Olympic Mountains Research Articles

Range Extension of Two Bumble Bee Species (Hymenoptera: Apidae) into Olympic National Park

Abstract Bumble bees (Hymenoptera: Apidae, Bombus) are cold-adapted insects, primarily known for their importance in providing ecosystem services to wild and cultivated flowering plants. Recent expeditions into the wilderness regions of the Olympic Mountains of Olympic National Park, USA discovered undocumented populations of two bumble bee species: Bombus sylvicola and B. vandykei. Application of species distribution models with range-wide locality records identified the Olympic Mountains to have high habitat suitability for B. sylvicola and low habitat suitability for B. vandykei. Our results suggest that Olympic National Park is a habitat island for B. sylvicola, isolated from the relatively contiguous distribution of the species in the Cascade and Sierra Nevada mountain ranges. Bumble bees are sensitive to environmental change, thus our discoveries will likely stimulate conservation-oriented investigations on these charismatic pollinators on the Olympic Peninsula and throughout the Pacific Northwest.

No seed zone effects on the survival, growth, and stem form of Pacific silver fir (Abies amabilis) in Britain

Pacific silver fir (Abies amabilis Douglas ex J. Forbes) was first introduced to Britain in 1830 but has not been widely planted and occupies a minute part of the forest estate. The results of six experiments established in the uplands of Britain examining material from 30 collection sites in 14 seed zones clearly demonstrate that its potential has not been recognised. The trials were assessed after 28 years and show that Pacific silver fir has the potential to be as productive as other common species options. There was little variation in performance between the 14 seed zones, and future seed collections could be carried out within a wide geographical range, including mainland British Columbia and Vancouver Island and the Olympic Mountains and western Cascades of Washington. The silvicultural characteristics of the species mean that it could be used more widely to diversify forests in Britain both as a plantation species and in the wider use of continuous cover management. More work is justified to determine its susceptibility to Annosum root rot (Heterobasidion annosum (Fr.) Bref), confirm its productivity on sites with rainfall below 800 mm·a–1 and (or) with a high peat content, and provide more detail on its wood properties.

Paleoseismic Evidence for Late Holocene Tectonic Deformation along the Saddle Mountain Fault Zone, Southeastern Olympic Peninsula, Washington

Abstract Trench and wetland coring studies show that northeast‐striking strands of the Saddle Mountain fault zone ruptured the ground about 1000 years ago, generating prominent scarps. Three conspicuous subparallel fault scarps can be traced for 15 km on Light Detection and Ranging (LiDAR) imagery, traversing the foothills of the southeast Olympic Mountains: the Saddle Mountain east fault, the Saddle Mountain west fault, and the newly identified Sund Creek fault. Uplift of the Saddle Mountain east fault scarp impounded stream flow, forming Price Lake and submerging an existing forest, thereby leaving drowned stumps still rooted in place. Stratigraphy mapped in two trenches, one across the Saddle Mountain east fault and the other across the Sund Creek fault, records one and two earthquakes, respectively, as faulting juxtaposed Miocene‐age bedrock against glacial and postglacial deposits. Although the stratigraphy demonstrates that reverse motion generated the scarps, slip indicators measured on fault surfaces suggest a component of left‐lateral slip. From trench exposures, we estimate the postglacial slip rate to be 0.2 mm/yr and between 0.7 and 3.2 mm/yr during the past 3000 years. Integrating radiocarbon data from this study with earlier Saddle Mountain fault studies into an OxCal Bayesian statistical chronology model constrains the most recent paleoearthquake age of rupture across all three Saddle Mountain faults to 1170–970 calibrated years (cal B.P.), which overlaps with the nearby M w 7.5 1050–1020 cal B.P. Seattle fault earthquake. An earlier earthquake recorded in the Sund Creek trench exposure, dates to around 3500 cal B.P. The geometry of the Saddle Mountain faults and their near‐synchronous rupture to nearby faults 1000 years ago suggest that the Saddle Mountain fault zone forms a western boundary fault along which the fore‐arc blocks migrate northward in response to margin‐parallel shortening across the Puget Lowland.

