Skill of ship-following large-eddy simulations in reproducing MAGIC observations across the Northeast Pacific stratocumulus-to-cumulus transition region
Christopher S. Bretherton — University of Washington
Jeremy McGibbon — University of Washington
Category
Warm low clouds, including aerosol interactions
Description
Time-height series of horizontal mean LES state and comparable observation for MAGIC Leg 15A. Green and yellow lines in (a) and (c) indicate simulated LCL at 150m and at 70% of inversion height, respectively. The corresponding circles in (b) and (d) indicate values from radiosondes. (e) shows horizontal mean LES-simulated radar reflectivity in SAM. (f) shows 5-minute mean WACR observed radar reflectivity (means taken of Z not dBZ).
During the Marine ARM GPCI Investigation of Clouds (MAGIC) in Oct. 2011-Sept. 2012, a container ship making periodic cruises between Los Angeles, CA and Honolulu, HI was instrumented with surface meteorological, aerosol and radiation instruments, a cloud radar and ceilometer, and radiosondes. Here, large-eddy simulation (LES) is performed in a ship-following frame of reference for 13 4-day transects from the MAGIC field campaign. The goal is to assess if LES can skillfully simulate the broad range of observed cloud characteristics and boundary-layer structure across the subtropical stratocumulus-to-cumulus transition region sampled during different seasons and meteorological conditions.
The SAM6.10.6 LES is forced by a combination of ECMWF operational analysis, ship-derived observations of aerosols, sea-surface temperature, and weak nudging toward sonde profiles of temperature and humidity. It simulates a small square doubly-periodic domain 6.4 km on a side with 35 m horizontal grid spacing and 5 m vertical grid spacing throughout the cloud layer, and it uses the RRTMG radiation parameterization and double-moment microphysics with an Abdul-Razzak scheme for nucleation of cloud droplets. It also includes a cloud radar simulator for comparison with ship radar observations.
Results from Leg 15A, which sampled a particularly well-defined stratocumulus-to-cumulus transition, demonstrate the approach. The LES reproduces the observed timing of decoupling and transition from stratocumulus to cumulus, and matches the observed evolution of boundary-layer structure, cloud fraction, liquid water path, and precipitation statistics remarkably well.
Considering the simulations of all 13 cruises, the LES skillfully simulates the mean diurnal variation of key measured quantities, including liquid water path (LWP), cloud fraction, measures of decoupling, and cloud radar-derived precipitation. The daily mean quantities are well represented, and daily mean LWP and cloud fraction show the expected correlation with estimated inversion strength. There is a -0.6 K-low bias in LES near-surface air temperature that results in a high bias of 6 W/m2 in sensible heat flux. This arises mainly from the stratocumulus regime. Simulations in which the cloud droplet number concentration is fixed at 50/cc have similar skill. Overall, these results build confidence in the ability of LES to represent the Northeast Pacific stratocumulus-to-trade-cumulus transition region.
