Coherent Structures are Ubiquitous in the Atlantic Trade Wind Boundary Layer in Both Near-Neutral and Unstable Stratification and Might Contribute to Surface Wind Bias in Reanalysis

The state of the marine atmospheric boundary layer (MABL) in the Atlantic trade wind region upwind of Barbados is analyzed using a uniform sampling of 2100 high resolution (5 m pixels) satellite C-band SAR images of sea surface roughness. Signatures of MABL coherent structures (CS): cells (27%), rolls (26%) and mixed roll/cell states (46%) are found in 99% of the images. ERA5 reanalyses are interpolated to each image and a sub-set of the SAR images are collocated to the NTAS surface flux buoy. Stratification, which we characterize with an air-sea Richardson number, is the ratio of buoyant production to shear production of turbulence in the boundary layer. These data show that the roll signature strengthens as the stratification approaches neutral and the cell signature strengthens as the stratification becomes more unstable. When cells dominate, ERA5 and buoy surface stress agree. When the roll signature emerges, the buoy stress trends higher than ERA5. The difference between reanalysis and buoy surface stress increases as the roll signature strengthens, i.e. as the stratification approaches neutral. A similar trend is seen in the surface latent heat flux, but not in the surface buoyancy flux. A persistent bias between observed and modeled winds in global forecast and reanalysis models in the trade wind region has been tied to too-little downward flux of horizontal momentum in the atmospheric boundary layer. Spatial patterns of ERA5 zonal surface wind biases in the trade wind regions roughly correspond to regions of persistent WS signature in SAR. This suggests a hypothesis that the PBL and convective parameterizations are not capturing the enhancement of the downward momentum flux induced by the persistent shear-generated roll circulations in near-neutral stratification.