sea level temperatures of 13 • C and 15 • C, with the former representing a typical AR in today's climate and the latter representing estimated warming by the end of the century [Dettinger, 2011]. Within ascending regions upstream of the ridge, the troposphere is assumed to be saturated and moist adiabatic, with a sea level pressure of 1000 hPa in both simulations.…”

“…These conditions, while not representative of orographic storms in general, are a reasonable approximation of "atmospheric rivers" (ARs) [e.g., Ralph et al, 2005], which are long bands of moist, tropical air flowing poleward and eastward within the warm sectors of midlatitude cyclones [Newell et al, 1992]. ARs are responsible for up to half the total annual precipitation-and virtually all major floods-in midlatitude coastal ranges like the Cascades and the Sierra Nevada and are therefore a natural starting point for understanding the overall impact of climate change on midlatitude orographic precipitation [Guan et al, 2010;Neiman et al, 2011;Dettinger et al, 2011].…”

Oceanic sources of continental precipitation and the correlation with sea surface temperature

“…[21] Furthermore, regions 1, 2, and 7 in the North Pacific Ocean and North Atlantic Ocean (Figure 2) are more important for continental precipitation in October-March, when strong advection over the relatively warm oceans brings large quantities of water to the colder continents, which is largely contributed to the existence of atmospheric rivers [see, e.g., Dettinger et al, 2011]. During AprilSeptember these oceanic source areas are weaker, and continental moisture recycling becomes important [see also, e.g., Bisselink and Dolman, 2008;Dominguez et al, 2009;van der Ent et al, 2010].…”

Seasonal flows of international British Columbia-Alaska rivers: The nonlinear influence of ocean-atmosphere circulation patterns

“…Broadly speaking, changes in streamflow directly impact water resources and also have a variety of indirect impacts on riverine ecosystem services, such as recreation and fish habitat. In basins that experience large interannual variations in hydroclimatic drivers, year-to-year variability in streamflow can be pronounced (e.g., [14]). Such variability complicates the precise management of water resources, particularly in basins where demands on water resources are increasing [67].…”