Introduction

Brightness temperature comparisons between MTSAT and the AVHRRs on the NOAA satellites are operationally conducted once a day. The comparisons are performed for the infrared channels between MTSAT IR1 and AVHRR channel 4, IR2 and channel 5, and IR4 and channel 3B. The spectral response functions of these channels are shown in the charts below. The data used in the comparisons are MTSAT HRIT data and AVHRR GAC data obtained from the FTP server of NOAA/NESDIS via the Internet.

• Notes:
  • The SRFs of NOAA/AVHRR were obtained from the NOAA KLM USER'S GUIDE website.
  • The brightness temperatures shown in the charts are computed using the LBLRTM line-by-line radiative transfer model with the HITRAN2000 line parameter database regarding the U.S. standard atmosphere. The LBLRTM code and the database were obtained from the Atmospheric and Environmental Research Inc. (AER) Website found at AER's Radiative Transfer Working Group.
    
    

Method of Comparison

Two comparisons are examined. One involves the comparison of brightness temperatures over clear sky and ocean to evaluate calibration differences between the two satellites over high-temperature regions. The other is a comparison over smooth cloud top to evaluate calibration differences over lower-temperature regions. The conditions for selection of the satellite data to be compared are shown below.

• Collocation conditions for clear sky and ocean comparison
  • Domain check: the observation points are between 30°N and 30°S over the ocean
  • Time check: the difference between the observation times is less than 30 minutes
  • Satellite zenith angle check: the secant difference between the satellite zenith angles is less than 3%
  • Unity check: the variation of brightness temperatures for a box of 5 × 5 pixels surrounding an evaluation pixel is within ±0.2 K
    
• Collocation conditions for smooth cloud top comparison
  • Domain check: the observation points are between 30°N and 30°S
  • Time check: the difference between the observation times is less than 5 minutes
  • Satellite zenith angle check: the secant difference between the satellite zenith angles is less than 3%
  • Range check: IR1 brightness temperatures are less than 260 K
  • Unity check: the variation of brightness temperatures for a box of 5 × 5 pixels surrounding an evaluation pixel is within ±3 K