1. Water-vapor contamination on Prelaunch SRF measurements
Na Xu, Xiuqing Hu
National Satellite Meteorological Center(NSMC), CMA
March 22, 2017
2017 GSICS Annual Meeting ,Wisconsin,US
March 19~24, 2017

2. Outline Background Prelaunch spectral measurement method
Assessement of SRF contamination impact and sensitivity to Lab ambient
Atmospheric contamination Correction to FY-2D WV SRF
Summary

3. Background Prelaunch measurement contamination
Prelaunch SRF of WV channels has been found to be contaminated by atmospheric absorption ,as the structures are similar to those shown by the atmospheric transmission
Prelaunch measurement contamination
Bréon et al (1999)
GMS-5
After correction
Atmospheric Contamination on the Measurement of the Spectral Response
not new problem
ilogical, in the past, fluctuant,
7.2μm（FY-3D MERSI）

4. Prelaunch spectral measurement of FY imagers
A monochromator and standard light sources system, monochromatic light change to parallel light from the collimator
The optical path length is increased in collimator
Lab ambient
collimator
standard light sources
monochromator
vacuum
imager

5. Assessement of SRF contamination impact and sensitivity to Lab ambient
Assessement based on sensitivity experiments
True (Ideal supposed) and contaminated SRF construction acording to FY-2 water vapor channel specification
True (Ideal supposed)
Contaminated
Ideal constructed spectral distribution of FY-2 water vapor channel, as well as the simulated top-of-atmosphere spectra of brightness temperature (BT) and transmittance over clear sky
Distribution comparison between SRFs before and after absorption contamination, as well as atmosphere transmittance for homogeneous path of 2m at a temperature 22℃, and humidity of 35%.

6. Assessement of SRF contamination impact and sensitivity to Lab ambient
sensitivity to Lab environment condition
sensitivity to WV column abundance of horizontal optical path
exponential correlation
Variation between TOA brightness temperature biases and water vapor column due to SRF contamination in different atmosphere conditions
Contour maps of TOA brightness temperature biases due to SRF contamination in different atmosphere conditions

7. Atmospheric contamination Correction to FY-2D WV SRF
Divide the original filter by the atmospheric transmission along atmospheric path at surface pressure .atmospheric transmission is estimate use the MODTRAN model .
Spectral resolution is an importance parameter. The simulation results must be spectrally smoothed at a resolution that is compatible with the instrument used for the filter measurement.
Accounting for this correction yield brightness temperatures colder by about 2K.
median filtering
(a)Distribution comparison between FY-2D SRF and atmosphere transmittance for homogeneous path of 1m at a temperature 25℃, and humidity of 35%. (b) Distribution comparison between FY-2D SRFs before and after atmospheric contamination correction.

8. Summary
Atmospheric contamination on prelaunch spectral responsemeasurement of FY water vapor channels is common phenomenon.
The SRF contamination shows great influences on the brightness temperatures calculations indicated from sensitivity testing. It should be corrected by deriving the original filter by the inferred atmospheric transmission during the measurement.
Either a drier environment (or a vacuum) or a shorter pathlength should be used for the SRF characterization.