The Application of Multi-Channels of EOS/MODIS to Detect Cirrus

摘要

Cirrus clouds exit in the upper troposphere and sometimes extend to stratosphere, as well as other kinds of clouds, exerting a serious influence on the energy budget in the earth-atmospheric system and the climate change on global and regional scale. The Moderate Resolution Imagine Spectroradiometer (MODIS) on the Terra (Aqua) Spacecrafts has channels: 1.38μm, 1.64μm, 3.7μm, 6.7μm, 8.7μm and 11μm. They all can be used to detect cirrus. The 1.38μm channel which is in the water vapor absorption band can screen the solar signals reflected by low and middle clouds, so the solar signals reflected by cirrus(high clouds ) is stronger than that by low and middle clouds; The 1.64μm channel is in the atmosphere window band. The combination of these two channels provides an effective means to distinguish ice and water clouds. In this paper, we used SBDART model to acquire the threshold value of these two channels' reflectivity-ratio that could distinguish cirrus and water clouds. Then, using this threshold value, we detected cirrus from MOD021KM and MOD35_L2 data. At last, we discussed the effect of 1.38μm and 1.64μm channel reflectivity-ratio algorithm. There is general agreement with MOD06_L2 (MODIS Cloud Products) result in the range and shape of cirrus distribution. Comparison of MODIS-based cirrus retrievals with surface-based observations, aircraft-based observations and GMS5/SVISSR data, The result proves that the means using reflectivity-ratio algorithm is better than that using two IR channels (8.5,11μm), which is in practice now. The means presented here lessen the possibility of misjudging water cloud as cirrus effectively.