A Description of the Atmospheric Large-scale Circulation during EUREC4A and its Impact on the Thermodynamic Vertical Profiles in the Trades

Abstract:

The clouds in the tropical winter trades of the North Atlantic in the vicinity of Barbados occur in different mesoscale organisation patterns. Each pattern is associated with a distinct cloud amount and radiative footprint. Therefore, the relative occurrence frequency of these patterns affects the global radiative budget. As shown by the recent study of Bony et al. (2019) in Geophysical Research Letters, the mesoscale cloud organisation in this region is controlled by near-surface wind speed and the strength of lower-tropospheric stability. However, it has not been investigated comprehensively to what extent the large-scale flow over the North Atlantic influences the thermodynamic conditions in the trades, which eventually constrain the cloud processes.

Tropospheric air parcels arriving in the trades follow different pathways depending on the prevailing large-scale circulation. Controlling factors are presumably the position of the Intertropical Convergence Zone, extratropical upper-level wave breaking events, and the position and strength of low-level subtropical anticyclones. The air parcels’ specific transport histories are associated with distinct diabatic processes such as radiation, phase changes in and below clouds and turbulent mixing. The diabatic processes encountered during transport modulate the thermodynamic properties of the air parcels and therefore influence the vertical thermodynamic structure of the atmosphere in the trades.

In this study, the large-scale circulation during EUREC⁴A and its impact on the thermodynamic profiles over Barbados is analysed with a time resolution of three hours. The large-scale circulation is assessed with the aid of backward trajectories from a vertical profile above the Barbados Cloud Observatory (BCO), which are calculated with three-dimensional wind fields from ECMWF analyses. Several variables are traced along the trajectories to investigate the evolution of the air parcels’ properties until they reach the BCO. It is shown that an extratropical upper-level wave breaking over the central North Atlantic at the beginning of EUREC⁴A leads to the development of an unusually deep cloud layer in the trades. Towards the end of the campaign, a coherent airstream ascending from the south caused the formation of an upper-level cloud layer covering the trade-typical low-level clouds. The robustness of the observed linkage between the transport characteristics and the anomalous cloud layer is tested over a 10-year period (2010-2020) using ERA5 reanalysis data. This study clearly reveals the important impact of anomalous large-scale transport, partially driven by dynamical processes in the extratropics, on the variability of vertical profiles of temperature, humidity and clouds in the North Atlantic winter trades.