WP 2000 Improved Identification of Clouds

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WP 2000 Improved Identification of Clouds 

• Jane Hurley, Anu Dudhia, Don Grainger
• University of Oxford

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Current Cloud Detection

• Colour Index (CI) Method is now used to detect
  cloud
• A couple of caveats however
• microwindows have not been optimized would be
  useful if CI EF
• fails to detect cloud with cloud fraction lt 30

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• Objective
• To create and analyze RFM-simulated cloudy
  spectrum of varying cloud effective fraction EF
  to formulate a new cloud detection method using
  Singular Vector Decomposition SVD.
• Singular Vector Decomposition SVD
• is statistical technique used for finding
  patterns in high dimensional data
• transforms a number of potentially correlated
  variables into a smaller number of uncorrelated
  variables (SINGULAR VECTORS)
• first SV captures the most variance and each
  successive SV captures increasingly less variance
• Idea is to find singular vectors that describe
  clear and cloudy atmospheres and use them in
  cloud detection

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Clear Singular Vectors
RFM-simulated spectrum with EF 0 and 9.0km
tangent height and the corresponding first 8
Clear Singular Vectors SVclear

• Use 2nd half of A band because more sensitive to
  cloud presence

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Need only first few SVs to well represent signal
First 3 SVs capture 90 of total variance
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Cloudy Singular Vectors

• Subtract off mean spectral radiance from Original
  signal
• Use SVclear to do a Least Squares Fit (LSF) on
  Original-Mean signal
• Subtract LSF from Original signal to get
  Cloud-Only signal.

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• Compare with Aerosol signature with same EF in
  the FOV
• SVD-calculated Cloud-only signal
• Aerosol signal

Do SVD on Cloud-only signal to get Cloudy
Singular Vectors SVcloud
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Application to MIPAS data
Use SVclear and SVcloud for given tangent height
to do a LSF to mean-subtracted Original signal
6 km
EF ? 0 ? Non-zero fit coefficient to SVcloud!
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Cloud Detection
Method 1 ?2 Ratio Use SVclear and SVcloud to do
LSF of arbitrary spectrum. Use ?2 error to
measure goodness of fit.
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Method 2 Ratio of Integrated Reconstructed
Radiances Use SVclear and SVcloud to do LSF of
arbitrary spectrum. Reconstruct cloudy and total
radiance using LSF.
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Comparison of Methods

• 4 Methods of Cloud Detection
• Radiance Thresholding
• Colour Index
• SVD ?2 Ratio
• SVD Ratio of Integrated Reconstructed Radiances

Methods applied to RFM Data Percent that
method gets prognosis right
Methods applied to MIPAS 2003 data Percent
agreement between methods
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Future Work
Finish selecting optimal microwindows for use
with Colour Index Method those that best
correlate with EF. Finalize choice of thresholds
for SVD Methods. Compare methods of cloud
detection on large MIPAS dataset against known
databases (ISCCP) etc to see what difference this
makes. Do SVD analysis of ice clouds and
implement this into an identification scheme.
Hopefully will then have a cloud type
identification scheme.
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For RFM-simulated spectra
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• For MIPAS data, not so sharp a distribution,
  obviously but clearly a bimodal distribution
• Can fit a Gaussian to clear peak and set a
  threshold
• thr peak 3st.dev.
• Should pick up 99.5 of cloud, if truly Gaussian

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Cloud Detection

• Method 1 ?2 Ratio
• Use SVclear and SVcloud to do LSF of arbitrary
  spectrum.
• Use ?2 error to measure goodness of fit.
• Fit given spectrum with SVclear ? ?2clear
• Fit given spectrum with SVclear and SVcloud ?
  ?2clearcloud
• Consider ratio of ?2clear / ?2clearcloud
• ?2clear / ?2clearcloud gt 1 for cloudy spectra
• ?2clear / ?2clearcloud 1 for clear spectra

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Application to RFM-simulated spectra
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• Method 2 Ratio of Integrated Reconstructed
  Radiances
• Use SVclear and SVcloud to do LSF of arbitrary
  spectrum.
• Radiance of cloud LSVcloud mean(Si (fit coeff)i
  SVcloud i), where i ranges over the cloudy SVs
  only and average over spectral points.
• LSVcloud 0 for clear spectra
• LSVcloud gt 0 for cloudy spectra
• Total radiance LSVall mean(Si (fit coeff)i
  SVi), where i ranges over all SVs and average
  over spectral points.
• Consider ratio LSVcloud / LSVall
• LSVcloud / LSVall 0 for clear spectra
• LSVcloud / LSVall gt 0 for cloudy spectra