The initiation, longevity and morphology of simulated convective storms as a function of free-tropospheric relative humidity

An eight-dimensional parameter space numerical simulation study shows how the initiation, longevity and morphology of convective storms depends on the ambient relative humidity above the moist layer. In these simulations, seven key environmental parameters are held constant except for the free tropospheric relative humidity, which is initialized at a constant prespecified value for all points in the troposphere above the level of free convection. Tests of the impact of relative humidity variations are conducted for a variety of vectors of the seven other key parameters.

It is found that, for a variety of the initial 7-vectors of key environmental parameters, storms consistently require warmer starting bubbles and have more difficulty sustaining themselves as the free tropospheric humidity is decreased. In addition, the storms that do survive in the lower humidity environments exhibit a greater tendency toward early outflow dominance as compared to their counterparts in moister environments aloft.

The simulations reveal that persistent convection cannot be supported when the free tropospheric relative humidity is 60 percent or less, for any reasonable choice of triggering bubble amplitude. All storms in such environments are of the form of single pulse updrafts that quickly decay after the first hour of the simulation. Entrainment of dry air from aloft is evidently the primary factor restricting secondary updraft development in the dry environments.

The findings are consistent with the findings of many observational studies, which indicate that sustained severe convection almost always occurs in zones of persistent low-level convergence. Such convergence is needed to ensure that the free troposphere is moistened sufficiently to prevent premature dissipation of potential new updrafts.