Temperature dependence of brightness temperature difference of AVHRR infrared split window channels in the Antarctic

摘要

One method to identify clouds from NOAA/AVHRR data is to use the difference in brightness temperature of infrared split window channels in the 10μm region. Under the low temperature over the Antarctic continent in winter, it is necessary to detect a slight difference in brightness temperature. In this paper, we investigate the temperature dependence of the brightness temperature difference of channel 4 (10.8μm) brightness temperature (T4), and channel 5 (12 μm) brightness temperature (T5) (T4-T5) of a cloud free scene. T4-T5 is about 0°C at low temperature around -80°C, and gradually increases up to a high of 1°C at high temperature around 0°C. The rates of increase in T4-T5 were almost constant for T4 lower than -40°C. For T4 higher than -30°C, T4-T5 remains almost unchanged. For T4 between -40°C and -30°C, T4-T5 increases rapidly. In order to explain this temperature dependence, the contribution of water vapor and surface emissivity to the difference in brightness temperature was calculated from in situ data using the radiation code MODTRAN. The result is shown below. About the contribution of water vapor, at T4 lower than -25°C, T4-T5 was nearly zero. From about -25°C to 0°C of T4, T4-T5 increases up to near 0.6°C. On the other hand, when the surface emissivity difference between CH4 and CH5 was set to 0.01, T4-T5 increased in all temperature ranges. The rate of increase was almost constant. In the temperature range lower than -40°C, T4-T5 conformed to T4-T5 of satellite data.