An Assessment of the Contribution of Non-Orographic Lifting in Atmospheric River Precipitation

Abstract:

Atmospheric Rivers (ARs) play an important role in shaping the regional hydroclimate of the United States (US) and have important implications for water resource management, emergency preparedness and response planning. In many US locations, the precipitation resulting from ARs is strongly related to the strength and duration of orographic lift. This property has been successfully used to design early warning and forecast diagnostic tools used by Western US water managers. However, observational records demonstrate that non-orographic lift is important in explaining AR precipitation, and may be critically important during extreme or difficult to forecast events. This observational study seeks to isolate the non-orographic term or large-scale lift in the expression for indexing orographic rain rate at several sites in both the western and eastern US. Hourly observations for sites along the West Coast, provided through the National Oceanic and Atmospheric Administration’s (NOAA) Earth System Research Laboratory (ESRL), are collected from a network of observing stations known as atmospheric river observatories (ARO) established to continuously monitor surface and upper-air meteorological conditions associated with landfalling ARs. Results aim to inform the Forecast Informed Reservoir Operations (FIRO) effort to improve hydrologic monitoring and management related to AR-driven precipitation. Understanding the importance of non-orographic precipitation in driving precipitation extremes during ARs and its variation with regional climates will enhance AR early warning systems and allow water managers to better anticipate flood events. Ultimately, data and results emerging from this study will improve AR and heavy precipitation forecasting through a better physical process understanding. The approach, leaning on existing monitoring resources, may also lend itself to the development of advanced decision-support tools that enhance the viability of FIRO at projects in multiple US climate regions.