Ship-following large-eddy simulation of a Sc-Cu transition observed during MAGIC Leg 15A

 

Authors

Christopher S. Bretherton — University of Washington
Jeremy McGibbon — University of Washington

Category

Warm Low Clouds and Interactions with Aerosol

Description

Large-eddy simulations (LES) are presented of a transition from a well-mixed stratocumulus (Sc) capped boundary layer to a cumuliform boundary layer observed in Leg 15A of the MAGIC AMF deployment. The simulation uses a small doubly-periodic domain. A ship-following approach is used to take advantage of the continuous in-situ and cloud remote sensing observations following the Horizon Spirit. In this approach, the initial thermodynamic profiles are specified from a radiosonde launch, then ECMWF analyses interpolated to the moving ship location are used to derive time-varying geostrophic wind, horizontal temperature/humidity, and vertical motion profile to force the LES. In deriving the advective tendencies, the air motion relative to the moving ship is used. In deriving these forcings, the authors uncovered spectral interpolation errors in the ECMWF MAGIC dataset that they discussed with Maike Alhgrimm of ECMWF, who has since produced a revised dataset that fixes the errors. Time-varying SSTs are used as surface boundary conditions. A 24-hour simulation was performed starting with a well-mixed Sc layer. The simulated boundary layer deepens due to entrainment, as observed, but so far the simulation does not accurately reproduce the observed transition to broken Cu as the ship moves westward, instead maintaining a Sc layer throughout the simulation. Sensitivity studies will be presented to investigate this discrepancy.