A127-02
Lagrangian simulations of cloud evolution in the subtropical Northeast Pacific and North Atlantic

Friday, 11 December 2020: 10:34
Virtual
Peter N Blossey1, Christopher S. Bretherton2, Johannes Mohrmann1, Isabel Louise McCoy3, Steven Boeing4, Leif Denby4 and Roel Neggers5, (1)University of Washington, Seattle, WA, United States, (2)Vulcan, Inc., Climate Modeling, Seattle, WA, United States, (3)University of Washington, Atmospheric Sciences, Seattle, WA, United States, (4)University of Leeds, Leeds, LS2, United Kingdom, (5)University of Cologne, Cologne, Germany
Recorded Presentation
Abstract:
Over subtropical oceans, marine boundary layer clouds evolve as the trade winds carry air masses westward and equatorward. Examples of such evolution include the stratocumulus to cumulus transition and the development of mesoscale organization in shallow cloud fields. Lagrangian frameworks for analyzing and modeling these phenomena are a natural choice because they minimize uncertainties in horizontal advective tendencies and can be simulated in smaller domains and at higher resolution than in a regional model. Here, we present simulations of Lagrangian case studies from two recent field experiments: CSET (Cloud System Evolution in the Trades) and EUREC4A (Elucidating the role of clouds-circulation coupling in climate). Each field experiment gathered a wealth of in situ and remote sensing data that provides insight into clouds, precipitation and circulations and can be used to constrain model simulations.

Large eddy simulations (LES) of stratocumulus to cumulus transitions have been performed for two Lagrangian case studies from CSET, with simulations along two neighboring trajectories sampling the variability of forcings within each air mass. The case studies offer a contrast between a relatively fast cloud transition in a boundary layer with low aerosol and cloud droplet concentration and a slower transition in a more strongly decoupled boundary layer with higher aerosol concentrations. The simulations show broad agreement with observations and reproduce the contrast in the speed of the transition between the two cases. However, simulations of the second case study tend to underestimate cloud cover early in the simulations and overestimate cloud top height later. Sensitivity studies reveal a strong sensitivity of cloud fraction and liquid water path to prescribed cloud droplet number concentrations and a weak overall sensitivity to domain size.

Results from initial simulations of Lagrangian case studies from EUREC4A will also be presented. Cases have been selected to sample different patterns of cloud organization and will test the ability of LES to reproduce both the observed meteorology and the organization of the cloud field in each case. One model includes the simulation of water isotopic tracers, which were observed during EUREC4A, and may provide information about moist physics.

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