UNDERSTANDING AND IMPROVING REPRESENTATIONS OF PROCESSES DETERMINING HIGH-LATITUDE MIXED-PHASE CLOUD PROPERTIES IN GCMS

Abstract

Mixed-phase clouds, which are composed of both supercooled liquid droplets and ice crystals, are ubiquitous over high-latitude regions. The crude representation of cloud processes generally leads to large uncertainties in modeled mixed-phase cloud properties in General Circulation Models (GCMs). In this dissertation, we aim to examine the sensitivity of modeled high-latitude mixed-phase cloud properties to different representations of cloud microphysical processes. Model results are validated against the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) observations and satellite retrievals. First, improved representation of heterogeneous distribution between cloud liquid and ice through modifying the Wegener-Bergeron-Findeisen (WBF) process is investigated in the Community Atmosphere Model version 5 (CAM5). Model results indicate that accounting for this heterogeneous distribution can significantly improve simulated Arctic mixed‐phase cloud properties. Biases in underestimated cloud liquid water mass are largely alleviated. Second, sensitivity of simulated Arctic mixed-phase clouds to introductions of the Classical Nucleation Theory (CNT) ice nucleation scheme, the Cloud Layers Unified By Binormals (CLUBB) parameterization, and the updated Morrison and Gettelman microphysics scheme (MG2) during the development of the DOE Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1) is examined. Results suggest that EAMv1 simulated Arctic mixed-phase clouds are overly dominated by supercooled liquid water and cloud ice water is largely underestimated, which is in dramatic contrast to CAM5. The underestimated ice crystal production from CNT heterogeneous ice nucleation and the missing ice condensate from CLUBB are primarily responsible for the underestimation of cloud ice water content. Last, hemispheric differences in mixed-phase cloud properties are examined between Utqiaġvik and McMurdo using ground-based remote sensing measurements and EAMv1 simulations. The impact of thermodynamics and aerosol on high-latitude mixed-phase cloud difference between two hemispheres is investigated.