Evaluating and constraining cirrus parameterizations in GCMs with SPARTICUS observations

 
Poster PDF

Authors

Xiaohong Liu — Texas A&M University
Kai Zhang — Pacific Northwest National Laboratory
Jennifer M. Comstock — Pacific Northwest National Laboratory
Minghuai Wang — Nanjing University
Gerald Mace — University of Utah
David L. Mitchell — Desert Research Institute

Category

Modeling

Description

Cirrus clouds composed of ice crystals play an important role in regulating the Earth's radiative budget and influencing the hydrological cycle. Although cirrus clouds are an important player in the global climate system, there are still large uncertainties in the understanding of cirrus cloud properties and processes and their treatments in global climate models. In this study we analyze the in situ cirrus measurements from various research campaigns, including the Small Particles in Cirrus (SPARTICUS) campaign (http://campaign.arm.gov/sparticus/). We use the derived statistical information to evaluate and constrain the cirrus cloud parameterizations in the Community Atmospheric Model version 5 (CAM5) with a focus on ice crystal formation through in situ ice nucleation and growth through water vapor deposition, and cirrus dynamics through vertical velocity. We compare modeled and observed ice crystal number concentration, ice water content, and relative humidity inside and outside cirrus and their covariance with temperature. We also evaluate the simulated vertical velocity in cirrus clouds and examine its impact on ice nucleation and the associated aerosol indirect forcing. Our results indicate that (1) homogeneous ice nucleation on sulfate aerosol may play a dominant role in the ice formation compared to the heterogeneous nucleation in the midlatitude cirrus with temperatures less than -40°C over the ARM Southern Great Plains (SGP) site, as revealed from the analysis of observation data; (2) heterogeneous ice nucleation has the potential to significantly perturb the cirrus cloud microphysical properties and radiative forcing if there is a sufficient number of ice nuclei (IN) in the upper troposphere, and thus a cooling of climate through the cirrus seeding of IN may be physically plausible; and (3) sub-grid vertical velocity is the most important factor for the large aerosol longwave indirect forcing in CAM5 through ice nucleation compared to other global climate models (GCMs).