GPS as an independent measurement to estimate terrestrial water storage variations in Washington and Oregon

AbstractThe Global Positioning System (GPS) measures elastic ground loading deformation in response to hydrological mass variations on or near Earth's surface. We present a time series of change in terrestrial water storage as a function of position in Washington and Oregon estimated using GPS measurements of vertical displacement of Earth's surface. The distribution of water variation inferred from GPS is highly correlated with physiographic provinces: the seasonal water is mostly located in the mountain areas, such as the Cascade Range and Olympic Mountains, and is much smaller in the basin and valley areas of the Columbia Basin and Harney Basin. GPS is proven to be an independent measurement to distinguish between hydrological models. The drought period of 2008–2010 (maximum in 2010) and the recovery period of 2011–2012 in the Cascade Range are well recovered with GPS‐determined time‐variable monthly water mass series. The GPS‐inferred water storage variation in the Cascade Range is consistent with that derived from JPL's GRACE monthly mass grid solutions. The percentage of RMS reduction is ~62% when we subtract GRACE water series from GPS‐derived results. GPS‐determined water storage variations can fill gaps in the current GRACE mission, also in the transition period from the current GRACE to the future GRACE Follow‐on missions. We demonstrate that the GPS‐inferred water storage variations can determine and verify local scaling factors for GRACE measurements; in the Cascade Range, the RMS reduction between GRACE series scaled by GPS and scaled by the hydrological model‐based GRACE Tellus gain factors is up to 90.5%.

Glacier status and contribution to streamflow in the Olympic Mountains, Washington, USA

AbstractThe Olympic Peninsula, Washington, USA, currently holds 184 alpine glaciers larger than 0.01 km2 and their combined area is 30.2 ± 0.95 km2. Only four glaciers are >1 km2 and 120 of the others are <0.1 km2. This represents a loss of 82 glaciers and a 34% decrease in combined area since 1980, with the most pronounced losses occurring on south-facing aspects and in the more arid northeastern part of the range. Annual rate of loss in glacier area for seven of the largest glaciers accelerated from 0.26 km2 a−1 (1900–80) to 0.54 km2 a−1 (1980–2009). Thinning rates on four of the largest glaciers averaged nearly 1 ma−1 from 1987 to 2010, resulting in estimated volume losses of 17–24%. Combined glacial snow, firn and ice melt in the Hoh watershed is in the range 63–79 ± 7 × 106 m3, or 9–15% of total May–September streamflow. In the critical August–September period, the glacial fraction of total basin runoff increases to 18–30%, with one-third of the water directly from glacial ice (i.e. not snow and firn). Glaciers in the Elwha basin produce 12–15 ± 1.3 × 106 m3 (2.5–4.0%), while those in the Dungeness basin contribute 2.5–3.1 ± 0.28 × 106 m3 (3.0–3.8%).

Luminescence dating of late Pleistocene proximal glacial sediments in the Olympic Mountains, Washington

Late Pleistocene glacial sediments from the South Fork Hoh River valley in the Olympic Mountains, Washington, USA, were dated using optically stimulated luminescence (OSL) on quartz sand. High sediment supply typical of glacial environments, short transport distances, and sediment newly eroded from bedrock sources were expected to pose problems for luminescence dating. Samples were collected from five distinct sedimentary facies, using approximated distances from the ice-front, to assess how luminescence results varied due to these factors and to determine which samples produced the most reliable age estimates. Results from the South Fork Hoh highlight the importance of transport environment and sedimentary facies on solar resetting. Sediments from better-sorted, ice-distal environments were determined to be more completely bleached and more reliable for OSL dating. Samples were collected as part of larger research goals to improve understanding of the glacial history of the South Fork Hoh River valley.

