The Effect of a Kona Low on the Eastern Pacific Valentine’s Day (2019) Atmospheric River

Abstract

A high-impact Atmospheric River (AR) event that made landfall on the US West Coast on Valentine’s Day, 2019 and produced widespread flooding in California is examined. The US Naval Research Laboratory cloud resolving and high-resolution Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) captures the main features impacting the life cycle and structure of the Valentine’s Day AR. Analysis of the model simulated AR reveals the complex processes leading up to the initial northeastward surge of the water vapor and enhanced near-surface flow associated with this AR. These include the pre-existence of a mesoscale cold-core Kona Low, a mesoscale anticyclone, and a strong low-level convergence in the corridor between the Kona Low and mesoscale anticyclone where the environment becomes supersaturated in a region of weak vertical wind shear. Model sensitivity experiments show that the eastward progression and magnitude of the AR water vapor surge is strongly sensitive to the magnitude of Kona Low circulation. Experiments with the Kona Low circulation amplitude reduced to less than 25% showed that the AR is not able to reach the US West Coast. These results help identify key new aspects of an important player – the Kona Low— and its significant contributions to the overall AR characteristics of this particular observed event.