Modelling of the time evolution of low tropospheric clouds capped by a stable layer
Bent Hansen Sass
Abstract
A numerical study is carried out on
the problem of simulating the time evolution of
low tropospheric clouds capped by a stable layer.
The fundamental processes governing the evolution of
boundary layer clouds are briefly reviewed.
The numerical investigation is carried out with the
physical parameterizations of a weather prediction model
(HIRLAM).
The role of
computing realistic fluxes of moisture and heat
near the interface between cloud top
and the stable overlying layer is investigated.
Two model formulations
of these heat- and moisture fluxes
are described and tested in 1-dimensional column experiments
with specified external forcing. It is shown that the formulation,
paying special attention to the exchange process from
shallow convection across the interface,
can produce realistic entrainment rates
of the stable air at the cloud
top when tested on observational data from the Atlantic Stratocumulus
Transition Experiment (ASTEX). Also the cloud structure and drizzle
appear to be better in agreement with observations when compared with
the default reference model version.
The model formulations are studied further
in idealized 1-dimensional tests
investigating the effect of different conditions including dynamical
forcing specified through vertical velocity varying with height.
It is concluded that both the individual physical
processes and dynamical forcing play a significant role
in the cloud prediction problem.
The experiments confirm that subsidence
and an upward surface sensible heat flux are strong cloud controlling
processes tending to dissipate
a cloud once the heating effects
have reached a sufficient magnitude.
Also the effect of wind shear near cloud top is significant in the
revised formulation providing increased heat and moisture
fluxes contributing to cloud dissipation.
The new formulation does in some situations
produce a significantly different
simulation of the cloud cover evolution which
has implications for weather forecasting
.
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