influences of ice particle model on ice cloud optical thickness retrieval
DESCRIPTION
Influences of ice particle model on ice cloud optical thickness retrieval. Zhibo ( zippo ) Zhang 03/29/2010 ESSIC. Outline. Background Importance of ice cloud Ice particle model and ice cloud retrieval Influence of ice particle model on t retrieval - PowerPoint PPT PresentationTRANSCRIPT
Influences of ice particle model on ice cloud optical
thickness retrievalZhibo (zippo) Zhang
03/29/2010ESSIC
Outline
• Background• Importance of ice cloud• Ice particle model and ice cloud retrieval
• Influence of ice particle model on t retrieval• Comparison of MODIS and POLDER ice t
retrieval• Influence on our understanding of ice cloud
seasonal variability
• Summary
Ice cloud: important
Ice clouds are important, because• Cover large portion of the Earth’s surface • Radiative effects• Water vapor budget • Cloud feedbacks
ISCCP day-time ice cloud amount
Earth
Albedo Effect
GreenhouseEffect (dominant)
Ice cloud: not well understood
Duane Waliser et al. 2009 JGR
Satellite-base remote sensing of ice cloud properties
Satellite remote sensing
In-situ measurements Scattering model
microphysics
GCMs
Ice Particle Model
Ice particle model
• Size distribution • Shape distribution• Orientation• Inhomogeneity & surface roughness
Ice Particle model
Ice particle size
• Size matters• Cloud life time (e.g., Heymsfield 1972, Jensen et
al.1996)• Cloud reflectance, radiative forcing,
heating/cooling rate (e.g., Ackerman et al. 1988; Jensen et al. 1994 )
• Cloud feedback (e.g., Stephens et al. 1990)Hard to measureShattering of large particlesGardiner and Hallett 1985; Gayet et al. 1996Field et al. 2003;
Earth Observing Laboratory NCAR
Num
ber d
ensit
y
Particle Size50 µm µm mm
Ice particle shape
• Why shape also matters?
wavelength
wavelength
Aerosol
Ice particle
From Bryan BaumComplicacy of ice particle shape must be acceptable by scattering models
Capabilities of current scattering models
Ice particle orientation
Horizontally orientated
Randomly orientated
Images from www.atoptics.co.uk
Ice particle orientationHorizontally orientated
Image credit: CNES
Inhomogeneity and surface roughness
Yang et al. 2008 JAMC Yang et al. 2008 ITGRS
Ice particle model
• Size distribution • Shape distribution• Orientation• Inhomogeneity & surface roughness
Ice Particle model
So many things to consider…not surprising that ice particle models are usually different from one another
Ice particle models: MODIS C5
Baum et al. 2005 JAMC• More than 1000 PSDs• Complicate habit/shape distribution• Random orientation• Homogeneous and smooth
Ice particle model: MODIS C5
Baum et al. 2005 JAMC
Baum et al. 2005 JAMC
IWC from MODIS C5 ice particle mode is consistent with in situ measurement
Ice particle model:POLDER
Inhomogeneous Hexagonal Monocrystal
Courtesy ofJerome Riedi
• Constant size (30µm)• One habit only• Random orientation• Internal inclusion of air bubbles
Scattering signature consistent with POLDER observationC.-Labonnote et al. 2000 GRL
Scattering phase function Baum05 VS IHM
Comparison of MODIS and POLDER ice cloud retrieval
Motivation
• How are MODIS and POLDER ice cloud retrievals different?
• What is the role of ice particle model?
• Any implications for climate studies?
• Is it possible to build up a long-term ice cloud property dataset from multiple missions?
Zhang, Z.et al. 2009: Atmos. Chem. Phys., 9, 1-15. (www.atmos-chem-phys.net/9/1/2009/)
MODIS POLDER
Resolution 1km 20kmCloud effective radius Retrieved AssumedIce particle model Baum05 IHMDirectionality Single Up to 16
Case for comparisonAqua-MODIS granule on July 22, 2007 (UTC 18:45)
NASA Langley TC4 team
Flight track
GOES IR image
Flight track of TC4 mission
CollocationCollocation of Level-1 radiance data
Collocation of Level-2 cloud products
6km6k
m
POLDER full resolution pixel
MODIS 1km pixel
POLDER 20km downscale to 6kmMODIS 1km aggregated to 6km
6km
6km
POLDER full resolution pixel
MODIS t vs POLDER t
tPOLDER/tMODIS follows the log- normal distribution
tPOLDER is substantially smaller than tMODIS
For more than 80% pixels tPOLDER < tMODIS
For more than 50% pixelstPOLDER < tMODIS by more than
30%
Same clouds; different t?Why?
Main reason for the difference
• Difference in resolution (Plane parallel albedo bias) ✗
• Difference in effective radius treatment ✗• Difference in ice particle model✔
R ~ (1−g)τ τ retrieval ~ Robs / (1 − g)
τ POLDER
τ MODIS~
1 − gBaum05
1 − g IHM= 0.7126 (0.6827)(From data: 0.68)
Implications for ice SW CRF Zonal mean ice optical thickness vs month (2006)
Implications for ice SW CRF
Instantaneous Shortwave CRF (FSW)
Implications for ice SW CRF
Wrong ice particle model
Wrong t retrieval
Wrong g used
“Not so wrong” FSWFSW ~ R ~ (1−g)τ
retrieval
FSW computation
Error cancellation
Ice particle model and seasonal variation of t retrieval
τ cIHM
τ cBaum05
IHM model is used for MODIS retrievalBaum05 model is used for MODIS retrieval
Difference in higher-order moment of P11
Difference in g
Angular signature of ice cloud reflectance
Angular signature is mainly determined by single-scattering
SatelliteSingle-scatteringMultiple-scattering
MODIS angular sampling
winter θ0
θs
θ0
θs
summer
MODIS angular sampling vs season
winter
summer
Impact on seasonal variation of t retrieval
winter
summer
Assume IHM to be the truth
Summary• The t of ice clouds retrieved from POLDER is substantially
smaller than that from MODIS retrieval. • This difference is mostly attributed to the difference in ice
bulk scattering models used in MODIS and POLDER retrievals• If a wrong bulk scattering model is used in the retrieval
algorithm, the error in g factor may lead to overestimation or underestimation of t . However, this error in t retrieval is largely cancelled in FSW computation by the error in g factor.
• The error in higher-order moment of P11 may lead to artificial seasonal variation of t and this error can NOT be cancelled in FSW computation
Questions?