Application of Ensemble Sensitivity during the 2022/2023 AR-Recon Season

Atmospheric Rivers (AR) are the source of significant precipitation along the West Coast of the United States. These features form over the open Pacific Ocean and previous research has shown that are often characterized by a lack of observations, including with satellites. As a consequence, these features may have significant initial condition uncertainty, which in turn can translate into higher variability and error in numerical model forecasts. Despite this, there have been relatively few studies that have quantitatively documented how uncertainty in specific aspects of the AR (e.g., wind, moisture content) and surrounding synoptic features (e.g., position of upper troughs) could impact the subsequent precipitation forecast. One method of quantifying this is through ensemble-based sensitivity analysis, which utilizes the statistics of a forecast ensemble to establish these associative relationships.

This study evaluates the sensitivity of California precipitation forecasts for cases that occurred during the active 2022/2023 winter season by applying this method to real-time ECMWF ensemble forecasts, which was used for flight planning during AR Recon. During this season, multiple new forecast metrics were employed for flight planning, including a landfall integrated vapor transport (IVT) metric and precipitation within river basins. Preliminary results indicate that the largest precipitation forecast sensitivity is associated with the position of shortwave troughs embedded within the upper level trough within Eastern Pacific during active periods. The evolution of these features subsequently modulated various aspects of the AR and in turn impacted where the precipitation was maximized.