LES Intercomparison Case of Precipitating Shallow Cumulus Margreet van Zanten, Bjorn Stevens, Pier Siebesma, Louise Nuijens A.S. Ackerman, H. Jiang, D. Mechem, D.C. Lewellen, S. Wang, B. Shipway, F. Couvreux, M. Khairoutdinov, A. Noda, P. Bogenschutz, J. Slawinska, A. Cheng, F. Burnet

A short reminder The RICO LES Intercomparison Case focuses on precipitating shallow cumulus convection and is based on observations taken during a three- week period in a relatively undisturbed trade-wind regime in the Caribbean. Fourteen participants provided output for 24-hour simulations with and without microphysics: explicit bin microphysics (3), one-moment bulk schemes (6), two-moment bulk schemes (5)

Models agree quite well!

When looking into detail: The character and amount of precipitation differs substantially among the simulations

Status First version of the paper has been written and distributed among participants (Margreet van Zanten) Work is in progress to put the paper into its final shape, that is:  to highlight the main results  shaping up the figures (combining model and observational data) In final form: weeks? The focus of this talk is to inform you about the main points we came up with after asking ourselves: what is the main message that we wish to bring across with this paper?

Our main points  Why did we have an intercomparison?  What did we learn from the intercomparison which we could not have learned from running our models individually? 1.How sensitive is precipitation formation to the microphysical versus numerical and dynamical representation of the flow? 2.Despite precipitation differences among models, does consistent behavior emerge when simulations start to precipitate? 3.Can observations from RICO constrain aspects of the simulations that are relatively robust? 4.What makes RICO an interesting case?

(1) Sensitivity to microphysics If dynamics would constrain precipitation, a positive relationship between in- cloud liquid water (for simulations without microphysics) and precipitation (for simulations with microphysics) may be expected

(2) Consistent behavior once it rains? Yes No

(3) Comparison with RICO data In the data as well as in LES rain seems to scale with echo fraction, which suggests that to a first approximation once a cloud begins to rain the micro- physical details do not matter

(3) Comparison with RICO data 'wet' 'dry' What threshold defines cloud?

(3) Comparison with RICO data When looking at intermittency of precipitation, one needs to keep in mind that the precipitation fraction or 'chances of finding rain somewhere in a domain', are very scale-dependent

(4) RICO versus BOMEX Mass flux decreasing less sharply with height : importance entrainment and properties of air entrained (Pier)

Summary What did we learn from the LES Intercomparison of precipitating shallow cumulus? 1.Precipitation formation appears very sensitive to microphysics 2.Notwithstanding precipitation differences among models, some consistent behavior does emerge once simulations begin to precipitate (different lapse rates, shallower clouds) 3.Given that past intercomparisons were cases without in-situ data to compare to, RICO data can help us constrain some robust aspects of the simulations 'To a first approximation rain is quite simple. Clouds rain or they don’t, and when they rain the character of the rain is universal. More rain then means more clouds are raining, or clouds raining for longer, but not that clouds are raining harder.'by: Bjorn Stevens