Isotopic constraints on moist processes over the tropics in NASA GISS ModelE2
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Isotopic constraints on moist processes over the tropics in NASA GISS ModelE2
Robert Field, Daehyun Kim, Gavin Schmidt, John Worden, Allegra LeGrande, Max Kelley
JPL
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Motivation
Climate projections with ModelE form a major part of the GISS contribution to IPCC.
Are measurements of the isotopic composition of water vapor useful for model evaluation? Can we use them to identify compensation errors in the parameterizations?
What physical processes might they be constraining, especially those that are hard to measure?
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Moisture in lower free troposphere
ERA-I
ModelE
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Model performance across different metrics
Pattern correlation
Ocean Land
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Aura TES HDO/H2O Simulator
Varying TES retrieval quality and vertical sensitivity must be taken into account.
See: Field, R.D., C. Risi, G.A. Schmidt, J. Worden, A. Voulgarakis, A.N. LeGrande, A.H. Sobel, R.J. Healy, A Tropospheric Emission Spectrometer HDO/H2O retrieval simulator for climate models, Atmospheric Chemistry and Physics, 12, doi:10.5194/acp-12-10485-2012, 10485-10504, 2012.
Retrieval quality (%)
Height of peak HDO retrieval sensitivity (hPa)
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Effects of TES operator on modeled δD fieldsRaw model δD (‰)
Change to raw model δD after applying ‘standard’ TES operator.Only works for prescribed meteorology.
Change to raw model δD after applying new TES operator.Works for arbitrary, free-running model configurations.
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δD over lower free troposphere
TES
ModelE with operator
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Pattern correlation
Model performance across different metrics
Ocean Land
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Model performance across different metrics
Pattern correlation
Ocean Land
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q as a constraint on convective recycling
Pattern correlation
Convective recycling ratio
Ocean Land
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δD as a constraint on convective recycling
Pattern correlation
Convective recycling ratio
Ocean Land
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Key points• Isotopic measurements provide a stronger
constraint than moisture amount or precipitation, especially over the ocean.
• They can provide guidance on the fidelity of hard-to-measure processes within the model.
• Next up:– Mechanistic constraints on model variability: MJO & ENSO– Land surface fluxes: evaporation / transpiration
partitioning– Observational error
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Convective moisture recycling• Ratio of re-evaporated to total convective
condensate• Not well constrained
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q and δD over lower free troposphere
HDO concentrations are expressed relative to an ocean water standard, in units of permil (‰).
qERA-I
δDTES
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Capturing variation in vertical sensitivity
More com
plicated categorizationsObs.
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GPCP
ModelE
Precipitation
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TES vs. ModelE δD
δDRaw - δDTES
δDTES
δDRetrAK - δDTES
δDCatAK - δDTES
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Stable isotopes (isotopologues) of water
16O
1H 1H
99.73%
18O
1H 1H
0.20%
16O
2H 1H
0.03%
The heavy isotopes of water evaporate less readily and condense preferentially.
There are now sufficient HDO measurements in the troposphere against which to evaluate models. We are currently working with HDO/H2O retrievals from the Tropospheric Emission Spectrometer (TES).
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