Current Changes in the Tropical Precipitation and Energy Richard P. Allan Department of Meteorology, University of Reading Thanks to Brian Soden, Viju.

Current Changes in the Tropical Precipitation and

EnergyRichard P. Allan

Department of Meteorology, University of ReadingThanks to Brian Soden, Viju John, William Ingram, Peter Good, Igor Zveryaev, Mark Ringer and Tony Slingo

http://www.met.reading.ac.uk/~sgs02rpa r.p.allan@reading.ac.uk

Sea Fishing 101

Course Convener

• Increased Precipitation• More Intense Rainfall• More droughts• Wet regions get wetter, dry

regions get drier?• Regional projections??

Precipitation Change (%)

Climate model projections (IPCC 2007)

Precipitation Intensity

Dry Days

NCAS-Climate Talk 15th January 2010 Trenberth et al. (2009) BAMS

Physical basis: energy balance

NCAS-Climate Talk 15th January 2010

Rad

iativ

e co

olin

g, c

lear

(W

m-2K

-1)

Allan (2009) J. Clim

Models simulate robust response of clear-sky radiation to warming (~2 Wm-2K-1) and a resulting increase in precipitation to balance (~2 %K-1)

e.g. Allen and Ingram (2002) Nature, Stephens & Ellis (2008) J. Clim

Trends in clear-sky radiation in coupled models

Clear-sky shortwave absorptionSurface net clear-sky longwave

Can we derive an observational estimate of surface longwave? Prata (1996) QJRMS

The energy constraint on global precipitation

Andrews et al. (2009) J Climate

NCAS-Climate Talk 15th January 2010

CC Wind Ts-To RHo

Muted Evaporation changes in models are explained by small changes in Boundary Layer:1) declining wind stress2) reduced surface temperature lapse rate (Ts-To)3) increased surface relative humidity (RHo)

Richter and Xie (2008) JGR

Evaporation

Pre

cip.

(%

)

Allan and Soden (2008) Science

Current tropical ocean variation in water vapour and precipitation

Current changes in tropical ocean column water vapour

…despite inaccurate mean state, Pierce et al.; John and Soden (both GRL, 2006)

- see also Trenberth et al. (2005) Clim. Dyn., Soden et al. (2005) Science

John et al. (2009)

models

Wat

er V

apou

r (m

m)

Thermodynamic constraint

1979-2002• Clausius-Clapeyron

– Low-level water vapour (~7%/K)– Intensification of rainfall: Trenberth et al. (2003) BAMS; Pall et al.

(2007) Clim Dyn

• Changes in intense rainfall also constrained by moist adiabat -O’Gorman and Schneider (2009) PNAS

• Could extra latent heat release within storms enhance rainfall intensity above Clausius Clapeyron?– e.g. Lenderink and van Meijgaard (2008) Nature Geoscience

Increases in the frequency of the heaviest rainfall with warming: daily data from models and microwave satellite data (SSM/I)

Allan et al. (2010) Environ. Res. Lett.Reduced frequency Increased frequency

• Increase in intense rainfall with tropical ocean warming (close to Clausius Clapeyron)

• SSM/I satellite observations at upper limit of model range

Model intense precipitation dependent upon conservation of moist adiabatic lapse rate but responses are highly sensitive to model-specific changes in upward velocities (see O’Gorman and Schneider, 2009, PNAS; Gastineau & Soden 2009).

Large-scale water cycle response

• Clausius-Clapeyron– Low-level water vapour (~7%/K)– Enhanced moisture transport (F)– Enhanced P-E patterns (below)

See Held and Soden (2006) J Clim

AR5

scaling

Models/observations achieve muted precipitation response by reducing strength of Walker circulation. Vecchi and Soden (2006) Nature

But see also Park and Sohn (2010) JGR in press

P~Mq

Circulation response

Contrasting precipitation response expected

Pre

cipi

tatio

n Heavy rain follows moisture (~7%/K)

Mean Precipitation linked to

radiation balance (~3%/K)

Light Precipitation (-?%/K)

Temperature e.g.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature

Contrasting precipitation response in wet and dry regions of the tropical circulation

Updated from Allan and Soden (2007) GRL

descent

ascentModelsObservations

Pre

cipi

tatio

n ch

ange

(%

)

Sensitivity to reanalysis dataset used to define wet/dry regions

Is the contrasting wet/dry response robust?

