A novel trajectory ensemble model of stratiform cloud and its possible applications

A novel trajectory ensemble model of stratiform cloud is described. In this model the boundary layer is fully covered by a great number of Largangian air parcels that can contain either wet aerosols or aerosols and droplets. In each parcel microphysical processes of diffusion growth of aerosols and droplets, as well as processes of collisions are accurately described. Droplet sedimentation is taken into account, which allows simulation of precipitation formation. The Lagrangian parcels are advected by a velocity field generated by a model of turbulent-like flow obeying turbulent correlation laws. A cloud-aerosol interaction is taken into account that allows to calculate successive increase in the aerosol and nucleated droplets size within Lagrangian parcels during their recirculation within the boundary layer. The output of the numerical model includes droplet and aerosol size distributions and their moments such as droplet concentration, droplet spectrum width, cloud water content, drizzle content, radar reflectivity, etc. calculated in each parcel as well as horizontally averaged values. We compare evolution of cloud microstructure in simulations that differ by the initial humidity value. While in case of low humidity drizzle did not develop, in case of high relative humidity intense drizzle develops at t=2 h. In non-drizzling case most droplet size distributions are narrow unimodal ones. In case of drizzle formation many droplet spectra are wide and bimodal. The latter indicates the existence of secondary droplet nucleation within the cloud layer caused by fluctuations of supersaturation in the cloudy parcels. The longer residence time of droplets within cloudy parcels in case of large humidity is another reason of drizzle formation. The increase in the air humidity leads also to the higher values of supersaturation (under similar dynamics) and to faster diffusion growth of droplets. It is shown that horizontally averaged droplet size distributions are wide not only because of spatial averaging, but largely because many droplet size distributions in individual cloud parcels are wide. In spite of the fact that aerosol size distributions were assumed similar in all parcels at t=0, the simulations indicate high spatial variability of droplet size distributions in parcels located at the same heights. Different applications of the model are discussed. The model dynamics can be tuned to observations performed using Doppler meteorological radars. Microphysical output can be compared with microphysical observations, first of all, with aircraft observations. Thus, the model can serve as a connecting link between the dynamical (turbulent) structure of the boundary layer and microphysical, radiative and geometrical properties of clouds. As an example of application of the model for purposes of remote sensing, we present scattering diagrams of radar-reflectivity-liquid water content (Z-LWC) relationships as well as radar-lidar signal relationships. The results of the calculations agree well with the observed data. Three different regimes (non-drizzling, transient and drizzling regimes) are clearly identified in the Z-LWC diagram. The reasons leading to a successive transfer from one regime to another are discussed.