12.6 Large Eddy Simulations of precipitating trade cumuli

Thursday, 13 July 2006: 4:45 PM
Hall of Ideas G-J (Monona Terrace Community and Convention Center)
Margreet C. Van Zanten, KNMI, De Bilt, Netherlands; and A. P. Siebesma and L. Nuijens

Although trade wind cumulus is the most dominant cloud type over the subtropical oceans, it is yet unknown how much precipitation is produced by trade wind cumuli and how this precipitation interacts with the cloud dynamics. In order to study this topic a Large Eddy Simulation (LES) will be made of a characteristic trade wind cumulus day during the RICO experiment.

The day chosen - Januari 11th 2005 - had in general patches of cumuli of moderate extent in which precipitation was prevalent; although precipitation below cloud was less widespread. Light winds prevailed in the (sub)cloud layer. Thermodynamically the atmosphere was rather moist in the cloud layer and it was somewhat surprising that only in localized areas deeper convection materialized.

During the eleventh of January 2005 an almost full suite of measurement platforms (the C-130 and King Air planes, the S-Pol radar and RV Seward Johnson) was available. The C-130 sampled a patch of cumuli in the neighbourhood of the ship. This combination of aircraft and ship data for the same cumuli field will help in putting better constraints on LES output than previous cumulus experiments have supplied.

The case study will be used as the 10th intercomparison study of the boundary layer clouds working group of the GEWEX Cloud System Study. Here, however, we would like to show the sensitivity of the case to various parameters. Besides basic tests such as the sensitivity to timestep and vertical and horizontal resolution, we plan to do simulations on very large domain sizes in order to study whether any mesoscale organization develops in the LES. Further we will study the influence of microphysical parameters on the development of the cumuli by switching precipitation on/off, changing the amount of cloud droplet numbers and varying the value of several parameters in the microphysical code.

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