Outstanding Issues Concerning GPS RO Measurements...
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Outstanding Issues Concerning GPS RO Measurements in the
Lower Troposphere
© 2012 California Institute of Technology. Government sponsorship acknowledged.
Chi O. Ao 1, Byron A. Iijima 1, Anthony J. Mannucci 1, Thomas K. Meehan 1, Feiqin Xie 1,2
1 Jet Propulsion Laboratory, Caltech, Pasadena, CA, USA 2 JIFRESSE, University of California, Los Angeles, CA, USA
March 28, 2012
IROWG2, Estes Park
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Three issues impacting retrievals in the lower troposphere & PBL
(mainly over the tropics)
1. Negative Refractivity (N) bias
2. Depth penetration
3. Retrieval nonlinearity (effect of random noise)
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Negative bias below ~ 2 km
Not all profiles reach surface due to ret/qc (worse for low SNR)
Positive bias when SNR is low? [Sokolovskiy et al. 2010]
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Three issues impacting retrievals in the lower troposphere & PBL
(mainly over the tropics)
1. Negative Refractivity (N) bias
2. Depth penetration
3. Retrieval nonlinearity (effect of random noise)
4
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5 Xie et al., GRL, 2010
(N-N
(EC
MW
F))/
N(E
CM
WF)
[%
]
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Causes of Negative N-Bias
1. Critical-refraction (CR) (aka super-refraction or ducting)
– When dN/dz < - 10^6/R ~ -157 per km, there is no unique refractivity solution given the bending angle profile.
2. Negative bending angle bias
– Insufficient tracking depth: The largest bending angle measurements (corresponding to lowest LSA) were not recorded.
– Noise affecting low LSA data.
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Critical Refraction vs. Insufficient Tracking Depth
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Insufficient tracking depth results only in a small bias compared to the CR bias
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8 Xie et al., GRL, 2010
CR Example from COSMIC-RAOB Collocation
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Three issues impacting retrievals in the lower troposphere & PBL
(mainly over the tropics)
1. Negative Refractivity (N) bias
2. Depth penetration
3. Retrieval nonlinearity (effect of random noise)
9
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Minimum Altitude Based on CT Amplitude
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Cutoff heights with current algorithm
Ideal response for SS atm
Refractivity CT amplitude
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Profile depth penetration appears anti-correlated with water vapor abundance
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Zmin gets better with higher SNR, up to a point
30 S-30 N JJA 2007
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Three issues impacting retrievals in the lower troposphere & PBL
(mainly over the tropics)
1. Negative Refractivity (N) bias
2. Depth penetration
3. Retrieval nonlinearity (effect of random noise)
13
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Key Questions
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• Can the results from Sokolovsky et al. 2010 be reproduced using JPL retrieval system? (Algorithm dependence)
• How does the bias vary geographically?
• Given an accuracy requirement for N, what is the desired SNR?
• Can we simulate this effect with end-to-end simulations?
• What filtering/smoothing method (if any) works best in reducing the bias?
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Additive Noise
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No positive bias from low SNR
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RO Simulations with LES Atmosphere (From George Matheou, JPL)
Cumulus Stratocumulus
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Additive Noise: 2D Simulations
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No positive bias from low SNR
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Zonal N vs Lat
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Latitutde [deg]
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Zonal N vs SNR
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• ECMWF (interpolated to COSMIC loc) shows similar SNR dependence to COSMIC!
• This can only be due to sampling differences between high and low SNR occultations.
• Could sampling bias account for the observed low SNR bias in COSMIC?
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Distribution of COSMIC OCC
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Summary
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• Most of the negative N-bias below 2 km is due to critical refraction that occurs mainly over the stratocumulus region. External information is needed to constrain the solution.
• Profile penetration in the lower 2 km over the tropics is still an issue. Higher SNRs and larger bandwidth should help.
• We did not find strong evidence of a positive bias at 3-4 km due to low SNR.