Extended Abstract (248K)5.2
Shallow cumulus convection: validation of large-eddy simulation against aircraft and LandSat observations
R. A. J. Neggers, KNMI, De Bilt, Netherlands; and P. G. Duynkerke and S. M. A. Rodts
Large-eddy simulation (LES) results of shallow cumulus convection are directly evaluated against in-cloud aircraft-measurements, as made during the Small Cumulus and Microphysics Study (SCMS). To this purpose an LES case is first constructed, based on available observations. Then the simulations are directly compared to the in-cloud measurements by using conditionally sampled fields. An advantage of the SCMS data-set is the combination of a range of different surface measurements, in-cloud measurements by an aircraft at many levels in the cloud layer, and the availability of high-resolution LandSat images.
The results show that given the correct initial and boundary conditions the LES concept is capable of realistically predicting the bulk thermodynamic properties of temperature, moisture and liquid water content of the cumulus cloud ensemble as observed in SCMS. Furthermore the in-cloud turbulent kinetic energy and the cloud size distribution in LES were in agreement with the observations. These results support the credibility of cloud statistics as produced by LES in general, and encourage its use as a 'virtual laboratory' to test hypotheses and parameterizations of cumulus cloud processes.
Several hypotheses which make use of conditionally sampled fields were tested on the SCMS data. The magnitudes and the decrease with height of the bulk entrainment rate following from the SCMS data confirm the typical values first suggested by Siebesma and Cuijpers (1995) using LES results on BOMEX. Secondly, applying the simplified equation for the cloud vertical velocity (Simpson and Wiggert 1969) to the in-cloud measurements results in a reasonably closed budget. Finally, the observations support the similarity theory for shallow cumulus convection as proposed by Grant and Brown (1999).
Session 5, Cloudy BLs
Tuesday, 16 July 2002, 8:30 AM-1:59 PM
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