Size decompositions of observed and simulated shallow convective cloud fields

Parameterizations of boundary-layer clouds remain at the heart of important problems in large-scale models for numerical weather and climate prediction. A long-standing problem is the unification of the representation of different boundary-layer cloud regimes. A new problem is that processes are becoming partially resolved at present-day resolutions, necessitating the adaptation of subgrid parameterizations. This is ideally done dynamically through the formulation of scale-dependent parameterizations, which can be achieved for boundary layer clouds through a formulation in terms of size distributions. This cloud size distribution then needs to be constrained by a generally applicable closure. This allows for unification of the various subgrid parameterization as well as a better physical foundation for those parameterizations. The aim of this proposal is to find a better understanding of how the cloud size distribution is being set, and how it impacts transport and precipitation.Our specific focus will be on how a variety of cumulus cloud properties scale as a function of cloud size. This includes cloud geometry, precipitation, condensate loading, dynamics, transport and mixing. By studying a wide range of different observable properties as a function of cloud size, at multiple sites and covering long periods, we can construct an entrainment closure that is best capable of reproducing all observed distributions combined. This holistic approach avoids parameterizations that do well in predicting one variable (e.g., cloud height), but less in others (e.g., precipitation). It has to be stressed that the cloud size distribution varies significantly between different LES models. Therefore, there is a critical need to focus on observations if the cloud size distribution is to be understood.Recent single-case LES studies have questioned the common assumption that entrainment is a function of cloud size (e.g. Dawe and Austin, 2013). While such LES studies of single cases do provide insight, any generally applicable conclusion requires analysis of a large variety of data sets, preferably from observations. Precipitation is also hypothesized to scale with cloud size, but only after a certain threshold size is met. We will link entrainment and precipitation to the cloud size, across a range of different cases, to cover a broad parameter space in terms of atmospheric state, surface fluxes and aerosol concentration.Research goalsThis project aims to investigate cloud size distributions under a wide range of meteorological conditions. High-frequency, multi-dimensional long-term ARM measurements are combined with semi-idealized LES of the same scenes. The main research goals are; to gain insight into the mechanisms that determine the shape and time-development of size distributions of shallow convective cloud properties; to understand relationships between cloud size, transport, and microphysics; to thus constrain a new class of parameterizations for weather- and climate models.A new boundary layer scheme based on size distributions (Neggers, 2015) will be calibrated against ARM data and tested in WRF. New insights resulting from the proposed activities will be directly implemented and tested on their impact on model climate.