The effect of wind velocity fluctuations on drop spectrum broadening in stratocumulus clouds

In this study, a Lagrangian adiabatic parcel model and a trajectory ensemble model (TEM) are used in a sequence of experiments to investigate effects of velocity fluctuations on drop spectrum broadening in stratocumulus clouds. Using the adiabatic parcel model, it is found that even with a weakly buoyant temperature profile, if the initial updraft velocity is low enough, new drop nucleation above the cloud base region can occur via updraft acceleration accompanied by an increase in supersaturation. New nucleation can also occur via parcel recirculation as drops that have not fully evaporated during downdraft reduce the cloud base supersaturation peak in subsequent updrafts. The new nucleation produces new modes in the drop spectrum, broadening the spectrum towards smaller drops. Spectral broadening is also reproduced in the TEM, where an ensemble of 680 parcel trajectories is derived from a large eddy simulation (LES) velocity field. In the TEM, the mechanisms of in-cloud nucleation and growth/evaporation asymmetry caused by updraft acceleration and parcel recirculation promote spectral broadening predominantly in the lower and intermediate levels of the cloud, where parcels with initially low updraft velocities reside. To explicitly evaluate the effect of subgrid scale turbulent velocity fluctuations on drop spectrum evolution, such fluctuations are added to the mean LES velocity field. The velocity fluctuations are simulated using the Langevin equation, where turbulent diffusion is represented as an autoregression random process of the 1st order. The velocity fluctuations alter parcel trajectories, allowing parcels with initially low vertical velocities to be swept into higher vertical velocity eddies and to penetrate deeper into the cloud. The resulting updraft acceleration and parcel circulation lead to new drop nucleation and to broadening of the upper level drop spectra. The added subgrid turbulent velocity fluctuations also increase the overall number of parcels that exhibit new drop nucleation, bringing the fraction to half of the total ensemble under the assumption of strong turbulence. The above mechanisms help explain the existence of quite broad and bimodal stratiform cloud drop spectra, as well as the variation in stratiform drop spectra over short spatial scales. The sensitivity of the above mechanisms to the shape of the CCN spectrum and to variation in the turbulence parameters is investigated.