Comparison of ship-following large-eddy simulations with cloud and boundary layer structure observed in MAGIC

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Description

The 2012-2013 MAGIC shipborne deployment of the ARM mobile facility sampled a broad range of subtropical marine stratocumulus (Sc), cumulus (Cu), and transition regimes during cruises between California and Hawaii. Ship-following large-eddy simulations (LES) of cruise legs of 4-5 days are compared with a broad suite of observations of cloud structure and radiative properties taken on the Horizon Spirit container ship. The goal of this quantitative comparison across a realistic range of synoptic and seasonal conditions assesses the suitability of LES for guiding the development of cloud and boundary layer parameterizations in global climate and weather forecast models and for simulating the sensitivity of such cloud regimes to climate perturbations. The System for Atmospheric Modeling (SAM) LES is used with a small, doubly-periodic domain and variable vertical resolution, initialized using thermodynamic profiles near the start of each cruise leg. Sea-surface temperatures are prescribed from observations, and ECMWF analyses are used to derive time-varying geostrophic wind, ship-relative large-scale advective forcing, and large-scale vertical velocity. ECMWF vertical velocities are adjusted to relax the temperature profile toward observations with a relaxation timescale of 1 day. The cloud droplet number concentration is specified based on the ship-measured accumulation-mode aerosol. The ship-following approach allows efficient comparison of model output with a broad suite of ship-based observations. The LES was run for all eleven suitable westward legs with adequate observations. The simulations cannot be expected to match the observations on timescales less than three hours because of cloud-scale and mesoscale sampling variability. The LES predicts daily mean cloud fraction and surface longwave radiation with little systematic bias and correlation coefficients of 0.33 and 0.53, respectively, but systematically underestimates liquid water path.