The article of record as published may be found at http://dx.doi.org/10.1175/MWR-D-16-0255.1This study investigates the influences of low-level atmospheric water vapor on the precipitation produced
by simulated warm-season midlatitude mesoscale convective systems (MCSs). In a series of semi-idealized
numerical model experiments using initial conditions gleaned from composite environments from observed
cases, small increases in moisture were applied to the model initial conditions over a layer either 600 m or
1 km deep. The precipitation produced by the MCS increased with larger moisture perturbations as expected,
but the rainfall changes were disproportionate to the magnitude of the moisture perturbations. The experiment with the largest perturbation had a water vapor mixing ratio increase of approximately 2 g kg2¯¹ over the
lowest 1 km, corresponding to a 3.4% increase in vertically integrated water vapor, and the area-integrated
MCS precipitation in this experiment increased by nearly 60% over the control. The locations of the heaviest
rainfall also changed in response to differences in the strength and depth of the convectively generated cold
pool. The MCSs in environments with larger initial moisture perturbations developed stronger cold pools, and
the convection remained close to the outflow boundary, whereas the convective line was displaced farther
behind the outflow boundary in the control and the simulations with smaller moisture perturbations. The high
sensitivity of both the amount and location of MCS rainfall to small changes in low-level moisture demonstrates how small moisture errors in numerical weather prediction models may lead to large errors in their
forecasts of MCS placement and behavior.National Science FoundationAGS-1359727AGS-PRF 152443
