Effects of Turbulent Mixing on the Structure and Macroscopic Properties of Stratocumulus Clouds Demonstrated by a Lagrangian Trajectory Model

Abstract

Abstract The role of turbulent mixing in the formation of the structure of stratocumulus clouds is investigated using a Lagrangian–Eulerian parcel cloud model. The model contains approximately 2000–5000 adjacent parcels with the linear size of 25–40 m, moving with a turbulent-like velocity field with observed energetic and statistical properties. The process of turbulent mixing of Lagrangian parcels is parameterized using the k-epsilon theory extended to the case of mixing nonconservative values. The model includes the interaction of cloud and the overlying inversion layer. The stratocumulus clouds observed during flight RF01 of the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS II) are simulated. Effects of turbulent mixing are analyzed by comparing simulations with and without mixing. When mixing between parcels is included, the thermodynamical and microphysical structure of the measured stratocumulus clouds is properly reproduced. Mixing leads to a more uniform cloud structure with well-defined borders. Good agreement is found between Paluch diagrams calculated in the model and those reproduced from measurements. The radius of correlation of liquid water content and other variables calculated in the model is on the order of several hundred meters and agrees well with observations. When mixing is not included, the radius of correlation is on the scale of a single parcel and the cloud layer contains dry entrained parcels, making the microphysical structure unrealistic. It is also shown that turbulent mixing leads to an increase in the effective radius and facilitates and accelerates drizzle formation. The time in which a 40-m air parcel preserves its identification is estimated from the results and is found to be on the order of 25 min.