The tropical precipitation pickup threshold and clouds in a radiative convective equilibrium model: 1. Column moisture

Abstract

AbstractA survey of published results indicates that a column relative humidity near 77% is consistently observed to separate raining and nonraining columns in a time and space mean in the tropics, but why this approximate value of humidity should initiate such a statistical state transition is not readily apparent. An investigation is conducted of the submesoscale cloud processes that link column relative humidity to this abrupt pickup in heavy precipitation and of the magnitude of humidity at which this transition occurs. A cloud system resolving model in radiative convective equilibrium is used. Precipitation statistics from this simulation indicate a switch in mean precipitation state at 77% relative humidity with infrequent heavy rainfall at lower humidity. These statistics are broadly insensitive to spatial scaling. Low‐level cloud fraction and convergent flux of moisture are shown to be sensitive to column humidity near 77%, while upper level cloud fraction is markedly less sensitive. Mean updraft mass flux increases with increasing humidity but only at values of humidity well above the pickup. Both warm rain processes and melting are shown to depend strongly on column humidity near the pickup but in different circumstances. No single process is determined to result in a pickup in precipitation. It is suggested that column humidity temporally leads precipitation and therefore causes its intensity.