Simulations of cirrus clouds using an explicit cloud model: integrating ARM water vapor and forcing data for analysis of cirrus microphysical properties

Understanding the atmospheric conditions required to initiate cirrus formation and produce observed microphysical properties is crucial to improving the representation of cirrus clouds in climate models. Ice formation in cirrus generally occurs at cold temperatures (below -30 „aC) and can take the form of either homogeneous or heterogeneous nucleation. The ice supersaturation required for ice formation is smaller for heterogeneous than homogeneous nucleation of unactivated aqueous aerosols, which requires an ice saturation as high as high as 1.45 to 1.6 at most cirrus temperatures. Although temperature and humidity are linked to ice formation, macroscopic factors, such as large-scale forcing and horizontal advection, also influence cirrus formation and evolution.

We use a 1-dimensional (1D) cloud model with an explicit microphysical scheme that treats both heterogeneous and homogeneous nucleation, to simulate cirrus clouds observed over the Department of Energy Atmospheric Radiation Measurement program's (ARM) Southern Great Plains (SGP) Climate Research Facility (CRF). Both ice crystals and H2SO4 aerosol particles are binned according to mass, allowing us to reconstruct the particle size distribution at each atmospheric level. The 1D nature of the model allows us to simulate a column of air with high vertical resolution.

A key factor in our simulations is the integration of water vapor profiles measured by the ARM Raman lidar located at the SGP site. The Raman lidar provides continuous profiles of water vapor mixing ratio up to ~12 km during nighttime at ~10 minute intervals, as compared with 6-hourly radiosonde profiles. We also couple the model with large-scale forcing data obtained through constrained variational analysis. This allows us to input a variable profile of vertical velocity and account for horizontal advection of water vapor and dry static energy. Direct radiative effects on ice crystal growth have also been included.

Through a series of model simulations, we will examine the value of using the ARM water vapor profiles for model initialization, and the effects of the large-scale forcing analysis on our simulations of cirrus microphysical properties. Using the model predicted particle size distribution, we calculate the equivalent radar reflectivity and extinction coefficient at 387 nm, and compare directly with ARM Millimeter Cloud Radar (MMCR) and Raman lidar observations. Through these comparisons, we will also discuss the contribution of small particles to the radar reflectivity and extinction observations using the simulated size distributions as a guide.