CONSIDERATIONS ON DAYLIGHT OPERATION OF 1.6-VERSUS 3.7-µm CHANNEL ON NOAA AND METOP SATELLITES

Abstract

The transition from the Advanced Very High Resolution Radiometer (AVHRR)/2 to AVHRR/3 on NOAA polar orbiters was associated with a switching from daylight operations of the 3.7- to 1.6-µm wave band, while retaining 3.7 µm for nighttime operations. Investigations of the daylight applicability of the two channels suggest that the 1.6-µm wave band for daylight operations does not prove to be the better choice, at least for cloud applications. The 3.7-µm wave band is much less affected by surface contamination, and measures more faithfully and unambiguously the particle effective radius near cloud tops. The 1.6-µm radiation penetrates deeper into the cloud, supplying an integrated signal throughout the inner portions of the cloud, including surface contribution. Therefore, a synergetic use of the two wave bands can provide an improved retrieval of cloud microstructure and precipitation than from any of the channels alone. However, when one channel must be selected for the AVHRR/3, 3.7 µm performs much better for these applications. Both wave bands identify equally well microphysical features in the anvils of severe storms. For other applications, such as detection of ice and snow over vegetated surfaces and desert dust aerosols, the 1.6-µm wave band does not present clear advantages with respect to 3.7 µm, except that it can be used directly as is, whereas the 3.7-µm wave band has to be corrected for the thermal emission and water vapor absorption. Anyway, the Moderate Resolution Imaging Spectroradiometer (MODIS) can be used instead for the applications to the relatively slowly changing surface properties, while prioritizing the AVHRR for the faster varying atmospheric applications. Finally, the 3.7-mm wave band is more effective in detecting fog, fires, and hot spots. All these factors need to be considered by the operators of AVHRR/3 making a justifiable choice of the channels for the maximum benefit of the user community.