75 Mesoscale Coastal SST Variability and Its Impact on a Modeled Atmospheric River Case Study in Southern California

Wednesday, 26 July 2017
Kona Coast Ballroom (Crowne Plaza San Diego)
Rachel Weihs, SIO, La Jolla, CA; and F. Cannon

Sea surface temperatures along the southern coast of California are subject to complex forcing
processes generated in response to the California current, bathymetry, seasonal wind flow, and
Ekman upwelling. During the late winter/early spring months, coastal upwelling along the
Southern California coast begins to dominate the coastal SST variability earlier in the year than
at higher latitudes. The timing of this transition coincides with relatively frequent atmospheric
river (AR) events in California. ARs are characterized by a relatively narrow region of high
moisture content and strong low-level wind speed preceding the cold front of a midlatitude
cyclone. Though they impact California typically during boreal winter, they are the primary
mechanism for annual precipitation. Previous studies have implied that SST variability
generated from seasonal-scale anomaly forcing, such as ENSO, can modify near-surface
stability, surface heat fluxes, and the amount of convective available potential energy prior to
AR landfall. This paper investigates the influence of SST variability, driven by coastal upwelling
and eddies, on orographic precipitation during an AR event that impacted the transverse
coastal ranges of Southern California in February 2017. The SST spatial patterns resulting from
upwelling are artificially damped to represent weaker upwelling and applied as boundary
conditions to a customized version of the Weather and Research Forecasting (WRF) model
(referred to as West-WRF). Simulated reflectivity from the model will be compared to GPM
along-track radar reflectivity and in-situ radar measurements to determine whether convection
is weakened in the presence of upwelling and cooler coastal SSTs. Furthermore, the boundary
layer profiles of moisture and winds will also be examined to identify potential physical
responses of the atmosphere to sea surface temperature forcing.
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