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

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.