Two-way transitions between closed and open cellular convection

Description

Along the precipitation rate, spatial distribution of precipitation is a key factor controlling the transition from closed cellular convection (CL) to open cellular convection (OP). For the first part of this study, simulations are conducted to evaluate the dependency of the transition on the areal coverage of rain, and to explore the role of interactions between multiple rainy areas in the OP formation. When rain is restricted to a small area, even substantial rain does not result in a transition. With increasing areal coverage of the rain, the transition becomes possible provided that the rain rate is sufficiently large. When multiple small rain regions interact with each other, the transition occurs and spreads over a wider area. The transition is faster for shorter distance between the rain regions. For much longer distances, the system is anticipated to remain in a CL state. Lastly we show that the transition occurs along a consistent path in the phase space of the mean vs. coefficient of variation of the liquid water path. This could be used as a diagnostic tool for global analyses of the statistics of CL and OP occurrence and transitions between them. In the second part of this study, the two-way transition between CL and OP is addressed. Simulations were conducted with a time-varying drop concentration (Nd) to control the transition. The transition between CL and OP states is asymmetrical: a rapid transition to the OP state and slower recovery to the CL state. The primary barrier to recovery is the loss in turbulence kinetic energy (TKE) associated with the loss in cloud water and radiative cooling, and the stabilization of the boundary layer during the OP period. The system faces the difficult task of replenishing cloud water fast enough to counter precipitation losses, so that it can generate radiative cooling and TKE. Sensitivity tests show that 1) the asymmetry in the two-way transition occurs even for very rapid Nd recovery, 2) recovery to the CL state is slower when radiative cooling is inefficient such as in the presence of free tropospheric clouds or after sunrise, and 3) a faster return to the CL state requires the Nd recovery accompanied by significant dynamical forcing, e.g., increase in surface thermodynamic flux. The observed closing of OP by ship effluent likely occurs when aerosol intrusions are large, when contact comes prior to the heaviest drizzle in the early morning hours, and when the free troposphere is cloud-free.