microwave radiative transfer in support of cloud and precipitation assimilation · 2016. 1. 23. ·...
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Microwave radiative transfer in support of cloud and precipitation assimilation
Ralf BennartzAtmospheric and Oceanic Sciences University of Wisconsin - Madison
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Outline
Fundamentals
Microwave RT for data assimilation
o Microwave properties of clouds and precipitationo Radiative transfer solvero How important is scattering? o Integration with NWP models
Status
Recommendations
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Fundamentals
H2O
H2OO2
O2
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Fundamentals
Fundamental observable: attenuation due to
liquid and rain column
Precipitation ice scattering: Increasing
effect at higher frequencies
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Modeling Chain
NWP/Cloud physics model
Optical properties model
RT-solver
Integration with NWP (e.g. overlap)
Simulated Tb
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Microwave Optical Properties Assessment
Spheres (Mie theory) JCSDA (snow, graupel, hail) Surussavadee and Staelin (2006) Dielectric mixing rules
Non-spherical (Discrete Dipole Approximation - DDA) Liu (2004,2008) Kim et al. (2007) Hong (2007)
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Microwave Optical Properties Assessment
Hong (2007)
Kim et al. (2007)
Liu (2004,2008)
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DDA
Unrealistic results -
spheres and too spherical
DDA
DDA produces realistic results
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RT Solvers:
Two community efforts:
o RTTOV (Supported by EUMETSAT’s NWP-SAF)
• Non-scattering RT• Two-Stream Delta-Eddington
o CRTM (Supported by JCSDA)
• Non-scattering RT• ADA (Advanced Doubling and Adding)• SOI (Successive Order of Interaction)
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RT solvers accuracy tests
Results from Greenwald
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Uses core CRTM routines
About 10 - 20 % speed increase due to truncated doubling and iteration instead of adding
RT solvers speed tests
Results from Greenwald
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RT solvers accuracy tests
Results from Greenwald
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How important is scattering?
Effective scattering optical depth gives an upper limit for the amount of scattering influencing the TOA radiance field
S (z) kS (z)dzz
TOA
S ,Eff (z) kS (z)eA (z)
z
TOA
dz
0 S,eff (z)S (z)
1
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Integration with NWP models: Overlap
From Geer et al. (2009)
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Going to higher spatial resolution
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15 km
3.5 km
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15 km
3.5 km
Warm Reflection off surface
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15 km
3.5 km
Starting to see direct emission by cloud
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15 km
3.5 km
Plane-parallel
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15 km
3.5 km
Still seeing cloud
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Status
• RT Solvers Various available and integrated into CRTM/RTTOV Work to be done to find good compromise accuracy vs. speed
• Optical properties Active/passive evaluation of various optical property models
ongoing Allows realistic estimate of observation error Inclusion into RT LUTs still outstanding
• Observation error characteristics Realistic estimates of observation error What to do when global NWP models reach resolution of passive
MW sensors or for mesoscale models in general? Abandon overlap for slant models?
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Recommendations
• Short term Various smaller improvements (e.g. assessment of effective
scattering optical depth for model selection, optical propertiesdatabases)
Pave way for models with higher spatial resolution.
• Longer-tem Treatment of higher frequencies, full use of scattering
information (Requires significant advances in interface to cloudphysics)
Error covariances