Mixed-phase cloud properties retrieval using MODIS reflectances and ARM NSA ground-based data

 

Authors

Loknath Adhikari — University of Wyoming
Zhien Wang — University of Colorado

Category

Cloud Properties

Description

The Arctic region is characterized by the high occurrence of low-level boundary-layer clouds and mid-level clouds, which often consist of complex three phase cloud systems of liquid, ice, and mixed phase. The coexistence of the different phases of cloud particles within a cloud layer often results in large uncertainties in the retrieved cloud microphysical properties by assuming a single phase (either liquid or ice) cloud layer because of the different refractive indices, particle sizes, and concentrations of ice and water particles. The current MODIS operational retrievals of effective radius (re) and optical depth assume a single water/ice phase clouds. The collocated MODIS retrievals and ARM NSA ground-based measurements show that MODIS-retrieved liquid water paths (LWPs) are systematically higher than ground-based measurements, especially at temperatures colder than -15°C. With ground-based retrievals as truths, we developed a two-layer mixed-phase cloud model (ice layer below water layer) to estimate the liquid phase re and optical depth with MODIS reflectances at 1.24 and 2.13 micrometers and ground-based measured ice properties. The Discrete Ordinate Radiative Transfer (DISORT) model is used to develop a look-up table for reflection function for mixed-phase clouds with different values of liquid water re and optical depth. Ice cloud properties, including effective particle size (Dge) and ice water content (IWC), calculated from measurements at the ARM NSA site are incorporated into radiative transfer calculations to estimate re and optical depth. Retrieved liquid water re and optical depth are validated against the ground-based measurements at the ARM NSA site and show significant improvements. The application of this new mixed-phase cloud retrieval algorithm to global MODIS measurements will be discussed too.