Using a novel coupled-model framework to reduce tropical rainfall biases

Nicholas KlingamanSteve Woolnough, Linda Hirons

National Centre for Atmospheric Science-ClimateDepartment of Meteorology, University of Reading

Outline

• Introduction to tropical biases in the MetUM• Biases in mean rainfall• Biases in intra-seasonal variability (Madden-Julian Oscillation)

• Introduction to the MetUM-GOML atmosphere—ocean-mixed-layer model

• Using MetUM-GOML to explore the sensitivity of tropical biases to coupling in the tropics andextra-tropics

• Summary and conclusions

Tropical rainfall biases in the MetUMMetUM DJFM precipitation TRMM DJFM precipitation

MetUM bias (MetUM-TRMM)

The MetUM produces a strong and southward displaced ITCZ in DJFM.

Tropical rainfall biases in the MetUMMetUM JJAS precipitation TRMM JJAS precipitation

MetUM bias (MetUM-TRMM)

Large wet biases over tropical oceans in JJAS, with dry biases over tropical land, particularly India.

● The MJO is the dominant mode of sub-seasonal (30-60 day) variability in the tropics.

● Events often form in the Indian Ocean, before propagating east to the Maritime Continent and the West Pacific.

● Suppressed convection precedes and follows each active event.

● In DJF (JJA) the MJO modulates the Australian (Asian, African) monsoon.

Composites of observed OLR (shading) and

850 hPa winds (vectors) for each MJO phase

(Source: Wheeler and Hendon, 2004)

Madden-Julian oscillation

Tropical rainfall biases in the MetUMNOAA CIRES observations

MetUM Control

Regressions of 20-100 day filtered OLR on a base point at 70°E.

The MetUM produces no eastward propagation in tropical convection, with very weak anomalies on sub-seasonal timescales.

Effects of convective entrainment

NOAA CIRES observations

MetUM Control MetUM 1.5x entrainment

Increasing entrainment and detrainment by 50% in the MetUM produces some eastward propagation.Klingaman and Woolnough (2014a, QJRMS)

Effects of convective entrainmentMetUM 1.5x entrainment JJAS 1.5x entrainment minus control

1.5x entrainment bias against TRMMControl bias against TRMM

See also Klingaman and Woolnough (2014a, QJRMS) and Bush et al. (2014, QJRMS).

MetUM-GOML (Ocean Mixed Layer) model

MC-KPP one-dimensional ocean model

MetUM AtmosphericGCM

OASIS coupler(sub-daily)

Key advantages:•Cheap: < 5% of the cost of the atmosphere, allowing high (1 metre) ocean vertical resolution.•Controllable: Easily constrainable to any desired ocean state (small SST biases).•Flexible: Air-sea coupling can be applied selectively in space and time to explore the role of coupling in a range of phenomena.•Adaptable: Works easily with any GCM configuration or grid.Climatological three-dimensional heat and salt tendencies are

applied to represent• (a) the mean advection in the ocean• (b) corrections for biases in atmospheric surface fluxes

MetUM-GC (NEMO 3D ocean) MetUM-GOML (KPP 1D ocean)

By using climatological heat and salt corrections, MetUM-GOML produces much smaller mean SST biases than a fully coupled GCM. Biases are typically smaller than +/- 0.5K.

The disadvantage is the lack of interactive ocean dynamics in MetUM-GOML, which are important for certain applications (e.g., ENSO)

MetUM-GOML (Ocean Mixed Layer) model

(1)IndianOcean

(2)Warm Pool

(3)Tropics-Wide

(4) 50N-50S

(4) 50N-50S

(5) Near-global

(5) Near-globalBecause the MC-KPP columns do not communicate, there is complete flexibility (except near sea ice) in where the atmosphere and ocean are coupled.

MetUM-GOML (Ocean Mixed Layer) model

Experiments

Experiment

Coupled?

Coupling region or SST specification

A-OBS No Climatological observed SSTs

K-30 Yes 30N–30S and 0–360E

K-50 Yes 50N–50S and 0–360E

A-K50(clim) No Climatological SSTs from K50

A-K50(15day)

No 15-day smoothed SSTs from K50

• All experiments use MetUM GA3.0 atmosphere with +50% to entrainment and detrainment for deep and mid-level convection.

• At least 25 years of data are analysed for each experiment.

Impacts on the Madden-Julian oscillation

NOAA A-OBS

K-30 K-50

Impacts on the Madden-Julian oscillation

NOAA K-50

A-50(clim) A-50(15day)

Effect of tropical coupling

K-30 JJAS precipitation K-30 minus A-OBS (tropical coupling)

K-30 bias against TRMM A-OBS bias against TRMM

K-50 JJAS precipitation K-50 minus K-30

K-50 bias against TRMM K-30 bias against TRMM

Effect of extra-tropical coupling

Effect of coupling 50N-50S (vs. clim SST)

K-50 JJAS precipitation K-50 minus A-K50(clim)

K-50 bias against TRMM A-K50(clim) bias against TRMM

Effect of coupling 50N-50S (vs. 15-day SST)

K-50 JJAS precipitation K-50 minus A-K50(15 day)

K-50 bias against TRMM A-K50(15 day) bias against TRMM

Mean seasonal cycle of all-India rainfall

Distributions of JJAS-mean all-India rainfall

Extra-tropical coupling (red, orange) consistently produces stronger monsoons than tropical coupling (brown) or atmosphere-only (purple, blue).

The strongest increases in rainfall come shortly after

monsoon onset in June. All other configurations have a delayed and weak monsoon

onset.

K-50 minus A-K50(15 day)

JJAS means of:•Colours: Precipitation•Vectors: 850-hPa winds•Contours: MSLP

A-K50(15 day) bias against TRMM and ERA-Int

A-K50 has a strong cyclonic bias with low MSLP in the West Pacific, both of which are reduced considerably in K-50.

April means of:•Colours: Precipitation•Vectors: 850-hPa winds•Contours: MSLP

K-50 minus A-K50(15 day)

A-K50(15 day) bias against TRMM and ERA-Int

In April, coupling prevents the erroneous northward migration of the Pacific ITCZ, focusing convection instead on the equator.

K-50 minus A-K50(15 day)

A-K50(15 day) bias against TRMM and ERA-IntMay means of:•Colours: Precipitation•Vectors: 850-hPa winds•Contours: MSLP

In May, coupling produces a strong extra-tropical Pacific jet and inhibits development of strong rainfall and cyclonic anomalies near the Philippines.

A-K50(15 day) “Wet India” composite

A-K50(15 day) “Wet minus dry”

K-50 “Wet India” composite K-50 “Wet minus dry”

Summary and conclusions• MetUM-GOML is an ideal framework for

examining sensitivities to tropical and extra-tropical coupling.

• Heat and salt corrections maintain the observed mean SST.

• Allows effects of regional coupling to be isolated in a model with very small SST biases.

• The MJO improves with tropical coupling, through better SST—surface-flux—rainfall relationships.

• Tropical rainfall biases reduce with extra-tropical coupling, through strengthening the subtropical high and delaying the seasonal progression of the ITCZ.