Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 3 4 2010 10.5194/amtd-3-3277-2010 http://www.atmos-meas-tech-discuss.net/3/3277/2010/ http://www.atmos-meas-tech-discuss.net/3/3277/2010/amtd-3-3277-2010.html http://www.atmos-meas-tech-discuss.net/3/3277/2010/amtd-3-3277-2010.pdf 3277 3291 2010-08-04 Possible shift of spectral response function of the MODIS 6.8 μm water vapor channel causing a cold bias up to 3 K B. J. Sohn sohn@snu.ac.kr B.-R. Kim S.-S. Lee School of Earth and Environmental Sciences, Seoul National University, Seoul, 151–747, Korea The calibration of the Moderate-resolution Imaging Spectroradiometer (MODIS) 6.8 μm water vapor (WV) channel was assessed by comparing Terra/MODIS measurements with the WV channel brightness temperatures equivalent to Infrared Atmospheric Sounding Interferometer (IASI) measurements for June 2007 and December 2007. IASI spectral information was transferred to the Japanese Multifunctional Transport Satellite (MTSAT)-1R WV channel and then to the MODIS WV channel. Results indicate that the MODIS WV channel is biased low up to 3 K, likely caused by the shift of the spectral response function by about +11 cm⁻¹. This finding is particularly important because the bias of −3 K can cause about +30% relative errors in the retrieval of upper tropospheric humidity (UTH). In other words, the current MODIS measurements may result in higher UTH values, showing relative errors up to +30%. Blumstein,~D.,~Tournier,~B., Cayla,~F R., Phulpin,~T., Fjortoft,~R., Buil,~C., and Ponce,~G.: In-flight performance of the Infrared Atmospheric Sounding Interferometer (IASI) on METOP-A, Proc. SPIE, 66840H, 1–12, 2007. Cao,~C., Xu,~H., Sullivan,~J., McMillin,~L., Ciren,~P., and Hou,~Y.: Intersatellite radiance biases for the High Resolution Infrared Radiation Sounders (HIRS) onboard NOAA-15, -16, and -17 from simultaneous nadir observations, J. Atmos. Oceanic Technol., 22, 381–395, 2005. EUMETSAT: IASI Level 1 product guide, EUM/OPS-EPS/MAN/04/0032, V2E. EUMETSAT, Darmstadt, Germany, (http://oiswww.eumetsat.org/WEBOPS/eps-pg/IASI-L1/IASIL1-PG-0TOC.htm), 2008. Larar,~A M., Smith,~W L., Zhou,~D K., Liu,~X., Revercomb,~H., Taylor,~J P., Newman,~S M., and Schlüssel,~P.: IASI spectral radiance validation inter-comparisons: case study assessment from the JAIVEx field campaign, Atmos. Chem. Phys., 10, 411–430, doi:10.5194/acp-10-411-2010, 2010. Moeller,~C C., Revercomb,~H E., Ackerman,~S A., Menzel,~W P., and Knuteson,~R O.: Evaluation of MODIS thermal IR band L1B radiances during SAFARI 2000, J. Geophys. Res., 108(D13), 8494, doi:10.1029/2002JD002323, 2003. Soden,~B J. and Bretherton,~F P.: Upper-tropospheric relative humidity from the GOES 6.7 μm channel: Method and climatology for July 1987, J. Geophys. Res., 98, 16669–16688, 1993. Soden,~B J.: Tracking upper tropospheric water vapor radiances: A~satellite perspective, J. Geophys. Res., 103, 17069–17081, 1998. Sohn,~B J., Schmetz,~J., Tjemkes,~S., Koenig,~M., Lutz,~H., Arriage,~A., and Chung,~E S.: Intercalibration of the Meteosat 7 water vapor channel with SSM/T-2, J. Geophys. Res., 105, 16673–16680, 2000. Sohn,~B J. and Schmetz,~J.: Water vapor-induced OLR variations associated with high cloud changes over the tropics: A~study from Meteosat-5 observations, J. Climate, 17, 1987–1996, 2004. Sohn,~B J., Park,~H S., Han,~H J., and Ahn,~M H.: Evaluating the calibration of MTSAT-1R infrared channels using collocated Terra MODIS measurements, Int. J. Remote Sens., 29, 3033–3042, 2008. Tobin,~D C., Revercomb,~H E., Moeller,~C C., and Pagano,~T S.: Use of atmospheric infrared sounder high-spectral resolution spectra to assess the calibration of moderate resolution imaging spectroradiometer on EOS aqua, J. Geophys. Res., 111, D09S05, doi:10.1029/2005JD006095, 2006. Xiong,~X. and Barnes,~W.: An overview of MODIS radiometric calibration and characterization, Adv. Atmos. Sci., 23, 69–79, 2006.