Current Changes in Tropical Precipitation - PowerPoint PPT Presentation

Loading...

PPT – Current Changes in Tropical Precipitation PowerPoint presentation | free to download - id: 7181ea-OWE1N



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Current Changes in Tropical Precipitation

Description:

... of the heaviest rainfall events is consistent with models but the models appear to underestimate the response to warming ... Change (%) Climate ... Global ... – PowerPoint PPT presentation

Number of Views:53
Avg rating:3.0/5.0
Slides: 22
Provided by: acuk
Learn more at: http://www.met.reading.ac.uk
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Current Changes in Tropical Precipitation


1
Current Changes in Tropical Precipitation
  • Richard P. Allan
  • Department of Meteorology, University of Reading
  • Thanks 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_at_reading.ac.uk

2
Climate model projections (IPCC 2007)
  • Increased Precipitation
  • More Intense Rainfall
  • More droughts
  • Wet regions get wetter, dry regions get drier?
  • Regional projections??

Precipitation Intensity
Dry Days
Precipitation Change ()
3
Physical basis energy balance
  • Radiative Convective balance
  • Enhanced radiative cooling (Allen and Ingram,
    2002)
  • Water vapour key conserve RH, MALR (Stephens et
    al 94)
  • Modulated by GHG/aerosol forcings fast and slow
    responses (Andrews et al. 2009 JGR Dong et al.
    2009 J Clim)
  • High cloud feedback
  • (Lebsock et al. 2010 J Clim)

Temperature-dependent responses (2-3/K) CO2
increases mute the transient response somewhat
(Andrews et al. 2010 ERL Wu et al. 2010 GRL)
Trenberth et al. (2009) BAMS
4
Physical basis water vapour
  • Clausius-Clapeyron
  • Low-level water vapour (7/K)
  • Intensification of rainfall OGorman
    Schneider, 2009 PNAS
  • Moisture transport enhanced P-E Held
    Soden 2006 J Clim
  • Radiative/thermodynamic constraints
  • Wet get wetter, dry regions gets drier
  • Reduced mass flux declining Walker circulation
    (Vecchi and Soden, 2006)

1979-2002
Water vapour (mm)
Temperature (K)
PMq
5
Observed increase in water vapour and
precipitation with warming
Precip. ()
Allan and Soden (2008) Science
6
Tropical ocean column water vapour constrained by
Clausius Clapeyron
John et al. (2009)
Water Vapour (mm)
models
despite inaccurate mean state, Pierce et al.
John and Soden (both GRL, 2006) decreases in RH
over land? (Simonds et al. 2009 JGR see also
Joshi et al. 08) Reanalyses struggle to capture
decadal changes in the water cycle. - see also
Trenberth et al. (2005) Clim. Dyn., Soden et al.
(2005) Science
7
Contrasting precipitation response in wet and dry
regions of the tropical circulation
ascent
Models
Observations
Precipitation change ()
descent
Sensitivity to reanalysis dataset used to define
wet/dry regions
Updated from Allan and Soden (2007) GRL
8
Is the contrasting wet/dry response robust?
GPCP Ascent Region Precipitation (mm/day)
John et al. (2009) GRL
  • Large uncertainty in magnitude of change
    satellite datasets and models time period

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

9
Precipitation changes in wettest 30 and driest
70 of grid boxes
  • 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

DRY WET
Models
10
Increases in the frequency of the heaviest
rainfall with warming daily data from models and
microwave satellite data (SSM/I)
Reduced frequency Increased frequency
Allan et al. (2010) Environ. Res. Lett.
11
  • 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 constrained by moist
adiabatic lapse rate responses highly sensitive
to model-specific changes in upward velocities
(OGorman Schneider, 2009, PNAS Gastineau
Soden 2009 Turner and Slingo, 2009 ASL).
12
Implications
  • Observations and simple physics help to confirm
    robust model projections
  • Increased precipitation (2/K)
  • Increased precipitation intensity (7/K)
  • Extratropics and wet regions of tropics get
    wetter
  • Dry regions of sub-tropics get drier
  • Transient response to GHG stabilisation
  • Outstanding Issues
  • Inaccurate simulation of precipitation events
  • Limitations of satellite and gauge data
  • Detecting and attributing signals
  • Cloud Feedback
  • Aerosol

13
END
14
Add in more about obs.
  • Add links to
  • Lebsock et al. (2010) J Clim rad/conv balance
    and feedback from obs Previdi (2010) ERL
  • Wu et al (2010) GRL Andrews et al. (2010) ERL
    transient changes in models
  • Enhanced moisture transport (dry to wet regions
    of tropics tropics to extra-tropics) Held and
    Soden (2006) and implications for wet/dry region
    precipitation (Chou et al. 2007 GRL)
  • Clausius Clapeyron contraint on intense
    precipitation (e.g. OGorman and Schneider 2009
    PNAS Allan and Soden 2008 Science Lenderink and
    van Mijgaard 2010 ERL)
  • Changes in surface evaporation (Richter and Xie
    2008 JGR) declining pan evaporation trends and
    wind speed (Roderick et al. 2007 GRL)
  • Large-scale tropical circulation weakening
    (Vecchi and Soden, 2007 Nature) and variability
    (Park and Sohn, 2010 JGR in press)

15
Thermodynamic constraint
  • 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
  • -OGorman 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

1979-2002
16
Top GFDL cm2.1 2080-2099 minus 1980-1999 (
precipitation) Bottom GFDL-GPCP precipitation
()
17
What do we expect?
  • Surface and atmosphere energy balance constraint
    (Allen and Ingram, 2002 Stephens and Ellis,
    2008 Lambert and Webb, 2008, Andrews et al.
    2009)
  • Moisture transport constraint (Held and Soden,
    2006)
  • Moisture convergence constraint (OGorman and
    Schneider, 2009 Lenderink and Van Meijgaard,
    2008)
  • Mass flux and moist adiabat arguments (e.g.
    Vecchi and Soden, 2007 Held and Soden, 2006)
  • Negative impact of greenhouse gases on transient
    precipitation response (Andrews et al. 2009 Wu
    et al. 2010)
  • See also special focus issue of Environmental
    Research Letters (No. 2, April-June 2010)

18
Observations
  • Daily estimates of column water vapour and
    precipitation from microwave retrievals (SSM/I)
    1987-2009 (e.g. Wentz et al. 2007 Science) and
    TRMM (1998-present) microwave and radar
  • Blended precipitation from infra-red and
    microwave radiance and rain gauge, Global
    Precipitation Climatology Project (GPCP Huffman
    et al. (2009) GRL monthly (1979-2009) and daily
    (1997-2009)
  • CERES Earth Radiation Budget measurements
    (2000-2006) ISCCP-based estimates (1983-2006)

19
Global precipitation changes constrained by
radiative cooling
Muted precip response achieved through reduced
Walker circulation (mass flux)
Enhanced tropical to extra-tropical moisture flux
Enhanced moisture flux from dry to wet regions of
tropics
Enhanced moisture flux
Enhanced tropical to extra-tropical moisture flux
Muted evaporation through subtle changes in BL
Reduced rainfall intensity and/or frequency
Enhanced rainfall intensity MALR scaling
20
Can we observe changes in atmospheric radiative
heating/cooling?
Changes in atmospheric longwave radiative
radiative cooling (Wm-2)
models
John et al. (2009) GRL
21
Avoid reanalyses in defining wet/dry regions
  • Sample grid boxes
  • 30 wettest
  • 70 driest
  • Do wet/dry trends remain?
About PowerShow.com