Size-dependence of surface-rooted three-dimensional convective objects
in continental shallow cumulus simulations
A clustering method is applied to high resolution simulations of shallow
continental convection to investigate the size dependence of coherent
structures in the convective boundary layer. The study analyses the
geometry of the clusters, along with their profiles of vertical velocity
and total water. The main science goal is to assess various assumptions
often used in spectral mass-flux convection schemes. Novel aspects of
the study methodology include i) a newly developed clustering algorithm,
and ii) an unprecedentedly large number of simulations being analysed.
In total 26 days of LASSO simulations at the ARM-SGP site are analyzed,
yielding roughly one million individual clusters. Plume-like
surface-rooted coherent convective clusters are found to be omnipresent,
the depth of which is strongly dependent on cluster size. The largest
clusters carry vertical structures that are roughly consistent with the
classic buoyancy-driven rising plume model, while
smaller clusters feature considerable variation in top height.
The cluster area is found to strongly vary with height and size, with
small clusters losing mass and large clusters gaining mass below cloud
Similar size dependence is detected in kinematic and thermodynamic
properties, being strongest above cloud base but much weaker below.
Finally the efficiency of the top-hat approach in flux parameterization
is investigated, found to be 80-85 \% including a weak
but well-defined dependence on cluster size. Implications of the results
for spectral convection scheme development are briefly discussed.