On the Climatology of Orographic Precipitation in the Mid-Latitudes Justin R. Minder A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2010 Program Authorized to Offer Degree: Department of Atmospheric Sciences University of Washington Graduate School This is to certify that I have examined this copy of a doctoral dissertation by Justin R. Minder and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Co-Chairs of the Supervisory Committee: Dale R. Durran Gerard H. Roe Reading Committee: Dale R. Durran Gerard H. Roe Mark T. Stoelinga Date: In presenting this dissertation in partial fulfillment of the requirements for the doctoral degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this dissertation is allowable only for scholarly purposes, consistent with \fair use" as prescribed in the U.S. Copyright Law. Requests for copying or reproduction of this dissertation may be referred to Proquest Information and Learning, 300 North Zeeb Road, Ann Arbor, MI 48106-1346, 1-800-521-0600, to whom the author has granted \the right to reproduce and sell (a) copies of the manuscript in microform and/or (b) printed copies of the manuscript made from microform." Signature Date University of Washington Abstract On the Climatology of Orographic Precipitation in the Mid-Latitudes Justin R. Minder Co-Chairs of the Supervisory Committee: Professor Dale R. Durran Department of Atmospheric Sciences Professor Gerard H. Roe Department of Earth and Space Science For mountainous regions, and regions downstream, the climatology of mid-latitude oro- graphic precipitation determines the susceptibility to hazards such as flooding and land- slides while also cotrolling the volume and timing of streamflow and fresh water resources. However, due to modeling and observational challenges, many aspects of the climatology of mountain precipitation remain poorly understood. This thesis uses a synthesis of nu- merical models, theory, and field observations, loosely focused on the Cascade and Olympic Mountains of western North America, to investigate in detail a number of general aspects of mid-attitude orographic precipitation. First, the climatology of ridge-valley scale precipitation patterns is investigated by anal- ysis of several years of data from the Olympic Mountains, both from archived operational mesoscale numerical model forecasts and a special dense observing network of precipitation gauges. By simulating and analyzing case studies, the physical processes responsible for the mean pattern and variations in the pattern are diagnosed. Large (> 50%) enhance- ment of precipitation over ridges relative to valleys a few kilometers away is found to be a very robust feature of the region's climate, and the climatological patterns are surprisingly well-simulated by a mesoscale model (despite frequent errors for individual storms). The impact of large climatological gradients in mountain precipitation, such as those found in the Olympic mountains, on patterns of landslide susceptibility is also investigated. This is accomplished using an idealized model of shallow landslides, forced by the climatol- ogy developed from mesoscale model forecasts. Results suggest that small-scale maxima in climatological precipitation may play an important role in making certain regions more sus- ceptible to slope failure. Furthermore, the use of unadjusted lowland precipitation data to characterize conditions on nearby mountain slopes may lead to a substantial underestimate of landslide hazard. Next, the controls on the sensitivity of mountain snowpack accumulation to climate warming are investigated using two idealized physically based models. Results suggest that the relationship between the climatological melting-level distribution and the topography is the principle control on the sensitivity of snowpack accumulation to climate warming. It is also shown that, while thermodynamically driven increases in precipitation with warming may moderate the loss of snowfall somewhat, for large amounts of warming increases in precipitation become unimportant, as the loss of accumulation area is too substantial. Finally, the physical mechanisms acting on the mesoscale to control the mountainside snow line are investigated. On the mountainside, the snow line is often located at an ele- vation hundreds of meters different from its elevation in the free air upwind. The processes responsible for this behavior are examined in semi-idealized simulations with a mesoscale numerical weather prediction model. Spatial variations in latent cooling from melting pre- cipitation, adiabatic cooling from vertical motion, and the melting distance of frozen hy- drometeors are all shown to make important contributions. The relative importance of these processes depends on properties of the incoming flow and terrain geometry. Results suggest an increased depression of the snow line below the upstream 0◦C level with increasing tem- perature, a relationship that, if present in nature, could act to buffer mountain hydroclimate against the impacts of climate warming. TABLE OF CONTENTS Page List of Figures........................................ iii List of Tables......................................... xiii Chapter 1: Introduction.................................1 1.1 Motivation.....................................1 1.2 Challenges......................................2 1.3 Previous work....................................3 1.4 An example: The Olympic and Cascade mountains..............5 1.5 Outline....................................... 10 Chapter 2: The climatology of small-scale orographic precipitation: Patterns and processes................................... 13 2.1 Introduction and Background........................... 13 2.2 Precipitation in the Olympic Mountains..................... 16 2.3 Case studies..................................... 29 2.4 Composite analysis of precipitation climatology................. 49 2.5 Discussion...................................... 52 2.6 Conclusions..................................... 55 Chapter 3: Spatial patterns of orographic rainfall and shallow landslide suscepti- bility..................................... 57 3.1 Introduction and Background........................... 57 3.2 Rainfall and Landslides over the Western Olympic Mountains......... 60 3.3 Methods....................................... 63 3.4 Results........................................ 67 3.5 Sensitivity Analysis................................. 74 3.6 Conclusions..................................... 77 i Chapter 4: The sensitivity of climatological mountain snowpack accumulation to climate warming.............................. 79 4.1 Introduction and background........................... 79 4.2 Focus and strategy................................. 80 4.3 Linear Theory (LT) Orographic Snowfall Model................. 83 4.4 Melting-Level (ML) Model............................. 95 4.5 Experiments: Controls on λS ........................... 98 4.6 Conclusions..................................... 107 Chapter 5: Mesoscale controls on the climatology of the mountainside snow line. 110 5.1 Introduction and background........................... 110 5.2 Numerical Model.................................. 119 5.3 Results: physical mechanisms........................... 123 5.4 Results: 2-D sensitivity experiments....................... 140 5.5 Results: 3D effects................................. 151 5.6 Discussion...................................... 156 5.7 Summary and Conclusions............................. 159 Chapter 6: Summary and conclusions......................... 161 Bibliography......................................... 165 ii LIST OF FIGURES Figure Number Page 1.1 Figure caption...................................6 2.1 Rainfall observations in the Olympic Mountains. (a) shows the terrain of the Olympic Peninsula (grayscale shading), with a maximum elevation of 2.43 km. The locations which regularly report hourly precipitation (from the RAWS, ASOS, and the SNOTEL networks) are shown with stars. Location of the Quillayute (KUIL) sounding, and the COOP stations (Forks and Sequim) discussed in the text are also denoted. (b) shows a detailed view of the Queets-Quinault gauge network. Mixed precipitation gauges are shown with white circles, while rain-only gauges are shown with black circles. The Queets and Quinault Valleys, and the Black Knob (BKBW) RAWS station are also denoted........................................ 17 2.2 MM5 annual precipitation climatology (color shading, in units of mm yr−1), and model topography (contours every 250 m) over Olympic, Cascade, and Coastal mountain ranges (water years 2001{2006)................ 18 2.3 Precipitation weighted wind roses from water years 2004{2006. The length of each radial line is proportional to the amount of precipitation falling at the BKBW station when winds are from each direction. (a) uses 6 hr 850 hPa winds from the gridpoint in the NCEP-NCAR reanalysis upwind of the site during average flow, (b) uses 6 hr averaged 10 m winds from the BKBW station........................................ 21 iii 2.4 Observed and modeled precipitation totals at sites along Queets-Quinault gauge transect for four rainy seasons (dates given in Table 2.1). Accumu- lated precipitation is plotted as