Surface temperature lapse rates over complex terrain : lessons from the Cascade Mountains

The typically sparse distribution of weather stations in mountainous terrain inadequately resolves temperature variability. Accordingly, high‐resolution gridding of climate data (for applications such as hydrological modeling) often relies on assumptions such as a constant surface temperature lapse rate (i.e., decrease of surface temperature with altitude) of 6.5°C km⁻¹. Using an example of the Cascade Mountains, we describe the temporal and spatial variability of the surface temperature lapse rate, combining data from: (1) COOP stations, (2) nearby radiosonde launches, (3) a temporary dense network of sensors, (4) forecasts from the MM5 regional model, and (5) PRISM geo‐statistical analyses. On the windward side of the range, the various data sources reveal annual mean lapse rates of 3.9–5.2°C km⁻¹, substantially smaller than the often‐assumed 6.5°C km⁻¹. The data sets show similar seasonal and diurnal variability, with lapse rates smallest (2.5–3.5°C km⁻¹) in late‐summer minimum temperatures, and largest (6.5–7.5°C km⁻¹) in spring maximum temperatures. Geographic (windward versus lee side) differences in lapse rates are found to be substantial. Using a simple runoff model, we show the appreciable implications of these results for hydrological modeling

2008: The climatology of small-scale orographic precipitation over theOlympicMountains: Patterns and processes.Quart

ABSTRACT: The climatology of small-scale patterns of mountain precipitation is poorly constrained, yet important for applications ranging from natural hazard assessment to understanding the geologic evolution of mountain ranges. Synthesizing four rainy seasons of high-resolution precipitation observations and mesoscale model output (from the Penn State/NCAR MM5), reveals a persistent small-scale pattern of precipitation over the ∼10 km wide, ∼800 m high ridges and valleys of the western Olympic Mountains, Washington State, USA. This pattern is characterized by a 50-70% excess accumulation over the ridge crests relative to the adjacent valleys in the annual mean. While the model shows excellent skill in simulating these patterns at seasonal time-scales, major errors exist for individual storms. Investigation of a range of storm events has revealed the following mechanism for the climatological pattern. Regions of enhanced condensation of cloud water are produced by ascent in stable flow over the windward slopes of major ridges. Synoptically generated precipitation grows by collection within these clouds, leading to enhanced precipitation which is advected by the prevailing winds. Instances of atypical patterns of precipitation suggest that under certain conditions (during periods with a low freezing level, or convective cells) fundamental changes in small-scale patterns may occur. However, case-studies and composite analysis suggest that departures from the pattern of ridge-top enhancement are rare; the basic patterns and processes appear robust to changes in temperature, winds, and background rainfall rates

The Chilean Coastal Orographic Precipitation Experiment: Observing the influence of microphysical rain regimes on coastal orographic precipitation

The Chilean Coastal Orographic Precipitation Experiment (CCOPE) was conducted during the australwinter of 2015 (May–August) in the Nahuelbuta Mountains (peak elevation 1.3 km MSL) of southern Chile(388S). CCOPE used soundings, two profiling Micro Rain Radars, a Parsivel disdrometer, and a rain gaugenetwork to characterize warm and ice-initiated rain regimes and explore their consequences for orographicprecipitation. Thirty-three percent of foothill rainfall fell during warm rain periods, while 50% of rainfall fellduring ice-initiated periods. Warm rain drop size distributions were characterized by many more and relativelysmaller drops than ice-initiated drop size distributions. Both the portion and properties of warm and ice-initiated rainfall compare favorably with observations of coastal mountain rainfall at a similar latitude inCalifornia. Orographic enhancement is consistently strong for rain of both types, suggesting that seeding fromice aloft is not a requisite for large orographic enhancement. While the data suggest that orographic en-hancement may be greater during warm rain regimes, the difference in orographic enhancement betweenregimes is not significant. Sounding launches indicate that differences in orographic enhancement are not easilyexplainable by differences in low-level moisture flux or nondimensional mountain height between the regimes