Investigating the Impact of Updraft Velocity on Cirrus Cloud Properties Using the CAM5 Model Constrained with Field Measurements

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. Updraft velocity in cirrus clouds determines the cooling rate for ice nucleation and thus influences the number concentration of ice crystals. It also plays an important role on the in-cloud supersaturation for ice crystal growth and thus the ice water content in cirrus clouds. However, there are still large uncertainties in the understanding of vertical velocity and its variability in cirrus cloud and its parameterization in global climate models, due to the scarcity of cirrus measurements. The DOE Atmospheric Radiation Measurement (ARM)'s Small Particles in Cirrus (SPartICus) campaign and the NASA's Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) conducted airborne measurements over central North America with special emphasis to investigate the properties of mid-latitude cirrus clouds, the processes affecting these properties and their impact on radiation. In this study we use the SPartICus/MACPEX observations to constrain and improve the parameterization of in-cloud subgrid updraft velocity in the Community Atmospheric Model version 5 (CAM5). Aircraft measurements of updraft velocities are mapped to 0.5x0.5 and 1x1 GCM grids to derive the sub-grid probability distribution frequency (sub-grid PDF) of velocities. The Liu and Penner (2005) ice nucleation scheme is called with each velocity sample inside a grid box to obtain the grid-box mean number of nucleated ice crystals. The characteristic updraft velocity (w*) for each grid box is inversely estimated by matching the grid-box mean ice crystal number. The relationship between w* and the mean and the standard deviation of sub-grid PDF of velocities inside a grid box is used to improve the current parameterization of w* in CAM5. Sensitivity tests with CAM5 are performed to assess the impact of parameterization of subgrid updrafts on cirrus cloud properties and radiative forcing through cirrus clouds.