Seasonal migration of cirrus clouds by using CALIOP observations

Cirrus clouds at heights of 10-18 km are investigated for the seasonal migrations based on 3 years observations of CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) in 2008-2012. Past studies have shown cirrus clouds migrate from their winter peaks in the equatorial regions to the maximum latitudes of +/- 30 degrees in the boreal summer as the migration of ITCZ (Intertropical Convergence Zone). However, this general picture does not show detailed distributions by including height, seasons and latitudinal dependences considering cirrus extended height and latitudinal coverages. We are also interested in understanding the occurrences of thin cirrus in the tropical tropopause and formation of all cirrus in the extratropics where little studies about their migration have been made. It is not well know if the summer enhancement in the extratropics is only related to the migration or by other factors. Cirrus distributions are studied in two height groups of low (< 15 km) or high cirrus (> 15 km), and in three regions of EQ (- 15 degrees to 15 degrees latitudes), NH (15 degrees-30 degrees), and SH (- 15 degrees to - 30 degrees) for different seasons. Our results show a major part of cirrus, the low cirrus, migrate to the extratropics in the summer. Like ITCZ, they stay mostly in the north (64%) less in the south (36%) following the same migration pattern. High cirrus cloud is decoupled from ITCZ with the major part showing a boreal winter peak spread to higher latitudes with seasons. In the extratropics, enhanced production of high cirrus can be attributed to the convective production in the summer monsoons. The summer convective production is used to estimate the formation of equatorial high cirrus in terms of rapid convection or slow ascending with a ratio determined.