Characterizing the role of blocking in frontal precipitation enhancement near and upstream of the Olympic Mountains

Analysis of OLYMPEX observations and high-resolution Weather Research and Forecasting (WRF) simulations upstream of the Olympic mountains from 17 December 2015 investigate low-level blocking’s role in precipitation distributions before and during a frontal passage near the Olympic Peninsula. Low-level blocking by stable air ahead of a front can modify precipitation distributions by frontal deformation, slowing, splitting, or merging. Observed coastal sounding-derived vertical stability profiles indicate high levels of low-level stability (~7.5 K km-1) and significant vertical wind shear (dU/dz= 0.025 s-1), which showed little change while the front propagated northeastward and stalled as the stable air mass likely dammed against the terrain. The resulting Froude number was sufficiently low ( 1) and the front well within the Rossby radius of deformation as the front bowed around the peninsula. Radial velocity from the NPOL radar and simulated wind fields indicate strong down-valley flow coupled with a frontal jet also contributed to long-lasting Kelvin-Helmholtz (KH) waves extending offshore. Simulated surface precipitation accumulation distributions divided the event into five regions upstream of NPOL and into timeframes relative to the fronts position. Results indicate that pre-warm frontal precipitation accumulations decrease with distance upstream of the coast with the highest accumulations present over the terrain. During the WF period, accumulations are highest far upstream of the coast before decreasing in a middle region and then increasing over the terrain. These results indicate that upstream precipitation enhancement upstream is an indirect effect of the terrain influencing the frontal shape and propagation, which resulted in enhanced near front precipitation accumulations.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Atmospheric Sciences

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• James, Scott Gregory

Contributors dc:contributor

• Hence, Deanna

• Atmospheric Science
• Mountain Meteorology
• Wrf Simulation
• Radar Meteorology

Statement dc:rights

• Copyright 2022 Scott James

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/117827

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F117827