Jrm. Pasquier et Pr. Jonas, TURBULENT TRANSPORT IN FIELDS OF WARM CUMULUS CLOUDS, Quarterly Journal of the Royal Meteorological Society, 124(546), 1998, pp. 363-387
Simulations of fields of warm (ice-free) cumulus clouds are made using
a large-eddy model based on observational soundings. Comparisons betw
een numerical and experimental results show good agreement. The cumulu
s clouds produced by the model are similar to those observed, with clo
ud base, cloud top and cloud scales all in good agreement. At cloud to
p, vertical velocities are found to be +/-5 m s(-1) and liquid-water m
ixing ratios have a maximum of 1.6 g kg(-1), which is consistent with
the observations. By considering fractional cloud cover and averaging
over time, the profiles of the turbulent kinetic energy and fluxes ave
raged over the horizontal level and in-cloud are derived. Much of the
turbulent energy and transport in the boundary layer is found to be pr
oduced in cloud. The dominant regions of turbulent kinetic energy resu
lt from the strong surface heating and the two in-cloud buoyancy sourc
es of latent heating through condensation at cloud base and evaporativ
e cooling following entrainment at cloud top. This is supported by the
upward transport of energy at mid-cloud levels and downward at cloud-
top. The terms in the turbulent kinetic-energy budget are derived, and
once again buoyant production is found to be dominant in both the clo
ud and the horizontal averages. The vertical turbulent fluxes of heat,
moisture and liquid are analysed and found to vary in accordance with
the turbulent processes driving the boundary layer. The penetration o
f the clouds above the main inversion shows that the clouds were effec
tive in coupling the surface to the base of the free troposphere by tr
ansporting the heat and moisture upwards. Overall, the kinetic-energy
profiles and turbulent fluxes in the cumulus case-study, both averaged
over the layer and the clouds, show good agreement with the observati
ons. This demonstrates the validity of the large-eddy simulation model
as a tool for studying cumulus clouds at greater vertical resolution
than is possible by current observations, and provides a basis for der
iving more accurate parametrization schemes of convective cloud-capped
boundary layers.