Disproportionate predation on endemic marmots by invasive coyotes

We investigated predation by nonnative coyotes (Canis latrans) on endemic Olympic marmots (Marmota olympus) in Olympic National Park, Washington, in 2005 and 2006. Although nearly the entire marmot range is protected within the park, declines and local extirpations of the species have been documented. Through analyses of carnivore scat across the range of the Olympic marmot we determined the distribution and relative density of coyotes and characterized the extent to which coyotes and native carnivores preyed on marmots. We used mitochondrial DNA analysis of scats to determine carnivore species, and microsatellite markers for individual coyote identification. Scat analysis indicated that invasive coyotes are widespread and the numerically dominant carnivore on sampled trails within the Olympic highlands—71% (301 of 426) of all scats verified to species arose from coyote. Out of all carnivore scats collected, 11.6% (111 of 958) contained marmot remains. For 85% of the samples with marmots, coyotes were confirmed as the predator. The remainder arose from bobcat (13%) and cougar (2%). Coyotes were the predominant marmot predator across all months and in most regions of the park. Twelve out of 13 coyote individuals identified with genetic markers preyed on marmots. Marmots ranked 5th in frequency of coyote diet items, after snowshoe hares (Lepus americanus), mountain beavers (Aplodontia rufa), voles, and cervids. Scat analysis indicated that in the Olympic Mountains, the coyote as an invasive generalist predator is subsidized by abundant multiple prey, and appears to be the primary terrestrial predator on the endemic Olympic marmot. We conclude that predation by coyotes on marmots is widespread and substantial across the marmot's species range, and therefore likely driving observed marmot declines and extinctions.

Recent Population Trends of Mountain Goats in the Olympic Mountains, Washington

Mountain goats (Oreamnos americanus) were introduced in Washington's Olympic Mountains during the 1920s. The population subsequently increased in numbers and expanded in range, leading to concerns by the 1970s over the potential effects of non-native mountain goats on high-elevation plant communities in Olympic National Park. The National Park Service (NPS) transplanted mountain goats from the Olympic Mountains to other ranges between 1981 and 1989 as a means to manage overabundant populations, and began monitoring population trends of mountain goats in 1983. We estimated population abundance of mountain goats during 18–25 July 2011, the sixth survey of the time series, to assess current population status and responses of the population to past management. We surveyed 39 sample units, comprising 39% of the 59,615-ha survey area. We estimated a population of 344 ± 72 (90% confidence interval [CI]) mountain goats in the survey area. Retrospective analysis of the 2004 survey, accounting for differences in survey area boundaries and methods of estimating aerial detection biases, indicated that the population increased at an average annual rate of 4.9% since the last survey. That is the first population growth observed since the cessation of population control measures in 1990. We postulate that differences in population trends observed in western, eastern, and southern sections of the survey zone reflected, in part, a variable influence of climate change across the precipitation gradient in the Olympic Mountains.

Climate change or climate cycles? Snowpack trends in the Olympic and Cascade Mountains, Washington, USA

Climate change could significantly influence seasonal streamflow and water availability in the snowpack-fed watersheds of Washington, USA. Descriptions of snowpack decline often use linear ordinary least squares (OLS) models to quantify this change. However, the region's precipitation is known to be related to climate cycles. If snowpack decline is more closely related to these cycles, an OLS model cannot account for this effect, and thus both descriptions of trends and estimates of decline could be inaccurate. We used intervention analysis to determine whether snow water equivalent (SWE) in 25 long-term snow courses within the Olympic and Cascade Mountains are more accurately described by OLS (to represent gradual change), stationary (to represent no change), or step-stationary (to represent climate cycling) models. We used Bayesian information-theoretic methods to determine these models' relative likelihood, and we found 90 models that could plausibly describe the statistical structure of the 25 snow courses' time series. Posterior model probabilities of the 29 "most plausible" models ranged from 0.33 to 0.91 (mean = 0.58, s = 0.15). The majority of these time series (55%) were best represented as step-stationary models with a single breakpoint at 1976/77, coinciding with a major shift in the Pacific Decadal Oscillation. However, estimates of SWE decline differed by as much as 35% between statistically plausible models of a single time series. This ambiguity is a critical problem for water management policy. Approaches such as intervention analysis should become part of the basic analytical toolkit for snowpack or other climatic time series data.

