Sensitivity of precipitation accumulation to small changes in low-level moisture in simulated elevated convective systems

Using a method for initiating a quasi-stationary, heavy-rain-producing elevated mesoscale convective system in an idealized numerical modeling framework, a series of experiments are conducted in which the near-surface moisture in the environment is altered slightly. The timing and overall evolution of the simulated convective systems are nearly identical, and the environment is still very moist in all of the experiments, with changes to the column-integrated precipitable water in the range of only 1-3%. However, the rainfall that accumulates at the surface is changed by a much larger fraction, primarily owing to changes in the evaporation rate in the shallow sub-cloud layer. Considering that these moisture differences are of the same order as observational uncertainties in low-level moisture and there are relatively few observations of moisture above the surface, the implications of these results on the potential predictability of extreme rainfall will be examined.