Mapping the thermal climate of the H.J. Andrews Experimental Forest, Oregon

The H. J. Andrews Experimental Forest in the Cascades of central Oregon provides a unique opportunity to study spatial climate patterns on a relatively small scale. Historical records at the 64 square-kilometer site provide a spatially dense 30-year dataset. Thermal regimes at the H. J. Andrews are generally known but the effects of its complex topography and canopy cover on temperatures have been poorly understood. In this study, 1971-2000 mean monthly maximum and minimum temperature maps of the H. J. Andrews were created over a 50- meter grid, accounting for several environmental factors affecting microclimates in forested, mountainous terrain. The effects of elevation, forest canopy, cloudiness, and topographic shading on radiation regimes were assumed to be the primary factors and the datasets were adjusted to account for them. Specifically, it was assumed that maximum temperatures were affected by shortwave daytime radiation regimes, and minimum temperatures were affected by surface longwave radiation emission at night. The Image-Processing Workbench (IPW) was used to estimate incoming shortwave solar radiation at all climate station sites, taking into account elevation, cloudiness and topographic shading. Using IPW, fisheye photographs, and the HemiView program, proportions of solar radiation and sky view factors blocked by the tree canopy were calculated at each site, and accounted for when calculating daily shortwave radiation values for each month. Sky view factors were calculated at each site accounting for canopy and surrounding topography. Specific site pairs were then analyzed by plotting observed monthly temperature differences against simulated radiation and sky view factors and computing monthly regression functions. Monthly maximum temperature/shortwave radiation regression functions were used to adjust maximum temperatures onto 'open, flat' terrain, (leaving only elevation effects on temperatures), and monthly minimum temperature/sky view factor regression functions were used to adjust minimum temperatures. Temperatures were spatially interpolated over the H. J. Andrews using the Parameter-elevation Regressions on Independent Slopes Model (PRISM) program, which calculates spatially-varying temperature/elevation gradients. Topographic effects of shortwave radiation and sky view factors were reintroduced to the PRISM temperature grids using the appropriate regression functions. To make the resulting maps as useful and applicable as possible for future research, temperatures were modeled to simulate open siting conditions common in NWS station networks. Overall, temperatures were most sensitive to elevation and topographic position. Maximum temperature
was sensitive to variations in shortwave radiation, especially in winter when solar radiation loads were small. Minimum temperature was sensitive to variations in sky view factors, particularly during clear summer months. Factors not accounted for in the project include small-scale effects of cold-air drainage, forest edge effects, topographic scale effects, and stream effects. These and other issues are summarized in a set of recommendations for future climate mapping research in the H. J. Andrews.