Rain-on-snow (ROS) occurs when warm, wet air moves into latitudes and/or elevations having vulnerable snowpacks, where it can alter water inputs to infiltration, runoff and erosion. The Pacific Northwest is particularly susceptible: winter storms off the Pacific cause locally heavy rain plus snowmelt almost annually, and disastrous flooding and landsliding intermittently. In maritime mountainous terrain, the effects seem more likely and hydrologically important where warm rains and seasonal snowpacks are liable to coincide, in middle elevations. Several questions arise: (1) In the PNW, does ROS affect the long-term frequency and magnitude of water delivery to the ground, versus total precipitation (liquid and solid), during big storms? Where and how much? (2) If so, can we determine which elevations experience maximum hydrologic effects, the peak ROS zone? Probabilistic characteristics of ROS are difficult to establish because of geographic variability and sporadic occurrence: scattered stations and short observational records make quantitative frequency analysis difficult. These problems dictate a modeling approach, combining semi-random selection of storm properties with physical rules governing snow and water behavior during events. I created a simple computer program to perform Monte Carlo simulation of large storms over 1000 "years", generating realizations of snowpack and storm-weather conditions; in each event precipitation falls, snow accumulates and/or melts, and water moves to the ground. Frequency distributions are based on data from the Washington Cascades, and the model can be applied to specific sites or generalized elevations. Many of the data sets were based on observations at Stampede Pass, where high-quality measurements of weather and snow at the Cascade crest have been made since the 1940s. These data were used to inform the model, and to test its reliability with respect to the governing data distributions. In addition, data from ROS events at Stampede, and at research sites in southwest Oregon, were used to confirm that the model's deterministic calculations of snow accumulation, snowmelt, and percolation (yielding water available for runoff) adequately simulate conditions observed in the field. The Monte Carlo model was run for elevations ranging from 200 to 1500 m, each over a hypothetical millennium. Results indicate that the presence of snow in some storms reduces the amount of water reaching the ground. This occurred more often in highlands but also at middle and lower elevations, affecting the long-term frequency-magnitude relations across the landscape. In these conditions, the rain-gauges overestimate the amount of liquid water actually reaching the ground. For many storms, however, ROS enhances water reaching the ground, most significantly at elevations between ~500-1100 m. At lower and higher elevations, the water available for runoff exceeds precipitation in ~2% of events, but this proportion rises to ~20-30% at ~800 m. Other metrics (e.g., series statistics, exponential regression coefficients, frequency-magnitude factors) also indicate that this middle-elevation band (around ~800 m) experiences ROS most often and with greatest water available for runoff. Of the west-central Washington Cascades study region, about one-third to one-half the landscape is susceptible to significant ROS influence. These results indicate areas where ROS currently has the greatest hydrologic consequence on ecosystems and human works, and possibly the greatest sensitivity to changes in land-use and climate.

Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 79-84). / Estuarine density stratification may be controlled primarily by cross-shore processes (analogous to longitudinal control in narrow estuaries), or by both cross- and alongshore processes (typical of coastal plumes). Here field observations and numerical modeling are used to investigate stratification on the low-sloped, periodically inundated Skagit Bay tidal flats. Advection of stratification by the depth-averaged velocity, straining of the horizontal density gradient by velocity shear, and turbulent mixing are shown to be the dominant processes. On the south-central flats (near the south fork river mouth) velocities are roughly rectilinear, and the largest terms are in the major velocity direction (roughly cross-shore). However, on the north flats (near the north fork river mouth), velocity ellipses are nearly circular owing to strong alongshore tidal flows and alongshore stratification processes are important. Stratification was largest in areas where velocities and density gradients were aligned. The maximum stratification occurred during the prolonged high water of nearly diurnal tides when advection and straining with relatively weak flows increased stratification with little mixing. Simulations suggest that the dominance of straining (increasing stratification) or mixing (decreasing stratification) on ebb tides depends on the instantaneous Simpson number being above or below unity. / by Vera L. Pavel. / Ph.D.