A dual mass flux scheme for shallow cumulus convection Results for GCSS RICO Roel Neggers, Martin Köhler, Anton Beljaars
Contents The DualM scheme An extension to EDMF for shallow cumulus convection A *VERY* short overview (more on Thursday) RICO results Mean state Equilibration Updraft profiles & fluxes Variances Cloudy state & precipitation Experiments with an entrainment efficiency closure for cumulus
Eddy diffusivity - mass flux (EDMF) decomposition for turbulent transport The current PBL scheme (cy30r1) Siebesma et al. (Submitted to JAS) Currently applied to dry PBL and stratocumulus topped PBL only Goal: upgrade EDMF to enable representation of i) shallow cumulus convection ii) transitions What is needed? K moist updraft K dry updraft
The dual mass flux scheme – main EDMF upgrades Moist updrafts Flexible area partitioning a up Dry updrafts K diffusion a 1 up a 2 up h z0z0 1 - a up I. Multiple updrafts N=2: dry updrafts stopping at cloud base, moist updrafts continuing as cumulus clouds. II. Flexible updraft area fractions Closure as a function of model state Facilitates representation of i) gradual regime transitions, ii) coupling of cumuli to mixed layer III. Extension of EDMF partitioning into the statistical cloud scheme A matching bimodal PDF (passive clouds and updraft clouds) Consistent treatment of PBL transport and cloudiness IV. Top-entrainment efficiency closures for cumulus V. … and many more … Explicit treatment of transport across strong unresolved gradients ( cu inversion, transition layer)
Flexible updraft area fractions facilitate representation of gradual regime transitions E LCL h h EE M2M2 M1M1 Stratocumulus Shallow cumulus Dry convection M1M1 M2M2 a2a2 a1a1 Moist and dry updrafts can co-exist at any time
Closure of the flexible updraft area fractions Explicit representation of the coupling between cumulus updrafts and mixed-layer III. Vertical integration of updraft budgets then reconstructs the joint-PDF I. First guess of cloud core fraction at cloud base, using local variance scaling II. These fractions are subsequently used in surface initialization of updrafts Knowledge of how many updrafts manage to reach LFC is `back-stitched` into the partitioning and initialization of the updrafts This carries information on cloud base transition layer properties
RICO results – mean state
RICO results – cloudy state DualM cy30r1
RICO results - equilibration The flexibility of a and its closure introduce a valve mechanism in M, always acting to equilibrate the mixed layer budgets Area fraction is retained in the updraft mass flux
RICO results - updrafts Lateral entrainment closure makes differently initialized updrafts diverge with height.
RICO results - fluxes LES
RICO results – variance I Used in statistical cloud scheme
RICO results – variance II qtqt ll Updraft PDF Diffusive PDF Lewellen & Yoh (JAS,1993) Extension of EDMF into the modelling of clouds – a bimodal distribution Relations between moments
RICO results – variance II Closure of updraft variance
RICO results – bimodal clouds
Passive clouds Updraft clouds
RICO results - precipitation
How important is it? RICOBOMEX
RICO results – entrainment efficiency experiments RICOATEX Wyant et al. (1997) All flux into cu inversion is explicitly modelled through the K-component of EDMF
Summary A shallow cumulus extension is formulated for EDMF More complexity is added to the system of equations, in various new ways: - multiple transporting updrafts - flexible updraft area fractions - EDMF partitioning is extended into the PBL cloud scheme (a bimodal PDF) Assuming flexible updraft area fractions facilitates representation of gradual regime transitions Closure of the updraft area fractions allows explicit representation of the coupling between cumuli and the mixed layer, introducing a valve mechanism in the cumulus mass flux The mean, turbulent and cloud structure of the RICO short composite case is reproduced Precipitation appears to be an important player in the SCM thermodynamic budgets A more detailed discussion of model physics on Thursday…