Why Does the Leeward Side of Mt. Tamalpais Experience a Climatological Precipitation Maximum?

Abstract:

Climatological records show annual precipitation maximum values on the northeast side of Mt. Tamalpais in the San Francisco Bay Area, which is typically the leeward side during winter precipitation events. The town of Kentfield, CA, about five kilometers to the northeast of the mountain, has a significantly higher annual-average rainfall than adjacent locations, particularly compared with stations to its north and south. One hypothesis regarding this phenomenon is that the topography if Mt. Tamalpais creates convergence on the leeward side, thereby enhancing precipitation. If prevailing southwesterly winds during rainfall events are interrupted by the mountain and then flow around either side, converging on leeward side, then enhanced cloud formation and precipitation would replace the divergent flow characteristic to leeward slopes. A second hypothesis is that Mt. Tamalpais is narrow relative to the advective scale of orographically triggered precipitation. If the mountain is relatively narrow, precipitation from clouds generated orographically on the upwind side might not develop and fall until air within the clouds has reached the leeward side, where Kentfield lies. With a network of surface weather stations, we analyze precipitation accumulations to quantify the Kentfield rainfall maximum over a series of rainfall events during 2015. For these rainfall events, we evaluate how well a 1.1-km resolution WRF-ARW model reproduces the precipitation observations. To the extent that the model agrees with observations, we investigate mechanisms underlying the rainfall maximum by analyzing patterns of low-level wind convergence downwind of Mt. Tamalpais and patterns of vertical velocity and cloud and rain-water mixing ratios in vertical cross sections across Mt. Tamalpais, parallel to the wind flow for each rainfall event. Cross-sectional mixing ratio analyses provide evidence of precipitation development and advection and test the second of the aforementioned hypotheses. Wind convergence and vertical velocity produced by the model downwind of the mountain help test the plausibility of the first hypothesis.