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The scheme: A short intro
Some relevant case results
Why a negative feedback?
EDMF-DualM results for the CFMIP-GCSS intercomparison case:
Impacts of a SST perturbation on updraft transport
Roel Neggers, Pier Siebesma
CFMIP-GCSS meeting, UBC Vancouver, 8-12 June 2009
Combining the diffusive and advective models in the parameterization of the turbulent-convective flux:
The Eddy Diffusivity Mass Flux (EDMF) approach
Transporting updraft
K diffusion
AK Aup
)φ(φMz
φKw´φ´Aw´φ´Aw´φ´ upup
upupKK
Applicable to well-mixed layers:
Implemented in:
ECMWF IFS KNMI RACMO etc.
The single updraft limit of EDMF
* The single transporting updraft of EDMF is further partitioned into a dry updraft and a moist (condensed) updraft
The dual updraft limit of EDMF: shallow cumulus
Dry updraft
Moist updraft
K diffusionau1
au2
AK Aup
Flexible area partitioning
* Continuous (flexible) updraft area partitioning is applied, as a function of moist convective inhibition at cloud base
Test parcel
* A non-transporting extreme test parcel is included to provide information about the variance amongst updrafts
* Reconstructing a double PDF in conserved variable space
Extending EDMF into the statistical modelling of clouds
Moist updraft PDF: tied to the test parcel and moist parcel
Diffusive PDF: residual of diagnostic variance budget and moist updraft PDF
Moist parcelTest
parcel
x
x
The Dual Mass flux scheme, Part I: Transport. Neggers et al. (JAS, 2009, vol. 66, 1465-1489) Part II: Clouds. Neggers (JAS, 2009, vol. 66, 1490-1506)
Submitted version of EDMF-DualM for CFMIP-GCSS
Developed for the ECMWF Integrated Forecasting System (IFS)
Status: Implementation completed, now in test phase (scores optimalization)
K diffusion model:
* K-profile method for the well-mixed layer
* Explicit flux at mixed-layer top
* Ri-diffusion in the cloud layer and at cloud top
StCu triggering criterion (single moist updraft mode):
* LTS (as in operational IFS)
SCM code details:
* IFS/RACMO version CY31R1
* Native deep convection scheme switched off: EDMF-DualM scheme does all convective transport
Case results - Cloud fraction
S6 S11 S12
S12: Stable equilibrium, single well-mixed layer
S11: Stable equilibrium, decoupled
S6: After an initial shallow cu period, the scheme tries to ‘go deep’ (updraft rain disturbes mixed layer, causing oscillation)
Humidity structure
S6: fair weather cu (very weak inversion)
S11: decoupled S12: well mixed
+2K SST: PBL depth increases
Cloud fraction – EDMF contributions
S6 S11 S12
+2K SST: Total cloud fraction doesn’t change much (no discrete regime change)
LCL unchanged for the cumulus regimes S6 and S11
Cloud condensate – EDMF contributions
S6 S11 S12
+2K SST: Condensate on the diffusive PDF increases for S11 and S12 Deeper condensate layer in S6 (carried by upraft PDF)
Boundary fluxes act on the jumps
A bulk mixed-layer interpretation: qsat(SST) and q+
qt+
qsat(SST)
qtPBL
Top entrainment flux
Surface evaporation
q
z
Inversion jump
Surface jump
qt+
qsat(SST)
qtPBL
Control +2K SST
Top entrainment flux
Evaporation
Both jumps increase: The new equilibrium is associated with stronger fluxes at the boundaries
Impacts of increased surface evaporation on
the EDMF transporting updrafts
Through the updraft initialization scheme, a stronger E leads to larger updraft initial qt excesses
As a result, the total updraft qt flux increases
variance updraft fraction
The updrafts are efficient in transporting moisture upwards to the top of the PBL
This favors capping cloud formation if the inversion remains strong enough
Summary
The negative cloud-climate feedback in EDMF-DualM seems related to the increase in surface evaporation
Through the updraft initialization scheme, the enhanced evaporation leads to increased updraft initial humidity excesses. This extra humidity is efficiently transported by the updrafts to the PBL top, which favors the buildup of condensate in the capping cloud layer
The PBL updrafts do not get more energetic; they just carry more humidity around
Remaining question: how and why does the top entrainment flux in the model change?
General remark:
In studying feedbacks of PBL clouds on climate, the change in free tropospheric humidity is as important as the change in SST
Should we try some experiments with a different change in qt+?