Considerations on daylight operation of 1.6 micron vs. 3.7 micron channels on NOAA and METOP satellites

Beginning with the NOAA-15 spacecraft, launched on May 13, 1998, a channel at 1.6 mm, called 3A, was added to the new generation Advanced Very High Resolution Radiometer (AVHRR/3) on board the National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites. The major purpose of the new channel is for improving snow and ice detection. Channel 3A is time shared with the preexisting 3.7 mm channel, now called channel 3B. Although AVHRR/3 is a six-channel radiometer, as with previous versions of the sensor the limitation of transmitting only five channels simultaneously remains. The spectral span of the AVHRR/3 channels and their main applications are presented in Table 1. Depending on the satellite channel 3A is operated during the daylight portion of the orbit and channel 3B at nighttime, or channel 3B is operated continuously. For example, NOAA-15 is the current backup morning satellite with an equator crossing time of 07:30 Local Solar Time (LST) and operates channel 3B continuously because of the poor solar illumination offered for 1.6 mm operations. The primary operational morning satellite, NOAA-17 which was launched on June 24, 2002, has an equator crossing time of 10 AM and switches between channels 3A and 3B as discussed above. The primary afternoon satellite, NOAA-16 launched on September 21, 2000, operates the 3.7 mm channel, 3B, continuously in support of global fire detection. All three satellites provide High Resolution Picture Transmission (HRPT) for direct readout and local use. The 1.6 and 3.7 mm wavebands can both be used for the following major cloud applications/retrievals: ·Cloud drops effective radius, re (Nakajima and King, 1990; Platnick, 1996; Baum et al., 2000). ·Cloud phase (ice or water) (Hutchison, 1999; Inoue, 2000; Inoue and Aonashi, 2000). ·Discrimination between clouds and ice or snow on the ground (Hutchison, 1999; Inoue, 2000). ·Identification of ice and snow on the ground, and sea ice (Hutchison et al., 1997). The 1.6 mm spectral band, together with the channels in the visible (VIS) spectral range, helps discriminate between small aerosols and desert dust, although a quantitative analysis of the 0.65 mm and 0.9 mm radiances can provide similarly unambiguous discrimination, as in the case of the Moderate Resolution Imaging Spectrometer (MODIS) (King et al., 1992, 2003). The perceived changes brought about by the new sensor can be summarized as follows: ·Solar reflection and thermal emission occur simultaneously within the 3.7 mm waveband and therefore need to be separated for quantitative applications, while the 1.6 mm waveband only detects reflected solar radiation. ·Cloud observations with the 3.7 mm channel are much less affected by surface contamination than those at 1.6 mm, thus the 3.7 mm channel measures more faithfully and unambiguously the effective radius of the particles near cloud tops. Contributions to the measured 1.6 mm radiation come from deeper in the cloud, producing an integrated signal from both cloud top and the inner portions of the cloud, including surface contributions. ·The 3.7 mm radiation field is affected by the atmospheric water vapor, so that the radiances have to be corrected for this effect. In contrast, the 1.6 mm radiation is negligibly affected by atmospheric water vapor. These apparent advantages of the 1.6 mm waveband made it readily applicable for rendering quite informative color composite images that highlight areas of ice and snow on the ground as well as the presence of glaciated clouds. However, as it will be shown here, the 3.7 µm waveband has advantages over the 1.6 mm waveband for several major cloud applications that can be realized by removing the thermal component from the waveband and correcting for the water vapor abs