Constraining ice cloud microphysics parameterizations in Community Atmospheric Model version 5 using SPARTICUS measurements

 

Authors

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

Category

Modeling

Description

Cirrus clouds composed of ice crystals play an important role in modifying the global radiative balance through scattering shortwave (SW) radiation and absorbing and emitting longwave (LW) terrestrial radiation. Cirrus clouds also modulate water vapor in the upper troposphere and lower stratosphere, which is an important greenhouse gas. 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, due to the scarcity of cirrus measurements and instrument artifacts of in situ ice crystal number measurements.

The Small Particles in Cirrus (SPARTICUS) campaign (http://campaign.arm.gov/sparticus/) measured routine number concentration and size distribution of small ice particles (i.e., < 50 µm diameter) over the DOE Atmospheric Radiation Measurement (ARM) South Great Plains (SGP) site from January to June 2010. With a new generation of probes designed to minimize artifacts due to shattering, SPARTICUS provides relatively long-term statistics (~150 hours of in situ data) of cirrus cloud measurements, including small ice particle concentrations.

In this study we use the SPARTICUS observations to constrain cloud ice microphysics parameterizations in the Community Atmospheric Model version 5 (CAM5) for the ice crystal formation through ice nucleation and ice crystal growth through water vapor deposition and autoconversion of cloud ice to snow. This is achieved by comparing modeled ice crystal number concentration, ice effective radius, ice water content, and relative humidity inside and outside cirrus, and their covariance with temperature with the statistics from SPARTICUS observations. Model sensitivity tests are performed with different ice nucleation mechanisms (homogeneous versus heterogeneous nucleation) and different vapor deposition coefficients to reflect the parameter uncertainties in cirrus parameterizations. Our results indicate (1) homogeneous ice nucleation may play a dominant role in the ice formation compared to the heterogeneous nucleation in the midlatitude cirrus with temperature less than -40°C over the SGP site, (2) the threshold size of ~250 μm is the parameterization for the autoconversion of ice crystals to snow, and (3) the deposition coefficient of water vapor on ice crystals is between 0.05 and 0.1.