Quaas_aerosol_cloud by gegouzhen12


									                                 Aerosol cloud-interactions
                           forcing estimate from satellite data

                                                                                            Introduction                                                                                 NASA’s TERRA satellite

                         greenhouse                                                                                                                                                                                                        MODIS
                              effect                                                                                                                                                                                                       - aerosols
                                                                                                                                     anthropogenic                                                                                         - clouds
                                                                                                                                     albedo change

                                                                                             F↓α                                F↓(α+Δα)
                                               plus aerosol

critical                                                                                                                                                                                                                                   CERES
                                                                                                                                                                                                                                           - radiative fluxes
value: +2°C                                                              F↓ : Incoming solar flux
                                                                         α : planetary albedo
                                                                                                                   Forcing: F↓ Δα
                                                                                                                   Measurable at the top
Aerosol-climate forcings still constitute one of the main                                                          of the atmosphere
uncertainties concerning global climate change.
The amount of greenhouse-gas-induced warming currently
masked by aerosol cooling effects determines the “climate               Aerosol-climate forcings can be considered as an increase in                                                     All necessary data are retrieved from
sensitivity” - the future global warming.                               planetary albedo due to anthropogenic aerosols.                                                                  instruments on board the same satellite.


                                                                                                                                                            Planetary albedo
                                   Aerosol measurements
                                   Cloud measurements
                                   No retrieval

                                                                                                                                                                                                                         CERES “ERBE-like” product:
                                Method adopted:                                                                                                                                                                          Four cloud scenes:
                                 > relate CERES planetary                                                                   Relationship                                       Aerosol concentration
                                  albedo to MODIS                                                                  60°N     established for:
                                  aerosol optical depth                                                            30°N     - Five regions                                                        1.) Cloud free                      < 5%
                                                                                                                    8°N                                                                                                                       Direct effect
                                                                                                                            - Four seasons                                                        2.) Partly cloudy                   5 – 50%
                Δxx              > daily values, 1° x 1°                                                            8°S       (MAM, JJA,                                                          3.) Mostly cloudy                  50 – 95% Indirect
                                 > five years of data March                                                        25°S
                                                                                                                               SON, DJF)                                                          4.) Overcast                        > 95%   effect*
                                   2000 to February 2005
                                                                                                                                                                                                  * No ice-cloud effects,
                                                                                                                                                                                                    second indirect effect partially

 Aerosol retrievals from satellite sensors are possible only if        Different regions and seasons are analysed separately, as are
 there are no clouds. Aerosols retrieved in the same grid-box on       land and ocean.                                                                      Different cloud „scenes“ help to differentiate direct and
 the same day in cloud-free pixels are assumed to be                                                                                                        indirect aerosol-climate forcings.
 representative for the aerosol concentrations at the base of the
 clouds in the same grid-box.

        Cloud free                   Partly cloudy
                                                                                                                                                                               α   tot

                                                                                                                                                                               α   tot


        Mostly cloudy                Overcast

                                                                                                                                                                               α   nat

                                                                                                                                                                                                                                          Forcing F↓ Δα

                 JJA, 30-60°N
                                                                                                                                                                                                     τ   nat
                                                                                                                                                                                                                           τ   tot
                                                                                                                                                                                                                                        τ : anthropogenic
                                                                                                                                                                                                               τ   ant
                                                                                                                                                                                                                                        AOD from MODIS
Statistical relationships between planetary albedo and column                                                                                                                                                                            (Bellouin et al., Nature 2005)
aerosol concentration (measured as aerosol optical depth,
AOD) are established for each season and region. The example                                                                                                            The relationships can be combined with a new dataset
of June-July-August, Northern hemisphere mid-latitutes is
                                                                       For completeness – all derived relationships. Light colours                                      of satellite-derived anthropogenic fraction of AOD (тaant) 
shown (blue-ocean, brown-land). AOD is shown on a
                                                                       indicate negative relationships, and dashed lines relationships                                  to obtain the change in planetary albedo due to 
logarithmic scale, and albedo in absolute % as the deviation
                                                                       not significant above the 90% level.
from the mean.                                                                                                                                                          anthropogenic aerosols. 
                                                                       Virtually all relationships are statistically significantly positive.
                                               0.0                                                                                                                      The radiative forcing then simply is the product of 
                                              -0.2   Global                                                                                                             incoming solar radiation (F↓) and the albedo change. 
                                              -0.4   annual mean:
                                              -0.8    direct effect:     #   Description                                  Direct effect   Indirect effect

                                              -1.0       -0.8 Wm-2                                                           [Wm-2]           [Wm-2]
                                              -1.2                       0   Baseline                                         -0.8             -0.3
                                              -1.4   indir. effect:
                                              -1.6                       1   Split time-series                            -0.8 to -0.7         -0.3
                                                        -0.3 Wm-2
                                              -2.0                       2   Stratification by liquid water path              -0.5             -0.2                 1.                   Method
                                              -2.2   total:
                                                                                                                                                                                                Significantly positive relationships CERES planetary
                                              -2.4       -1.1 Wm-2       3   Different choice of seasons/regions           -0.8 to -0.7    -0.4 to -0.3
                                                                                                                                                                                                albedo – MODIS aerosol optical depth
                                              -2.6                       4   Different constraints on aerosol                 -0.7         -0.3 to -0.2
                                              -2.8                           homogeneity / liquid cloud fraction
                                              -3.0                                                                                                                  2.                   Estimate of aerosol radiative forcings
                                              -4.0                       5   Aqua instead of Terra cloud data                 -0.8             -0.3
                                              -5.0                                                                                                                                         a)   Direct effect: -0.8 Wm-2
                                                                         6   Uncertainty on anthropogenic                  -0.9 to -0.6    -0.4 to -0.3
                                                                             aerosol fraction                                                                                              b)   Indirect effect: -0.3 Wm-2
                                                                                                                          -0.9 to -0.5     -0.4 to -0.2
   The derived forcing by the direct effect is similar to
   previous satellite-derived values.                                                                                                                               3.                   Uncertainty estimate
   The indirect forcing is lower than most model estimates,             The uncertainty inherent in our method can be estimated                                                            a)   Direct effect: ± 0.3 Wm-2
   but agrees very well with our previous studies                       by sensitivity studies. It is of roughly 30%.
   constraining global climate model parameterisations of the                                                                                                                              b)   Indirect effect: ± 0.1 Wm-2
   aerosol indirect effect with satellite data.

                                                              1                                              2                                                                     2                                                            1
                  Johannes Quaas , Olivier Boucher , Nicolas Bellouin & Stefan Kinne
                           1                                                                                                      2
                               Max Planck Institute for Meteorology, Hamburg | Hadley Centre, Met Office, Exeter

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