A Gibbs sampling disaggregation model for orographic precipitation

Hydrological applications in complex topographic areas need high spatial resolution precipitation data. Some daily high-resolution products are now available for recent past data, even in complex terrain. While the spatial resolution of Regional Climate Models (RCMs) and operational meteorological models are becoming increasingly fine, there still exists a mismatch between the spatial resolutions of observed or estimated recent past data and simulated or forecasted precipitation.Statistical disaggregation models can generate precipitation on a high-resolution grid using as input a mesoscale precipitation grid (e.g., RCM or meteorological grid). In this paper, a Gibbs sampling disaggregation model previously developed for flat areas is adapted to account for topography. Only one variable, the topographic anomaly, is added to the original model. The model is applied on a 300km×300km area in the northwestern United States, covering the Olympic Mountains and the Cascade Range. Daily high-resolution precipitation data for the 2002–2005 period are used to estimate the model parameters. Using 750 days taken from the 2006–2008 period, 36, 52-km grid boxes are disaggregated on 4.3-, 8.7-, 13-, 17.3- and 26-km grids; each day being simulated nine times. Thank to the Gibbs sampling algorithm, the original model, which does not account for topography, is able to capture the mesoscale topographic structure of the daily precipitation, while the adapted model accounting for topography is better suited to recreate the local impact of topography on interannual means, interday standard deviation, and maximum values. The model outputs could be used by hydrological modelers who need high-resolution precipitation data in complex topographic area application.

Flooding in Western Washington: The Connection to Atmospheric Rivers*

AbstractThis study utilizes multiple decades of daily streamflow data gathered in four major watersheds in western Washington to determine the meteorological conditions most likely to cause flooding in those watersheds. Two are located in the Olympic Mountains and the other two in the western Cascades; and each has uniquely different topographic characteristics. The flood analysis is based on the maximum daily flow observed during each water year (WY) at each site [i.e., the annual peak daily flow (APDF)], with an initial emphasis on the 12 most recent water years between WY1998 and 2009, and then focusing on a 30-year interval between WY1980 and 2009. The shorter time period coincides with relatively complete passive microwave satellite coverage of integrated water vapor (IWV) over the Pacific basin. The combination of IWV imagery and streamflow data highlights a close link between landfalling atmospheric rivers (ARs) and APDFs (i.e., 46 of the 48 APDFs occurred with landfalling ARs). To complement this approach, the three-decade time series of APDFs, which correspond to the availability of the North American Regional Reanalysis (NARR) dataset, are examined. The APDFs occur most often, and are typically largest in magnitude, from November to January. The NARR is used to assess the composite meteorological conditions associated with the 10 largest APDFs at each site during this 30-year period. Heavy precipitation fell during the top 10 APDFs, and anomalously high composite NARR melting levels averaged ~1.9 km MSL, which is primarily above the four basins of interest. Hence, on average, mostly rain rather than snow fell within these basins, leading to enhanced runoff. The flooding on the four watersheds shared common meteorological attributes, including the presence of landfalling ARs with anomalous warmth, strong low-level water vapor fluxes, and weak static stability. There were also key differences that modulated the orographic control of precipitation. Notably, two watersheds experienced their top 10 APDFs when the low-level flow was southwesterly, while the other two basins had their largest APDFs with west–southwesterly flow. These differences arose because of the region’s complex topography, basin orientations, and related rain shadowing.

Sedimentary basins reconnaissance using the magnetic Tilt-Depth method

We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3 km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to basement). The method is especially valuable as a reconnaissance tool in regions where drillhole or seismic information are either scarce, lacking, or ambiguous.

Stable water isotope climate archives in springs from the Olympic Mountains, Washington

The 18O and 2H (HDO) compositions are summarized for sampled springs (n = 81) within the Elwha watershed (≈692 km2) on the northern Olympic Peninsula. Samples, collected during 2001–2009, of springs (n = 158), precipitation (n = 520), streams (n = 176), and firn (n = 3) assisted the determinations for meteoric composition of recharge waters. The local mean water line (LMWL) is defined as δ2H = 8.2δ18O − 9.3 for the watershed. Recharge history is surmised from groundwater ages ranging from 5 ± 3 years (apparent 85Kr) to 9,490 ± 420 14C cal years BP. About 56% of the springs were recharged over the last 1,000 years while 13% of springs were recharged over 5,000 years ago. Spring HDO values fluctuate between −11.8 to −15.6‰ δ18O and −90.9 to −119.4‰ δ2H. Deuterium excess values predominate around 4–6‰. The HDO proxy records from springs suggest a pronounced paleoclimate shift in air masses near 5,000 year BP on the Peninsula.

