250 Evaluation of the Physical Properties of Mid-Latitude Cirrus in Six Months of Cloud Resolving Model Simulations over the ARM Southern Great Plains

Wednesday, 9 July 2014
Chen Zhang, University of Washington, Seattle, WA; and A. Muhlbauer and T. P. Ackerman

Cirrus clouds have a strong impact on the radiative energy balance of the Earth's atmosphere. The radiative effect of cirrus clouds is largely controlled by the microphysical (e. g., particle size distribution, ice number concentration, ice water content) and macrophysical (e. g., cloud height, cloud fraction, ice water path) properties of cirrus. In this study, we perform a detailed comparison of the physical properties of mid-latitude cirrus in six months of cloud resolving model simulations with in situ microphysical observations collected during the Department of Energy Small Particles in Cirrus (SPARTICUS) field campaign and observations from the ground-based millimeter-wavelength cloud radar (MMCR) at the Atmospheric Radiation Measurements (ARM) Southern Great Plain (SGP) site.

A synoptic classification technique is employed to objectively define a set of atmospheric states, each representing distinct environmental conditions, and is based on a combination of ECMWF ERA-Interim reanalysis data with 14 years of continuous observation from the MMCR at the ARM SGP site. Analysis of the observations and comparisons with the model simulations are carried out based on the synoptic states, which allows for better understanding the relationships between the microphysical properties of cirrus clouds and the large-scale synoptic conditions.

SPARTICUS observations provide a long-term (Jan – Jun 2010) dataset of in situ microphysical observations collected by aircraft over the ARM SGP site, aiming at enhancing the scientific understanding of microphysical and dynamical processes controlling the evolution and life cycle of mid-latitude cirrus. Bulk microphysical variables are obtained using data from the Two-Dimensional Stereo (2-DS) probe. Other cirrus properties (e. g., cloud fractions, vertical velocities) are evaluated using the AIMMS-20 turbulence probe and the ground-based MMCR observations.

The numerical model utilized in this study is the Consortium for Small-Scale Modeling (COSMO). It is driven with initial and boundary conditions from the ECMWF analysis archive on the model's outermost course resolution domain, which employs a grid spacing of 10km. Simulations on the inner-domains with higher resolutions (4km and 2km grid spacing) are performed using a grid nesting technique. The duration of the simulations is set to meet the SPARTICUS campaign period from January to June 2010.

Probability distribution functions (PDFs) of cirrus properties, including cirrus cloud fraction, ice number concentration and ice water content, demonstrate significant distinctness among different states. Model simulated cirrus cloud microphysical and macrophysical properties show characteristic patterns that are consistent with the observations under these different synoptic conditions. Additionally, simulated cirrus microphysics is sensitive to the model spatial resolution. The preliminary results allow us to link the large scale and meso-scale dynamical processes triggering cirrus cloud formation and the microphysical properties of mid-latitude cirrus.

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