• Large uncertainty in magnitude of change: satellite datasets and models & time period

TRMM

GPCP Ascent Region Precipitation (mm/day)

John et al. (2009) GRL

• Robust response: wet regions become wetter at the expense of dry regions. Is this an artefact of the reanalyses?

Avoid reanalyses in defining wet/dry

regions

• Sample grid boxes:– 30% wettest– 70% driest

• Do wet/dry trends remain?

Current trends in wet/dry regions of tropical oceans

• Wet/dry trends remain– 1979-1987 GPCP

record may be suspect for dry region

– SSM/I dry region record: inhomogeneity 2000/01?

• GPCP trends 1988-2008

– Wet: 1.8%/decade– Dry: -2.6%/decade– Upper range of model

trend magnitudes

Models

DR

Y

WE

T

Outstanding Issues

Can we understand and predict regional climate change?

Could aerosols short-circuit the changing water cycle?

Are the cloud feedback and water cycle issues linked?

One of the largest challenges remains improving predictability of

regional changes in the water cycle…Changes in circulation systems are crucial to regional changes in water resources and risk yet predictability is poor.

How will catchment-scale runoff and crucial local impacts and risk respond to warming?

What are the important land-surface and ocean-atmosphere feedbacks which determine the response?

Top: GFDL cm2.1 2080-2099 minus 1980-1999 (% precipitation)

Bottom: GFDL-GPCP precipitation (%)

Pre

cipi

tatio

nCurrent changes in precipitation

for Europe-Atlantic region

Could changes in aerosol be imposing direct and indirect changes in the hydrological cycle? e.g. Wild et al. (2008) GRL

Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim

Mishchenko et al. (2007) Science

Can we observe atmospheric radiative heating/cooling?

John et al. (2009) GRL

Are the issues of cloud feedback and the water cycle linked?

2006

Allan et al. (2007) QJRMS

How important are cloud microphysical processes in stratocumulus and large-scale processes involving cirrus outflow? e.g. Ellis and Stephens (2009) GRL; Stephens and Ellis (2008) J Clim. Zelinka and Hartmann (in prep) “FAT/FAP hypothesis”; Stephens et al. (2010) JAS in prep

• Robust Responses– Low level moisture; clear-sky radiation

– Mean and Intense rainfall

– Observed precipitation response at upper end of model range?

– Contrasting wet/dry region responses

• Less Robust/Discrepancies– Moisture at upper levels/over land and mean state

– Inaccurate precipitation frequency distributions

– Magnitude of change in precipitation from satellite datasets/models

• Further work– Decadal changes in global energy budget, aerosol forcing effects

and cloud feedbacks: links to water cycle?

– Precipitation and radiation balance datasets: forward modelling

– Surface feedbacks: ocean salinity, soil moisture (SMOS?)

– Boundary layer changes and surface fluxes

Conclusions

Radiative effects of persistent aircraft contrails: a case study

Richard AllanEnvironmental Systems Science Centre

Courtesy of Jim Haywood

Met Office NAME model

NOAA17 satellite image 20 March 2009 10:06

Courtesy of Jim Haywood

Courtesy of Jim Haywood

Courtesy of Jim Haywood

Courtesy of Jim Haywood

Courtesy of Jim Haywood

Courtesy of Jim Haywood

Using GERB-like/SEVIRI to quantify radiative effects of persistent contrail cirrus

More details in Haywood et al. (2009) JGR

SW

NET

LW

Radiative Effect

Estimated effect as large as 7% of radiative forcing of entire aircraft fleet for that day. Future work: Icelandic volcano influence on cirrus contrails?