Reassessment of the Zemorrian foraminiferal Stage and Juanian molluscan Stage north of the Olympic Mountains, Washington State and Vancouver Island

Reassessment of the Zemorrian foraminiferal Stage and Juanian molluscan Stage north of the Olympic Mountains, Washington State and Vancouver Island

Saddle Mountain fault deformation zone, Olympic Peninsula, Washington: Western boundary of the Seattle uplift

The Saddle Mountain fault, first recognized in the early 1970s, is now well mapped in the Hoodsport area, southeastern Olympic Peninsula (northwestern United States), on the basis of light detection and ranging (LIDAR) surveys, aerial photography, and trench excavations. Drowned trees and trench excavations demonstrate that the Saddle Mountain fault produced a M W 6.5–7.0 earthquake 1000–1300 yr ago, likely contemporaneous with the M W 7.5 Seattle fault earthquake 1100 yr ago and with a variety of other fault and landslide activity over a wide region of the Olympic Peninsula and Puget Lowland. This near synchroneity suggests that the Saddle Mountain and Seattle fault may be kinematically linked. Aeromagnetic anomalies and LIDAR topographic scarps define an en echelon sequence of faults along the southeastern Olympic Peninsula of Washington, all active in Holocene time. A detailed analysis of aeromagnetic data suggests that the Saddle Mountain fault extends at least 35 km, from 6 km southwest of Lake Cushman northward to the latitude of the Seattle fault. A magnetic survey over Price Lake using a nonmagnetic canoe illuminated two east-dipping reverse faults with 20 m of vertical offset at 30 m depth associated with 2–4 m of vertical displacement at the topographic surface. Analysis of regional aeromagnetic data indicates that the Seattle fault may extend westward across Hood Canal and into the Olympic Mountains, where it terminates near the northward terminus of the Saddle Mountain fault. The en echelon alignment of the Saddle Mountain and nearby Frigid Creek and Canyon River faults, all active in late Holocene time, reflects a >45-km-long zone of deformation that may accommodate the northward shortening of Puget Lowland crust inboard of the Olympic massif. In this view, the Seattle fault and Saddle Mountain deformation zone form the boundaries of the northward-advancing Seattle uplift.

Spatial patterns of rainfall and shallow landslide susceptibility

We quantify the effect of spatial patterns in climatological rainfall on shallow landslide susceptibility by forcing a physically based model of slope stability (SHALSTAB) with the rainfall pattern produced by a high‐resolution atmospheric model (MM5) over the western Olympic Mountains of Washington State. Our results suggest that for two small basins in the Olympics, 10 km–scale variations in rainfall have a nontrivial effect on landslide susceptibility. Assuming uniform rainfall equal to the average rainfall over the basins results in a moderate underestimate of landslide susceptibility. Using climatological data from a lowland station to characterize the rainfall over the basins results in a substantial underestimate of susceptibility. The effect of spatial variability in rainfall on variations in stability is comparable to the effect of moderate‐to‐large variability in soil parameters (such as ±30% variations in soil thickness). At a practical level, these results imply that accounting for persistent patterns of rainfall may aid in discerning regions within the same watershed where similar land use practices will lead to differing landslide risk.

