Convective response to moisture and stability changes in the large-scale environment: Simulating observed mass fluxes

We use spectral Weak Temperature Gradient (WTG) approximation - in a context of a cloud resolving model - to represent the large scale environment. The perturbations that we apply to the reference profile, for initiating WTG simulations, correspond to perturbations observed over the Atlantic and the West Pacific.

In previous studies - theoretical and observational - it has been hypothesized that the vertical mass flux profile is largely determined by the thermodynamic stratification of the troposphere; more stable troposphere is conducive to convection that results in bottom-heavy mass fluxes, whereas more unstable troposphere favors convection which produces top-heavy mass flux profiles.

In this study we wish to simulate observed mass flux profile. In doing so, we want to separate the effects of tropospheric stability on convection from the effects of surface fluxes and moisture on convection. We use observed thermodynamic stratification to perform series of simulations in which we vary surface forcing, and series of simulations in which we add moisture at different elevations.

The observed mass flux profiles were not exactly reproduced in these experiments, however, the simulated mass flux profiles vary from case to case in a similar fashion as the respective observed mass fluxes vary. The discrepancies of the results are likely due to our choice of a reference state.

The main findings of this study are that the shape of the resulting mass flux profile is entirely governed by the vertical profile of the potential temperature. Changes in the surface forcing affected only the magnitude of the mass flux profile, and not the elevation at which the maximum vertical mass flux occurs. Moisture added in the boundary layer, as expected, also contributed to increase of the mass flux. Moisture added at elevations higher than 3 kilometers did not affect the mass flux profile significantly, unless the amount added was beyond any reasonable value.