Impacts of heterogeneous distribution of cloud hydrometeors on mixed-phase cloud phase partitioning
Submitter
Liu, Xiaohong — Texas A&M University
Area of research
Cloud Processes
Journal Reference
Science
Mixed-phase cloud phase partitioning has substantial impacts on the surface energy budget, air temperature, and regional climate. The Wegener-Bergeron-Findeisen (WBF) process, which determines the growth rate of ice particles at the expense of liquid droplets, is an important process for the partitioning of condensed cloud water in mixed-phase clouds. Although homogeneous distribution of cloud liquid and ice is commonly assumed in the WBF process calculation in global climate models (GCMs), in situ measurements indicate that supercooled liquid droplets and ice crystals may not be homogeneously distributed inside mixed-phase clouds. As the WBF process is dependent on the spatial distribution of cloud liquid and ice, the heterogeneous cloud structures can therefore largely modulate the mixed-phase cloud phase partitioning.
Impact
This study investigates the impacts of representing heterogeneous distribution of liquid droplets and ice particles on the arctic mixed-phase cloud phase partitioning in the NCAR Community Atmosphere Model (CAM). Results suggest that modeled mixed-phase cloud properties are highly sensitive to the perturbation of the WBF process. A physically based representation of heterogeneous cloud structure in the WBF process can reduce the low bias in simulated liquid water and improve the phase partitioning pattern of simulated arctic mixed-phase clouds. We also find that the heterogeneous spatial distribution between cloud hydrometeors should not only be reflected in one single microphysical process, but also be consistently considered in all parameterizations of cloud microphysical processes, including the snow accretion of liquid drops. The high model vertical resolution is also important for simulating the liquid water maintenance in mixed-phase clouds.
Summary
In this study, three sensitivity experiments with heterogeneous cloud structures represented through different perturbations to the WBF process are conducted using the Community Atmosphere Model version 5 (CAM5). In one experiment, the perturbation factor is based on the assumption of pocket structure, and the second experiment is based on the partial homogeneous cloud volume derived from observational data obtained in the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observation (HIPPO) campaign. In the third experiment, a liquid and ice mass-weighted assumption is used in the calculation of the WBF process to mimic the appearance of unsaturated area in mixed-phase clouds as the result of heterogeneous distribution. Model experiments are tested in both single-column and weather forecast modes and evaluated against observational data from the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) user facility’s Mixed-Phase Arctic Cloud Experiment (M-PACE) field campaign and long-term, ground-based, multi-sensor measurements. Model results indicate that perturbations to the WBF process can significantly modify simulated microphysical properties of arctic mixed-phase clouds. The improvement of simulated cloud water phase partitioning tends to be linearly proportional to the perturbation magnitude that is applied in the three sensitivity experiments. Moreover, this study indicates that heterogeneous distribution between cloud hydrometeors should be treated consistently for all cloud microphysical processes in the model cloud microphysical parameterization. Meanwhile, the high model vertical resolution also improves the modeled phase partitioning of arctic mixed-phase clouds.
Liquid water path (LWP) from single-column model (SCM) simulations compared with the ground-based observation at ARM's North Slope of Alaska Barrow site during the M-PACE campaign. Grey crosses and straight lines represent remote-sensing retrievals of LWP and LWP one standard deviation, respectively. CTL is the default experiment assuming the homogeneous distribution in the WBF process; WBF_TS16 is the experiment with pocket structure; WBF_HIPPO uses the partial homogeneous cloud volume derived from the HIPPO campaign; WBF_MSWT is the experiment using liquid and ice mass-weighted water vapor mixing ratio treatment in the WBF process calculation.
Vertical profiles of cloud fraction, cloud liquid water content, and cloud ice water content averaged between 1200 UTC 9 October and 1200 UTC 10 October during M-PACE. Black dashed lines are for ground-based observations. Solid black lines represent the CTL, which show the default model substantially underestimates the cloud liquid water content. Solid blue lines are for WBF_TS16. Solid green lines are for the experiment (WBF_TS16_ACC) with a consistent treatment of heterogeneous cloud structure in the snow accretion processes in addition to WBF_TS16. Dashed lines are for two sensitivity experiments with different model vertical resolutions, as red for 60 vertical layers (WBF_TS16_ACC_L60) and blue for 120 vertical layers (WBF_TS16_ACC_L120). The increased model vertical resolutions improve the simulation of cloud liquid and ice water contents and thus phase partitioning in mixed-phase clouds, compared to the model experiment with 30 layers (WBF_TS16_ACC). The modeled cloud base and top are still not correct due to too wet large-scale forcing in the boundary layer used to drive the model simulations.