Skill of ship‐following large‐eddy simulations in reproducing MAGIC observations across the northeast Pacific stratocumulus to cumulus transition region

• Published in: Journal of advances in modeling earth systems Vol. 9; no. 2; pp. 810 - 831
• Main Authors: McGibbon, J., Bretherton, C. S.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.06.2017
• Subjects: Aerosols; Air temperature; Atmosphere; Atmospheric precipitations; Boundary layer structure; Cargo; Cargo handling; Cloud investigations; Cloud properties; Clouds; Container ships; Correlation; Cruises; cumulus; Diurnal cycle; Droplets; Evolution; Heat flux; Heat transfer; Instruments; large‐eddy simulation; LES; marine boundary layer; Meteorological conditions; Oceanic eddies; Precipitation; Properties; Radar; Radiation; Radiation instruments; Radiosondes; Rainfall; Seasons; Sensible heat; Sensible heat flux; Sensible heat transfer; Ships; Simulation; Statistical methods; stratocumulus; Surface-air temperature relationships; Temperature; Testing; Variability; Water temperature; Los Angeles California; United States > US
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1002/2017MS000924

During the Marine ARM GPCI Investigation of Clouds (MAGIC) in October 2011 to September 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 four 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. 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 5.6 W m−2 in sensible heat flux (SHF). Overall, these results build confidence in the ability of LES to represent the northeast Pacific stratocumulus to trade cumulus transition region. Plain Language Summary During the Marine ARM GPCI Investigation of Clouds (MAGIC) field campaign in October 2011 to September 2012, a cargo container ship making regular cruises between Los Angeles, CA, and Honolulu, HI, was fitted with tools to measure aspects of the clouds and atmosphere above the ship. We used some of these observations to perform high‐resolution computer simulations of the atmosphere in the region around the ship, with the goal of testing how well the simulation produces clouds and atmosphere similar to what was observed. Simulations of 13 one‐way cruises to Honolulu, HI, were performed. We see the simulations skillfully produce changes in cloud properties that occur at different times of day and have average properties that match well with the observations. One error is that the air near the surface is slightly too cold in the simulations, meaning more heat is transferred up from the surface. Overall, this result builds confidence and trust in the ability of this type of simulation to produce realistic cloud properties in the northeast Pacific. Key Points LES skillfully reproduces day‐to‐day variability and the diurnal cycle of NE Pacific boundary layer clouds during the MAGIC field campaign A novel ship‐following approach is used to force the model The LES responds as expected to specified changes in cloud droplet number, but cloud variability is much more correlated with EIS