Terrain-trapped airflows and orographic rainfall along the coast of northern California: Horizontal and vertical structures of kinematics and precipitation

Abstract

Land-falling extratropical cyclones are responsible for the majority of precipitation that falls in the western United States. The spatial distribution of precipitation from these storms is strongly influenced by the regions’ complex terrain. A narrow channel of concentrated horizontal water vapor flux in the lowest 3-4 km MSL is often present immediately ahead of the cold fronts associated with extratropical cyclones. Upon impacting the terrain, these statically-neutral atmospheric rivers (ARs) can facilitate moist orographic uplift that leads to enhanced precipitation. While this relatively simple conceptual model can explain a significant fraction of orographic precipitation that falls in the region, it does not take into account the fact that mountains can produce their own mesoscale circulations that modify the spatial distribution of precipitation. One example is the presence of a terrain-trapped airflow (TTA), which is defined as a relatively narrow air mass flowing in close proximity and approximately parallel to the windward slope of a mountain barrier. TTA impacts on orographic precipitation have been studied extensively in association with several large-scale mountain ranges such as the European Alps, the Southern Alps of New Zealand, the Rocky Mountains of Colorado and the Sierra Nevada of California. In contrast, TTA impacts on orographic precipitation occurring in association with relatively small-scale mountains (altitudes below ~1 km MSL) has received much less attention. It is notable that orographic precipitation over small-scale mountains has the potential to produce rapid runoff and flooding due to the prevalence of precipitation in the form of rain (rather than snow) and the relative scarcity of flood control infrastructure. This is a particularly relevant issue along the coastal mountains of northern California, where unobstructed ARs can directly interact with the coastal terrain (~0.5 km MSL and oriented northwest to southeast) to produce intense rainfall that leads to significant economic impacts. This study employs observations collected along the California coast north of San Francisco as part of the California Land-Falling Jets (CALJET), Pacific Land-Falling Jets (PACJET) and Hydrometeorology Testbed experiments operated by the National Oceanic and Atmospheric Administration’s (NOAA) Earth System Research Laboratory (ESRL). One of the main instruments is a 915-MHz wind-profiling radar located on the coast at Bodega Bay (BBY, 15 m MSL) that provided hourly, high-resolution (~100 m) vertical profiles of horizontal wind up to ~4 km MSL over 13 winter seasons. Supporting information is provided by surface meteorology and rain gauge observations at BBY and in the adjacent coastal mountains at Cazadero (CZD, 478 m MSL). These data allow documentation of the longterm kinematic and precipitation characteristics of TTAs in this area. Mean wind direction in the lowest 500 m MSL (WDIR500) less than 140° is used as the initial criterion to identify TTA conditions based on the average orientation and altitude of topography near BBY and CZD. Employing this threshold reveals a distinct easterly jet structure of zonal-component winds at ~250 m MSL and enhanced meridional-component winds, especially above 500-m MSL. TTA-regime duration varies seasonally between 1.9 h and 3.2 h, with an average duration of 2.5 h. The mountain-to-coast ratio (i.e., CZD/BBY) of rainfall during TTA conditions is 1.4, which is significantly lower than the ratio of 3.2 observed when TTA conditions are not present. A more detailed analysis of the relationship between WDIR500 and orographic rainfall reveals that a threshold of 150° more precisely divides the two regimes of orographic enhancement. Additionally, a TTA-duration threshold of at least 2 h filters out insignificant events.