Dropsonde Observations of the Stable Marine Boundary Layer beneath Atmospheric Rivers

Abstract An important source of errors in predicting landfalling atmospheric rivers (ARs) and their associated extreme precipitation and streamflow are inaccuracies in model initial conditions in ARs offshore. These inaccuracies are particularly evident in the marine boundary layer (MBL) where the tendency of ARs to transport warm air poleward over progressively cooler sea surface temperatures (SSTs) generates a stable MBL (SMBL). The SMBL’s vertical structure and key modulating processes are documented using >1000 dropsondes along 99 transects of ARs from the AR reconnaissance and CalWater field campaigns. The SMBL depth, modulated by sensible heat loss to the ocean, is typically 300–800 m in the AR core, with vertical wind shears ranging from 5 to 50 m s −1 km −1 , representing a highly variable decoupling of the AR aloft from conditions at the ocean surface. Simulated backward air parcel trajectories originating from dropsonde locations within the AR core are used to calculate the 24-h change in SST experienced by an air parcel (DSST24) beneath each AR. The DSST24 varies from −13° to +2°C and is directly related to the strength of the AR and its orientation relative to the SST gradient. The DSST24, therefore, distinguishes weak and strong decoupling regimes (WDR and SDR). In SDR cases, relative to WDR cases, the SMBL is characterized by greater sensible heat loss to the ocean, as well as stronger static stability, vertical wind shear, low-level jet, and horizontal water vapor transport. In SDR cases, the SMBL is deeper in the core than in adjacent warm and cold sectors.