Changes in clear-sky longwave radiative cooling in the atmosphere 1979-2004

Richard Allan

Environmental Systems Science Centre, University of Reading, UK

Introduction

• Clear-sky radiative cooling: – radiative convective balance– atmospheric circulation

• Earth’s radiation budget– Understand clear-sky budget to understand cloud radiative effect

• Datasets:– Reanalyses – observing system– Satellites – calibration and sampling

• GERB comparisons with UKMO forecast model• Met Office modelling activities

Datasets used• Surface and Top of Atmosphere clear-sky LW fluxes• Column integrated water vapour (CIWV)

• Reanalyses:– ERA-40 (1979-2001)– NCEP-1 (1979-2004)

• Satellite data– ERBS, ScaRaB, CERES (clear-sky OLR)– SMMR, SSM/I V5 (CIWV)

• Combination datasets:– SRB Rel. 2(1983-1994)…reanalysis?

Global variability

1980 1985 1990 1995 2000 2005

Ts

CIWV

LWc

SFC

ERA40

NCEP

SRB

Global variabilityERA40

NCEP

SRB

Surface Net LWc

Clear-sky OLR

Clear-sky Atmos LW cooling

QLWc

Surface LWc and CIWV

dSNLc/dCIWV ~ 1.5 Wkg-1

ERA40 NCEP

Atmospheric LWc and Ts

dQLWc/dTs ~ 3 Wm-2K-1

ERA40 NCEP

Clear-sky net LW over oceans

• SMMR and SSM/I to prescribe CWV

• HadISST SST and da Silva climatology to estimate T0

• Prata (1996) clear-sky LW formula:

SNLc=εs(SDLc-σTs4)

Tropical Oceans

ERA40

NCEP

SRB

OBS

1980 1985 1990 1995 2000 2005

Tropical Oceans: OLRc and QLWc

ERA40

NCEP

SRB

OBS

Clear-sky LW and CIWV scatter with Ts: SRB and OBS

Tropical Oceans: descent QLWc

ERA40

NCEP

SRB

OBS

Linear least squares fit

• Tropical ocean: descending regime

• Dataset dQLWc/dTs Slope

• ERA-40 3.7±0.5 Wm-2K-1

• NCEP 4.2±0.3 Wm-2K-1

• SRB 3.6±0.5 Wm-2K-1

• OBS 4.6±0.5 Wm-2K-1

Implications for tropical precipitation (GPCP)?

ERA40 QLWc

GPCP P

OBS QLWc

Conclusions

• Intercomparisons of datasets: clear-sky LW at SFC, TOA, ATM

• Reanalyses: observing system changes

• Satellites: calibration, sampling

• Increase in clear-sky LW cooling of atmosphere of ~3-5 Wm-2K-1

• All-sky changes? Models?

Initial comparisons of GERB data with UK Met Office global

forecast model

Harries et al.(2005) BAMS 86 p.945

Allan et al. (2005) JGR 110, D14111

http://www.nerc-essc.ac.uk/~rpa/GERB

With thanks to GERB International Science Team including Jacqui Russell (Imperial College, London), Nicolas Clerbaux (RMIB, Belgium)

July 2004 monthly-diurnal mean

GERB prelim version CERES FM1+FM4

OLR

Albedo

See Jacqui Russell’s talk for more detailed analysis

“Global”, 1200 UTC, GERB vs UKMO forecast model

Model change

GERB change

CLEAR-SKY differences (model minus GERB)

“Global” Ocean, 1200 UTC data

Clear-sky OLR

Clear-sky Albedo

Current activities at UK Met Office

Alejandro Bodas

Mark Ringer

HadGAM1 – SRB2

SW

LW

HadGAM1-ISCCP D2

Cloud

HadGAM1- 4D DAO CIWV