Hybrid speciation in Erythronium (Liliaceae): a new allotetraploid species from Washington State

Abstract Erythronium populations from the southwestern Olympic Mountains of Washington, U.S.A. are tetraploid (2n = 48) and are morphologically intermediate between two widespread diploid species of this region, the subalpine E. montanum and the low-elevation E. revolutum. The Olympic Mountains populations lie between the geographic and elevational ranges of E. montanum and E. revolutum and exhibit a combination of the isozyme characteristics of these two species, possessing 11 alleles in common with E. montanum and four in common with E. revolutum. The evidence supports their derivation from the above two species through allopolyploid hybridization, and they are described here as the new species Erythronium quinaultense. Communicating Editor: Aaron Liston

Two Post-Glacial Earthquakes on the Saddle Mountain West Fault, Southeastern Olympic Peninsula, Washington

Abstract Two earthquakes ruptured the western strand of the Saddle Mountain fault after the Cordilleran ice sheet began its retreat from the Puget Sound Lowland about 17 ka. The fault system includes multiple northeast-trending thrust faults on the southeastern Olympic Peninsula delineated on light detection and ranging topography by prominent, east-side-up scarps that deform Vashon till and extend over 6–7 km. The Cargill Creek Trench on the Saddle Mountain West fault exposed faulted drift and postglacial colluvium below a 1–2 m high scarp. Steeply dipping northeast-striking faults form a 10 m wide zone beneath the scarp. Structural relations in the trench indicate that two earthquakes produced at least 1 m of vertical separation. Additional folding or faulting beyond the trench raised the scarp to a total height of 1.7 m. Aligned cobbles dipping approximately 40° to the north attest to earthquake-related tilting. A cobble firmly lodged in till and split by a secondary fault indicates that hanging-wall deformation involved a dextral component of slip. However, slip on the master fault may have involved a left-lateral component based on slip indicators at adjacent trench sites in the region. A single charcoal age and estimates of the onset of ice retreat limit the time of the oldest earthquake to 17–7.7 ka. The most recent event occurred after 1.7 ka, and may be the same earthquake that dammed Price Lake about 1100 yr ago known from related studies. Empirical relations suggest that the most recent earthquake was about M 7±0.2 and produced an estimated total slip of 1.3–1.7 m. The total postglacial slip rate ranges from 0.1 to 0.3 m/k yr. These results support the hypothesis that the Saddle Mountain fault is part of a larger fault system that accommodates transpressive strain as the Puget Lowland crust migrates around the eastern flank of the Olympic Mountains.

Varied climatic and topographic influences on Late Pleistocene mountain glaciation in the western United States

AbstractThe timing of Late Pleistocene mountain glacier fluctuations varied widely across the western United States. Glaciers in the maritime Olympic Mountains reached local maxima 125–60 ka and 34–31 ka, and were far less extensive during the global Last Glacial Maximum (LGM). Glaciers in the Cascade Mountains showed a similar general pattern but with contrasts in detailed timing. Most mountain glacier systems further inland exhibited maximum ice extent during Marine Isotope Stage (MIS) 2. In the Wallowa, Wind River, and possibly the Sawtooth Mountains, maximum ice extent occurred ca. 20–23 ka. Mountain glacier systems of the Cordilleran Ice Sheet (Puget Lobe), greater Yellowstone region and Uinta Mountains reached maximum extent ca. 19–16 ka, and those in the Wallowa, Sawtooth and Wind River Ranges were at near‐maximum positions ca. 17 ka. The variable timing of maximum and near‐maximum ice advances appears to be related to climatic setting and glacier mass balance characteristics. Mountain glacier systems under the strongest maritime influences reached maximum extent at times of strong westerly atmospheric flow. Olympic and Cascades glaciers appear to have been negatively influenced by weakened westerly flow at the time of the global LGM, perhaps associated with ice‐sheet‐induced anticyclonic flow. Glacier systems with maximum advances correlative with the global LGM appear to have responded more strongly to depressed temperature. The ca. 17 ka ice advances of several glacier systems are broadly correlative with Heinrich Event 1 in the North Atlantic region, and imply a cooling event and reinvigoration of moisture transport into the region. Post‐LGM advances of Uinta Mountain glaciers appear to have arisen from moisture influences of pluvial Lake Bonneville. Copyright © 2008 John Wiley & Sons, Ltd.

The climatology of small‐scale orographic precipitation over the Olympic Mountains: Patterns and processes

AbstractThe 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. Copyright © 2008 Royal Meteorological Society