Parameterizing convective organization to escape the entrainment dilemma

Since buoyant ascent involves natural selection, subgrid structure makes convection systematically deeper and stronger than the pure unorganized case: plumes of average (or randomly sampled) air rising in the average environment. To reflect this, org is nonnegative, but we leave it dimensionless. A time scale characterizes its behavior (here ~3 h for a 2° model).

Currently its source is rain evaporation, but other sources can be added easily. We also let org be horizontally transported by advection, as a mass-weighted mean over the convecting layer. Linear coefficients link org to a plume ensemble, which it assists via: 1) plume base warmth above the mean temperature 2) plume radius enhancement (reduced mixing), and 3) increased probability of overlap in a multi-plume scheme, where interactions benefit later generations (this part has only been implemented in an offline toy column model).

Since rain evaporation is a source for org, it functions as a time-lagged but positive feedback on deep convection development. This evades the entrainment dilemma, since fully developed org - enhanced convection is not overly dilute, avoiding stability bias, while the pioneering updrafts of new convection are suppressed by entrainment, encouraging more large-scale variability.