Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

David Kingsmill

Second Advisor

Katja Friedrich

Third Advisor

John Cassano

Fourth Advisor

Matthias Steiner

Fifth Advisor

Albin Gasiewski

Abstract

While several studies have documented kinematic and precipitation structures associated with orographic effects in large-scale mountains (e.g. altitudes > 1.0 km MSL), surveys on small-scale mountains are still relatively rare. Given their lower altitude, these mountain are usually exposed to rain (instead of snow) during the approaching of baroclinic waves and concomitant warm and cold fronts, thus they are prone to flash floods and hydrological disasters during heavy rain episodes.

One factor that influence kinematic and precipitation structures over mountain ranges is associated with airflows induced by the terrain itself, such as during low-level blocking and gap flow episodes. Force balance make these airflows move relatively parallel and in close proximity to the terrain, thus they can be categorized as terrain-trapped airflows (TTAs). TTAs can cause the lifting of incoming synoptic airflows upstream the terrains foothills, initiating and enhancing precipitation well before it would be observed by upslope forcing over the terrain. TTAs and orographic precipitation forcing along the small-scale coastal mountains of northern California have been studied from a 1-dimensional perspective; yet, details of the TTAs 3-dimensional kinematic and precipitation structure, especially in the lowest 500-m MSL, has not being addressed.

In this doctoral thesis I examine physical characteristics and impacts of TTAs on orographic precipitation along the coastal mountains of northern California using a 13-winter season dataset. Selected case studies are employed to document in detail 3-dimensional kinematic and precipitation structures associated with pre-cold-frontal low-level jets (LLJs) and TTAs. The main observational asset is a ground-based X-band dual-polarization scanning Doppler radar located at the coast. Results show that TTAs are normally present during winter time, although with variable duration and offshore extension. In average, they account for 20% and 10% of the long-term rainfall along the coast and over the coastal mountains, respectively. Doppler radar observations depict the lifting of LLJs offshore forced by TTAs, creating an area of enhanced precipitation about 20 km from the coast. Forcing of the TTA seems to be most commonly associated with gap flows coming from Petaluma Gap and, to a shorter extent, with low-level blocking.

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