Dynamical Modeling of Marine Boundary Layer Convection.

This dissertation investigates the interaction between dynamics and infrared radiation as well as the problem of entrainment instability in the cloud-topped marine boundary layer. A two-dimensional Boussinesq moist model with a numerical technique (Fourier-Chebysheve tau method) and resolution sufficient to simulate cloud top processes has been developed. Previous measurements suggest that the cloud-top radiative cooling is likely to undergo significant horizontal and vertical variability. To investigate the impact of infrared cooling on the boundary layer dynamics, numerical experiments on marine boundary layer convection under various radiative forcings are performed. The results indicate that the model steady state does not depend on the horizontal and the vertical distribution of the cooling when the cooling is confined to the turbulent region. The sensitivity of the model to infrared cooling appears to be primarily in the vertical placement of the cooling relative to the turbulent cloud top region. The thermodynamic theory and observations taken during the last fifteen years are summarized. The results indicate that stratocumulus remain solid even when the theoretical entrainment instability criterion is satisfied. From an initial value problem with an initial cold anomaly, we conclude that insufficient evaporative cooling of the entraining air may be the key missing ingredient in the classical entrainment instability argument. Because of insufficient evaporation, the mixed parcel will not be colder than the surrounding environment even when the equivalent potential temperature jump is negative. Radiatively forced numerical simulations are performed. The simulated stratocumulus contain dome-shaped convective structures with sharp liquid water gradients on the sides. The cloud cells can decay and reform in a finite time. The appearance and disappearance of cloud holes may have nothing to do with entrainment instability. We do not observe signs of stratocumulus breakup even in the situation of strong entrainment. The flux profiles averaged over a one hour period suggest that different soundings under different radiative forcings are in the same equilibrium state. Unless there are multiple equilibria, we should not expect the breakup of marine stratocumulus by the entrainment instability mechanism.