analysis of d band cloud flag jane hurley anu dudhia graham ewen university of oxford
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Analysis of D Band Cloud Flag
Jane Hurley
Anu Dudhia
Graham Ewen
University of Oxford
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Background• Presence of clouds in the field of view (FOV) of remote
sensing instruments influence observations
• Retrievals can normally deal with small amounts of cloud in the FOV by fitting a continuum term in parallel with that of the retrieved species
Important to recognize and reliably be able to identify presence of clouds
• A couple of common and simple techniques for cloud detection …
Simple Radiance Thresholding (basic)Color Index Thresholding (improvement, as
reduces influence of variations in p and T)
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Colour Indices (CI) in a Nutshell• CIs work on the principle of ratios of mean radiances
between two spectral microwindows which respond differently to cloud.
• CI = Lav MW1 / Lav MW2
Large CI (ie. CI > 4) → cloud freeSmall CI (ie. CI ~ 1) → thick cloud Range of CIs represents range of optical
thicknesses of clouds present in FOV
• Presence of cloud determined by setting a threshold on the CI such that
for all CI > CIthresh → cloud free FOV
for all CI < CIthresh → cloudy FOV
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• Cautions: Definition of CI breaks down above ~ 30 km with
decreased SNRAbove threshold (“cloud free”), cloud can still occur
if the cloud is optically thin or partially filling the FOV …
• MIPAS has CIs defined for 3/5 bands:
Colour Index MW1 (cm-1) MW2 (cm-1) Threshold
CI-A
CI-B
CI-D
788.0 - 796.0
1246.3 -1249.1
1929.0 -1935.0
832.0 - 834.0
1232.3 -1234.4
1973.0 -1983.0
1.8
1.2
1.8
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Issues for Discussion
• Anomalous situations where the D band flags cloud but the A band does not – is it possible that the D band is better suited to detect certain cloud types?
• A correlation between these anomalous events (where the D band flags cloud while the A band doesn’t) and extremely low H2O volume mixing ratio (vmr)
• Day/night difference in behaviour of the D band
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Correlation of Low H2O VMR and D/A Cloud Flagging Anomaly
• Previous work (Remedios) implied a correlation between anomaly and low H2O vmr values (10-10ppmv).
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• These low H2O vmrs are markers for when retrieved H2O vmr is negative → a bad H2O vmr retrieval
• Using arbitrary day (15 August 2003), in keeping with previous work, found that in the range of definition of CI:
if D band flags and A band doesn’t, ~ 8% of the occurrences have low H2O vmr (are bad retrievals)
if have low H2O vmr, ~ 20% of the occurrences have the D band flagged but the A band unflagged.
A WEAK CORRELATION AT BEST. Not trustworthy enough to risk throwing away quite a bit of useful data …
If were to use D band as a filter for poor H2O retrieval, could potentially lose 92% of such points which could be good data
# flagging anomaly / # total points = 353 / 12041 ~ 3%# low H2O vmr / # total points = 173 / 12041 ~ 1.5%
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Day/Night Difference in D Band
• Remedios found that anomaly of D band flagging and A band not flagging occurred for winter daytime points (60-75 S day, 60 – 75 S night, 75 – 90 S day);
• Could not replicate this result – but definitely found concentration of such anomalous winter daytime points in winter south pole;
• Difference in day/night D band spectra discussed later …
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D Band Sensitive Cloud Detector?
• Mostly winter daytime points (but quite a few mid-lat nighttime points as well …)
• Polar points attributable to PSCs (A band close enough to threshold)
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Standard Spectra
• A Band Clear: level zero-magnitude baseline; day/night spectra have same relative shape.
• A Band Cloudy: slanted baseline heightened in magnitude above zero; day/night spectra have same relative shape.
• D Band Clear: level zero-magnitude baseline; day/night spectra have same relative shape.
• D Band Cloudy: slanted baseline heightened in magnitude above zero; daytime spectra exhibit large non-LTE feature at ~ 2350 cm-1.
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Anomaly Spectrum• Low altitude, mid-latitude daytime example of where D
band flags but A band doesn’t. CI-D = 0.94, CI-A = 1.88
• Both spectra exhibit cloudy features – there is cloud, but no surprise since A band quite close to threshold.
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• Pick situations where flagging anomaly occurs but can’t be attributed to cloud – ie. mid-lats, 20 – 30km altitude.
• For a period of 15 July – 15 August 2003, such anomalous points (D flagged/ A unflagged) looked clear in the A band and had strange negative feature in D band at ~ 2350 cm-1. ANOMALY NOT CAUSED BY CLOUD PRESENCE!
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• Thick cloud spectra should limit to Planck blackbody function at brightness temperature
Planck function at appropriate TB should flag as cloud
• Using ECMWF temperature profiles:
• A band reliably flags thick cloud but D band stops flagging cloud at higher altitudes/ lower brightness temperatures
15 km 18 km 22 km 25 km 28 km
CI-D 1.29 1.32 1.28 1.26 1.25
CI-A 1.15 1.17 1.14 1.13 1.12
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• D band moves away from flagging Planck function at colder temperatures.• Suggested threshold of 1.2 would fail for most atmospheric temperatures.
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RFM Atmospheric Models• Reference Forward
Model (RFM) used to simulate clear and thick cloudy atmospheric conditions
• A more realistic model than a blackbody, taking atmospheric absorptions/emissions etc into account
• Using a ‘step function’ cloud …
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• Sample spectra at a tangent height of ~ 15 km for a cloudy atmosphere
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• RFM shows range over which can confidently use CI-A and CI-D
• Clearly D band flag acts unreliably at most altitudes
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General Behaviour of CI-D & CI-A
• A band behaves as expected, detecting cloud at low altitudes (5-20km) and not at higher altitudes (20-30km);
• D band less convincing, detecting cloud even where there should be none from a statistical standpoint;
A band seems a more reliable cloud flag than the D band!!
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Conclusions• Not a high enough correlation between low H2O vmrs
(poor retrievals) and flagging anomaly to warrant using anomaly as a detection mechanism for poor data;
• Day/night difference between D band spectra due to a large non-LTE feature appearing in the daytime at ~ 2350 cm-1;
• D band more sensitive to clouds? Maybe, but probably not – and certainly not reliably so!D band flags cloud where there simply isn’t anyD band doesn’t consistently flag a blackbodyStrange features in anomalous spectra (non-LTE and
negative)
Recommendations: Stick with tried-and-true A band. D band simply not reliable.