OCTOBER 2007 D A L Y E T A L . 1565 High-Resolution Spatial Modeling of Daily Weather Elements for a Catchment in the Oregon Cascade Mountains, United States CHRISTOPHER DALY,JONATHAN W. SMITH, AND JOSEPH I. SMITH Oregon State University, Corvallis, Oregon ROBERT B. MCKANE U.S. Environmental Protection Agency, Corvallis, Oregon (Manuscript received 7 August 2006, in final form 8 January 2007) ABSTRACT High-quality, daily meteorological data at high spatial resolution are essential for a variety of hydrologic and ecological modeling applications that support environmental risk assessments and decision making. This paper describes the development, application, and assessment of methods to construct daily high- resolution (ϳ50-m cell size) meteorological grids for the 2003 calendar year in the Upper South Santiam Watershed (USSW), a 500-km2 mountainous catchment draining the western slope of the Oregon Cascade Mountains. Elevations within the USSW ranged from 194 to 1650 m. Meteorological elements modeled were minimum and maximum temperature; total precipitation, rainfall, and snowfall; and solar radiation and radiation-adjusted maximum temperature. The Parameter–Elevation Regressions on Independent Slopes Model (PRISM) was used to interpolate minimum and maximum temperature and precipitation. The separation of precipitation into rainfall and snowfall components used a temperature-based regression function. Solar radiation was simulated with the Image-Processing Workbench. Radiation-based adjust- ments to maximum temperature employed equations developed from data in the nearby H. J. Andrews Experimental Forest. The restrictive terrain of the USSW promoted cold-air drainage and temperature inversions by reducing large-scale airflow. Inversions were prominent nearly all year for minimum tem- perature and were noticeable even for maximum temperature during the autumn and winter. Precipitation generally increased with elevation over the USSW. In 2003, precipitation was nearly always in the form of rain at the lowest elevations but was about 50% snow at the highest elevations. Solar radiation followed a complex pattern related to terrain slope, aspect, and position relative to other terrain features. Clear, sunny days with a large proportion of direct radiation exhibited the greatest contrast in radiation totals, whereas cloudy days with primarily diffuse radiation showed little contrast. Radiation-adjusted maximum tempera- tures showed similar patterns. The lack of a high-quality observed dataset was a major issue in the inter- polation of precipitation and solar radiation. However, observed data available for the USSW were superior to those available for most mountainous regions in the western United States. In this sense, the methods and results presented here can inform others performing similar studies in other mountainous regions. 1. Introduction mainly to difficulties in extrapolating data collected from sparse networks of climate stations, high-resolu- There is a growing need for high-quality, daily me- tion meteorological data are not available for many re- teorological data at fine spatial scales as scientists, land gions, especially for mountainous areas where the ef- managers, and policy makers increasingly rely on spa- fects of terrain create spatially and temporally complex tially distributed simulation models to assess the effects climatic patterns. This paper describes the develop- of human activities on ecosystem services. Owing ment, application, and assessment of methods to con- struct daily high-resolution meteorological grids for the 2003 calendar year in the Upper South Santiam Water- Corresponding author address: Christopher Daly, PRISM 2 Group, Department of Geosciences, 326 Strand Agriculture Hall, shed (USSW), a 500-km mountainous catchment in the Oregon State University, Corvallis, OR 97331. Cascade Range in western Oregon. Located within the E-mail: [email protected] Willamette National Forest, the USSW is managed pri- DOI: 10.1175/JAM2548.1 © 2007 American Meteorological Society JAM2548 1566 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 46 marily for forest products and wildlife habitat. The me- The USSW, located on the western, or windward, teorological grids described here were developed to slopes of the Cascade Range, receives orographically support a U.S. Environmental Protection Agency enhanced precipitation that typically increases with el- (EPA) modeling framework that combines a model evation. Observed annual precipitation totals for simulating the effects of land use and climate on the 1971–2000 range from 1388 mm yrϪ1 at Foster Dam structure of plant communities, which serve as wildlife [Cooperative Observer Program (COOP) station habitat, with a spatially explicit