A simple parameterization for detrainment in shallow cumulus Hirlam results for RICO Wim de Rooy & Pier Siebesma Royal Netherlands Meteorological Institute (KNMI)

Hirlam 1D Hirlam but with: Statistical cloud scheme Tiedtke mass flux convection scheme with updates: –Mass flux closure at cloud base (Neggers et al. 2002) –Triggering (Jacob & Siebesma 2003) –versions with a conventional or a new lateral mixing concept

Mass flux concept M MM MM Fixed  and  : No dependence on environmental humidity conditions Buoyancy sorting concept Complex, fundamental problems Fraction of environmental air Courtesy: Stephan de Roode

Conventional fixed  =z -1 and  = m -1 okay for BOMEX but for RICO? What’s going on?

LES results for BOMEX, ARM and RICO show: Not much variation in . For a correct simulation of the Mass flux profile,  =z -1 is good enough. Much more variation in . The value of  mainly depends on: - Cloud Layer Height - Environmental conditions

(e.g. BOMEX) Cloud layer depth=1000m Mass flux profiles with  =z -1 and  = Cloud layer depth=200m Cloud layer depth=2000m e.g. RICO Cloud ensembles z M M M z z bot z top z bot Cloud Layer Height dependence

LES Non-dimensionalized mass flux profiles z*z* ARM case LES Eliminate cloud height dependence by looking at a non- dimensionalized mass flux profile Dependence on environmental conditions

Suppose we would know the non-dimensionless mass flux m* halfway the cloud layer at height z* Z* Z bot Z top

From LES, the non-dimensionalized mass flux half way the cloud layer as a function of  c

Good results with the new parameterization for ARM, BOMEX and:

Conclusions The proposed detrainment parameterization is simple but includes two important dependencies: Cloud layer height dependence Current mass flux schemes ignore this dependence which evidently can lead to large discrepancies with observed mass flux profiles. Environmental conditions With the  c dependence the new scheme can be seen as an alternative for more complex buoyancy sorting schemes (without some of the disadvantages)

Conclusions Good results for a wide range of shallow convection cases (BOMEX, ARM, RICO) Easy to incorporate in existing mass flux schemes (and will be incorporated in an EDMF dual mass flux environment)

LES: The non-dimensionalized mass flux half way the cloud layer as a function of RH

LES Non-dimensionalized mass flux profiles Non-dimensionalized mass flux profiles with fixed  and  ARM case Eliminate cloud height dependence by looking at a non- dimensionalized mass flux profile Dependence on environmental conditions