Terrain trapped airflows and orographic rainfall along the coast of northern california. part I: kinematic characterization using a wind profiling radar

Abstract

This study develops an objective method of identifying terrain-trapped airflows (TTAs) along the coast of Northern California and documenting their impact on orographic rainfall. TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. A 13-winter-seasons dataset is employed, including observations from a 915-MHz wind profiling radar along the coast at Bodega Bay (BBY, 15m MSL) and surface meteorology stations at BBY and in the coastal mountains at Cazadero (CZD, 478m MSL). A subset of rainy hours exhibits a profile with enhanced vertical shear and an easterly wind maximum in the lowest 500m MSL, roughly the same depth as the nearby coastal terrain. Both flow features have a connection to TTAs along the coast of Northern California. Based on the average orientation (320 degrees-140 degrees) and altitude of nearby topography, mean wind direction in the lowest 500m MSL ((WDIR) over bar (500)) between 0 degrees-140 degrees is used as the initial criterion to identify TTA conditions. Application of this threshold yields a CZD/BBY rainfall ratio of 1.4 (3.2) for TTA (NO TTA) conditions. More detailed analysis of the relationship between (WDIR) over bar (500) and orographic rainfall reveals that an upper threshold of 150 degrees more precisely divides the TTA and NO-TTA regimes. A sensitivity analysis and comparison with a TTA documented in a previous case study show that the best TTA identification criteria correspond to 0 degrees <= (WDIR) over bar (500), 150 degrees with a duration of at least 2 h. This objective identification method is applied to seven case studies in Part II of the present study.