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Linking Water Vapor Channels of GOES-11 and GOES-12 Imagers Using IASI Hyperspectral Observations Likun Wang 1 & Changyong Cao 2 1. Perot System Government Service 2. NOAA/NESDIS/STAR. Motivation. Global Water Vapor Composite Image from CIMSS.

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Linking Water Vapor Channels of GOES-11 and GOES-12 Imagers Using IASI Hyperspectral ObservationsLikun Wang1 & Changyong Cao21. Perot System Government Service2. NOAA/NESDIS/STAR

motivation
Motivation

Global Water Vapor Composite Image from CIMSS

  • Created from water vapor channels on operational geostationary imagers
  • Important products for upper troposphere water vapor monitoring

2008 SPIE

discrepancy between goes 11 and goes 12

GOES-11

GOES-12

Discrepancy between GOES-11 and GOES-12

2-5 K

warmer

than

GOES11

2008 SPIE

Note that 15 minute observational discrepancy

after limb correction
After limb correction

GOES-11

GOES-12

MOTRAN simulation for

Tropical Atmosphere profile

Limb correction does not help!

Difference is still ~2-5 K!

2008 SPIE

From liu et al. 2008

possible causes for discrepancy
Possible Causes for Discrepancy
  • Radiometric calibration difference:
      • Both have onboard calibration: space view and blackbody view
      • Need to evaluate it using a common standard
  • Spectral response difference
      • GOES-12 has a wider SRF in order to improve signal-noise ratio due to the increased spatial resolution (from 8 km to 4 km)
      • Major root cause

GOES-11 imager CH3

Water

Vapor

Channels

2008 SPIE

GOES-12 imager CH3

From CIMSS website

similar problems for other geostationary imagers spectral difference
Similar problems for other geostationary imagers – Spectral difference

If we can link GOES-11 and GOES-12, the same method can be used to resolve the spectral differences for other geostationary imager water vapor channels.

2008 SPIE

hirs ch12 time series
HIRS Ch12 time series

HIRS/2

10K

HIRS/3

If it works for GEO instruments, the same method can be extended to LEO instruments to resolve their spectral response difference.

HIRS/3

HIRS/2

2008 SPIE

can iasi play as a transfer radiometer to link the goes 11 and goes 12 water vapor channels
Can IASI play as a transfer radiometer to link the GOES-11 and GOES-12 water vapor channels?
  • Launched on Oct. 19 2006
  • Local Solar Time: 09:30 (descending node)
  • Interferometer sounder:
    • 645 – 2760 cm-1 (i.e., 3.6-15.5 μm)
    • spectral sampling interval : 0.25 cm-1
    • 8461 channels
    • No spectral gaps

IASI

AIRS

GOES11 &12

HIRS

  • Two steps:
    • Assessing GOES imager onboard calibration using IASI and tying their calibration to a common reference like IASI
      • IASI-simulations vs. GOES observations
    • Using a wide variety of IASI spectra to simulate GOES-11 and GOES-12 water vapor channels and find how they are correlated.
      • IASI-simulated GOES-11 vs. IASI-simulated GOES-12

2008 SPIE

From CIMSS website

how good is iasi
How good is IASI?

IASI versus SHIS, NASTI

AIRS versus IASI

Water vapor absorption region

2008 SPIE

Courtesy of D. Tobin and D.Blumstein

Courtesy of David Tobin

temporal and spatial collocation
Temporal and Spatial Collocation

Metop-A Orbit

IASI FOR

GOES satellite

Sub-point

IASI pixel (12 km)

Four IASI pixels are averaged to compare with GOES pixels

(~200) falling into the corresponding IASI FOR

Metop-A orbit passes the GOES satellite sub-point several times each month

2008 SPIE

spectral convolution
Spectral Convolution

R is the IASI radiance

S is the GOES imager spectral response function

L is the IASI-convolved GOES imager radiance

i is the channel number

ν is the wavenumber

2008 SPIE

sensitivity testing
Sensitivity Testing

How does the BT difference vary with the scene uniformity?

Stdev/Mean

2008 SPIE

sensitivity testing1
Sensitivity Testing

How does the BT difference vary with the observational time difference?

2008 SPIE

iasi vs goes 11
IASI vs. GOES-11

IASI-GOES : -0.271±0.170 K

Seasonal variation of BT difference between IASI and GOES.

2008 SPIE

iasi vs goes 12
IASI vs. GOES-12

0.202±0.140K

-0.302±0.137K

GOES-12 decontamination on 07/02/07

A GOES-12 imager “decontamination procedure” was performed on 02 July 2007, where certain internal optical components were heated in an attempt to drive off contaminants that had been accumulating for several years.

2008 SPIE

the discrepancy is not caused by the radiometric calibration
The discrepancy is not caused by the radiometric calibration
  • GOES11 & GOES12 vs. IASI

<= 0.3K at the BT range of 240-260 K with a standard deviation of ~0.2 K

  • The discrepancy is mostly caused by the spectral response difference.
  • IASI simulations can well represent the GOES imager observations.
  • Using IASI spectra to simulate GOES imager radiance to study SRF-difference induced bias.

2008 SPIE

step 2 iasi simulated goes 11 vs iasi simulated goes 12
Step 2: IASI-simulated GOES-11 vs. IASI-simulated GOES-12
  • Using the IASI spectra to simulate the GOES-11 and GOES-12 water vapor channels
  • Same dataset but extend to [70S, 70N]
  • Nadir pixels only
  • One-year IASI spectra
    • 05/2007-08/2008
  • Samples:
    • GOES-11: 12654

2008 SPIE

bt relationship between goes 11 and goes 12 water vapor channels

Y=1.065*X-12.714

R=0.996

Stdev (residual)=0.6 K

GOES11 BT (K)

BT relationship between GOES-11 and GOES-12 water vapor channels

0.6 K (~0.3%) uncertainty when one predicts the GOES-12 BT using the GOES-11 BT in the temperature range of 200-265K.

2008 SPIE

merging observations from goes 11 and goes 12 water vapor channels
Merging observations from GOES-11 and GOES-12 water vapor channels
  • Limb Correction
    • Normalize off-nadir observations to nadir-like ones
  • Tying GOES measurements to IASI calibration
    • Adjust each instrument’s calibration to a common reference
  • Resolving the SRF-difference-induced observation difference
    • Using the linear relationship proposed in the previous slide

2008 SPIE

before and after correction
Before and after correction

Before correction

After Correction

GOES-12

GOES-11

GOES-12

GOES-11

Note that 15-minites observational difference

2008 SPIE

summary
Summary
  • IASI hyperspectral measurements can successfully access the calibration accuracy of narrow- or broad band instruments.
  • IASI hyperspectral measurements accurately resolve the observation difference caused by the spectral response difference.
  • IASI can play as a transfer radiometer to link the observations from different instruments to create consistent, seamless global products or time series.

2008 SPIE