Size‐Dependent Characteristics of Surface‐Rooted Three‐Dimensional Convective Objects in Continental Shallow Cumulus Simulations.

A segmentation algorithm is applied to high resolution simulations of shallow continental convection to identify individual convective 3D objects within the convective boundary layer, in order to investigate which properties of the objects vary with the object width. The study analyses the geometry of the objects, along with their profiles of vertical velocity and total water, to assess various assumptions often used in spectral mass‐flux convection schemes. The methodology of this paper is unique in that we use (a) a novel application of the watershed algorithm to detect individual objects in the constantly evolving continental boundary layer efficiently, and (b) an unprecedentedly large number of simulations being analyzed. In total, 26 days of LASSO simulations at the Atmospheric Radiation Measurement Southern Great Plains site are analyzed, yielding roughly one million objects. Plume‐like surface‐rooted objects are found to be omnipresent, the vertical extent of which is strongly dependent on the object width. The vertical velocity and moisture anomaly profiles of the widest objects are roughly consistent with the classic buoyancy‐driven rising plume model. The kinematic and thermodynamic properties of the objects vary with object width. This width dependence is strongest above cloud base, but much weaker below. Finally the impact of neglecting the contribution of covariances to the vertical moisture flux, which is commonly used in mass‐flux parameterizations, is investigated. The average effect of neglecting covariances increases linearly with object width, leading to a 20% flux underestimation for 2 km wide objects. Implications of the results for spectral convection scheme development are briefly discussed. Plain Language Summary: This paper studies updrafts, which are volumes of warm air traveling upwards in the atmosphere, using high‐resolution simulations. These simulations have sufficient resolution to capture how the sun heating the surface causes warm air to travel upwards. These updrafts are responsible for the formation of cumulus clouds. What separates the methodology of this study from others is the newly developed method used to detect these updrafts in the model output, and the comparatively large amount of simulations analyzed. The model domain is large enough to contain thousands of updrafts. 26 days of simulations over the great plains in Oklahoma are used, yielding about a million simulated updrafts in total for us to study. From our results we conclude that there is a clear positive relationship between updraft width and height. Thinner updrafts seldom reach up to the cloud base, and they tend to get thinner the further away they are from the surface. In contrast, we find that the thicker updrafts are wider at cloud base than at the surface. Other aspects of the updrafts, such as their vertical velocity and moisture transport, are studied to determine how these properties vary with the width of the updrafts. Key Points: A 3D segmentation analysis is applied to large‐eddy simulations of shallow cumulus cloud fields as observed at the Atmospheric Radiation Measurement Southern Great Plains sitePlume‐like surface‐rooted convective objects are omnipresent, with object height being strongly dependent on object widthWe find that the thermodynamic, geometric, kinematic, and flux properties of the objects also depend on object width [ABSTRACT FROM AUTHOR]