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)

Why two updrafts? One scheme fits all:

1 updraft 1 updraft2 updrafts

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)

(Albrecht 1996)

S6S11S12

Where do our simulations sit in the transition?

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)

Cloud-radiative impacts

Why?

Surface heat fluxes

The surface buoyancy forcing remains more or less unchanged

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

+2K SST: qsat(SST) changes much more than qt

+

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+?