Studying the Modification of Extratropical Cyclones as they Traverse Professor Houze's 'Backyard': The Mountainous West Coast of North America

The mountainous west coast of North America is frequented by extratropical cyclones from the Pacific Ocean.  Precipitation from these storms is important for building the winter snowpack for summer water supply, but also negatively affects the region through significant flooding every winter. As a natural laboratory for studying the modification of precipitation processes by topography, it is no surprise that this region has been the focus of intensive research and observations.  For well over 40 years, Prof. Robert A. Houze, Jr., has been involved with or led field campaigns in his own ‘backyard’ of the coastal mountainous regions of Washington and Oregon to study the meso- and microscale structure of mid-latitude cyclones and how they are modified when they approach and traverse across complex terrain.

His early work during the CYClonic Extratropical Storms (CYCLES) campaign along the Washington coast in the 1970s and early 1980s used Doppler radar and airborne in situ cloud measurements to describe various types of mesoscale rainbands that are embedded within the general frontal structures. In December 1993 and again in December 1995, the Coastal Observation and Simulation with Topography (COAST) project took place over the coastal and mountainous west coast of North America from California to Vancouver Island. Its purpose was to document the nature, magnitude, and offshore extent of the modification of the incoming flow as storms approached topographical barriers and how mesoscale structures of fronts and cyclones were modified as they approached coastal terrain. Using data from a helically scanning tail-mounted Doppler radar and cloud microphysical instruments, Prof. Houze and co-authors described how the near-shore evolution of rainbands was significantly affected by the upstream influence of the coastal topography.

Later, the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE) took place from January – February 2001 on the Washington coast (IMPROVE I) and again from late November 2001 through late December 2001 in the Oregon Cascades (IMPROVE II). The primary goal of these campaigns was to improve model microphysics by using detailed field measurements to inform parameterizations and to verify model-produced quantitative precipitation forecasts. The instrument platforms included aircraft equipped with microphysical instrumentation, S-band dual-polarization Doppler radar, wind and S-band profilers, and ground instrumentation. Many influential research studies by Houze and colleagues resulted from the analysis of this data. Seminal results included documentation of the vertical structure of precipitation in different sectors of extratropical storms and describing the role of turbulence generated at a shear layer between blocked low-level flow and upper-level strong cross barrier flow in contributing to precipitation generation on the windward slopes of complex terrain.

The most recent field campaign took place on the Olympic Peninsula in the northwest corner of Washington State in the fall 2015 through winter 2016 (OLYMPEX) and was led by Prof Houze. This effort expanded upon previous campaigns with an unprecedented collection of state-of-the-art ground-based and aircraft radars, a vast array of rain gauges and disdrometers, and special soundings documenting mid-latitude cyclones as they traversed from the Pacific Ocean to the coast to high terrain and the leeside. Preliminary results will be presented in other talks.

In all of these field campaigns, Prof. Houze’s influence was instrumental in their success. Many of the resulting publications were groundbreaking and are adaptable to many regions of the world, far beyond Prof. Houze’s own ‘backyard’ of the west coast of North America.