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1. Analyzing Surface Precipitation Accumulations Upstream of the Olympic Mountains using High-Resolution Simulations and Observations: An OLYMPEX Case Study

   James, S; Hence, D A (June 2022, 20th Conference on Mountain Meteorology)

   Analysis of surface precipitation accumulations upstream, near-shore, and adjacent to the Olympic mountains from the 17 December 2015 case during OLYMPEX using high-resolution Weather Research and Forecasting (WRF) simulations, and observations from the NPOL dual-polarization radar show that strong down-valley flow coupled with a frontal jet led to the formation of long-lasting Kelvin-Helmholtz (KH) waves. High-resolution soundings investigate the role of low-level blocking on upstream precipitation enhancement. Past work shows that frontal systems often slow while approaching complex terrain if the Froude number is sufficiently low. Low-level blocking by stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding profiles from OLYMPEX indicate little change in surface thermodynamic characteristics as the front propagated northeastward and stalled, a possible indication of blocking by the terrain. Initial findings from sounding-derived vertical stability parameters show high levels of low-level stability (~7.5 k km-1) and high vertical wind shear (dU/dz = 0.025 s-1). Precipitation accumulations from WRF simulations and OLYMPEX observations divide the event into five regions 100 km, 80 km, 40 km, 15 km, and 0 km upstream of NPOL and into timeframes relative to landfall to analyze the distribution of precipitation enhancement associated with a warm front. Preliminary results indicate that 40 km upstream of the coast saw nearly double 5-min precipitation accumulations compared to 15 and 0 km even though radar indicates that precipitation was stratiform across the region. We will isolate the impact of the KH waves and frontal dynamics on precipitation enhancement by analyzing each region’s respective model output precipitation accumulation distributions. Further analysis of the frontal characteristics, thermodynamic environment, and stability parameters will assist with determining if blocking had a role in a potential upstream shift in accumulations. 

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2. Analyzing Warm-Frontal Precipitation Distributions and Accumulations Upstream of the Olympic Mountains using Observations and High-Resolution Simulations: An OLYMPEX Case Study

   James, S; Hence, D A (January 2023, Third Symposium on Mesoscale Processes, 103rd American Meteorological Society Annual Meeting)

   Analysis of surface precipitation accumulations upstream, near-shore, and adjacent to the Olympic mountains from the 17 December 2015 case during OLYMPEX using high-resolution Weather Research and Forecasting (WRF) simulations, and observations from the NPOL dual-polarization radar show that strong down-valley flow coupled with a frontal jet led to the formation of long-lasting Kelvin-Helmholtz (KH) waves. High-resolution soundings investigate the role of low-level blocking on upstream precipitation enhancement. Past work shows that frontal systems often slow while approaching complex terrain if the Froude number is sufficiently low. Low-level blocking by stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding profiles from OLYMPEX indicate little change in surface thermodynamic characteristics as the front propagated northeastward and stalled, a possible indication of blocking by the terrain. Initial findings from sounding-derived vertical stability parameters show high levels of low-level stability (~7.5 k km ) and significant vertical wind shear (dU/dz = 0.025 s ). As a result, the Froude number was sufficiently low (Fr < 1) and the front was well within the Rossby radius of deformation for most of the case. Precipitation accumulations from WRF simulations and OLYMPEX observations divide the event into five regions 100 km, 80 km, 40 km, 15 km, and 0 km upstream of NPOL and into timeframes relative to the fronts position to analyze the distribution of precipitation enhancement associated with a warm front. Results indicate that 40 km upstream of the coast saw nearly double 5-min precipitation accumulations compared to 15 and 0 km even though radar indicates that precipitation was stratiform across the region. We assess that the precipitation enhancement upstream is not a direct result of orography, but more likely that the terrain influences the frontal shape and propagation which then results in enhanced near- front precipitation accumulations. 

