Synoptic Control over Orographic Precipitation Distributions during the Olympics Mountains Experiment (OLYMPEX)

Purnell, David; Kirshbaum, Daniel J.

doi:10.1175/mwr-d-17-0267.1

Cited by 29 publications

(32 citation statements)

References 40 publications

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“…The time of day/year was suggested by Tachibana (1995) to have an effect upon the LPE distribution during periods of weak flow, but we found the effect to be relatively minor. A number of studies show that CBVT and similar variables modulate the distribution and intensity of orographic precipitation (e.g., Minder et al 2008;Rutz et al 2014;Purnell and Kirshbaum 2018), but we found its effect on the LPE distribution was nearly identical to that of U due to their strong correlation (Fig. 3c).…”

Section: B Environmental Conditionsmentioning

confidence: 56%

Factors Affecting the Inland and Orographic Enhancement of Sea-Effect Snowfall in the Hokuriku Region of Japan

Veals

Steenburgh

Nakai

et al. 2019

Monthly Weather Review

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The Hokuriku region along the west coast of the Japanese island of Honshu receives exceptionally heavy snowfall accumulations, exceeding 500 cm from December to February near sea level and 1300 cm at high elevation sites, much of which is produced by sea-effect systems. Though the climatological enhancement of snowfall is large, the lowland–upland snowfall distribution within individual storms is highly variable, presenting a challenge for weather forecasting and climate projections. Utilizing data from a C-band surveillance radar, the ERA5 reanalysis, and surface precipitation observations, we examine factors affecting the inland and orographic enhancement during sea-effect periods in the Hokuriku region during nine winters (December–February) from December 2007 to February 2016. The distribution and intensity of precipitation exhibits strong dependence on flow direction due to three-dimensional terrain effects. For a given flow direction, higher values of boundary layer wind speed and sea-induced CAPE favor higher precipitation rates, a maximum displaced farther inland and higher in elevation, and a larger ratio of upland to lowland precipitation. These characteristics are also well represented by the nondimensional mountain height H^, with H^<1 associated with a precipitation maximum over the high elevations and a larger ratio of upland to lowland precipitation, and H^>1 having the opposite effect. Nevertheless, even in high enhancement periods, precipitation rates decline as one moves inland from the first major mountain barrier, even over high terrain. These results highlight how the interplay between sea-effect and orographic processes modulates the distribution and intensity of precipitation in an area of complex and formidable topography.

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Section: B Environmental Conditionsmentioning

confidence: 56%

Factors Affecting the Inland and Orographic Enhancement of Sea-Effect Snowfall in the Hokuriku Region of Japan

Veals

Steenburgh

Nakai

et al. 2019

Monthly Weather Review

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“…Even mountains of more moderate scale may produce blocking in certain synoptic situations. For example, Purnell and Kirshbaum () demonstrated for the Olympic mountains that blocking tends to occur in the air ahead of the surface warm front due to the frontal inversion aloft. As the SF scheme will be active across the globe in a range of situations, it is necessary to account for this low‐level blocking.…”

Section: The Seeder–feeder Schemementioning

confidence: 99%

“…The SF process has been shown to be responsible for producing small‐scale precipitation variability over larger mountain ranges such as the Alps (Zängl, ). By comparing quasi‐idealized simulations with and without large‐scale forcing, Purnell and Kirshbaum () demonstrated that much of the orographic precipitation enhancement produced over the Olympic Mountains in the U.S. Pacific Northwest during the passage of a number of frontal systems was due to the SF process. These two mechanisms both occur within laminar flow as a result of mechanical lifting over orography.…”

Section: Introductionmentioning

confidence: 99%

Verification of a seeder–feeder orographic precipitation enhancement scheme accounting for low‐level blocking

