Frontal Gradients of CO2 as sampled by ACT-America Flights and a Box Model

Meridional transport along frontal surfaces plays an important role in modulating the seasonal cycle of CO2 in midlatitudes and is a dominant source of CO2 seasonality in the Arctic. Frontal zones are typically embedded in clouds so these strong CO2 gradients are poorly observed by satellite instruments. Evaluation of simulated frontal CO2 processes is therefore critical to inverse modeling and is a major objective of the ACT-America campaign. Shearing and deformation, horizontal and vertical advection as well as surface flux all combine to determine the nature of these frontal boundaries.

The ACT-America campaign provides a unique opportunity to observe CO2 dynamics across frontal boundaries. Modeling can help quantify and understand the mechanisms that produce frontal gradients, but coarse model output 'smears out' sharp frontal features.

Here, we perform an experiment in an idealized framework as a means to reproduce observations and attribute relative importance of component terms to the total gradient. Surface fluxes are simulated for Lagrangian pre- and post-frontal regions using high resolution reanalyses from several case studies as drivers, and we use a simple box model to represent transport and generate CO2 fields that can be compared to data collected during ACT-America flights. Surface flux anomalies over several days produce even larger cross-frontal gradients than observed, and substantial vertical mixing across the frontal surface is indicated.