UC San Diego

Atmospheric rivers (AR) have been found to substantially contribute to the annual precipitation in many mid-latitude regions, but while their synoptic scale dynamics have been studied for a century (through extratropical cyclone research), the mesoscale dynamics are less understood and present forecasting challenges. In this dissertation we focus on i) the pre-cold-frontal low-level jet (LLJ), ii) moisture effects on the orographic and dynamic precipitation components, and iii) physical processes associated with AR initial condition sensitivity.

An analysis using dropsonde observations from 24 cross-AR transects found that the LLJ is strongly supergeostrophic with about 20% of the wind being ageostrophic. The ERA5 reanalysis product is used to investigate the ageostrophic forcing mechanism finding that the pressure tendency term serves to accelerate the ageostrophic jet, and the Coriolis torque and advective tendency terms serve to propagate the LLJ. Therefore, to accurately simulate an LLJ, a model must adequately resolve the pressure tendencies along the cold front.

Next, an analysis of a strong landfalling AR is presented comparing the evolution of a control simulation with an optimally perturbed simulation using the moist adjoint tool from the Coupled Ocean/Atmosphere Mesoscale Prediction System. The perturbed simulation shows a strengthened role of the orographic and dynamic components of the precipitation both resulting from enhanced latent heating effects by which: i) a stronger diabatically driven low-level potential vorticity anomaly strengthens the water vapor transport and thereby the orographic precipitation, and ii) a greater moist diabatic forcing enhances the transverse circulation thereby increasing ascent and the dynamic precipitation.

An idealized two-dimensional moist frontogenesis model is then used to quantify the transverse circulation response to varying amplitudes of optimal moisture perturbations demonstrating that small changes in moisture leads to comparatively larger changes in the frontal ascent. This non-linear growth is due to a feedback process in the transverse circulation by which small changes in moisture cause greater latent heating which ultimately lead to a strengthened transverse circulation and the cycle repeats.

The research presented in this dissertation improves the understanding of AR mesoscale dynamics through investigation of LLJs, effects of moisture perturbations, and by identifying an important feedback process.