Chapter 3 - Detection of Mixed-Phase Clouds From Shortwave and Thermal Infrared Satellite Observations

Abstract

Multispectral band measurements from satellite radiometers have been widely used to detect and characterize the optical and microphysical properties of global clouds. These passive radiometer observations also provide information about cloud top phase. In situ observations have shown that clouds often contain a mixture of both liquid and ice at temperatures below 0°C. These mixed-phase clouds remain an important source of uncertainty in numerical models and satellite retrieval algorithms. Cloud phase information can be inferred from conventional satellite radiometers using thermal-infrared bands, but the sensitivity is biased strongly toward cloud top due to strong absorption in these bands. In situ observations of many mixed-phase clouds reveal a profile of supercooled liquid water at or near cloud top and a predominantly ice phase residing below, contrary to typical assumptions used in many retrieval algorithms. Based on cloud top sensitivity bias, conventional thermal infrared-based satellite algorithms would identify these clouds simply as supercooled liquid without any further information regarding the presence of a sub-cloud top phase. Here, we introduce a daytime detection method of liquid-top mixed-phase clouds from passive radiometer observations which utilizes reflected sunlight in narrow bands at 1.6 and 2.25μm to “optically probe” within liquid-topped mixed-phase clouds in an attempt to detect sub-cloud top phase. The basis of the algorithm is established on differential absorption properties between liquid and ice particles, accounting for varying sun/sensor geometry and cloud optical properties. The algorithm has been applied experimentally to the S-NPP VIIRS and Himawari-8 AHI, and evaluated based on active sensor measurements from CloudSat and CALIPSO. The results showed that the algorithm has promising potential for distinction between supercooled liquid-top phase clouds with and without an underlying mixed phase component.