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3. Characterizing the Role of Blocking in Frontal Precipitation Accumulation Upstream of the Olympic Mountains: An OLYMPEX Case Study

   James, S; Hence, D A (January 2021, 20th Student Conference, 101st American Meteorological Society Annual Meeting)

   This case study examines the impacts of terrain on the precipitation and thermodynamic characteristics of the 17 December 2015 occluded front during the Olympic Mountains Experiment (OLYMPEX) field campaign using dual-polarization radar, in-situ aircraft, high resolution soundings, synoptic field reanalysis using the University of Washington’s Weather Research and Forecasting (UW WRF) model, and surface-based precipitation instruments. Frontal precipitation processes in the Pacific Northwest are often modified by terrain blocking due to the high mountain heights of the Olympic Mountain range, resulting in enhanced precipitation on the windward slopes (west/southwest). Many studies have observed orographic enhancement over the high terrain, but often neglect how these processes extend upstream in rapidly-shifting flow regimes. This study bridges the gap between upstream enhancement of frontal passages and the high terrain enhancement by characterizing the role that blocking has on a complex occluded front that produced intense Kelvin-Helmholtz (KH) waves. The occluded front will be broken into two different frontal regimes, warm-frontal and postfrontal. Each regime will be characterized by a temporal and spatial stability analysis, precipitation characterization using dual-polarization radar, and thermodynamic profile analysis from aircraft dropsondes and surface-based radiosondes. Further analysis will be done on the remarkable KH wave event during the period to see if KH waves play a role in upstream orographic precipitation enhancement. 

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4. Orographically Modified Ice-Phase Precipitation Processes During the Olympic Mountains Experiment (OLYMPEX)

   https://doi.org/10.1175/JAS-D-21-0091.1  

   DeLaFrance, Andrew; McMurdie, Lynn; Rowe, Angela (September 2021, Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract Over mountainous terrain, windward enhancement of stratiform precipitation results from a combination of warm-rain and ice-phase processes. In this study, ice-phase precipitation processes are investigated within frontal systems during the Olympic Mountains Experiment (OLYMPEX). An enhanced layer of radar reflectivity (Z H ) above the melting level bright band (i.e., a secondary Z H maximum) is observed over both the windward slopes of the Olympic Mountains and the upstream ocean, with a higher frequency of occurrence and higher Z H values over the windward slopes indicating an orographic enhancement of ice-phase precipitation processes. Aircraft-based in situ observations are evaluated for the 01-02 and 03 December 2015 orographically-enhanced precipitation events. Above the secondary Z H maximum, the hydrometeors are primarily horizontally oriented dendritic and branched crystals. Within the secondary Z H maximum, there are high concentrations of large (> ~2 mm diameter) dendrites, plates, and aggregates thereof, with a significant degree of riming. In both events, aggregation and riming appear to be enhanced within a turbulent layer near sheared flow at the top of a low-level jet impinging on the terrain and forced to rise above the melting level. Based on windward ground sites at low-, mid-, and high-elevations, secondary Z H maxima periods during all of OLYMPEX are associated with increased rain rates and larger mass-weighted mean drop diameters compared to periods without a secondary Z H maximum. This result suggests that precipitation originating from secondary Z H maxima layers may contribute to enhanced windward precipitation accumulations through the formation of large, dense particles that accelerate fallout. 

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5. Investigating Frontal Precipitation Enhancement Upstream of the Olympic Mountains

   Garcia, B G; Eidhammer, T; Hence, D A; Rauber, R M (January 2021, 20th Student Conference, 101st American Meteorological Society Annual Meeting)

   Orographic precipitation enhancement is the tendency of mountains to cause clouds to produce more precipitation than they would otherwise, which greatly affects the total rainfall in mountainous regions and plays a major role in flooding and mudslides. Most past studies have examined this enhancement close to the mountains themselves. The present study examined the orographic enhancement of frontal precipitation upstream of the Olympic Mountains of Washington State, using data from a Weather Research and Forecasting (WRF) regional climate simulation. Using this simulation, we strived to determine how thermodynamic and dynamic conditions affect the enhancement of frontal precipitation upstream of the Olympic Mountains. To do so, the characteristics of frontal passages were analyzed, including frontal type, orientation, velocity, warm-air moisture content, and accumulated precipitation. Of the five fronts analyzed to date, there were two cold fronts, two warm fronts, and one occluded front. We then analyzed each characteristic by comparing each event to each other and as a function of distance from the mountains. We found that all fronts slowed down as they approached the mountains, four of which to almost half of their original speeds, and some even beginning to decelerate upwards of 700 km upstream. Areas of enhanced precipitation were observed along the two cold fronts, as far upstream as 560 km. These results document upstream impacts on frontal passages, but further analysis is needed to examine additional frontal passages and determine if the observed precipitation enhancement and frontal deceleration were caused by orographic effects from the Olympic Mountains. 

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