A GCSS model intercomparison for a tropical squall line observed during TOGA-COARE. I: Cloud-resolving models

Results from eight cloud-resolving models are compared for the first time for the case of an oceanic tropicalsquall line observed during the Tropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. There is broad agreement between all the models in describing the overall structure and propagation of the squall line and some quantitative agreement in the evolution of rainfall. There is also a more qualitative agreement between the models in describing the vertical structure of the apparent heat and moisture sources. The three-dimensional (3D) experiments with an active ice phase and open lateral boundary conditions along the direction of the system propagation show good agreement for all parameters. The comparison of 3D simulated fields with those obtained from two different analyses of airborne Doppler radar data indicates that the 3D models are able to simulate the dynamical structure of the squall line, including the observed double-peaked updraughts. However, the second updraught peak at around 10 km in height is obtained only when the ice phase is represented. The 2D simulations with an ice-phase parametrization also exhibit this structure, although with a larger temporal variability. In the 3D simulations, the evolution of the mean wind profile is in the sense of decreasing the shear, but the 2D simulations are unable to reproduce this behaviour.