Subduction, accretion, and exhumation of coherent Franciscan blueschist-facies rocks, northern Coast Ranges, California

AbstractNon-brightband (NBB) rain is a shallow, orographic precipitation that does not produce a radar brightband as a result of melting ice crystals. However, NBB rain is not the same as warm rain, which excludes ice from being involved in the microphysical growth of precipitation. Despite this difference, NBB rain is often treated as warm rain in the literature, and past studies have mostly ignored the role of ice. Here, we use two wet-seasons (2015-16 and 2016-17) at four precipitation observing sites in the Northern Coast Ranges of California to show the role of echo top height and ice in determining NBB rain intensity. It was found that NBB rain was only absent of brightbands 32-46% of the time depending on location of the site. Additionally, all NBB rain rates that exceeded 10 mm hr−1 exhibited observable brightbands during the hour period. We also define Growth Efficiency (GE) as the ability of shallow rain clouds to produce raindrops larger than drizzle size (D > 0.5 mm). High-GE rain drop size distributions were composed of fewer small drops and more large drops than low-GE rain, which was mostly drizzle. High-GE rain occurred with echo top heights above the freezing level where rapid growth of precipitation was observed by radar. Echo tops that only extended 1 km or less above the freezing level suggested hydrometeor growth from mixed-phase processes, indicating that ice may be present in coastal precipitation at warmer temperatures than previously considered.

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Moist Dynamics and Orographic Precipitation in Northern and Central California during the New Year’s Flood of 1997

Monthly Weather Review ◽

10.1175/mwr2943.1 ◽

2005 ◽

Vol 133 (6) ◽

pp. 1594-1612 ◽

Cited By ~ 44

Author(s):

Joseph Galewsky ◽

Adam Sobel

Keyword(s):

Sierra Nevada ◽

Mesoscale Model ◽

Atmospheric Stability ◽

Central Valley ◽

Heavy Precipitation ◽

Horizontal Distribution ◽

Northern Coast ◽

Central California ◽

Low Level ◽

Coast Ranges

Abstract The dynamics of moist orographic flows during the January 1997 floods in northern and central California are investigated using numerical simulations computed with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Early in the event (31 December 1996–1 January 1997), the low-level winds offshore of California’s central coast were blocked by the topography of the Santa Lucia Range, and the low-level winds in the Central Valley were blocked by the topography of the central Sierra Nevada Range. In contrast, moisture-laden winds along the northern Coast Ranges and the northern Sierra Nevada flowed over topographic barriers. As the core of humid air migrated to the south over 24 h, the low-level barrier jets weakened as the atmospheric stability decreased, bringing heavy rainfall to the central and southern Sierra Nevada at the end of the event. The heavy precipitation during this event was largely controlled by the interaction of the flow with topography, with little contribution from non–topographically forced dynamical uplift. Latent heating was essential for lowering the effective stability of the flow and allowing the winds to flow over mountainous terrain, particularly in the northern Coast Ranges, and for enhancing the low-level jet and associated moisture transport. The horizontal distribution of static stability played a key role in the event by setting up a complex combination of flow-over and flow-around regimes that enhanced uplift in the northern Sierra Nevada during the period of heaviest rainfall.

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