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The diurnal variation of tropical ice clouds has been well observed and examined in terms of the occurring frequency and total mass but rarely from the viewpoint of ice microphysical parameters. It accounts for a large portion of uncertainties in evaluating ice clouds' role on global radiation and hydrological budgets. Owing to the advantage of precession orbit design and paired polarized observations at a high-frequency microwave band that is particularly sensitive to ice particle microphysical properties, 3 years of polarimetric difference (PD) measurements using the 166GHz channel of Global Precipitation Measurement Microwave Imager (GPM-GMI) are compiled to reveal a strong diurnal cycle over tropical land (30 degrees S-30 degrees N) with peak amplitude varying up to 38%. Since the PD signal is dominantly determined by ice crystal size, shape, and orientation, the diurnal cycle observed by GMI can be used to infer changes in ice crystal properties. Moreover, PD change is found to lead the diurnal changes of ice cloud occurring frequency and total ice mass by about 2h, which strongly implies that understanding ice microphysics is critical to predict, infer, and model ice cloud evolution and precipitation processes.

Plain Language Summary Along with the rising and setting of the Sun, our Earth's clouds vary between day and night. For tropical ice clouds, there are many evidences showing that the cloud cover and thickness change dramatically during a day (referred to as the "diurnal cycle") but few hints of what happens to individual ice crystals. Using a new spaceborne satellite instrument called Global Precipitation Measurement Microwave Imager, we found that ice crystal size or shape also exhibits a strong diurnal cycle over tropical land but not over tropical ocean. We can further infer that the microscopic variation of ice crystals may be an important reason for the change of ice cloud cover and thickness later on.