A New Conceptual Picture of the Trade Wind Transition Layer

Abstract The transition layer in the trades has long been observed and simulated, but physical processes producing its structure remain little investigated. Using extensive observations from Elucidating Role of Clouds–Circulation Coupling Climate (EUREC 4 A) field campaign, we propose a new conceptual picture trade wind layer, occurring between mixed-layer top (around 550 m) subcloud-layer 700 m). theory cloud-free convective boundary layers suggests transition-layer with strong jumps at top, yet such are only rarely. Despite cloud-base cloud fraction measured as 5.3% ± 3.2%, canonical is infrequent confined to large [ O (200) km] areas. We show that majority bases form within instead above it, cloud-top height distribution bimodal, first population very shallow clouds (tops below 1.3 km) second deeper (extending 2–3 km depth). then life cycle this maintains structure. That is, smooth vertical thermodynamic gradients by condensation–evaporation mechanism, which fully coupled mixed layer. Inferences mixing diagrams, moreover, suggest does not affect rate entrainment mixing, rather properties air incorporated into primarily enhance moistening. Significance Statement thin atmospheric thought be important for regulating convection, well understood. recent find structure, an abrupt discontinuity variables, infrequent, despite being small. both forming dissipating compared jump, except This appear