Objective Identification of Atmospheric Rivers, and Implications for Extreme Precipitation at the Basin Scale

Abstract:

Atmospheric rivers (ARs) are narrow, filamentary structures of water vapor in the atmosphere often present in the warm sector of extratropical cyclones. ARs were present and were an important contributor to major flooding events along the U.S. west coast. However, several aspects of the complete relationship between ARs and flooding events remain uncertain. Given the infrequent nature of flooding events, a long time series of ARs, orographic precipitation processes, and flood occurrence is required to yield statistically robust results characterizing conditions necessary to lead to significant precipitation.

This project analyzes the mesoscale to synoptic processes relevant for controlling extreme precipitation from atmospheric rivers at the regional scale, crucial for decision making about disaster planning and water budgeting in a changing climate system. First, a 30-year catalog of atmospheric rivers is constructed with an objective identification tool applied to the Climate Forecast System Reanalysis. A complementary catalog of extreme precipitation events across California is then identified in gridded daily observation-based data. Synoptic characteristics of the subset of these atmospheric rivers that are extreme-precipitation-producing are compared against similar statistics for non-extreme-precipitation-producing atmospheric rivers. A subset of both extreme and non-extreme events are then investigated in high resolution regional climate model simulations generated with the Weather Research and Forecast model run at 6km horizontal grid spacing across California. These model simulations are compared to results from two other models of varying complexity, including a simple linear model of orographic precipitation and a simplified dynamical weather model. The high-resolution model simulations will help identify the mesoscale processes responsible for determining basin-scale response to the larger-scale water vapor fluxes associated with atmospheric rivers, as well as the model complexity necessary to properly represent these processes. The latter results will be used to inform a future experiment downscaling atmospheric rivers in global climate model output.