Correction for the Cirrus Cloud Top Height of MODIS Based on CALIPSO in Beijing-Tianjin-Hebei Region
Cirrus plays an important role in atmospheric radiation. It affects weather system and climate change. Satellite remote sensing has great advantage in cirrus detection, relative to traditional observation. As a passive remote sensing instrument, large deviations are found at thin cirrus cloud top height from MODIS. Comparatively, CALIOP as an active remote sensing instrument can acquire more accurate characteristics of thin cirrus cloud.
There are total 5 years matched data from 2013 to 2017. Four years data were used as training data for modeling, and one year was used as testing data for validation. Linear regression and cross-validation were used to analysis the relationship between cloud top heights of MODIS and CALIPSO and to establish correction model. Based on the model, MODIS cirrus cloud top height was corrected. The results showed that the linear regression of cirrus cloud top height between MODIS and CALIPSO was well. The model got the 0.01 significance level. The probability distribution demonstrated that the cloud top height of CALIPSO was higher than MODIS, which respective highest frequency was around 10～11 km(CALIPSO) and 9～10 km(MODIS). After correction, probability distribution of corrected MODIS was closer to CALIPSO and the deviation between the two is obviously reduced. The distribution range of the difference between MODIS original value and CALIPSO is – 3 ~ 2km, and the peak value is about – 0.8km. After correction, the distribution curve of MODIS cloud top height and CALIPSO difference shifted to the right, with the distribution range of – 2.0-2.5 km, and the peak value concentrated near 0.2 km. The correction effect of cirrus cloud top height varies with cloud top height and cloud optical thickness. The correction effect is obvious at different heights, and the correction range of cirrus cloud in lower layer is larger. For the thin cirrus cloud with small optical thickness, the correction effect was more obvious.