model that simulates 353047, 168 m; Fig. 1] to 2193 mm yrϪ1 at Jump Off Joe animal population responses to changes in habitat dis- [snow telemetry (SNOTEL) station 22E07S, 1067 m; tribution and quality (Busing et al. 2007; McRae et al. Fig. 1]. Temperatures are typically mild throughout the 2006, manuscript submitted to Ecol. Modell.). year, owing to the moderating influence of marine air Meteorological elements modeled were minimum from the Pacific Ocean, brought inland on persistent, and maximum temperature; total precipitation, rainfall, onshore winds. The Cascade crest serves as an effective and snowfall; and solar radiation and radiation-ad- barrier to cold-air outbreaks originating in interior justed maximum temperature. The study area and digi- Canada to the northeast. Snow is relatively rare below tal elevation model (DEM) are discussed in section 2. about 500 m, but a substantial seasonal snowpack ac- Processing and quality control of station data are dis- cumulates above about 900 m. Mean 1971–2000 Janu- cussed in section 3. Section 4 presents methods for in- ary minimum temperatures at Foster Dam and Jump terpolating daily precipitation and temperature, and Off Joe are 1° and Ϫ2.6°C, respectively, and July maxi- the separation of precipitation into rainfall and snow- mum temperatures at Foster Dam and Jump Off Joe fall. Section 5 presents methods for modeling solar ra- are 26.7° and 24.3°C, respectively. diation. Section 6 discusses adjustments to daily maxi- Daily weather interpolation was performed on a mum temperature to account for solar radiation expo- 2-arc-s (ϳ50 m) DEM in geographic (latitude/longi- sure. Results and discussion are presented in section 7, tude) coordinates. The source for this DEM was the and a summary and conclusions are presented in sec- U.S. Geological Survey (USGS) 1-arc-s (ϳ25 m) DEM tion 8. (obtained online at http://gisdata.usgs.gov/NED/). A modified Gaussian filter (Barnes 1964) was applied to ϳ 2. Study area and digital elevation model obtain 2-arc-s ( 50 m) grid resolution (Fig. 1a). This 2-arc-s DEM was used to supply gridded elevations for The USSW is located on the western slope of the modeling maximum and minimum temperature and so- Cascade Range, approximately 80 km east-northeast of lar radiation. For precipitation, the Gaussian filter Eugene, Oregon (Fig. 1). The watershed is approxi- (Barnes 1964) was applied to the 2-arc-s DEM to filter mately 508 km2 in size, with elevations ranging from out terrain features up to 2.5 arc min (ϳ4 km) in extent 1650 m at the headwaters region in the east to 194 m at while retaining the 2-arc-s grid resolution (Fig. 1b). The Foster Lake, a reservoir that marks the lower terminus direct effects of elevation on precipitation do not ap- of the USSW in the west. The USSW is in the south- pear to be strong below scales of about 5–10 km, be- eastern corner of the larger Santiam River Watershed, cause of a number of mechanisms, including the advec- which encompasses 4737 km2 of the eastern portion of tive nature of moisture-bearing airflow, the viscosity of the Willamette River basin and drains the Cascade the atmosphere, delays between initial uplift and sub- Range. sequent rainout, and the movement of air around ter- The USSW is characterized by steep slopes and rain obstacles (Daley 1991, 2006; Daly et al. 1994; deeply incised drainages. Generally representative of Sharples et al. 2005). Larger-scale uplift seems to be the rugged mountainous landscape of the Pacific North- especially characteristic of marine-based systems, such west, the USSW contains representative examples of as those of western Oregon, because of the dominance the region’s conifer-dominated forest ecosystems. The of synoptic-scale storm systems over small-scale con- species composition of forest communities is strongly vective systems. To increase clarity, grid resolutions will associated with moisture and temperature gradients at hereinafter be given in approximate kilometers or different elevations (Franklin and Dyrness 1988). meters, rather than geographic arc distances. The climate of the USSW is Mediterranean, charac- terized by copious winter precipitation and by summer 3. Station data drought; approximately 70%–75% of the annual pre- Daily data for maximum and minimum temperature, cipitation falls during November–April. During winter, precipitation, and solar radiation for stations within the the polar jet stream steers a series of moist frontal sys- modeling region were obtained for the 2003 calendar tems from the Pacific Ocean onshore into the region. year (Table 1; Fig. 1). Networks included the U.S. De- OCTOBER 2007 D A L Y E T A L . 1567 FIG. 1. The 2-arc-s DEM (a) used for temperature modeling and (b) low-pass filtered to a 2.5-arc-min effective