Influences of environmental relative humidity and horizontal scale of subcloud ascent on deep convective initiation

This study examines two factors impacting initiation of moist deep convection: free-tropospheric environmental relative humidity (phi(E)) and horizontal scale of subcloud ascent (R-sub), the latter exerting a dominant control on cumulus cloud width. A simple theoretical model is used to formulate a "scale selection" hypothesis: that a minimum R-sub is required for moist convection to go deep, and that this minimum scale decreases with increasing phi(E). Specifically, the ratio of R-sub(2) to saturation deficit (1 - phi(E)) must exceed a certain threshold value that depends on cloud-layer environmental lapse rate. Idealized, large-eddy simulations of moist convection forced by horizontally varying surface fluxes show strong sensitivity of maximum cumulus height to both phi(E) and R-sub consistent with the hypothesis. Increasing R-sub by only 300-400 m can lead to a large increase (>5 km) in cloud height. A passive tracer analysis shows that the bulk fractional entrainment rate decreases rapidly with R-sub but depends little on phi(E). However, buoyancy dilution increases as either R-sub or phi(E) decreases; buoyancy above the level of free convection is rapidly depleted in dry environments when R-sub is small. While deep convective initiation occurs with an increase in relative humidity of the near environment from moistening by earlier convection, the importance of this moisture preconditioning is inconclusive as it is accompanied by an increase in R-sub. Overall, it is concluded that small changes to R-sub driven by external forcing or by convection itself could be a dominant regulator of deep convective initiation.