Dropsonde Observations of the Ageostrophy within the Pre-Cold-Frontal Low-Level Jet Associated with Atmospheric Rivers

The pre-cold-frontal low-level jet (LLJ) is an important contributor for water vapor transport within atmospheric rivers, though its dynamics are not completely understood. The present study investigates the LLJ using dropsonde observations from 24 cross-atmospheric river transects taken during the CalWater-2014, 2015 and the AR-Recon 2016, 2018 field campaigns. It is found that the LLJ, located at similar to 1-km elevation ahead of the cold front, has an average maximum wind speed of 30 m s(-1) and is strongly supergeostrophic with an average ageostrophic component of 6 m s(-1). The alongfront ageostrophy occurs within the atmospheric layer (750-1250 m) known to strongly control orographic precipitation associated with atmospheric rivers. The ERA5 reanalysis product is used to both validate the observed geostrophic winds and investigate the supergeostrophic jet dynamics. The comparison demonstrates that there is no systematic bias in the observed geostrophic wind but that the ERA5 LLJ total wind field is generally biased low by an amount consistent with the observed ageostrophy. One of the few cases in which the ERA5 produces an ageostrophic LLJ occurs on 13 February 2016, which is used to investigate the dynamical processes responsible for the ageostrophy. This analysis demonstrates that the isallobaric (pressure tendency) term serves to accelerate the ageostrophic jet, and the Coriolis torque and advective tendency terms serve to propagate the jet normal to the LLJ. Therefore, if a model is to accurately represent the LLJ, it must adequately resolve processes contributing toward the pressure tendencies along the cold front.