Speaker: George Matheou (UConn)
Title: Mesoscale organization development in precipitating shallow cumulus convection

Abstract:
Horizontal organization or mesoscale variability is an important mechanism in the interaction of the boundary layer with the large-scale conditions. The development of organization in a precipitating cumulus trade wind boundary layer is studied using large-eddy simulations with extensive horizontal domains, up to 320 km, and fine grid resolution (40 m). The large computational domains blend the boundary between local convective circulations and mesoscale horizontal motions leading to the dependence of horizontal TKE on the LES domain size. Energy-containing horizontal length scales are defined based on the premultiplied spectra. When large-scale organization develops, the premultiplied spectra develop multiple peaks corresponding to the characteristic horizontal scales in the boundary layer. All flow variables have a small length scale of 1–2 km, which corresponds to local convective motions, e.g., updrafts and cumulus clouds. Organization development creates additional larger length scales. The growth rate of the large length scale is linear and it is about 3–4 km/h, which agrees well with the growth rate of the cold pool radii. A single energy containing length scale is observed for vertical velocity for the entire run (even after organized convection develops) that is constant with height. A computational domain sensitivity study suggests that for the idealized and horizontally homogenous conditions of the LES, domains of at least 160 km are required to realistically capture the mesoscale structure of the boundary layer.