Sensitivity Study on the Influence of Cloud Microphysical Parameters on Mixed-Phase Cloud Thermodynamic Phase Partitioning in CAM5

Abstract

The influence of six CAM5.1 cloud microphysical parameters on the variance of phase partitioning in mixed-phase clouds is determined by application of a variance-based sensitivity analysis. The sensitivity analysis is based on a generalized linear model that assumes a polynomial relationship between the six parameters and the two-way interactions between them. The parameters, bounded such that they yield realistic cloud phase values, were selected by adopting a quasi-Monte Carlo sampling approach. The sensitivity analysis is applied globally, and to 208-latitude-wide bands, and over the Southern Ocean at various mixedphase cloud isotherms and reveals that the Wegener-Bergeron-Findeisen (WBF) time scale for the growth of ice crystals single-handedly accounts for the vast majority of the variance in cloud phase partitioning in mixedphase clouds, while its interaction with the WBF time scale for the growth of snowflakes plays a secondary role. The fraction of dust aerosols active as ice nuclei in latitude bands, and the parameter related to the ice crystal fall speed and their interactions with the WBF time scale for ice are also significant. All other investigated parameters and their interactions with each other are negligible (,3%). Further analysis comparing three of the quasi-Monte Carlo-sampled simulations with spaceborne lidar observations by CALIOP suggests that the WBF process in CAM5.1 is currently parameterized such that it occurs too rapidly due to failure to account for subgrid-scale variability of liquid and ice partitioning in mixed-phase clouds.

Why are mixed‐phase altocumulus clouds poorly predicted by large‐scale models? Part 1. Physical processes

“…, ; McCoy et al. , ; Tan and Storelvmo , ] for different clouds. A GCM with a more sophisticated ice microphysics scheme (separate prognostic variables for ice and liquid) had the least supercooled liquid water, less than the models with diagnostic mixed‐phase representation and significantly less than observed. Although a prognostic mixed‐phase scheme is in principle able to capture the correct vertical structure of the layer cloud with supercooled liquid water at cloud top, the glaciation was too rapid and the details of assumptions in the microphysics are important. The presence of supercooled liquid water is very sensitive to the rate at which ice grows by vapor deposition due to the Wegener‐Bergeron‐Findeisen mechanism, where ice grows at the expense of water droplet evaporation.…”

The Role of Thermodynamic Phase Shifts in Cloud Optical Depth Variations With Temperature

“…Thermodynamic phase changes due to the WBF process occur on relatively fast timescales and generally act to decrease τ by growing ice crystals at the expense of liquid droplets; a mixed‐phase cloud can completely glaciate within hours, although they have been observed to exist for days or even weeks (Morrison et al, ). Nonetheless, the longer‐term impact of the WBF process on the climatological partitioning of liquid and ice in mixed‐phase clouds and climate may be substantial (Tan & Storelvmo, ; Tan & Storelvmo, 2019; Tan et al, ).…”

Vertical distribution of microphysical properties of Arctic springtime low-level mixed-phase clouds over the Greenland and Norwegian seas

“…The threshold values are questionable. The standard assumption in climate models is that liquid and ice are uniformly mixed throughout each entire model grid box (with a typical horizontal resolution of 100 and 1 km in the vertical; Tan and Storelvmo, 2016). However, some field measurements (see, among others, Rangno and Hobbs, 2001 or suggest that different pockets of solely water or ice in mixed-phase regions coexist with typical scale of tens of meters.…”