Effect of Wind Shear in Marine Stratocumulus Drizzle Production

Marine stratocumulus (Sc) frequently produces drizzle which is shown to be a strong modulator in stratocumulus to cumulus transition (Yamaguchi et al. 2017). The rapid drizzle onset is not fully understood and it is poorly represented in climate models; the latter causes large uncertainties in estimating low cloud cooling effect. A set of large eddy simulations (LES) are used to explore the effect of horizontal wind shear in promoting drizzle initiation in marine Sc based on a case over the eastern north Atlantic (Wu et al. 2017). Simulations from WRF represent the observed drizzling cloud relatively well. In the next step, the atmospheric profiles will be taken from WRF output and be used as initial conditions to drive LES. Autoconversion and accretion rates as well as boundary layer turbulent kinetic energy (TKE) will be compared for periods with and without shear to show the effect of wind shear on drizzle initiation. Additionally, prescribed droplet number concentrations (N_c) will be used as a proxy of different aerosol loadings. A threshold of N_c is expected to be found below which the shear-induced drizzle leads to close to open cell transition and above the N_c threshold, the cloud persists through the drizzling phase because of a continued condensational growth of cloud droplets. In this set of experiment, the effects of shear-induced drizzle on cloud structure will be explored under various aerosol conditions. This study will shed light on the impact of dynamics on cloud and precipitation microphysics in boundary layer clouds.

Reference:

Yamaguchi, T., Feingold, G., & Kazil, J., 2017: Stratocumulus to cumulus transition by drizzle. Journal of Advances in Modeling Earth Systems, 9, 2333–2349, https://doi.org/10.1002/2017MS001104.

Wu, P., X. Dong, B. Xi, Y. Liu, M. Thieman, and P. Minnis (2017), Effects of environment forcing on marine boundary layer cloud-drizzle processes, J. Geophys. Res. Atmos., 122, 4463–4478, doi:10.1002/2016JD026326.