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Abstract:
The trade-wind subcloud layer is an important structural component of the atmosphere. Its thermodynamic variability has long been characterized using simple frameworks, of which mixed layer theory is the simplest kind. Past studies qualitatively support such a description, yet the adequacy of mixed layer theory as a quantitative description has not been tested. Here we use observations collected during the EUREC 4 A ( Elucidating the role of clouds–circulation coupling in climate ) field campaign to test this framework and evaluate our understanding of the trade-wind subcloud layer. We find evidence for a transition layer separating the mixed layer and subcloud layer tops. The presence of such a finitely-thick transition layer with vertical gradients complicates the application of mixed layer theory, which assumes an abrupt gradient, or ‘jump’ at the subcloud layer top. This ambiguity introduces effective parameters and motivates their estimation through a Bayesian methodology. Results from this Bayesian inversion further reflect a finite-depth entrainment zone. We find that subcloud layer moisture and heat budgets close for synoptic variability and a monthly campaign-mean, yielding a campaign-mean residual of 3.6 Wm −2 for moisture and 2.9 Wm −2 for heat. Surface wind speed variability influences the subcloud layer depth and fluxes, yet thermodynamic variability above the subcloud layer top emerges as the primary control on subcloud layer moisture and heat variability. Given that this simple theoretical framework can explain observed variability, it offers an appealing framework for evaluating larger-scale models that must parameterize the processes regulating this fundamental part of the atmosphere.