Smith

Field

Derbyshire

2019

Quart J Royal Meteoro Soc

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A subgrid parametrization scheme representing the enhancement of precipitation due to subgrid orography via the seeder–feeder (SF) effect has been modified to account for flow blocking in small Froude number situations. The scheme was validated in a set of limited‐area model simulations with a 1.5 km grid spacing, in which the orography was degraded by varying amounts. For simulations in which the largest orographic scales are still fairly well represented, the SF scheme was able to reduce the precipitation deficit by 30 to 70%. For simulations where the hills were completely subgrid, the SF scheme was still able to reduce the precipitation deficit by 10 to 30%. As well as increasing the integrated precipitation in global simulations with various grid spacings, some of the precipitation production was shifted from the convection scheme, with its very simple microphysical representation, to the microphysics scheme. In a long‐duration climate simulation, the SF scheme enhancements of orographic precipitation perturb the large‐scale hydrological cycle, as evidenced by the far‐field changes in both microphysical and convective precipitation. Changes over major mountain ranges were similar to those described for the case‐studies, so long as the upstream precipitation impinging on the mountains was not reduced by these changes in the global hydrological cycle. Increased atmospheric drying reduces column‐integrated resolved cloud water mixing ratios over mountains, reducing a positive bias in the amount of optically thick cloud with low‐ to mid‐level tops compared to ISCCP satellite observations. The associated reductions in cloud albedo slightly reduced the RMS error in the top‐of‐atmosphere outgoing short‐wave radiative fluxes over mountains compared to CERES satellite observations.

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“…Zagrodnik et al (2018) analyzed gauges and disdrometers located from the coast to windward slopes during a strong AR event from 12 to 13 November 2015, in which there were large concentrations of small to medium raindrops, in addition to highly varying concentrations of large drops, suggesting both warm rain (e.g., collision-coalescence) and cold rain processes (e.g., melting). Purnell and Kirshbaum (2018) noted the presence of cold rain via an active seederfeeder process during warm frontal and sector conditions throughout OLYMPEX from the synthesis of observations and model simulations in which ''seeder'' clouds initiate precipitation growth that falls into orographically enhanced (''feeder'') clouds at lower levels (Cotton et al 2011). McMurdie et al (2018) provided further evidence of this seeder-feeder process after documenting a larger reflectivity signature above the melting layer over the windward slopes compared to the ocean throughout OLYMPEX.…”

Section: Introductionmentioning

confidence: 97%

Evaluating Warm and Cold Rain Processes in Cloud Microphysical Schemes Using OLYMPEX Field Measurements

Naeger

Colle

Zhou

et al. 2020

Monthly Weather Review

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Field observations from the Olympic Mountain Experiment (OLYMPEX) around western Washington State during two atmospheric river (AR) events in November 2015 were used to evaluate several bulk microphysical parameterizations (BMPs) within the Weather Research and Forecasting (WRF) Model. These AR events were characterized by a prefrontal period of stable, terrain-blocked flow with an abundance of cold rain over the lowland region followed by less stable, unblocked flow with more warm rain, and a shift in the largest precipitation amounts to over the windward Olympic slopes. Our WRF simulations underpredicted the precipitation by 19%–36% in the Morrison (MORR) and Thompson (THOM) BMPs and 10%–23% in the predicted particle properties (P3) BMP, with the largest underpredictions over the windward slopes during the more convective, unblocked flow conditions. Several important processes related to the BMPs led to the differences in simulated precipitation. First, the prognostic single ice category parameterization in the P3 scheme promoted a more realistic evolution of rimed particles and larger cold rain production, which led to the lowest underpredictions in precipitation among the schemes. Second, efficient melting processes associated with the production of nonspherical ice and snow in the P3 and THOM BMPs, respectively, promoted a more realistic transition to rain fall speeds within the warm layer compared to the spherical snow assumption in MORR. Last, all BMPs underpredict the contribution of warm rain processes to the surface precipitation, particularly during the unblocked flow period, which may be partly explained by too weak condensational and collisional growth processes due to the neglect of turbulence parameterizations within the schemes.

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Synoptic Control over Orographic Precipitation Distributions during the Olympics Mountains Experiment (OLYMPEX)

Cited by 29 publications

References 40 publications

Factors Affecting the Inland and Orographic Enhancement of Sea-Effect Snowfall in the Hokuriku Region of Japan

Factors Affecting the Inland and Orographic Enhancement of Sea-Effect Snowfall in the Hokuriku Region of Japan

Verification of a seeder–feeder orographic precipitation enhancement scheme accounting for low‐level blocking

Evaluating Warm and Cold Rain Processes in Cloud Microphysical Schemes Using OLYMPEX Field Measurements

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