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Much has been learned about the synoptic forcings and
mesoscale internal gravity wave amplifications, hydraulic jumps and rotors
common to the strong Santa Ana events in Southern California. Less well understood are so-called
"Sundowner" events in the vicinity of Santa Barbara and the Santa Ynez
Mountains. The modest body of
research on Sundowners has not yet allowed for robust forecasting of such
events, which can include strong downslope winds and/or strong low-level
warmings that dramatically increase fire danger in Santa Barbara and Ventura
counties.
Open scientific questions, include, but are not limited to: 1) the degree to
which regional transports at low- to mid-tropospheric levels condition the
upstream stability profile such that strong and nonlinear internal gravity
wave (IGW) dynamics are in play: 2) the impacts of boundary-layer/lower
tropospheric offshore eddies in modulating an onshore sea breeze circulation
that can disrupt the downslope Sundowner flows; (3) whether differences in
the environment leading to trapped versus propagating IGWs are
significant in Sundowner evolution.
To address the open scientific questions, as well as to better determine the
predictability of the meso- and upper microscale characteristics of Sundowner
events, we
present a series of WRF
simulations of several Sundowner events, focusing on the open science
questions as well as sensitivity of the simulations to varying the initial
conditions, physical parameterization schemes, and other aspects of the model
configuration (including upper ocean initialization and treatment and a few
basic data assimilation options).
We also intend to briefly explore the possible use of an ensemble
approach for Sundowner forecast simulations.
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