Sensitivity Study of Deep Convection Dynamics to Tropospheric Humidity Using Large-Eddy Simulations

Large-Eddy Simulations performed with the Meso-NH research model (Lac et al 2018) are used to assess the impact of environmental moisture on convective cloud development. These idealized simulations with fine resolution (50 m) are initialized with temperature and humidity profiles from Weisman and Klemp (1982). The control simulation is based on an analytical humidity profile with a relative humidity of 25% at the tropopause. A sensitivity study to humidity is then conducted with other simulations by varying the relative humidity from 5 to 45% at the tropopause. It is important to note that the low-level humidity and instability are the same for all simulations.

By examining this set of simulations, we notice that there are marked differences in the dynamics of the clouds and in the exchanges with their environment. Thus, the more humid the environment, the stronger the vertical velocities in the mid-troposphere within the convective clouds. It is the opposite in the upper troposphere. Cloud tops rise faster and the updraft mass flux is stronger when the environment is more humid. Buoyancy inversion at cloud top is stronger when the environment is drier. Entrainment increases with humidity whereas detrainment tends to decrease. The resolved kinetic energy is stronger in the mid-troposphere in humid environment, related to the more intense ascents. The kinetic energy (subgrid and resolved) is stronger in the upper troposphere when the environment is drier suggesting more intense exchanges in this case.

Two additional experiments are carried out in order to investigate the impact of a dry layer on convection development. All these results will be discussed during the presentation.

Figure: Time averaged (240-300 min) vertical mean profiles of vertical velocity (m s^-1 ) in descending cloud
(left panel) and ascending cloud (right panel) for relative humidity varying from 5 to 45 % at the tropopause (color lines).