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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?
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