Characterizing Cloud-top Relative Humidity with Respect to Ice in Midlatitude Cirrus Clouds as a Function of Synoptic State

Cirrus clouds have long been known to impact the earth's radiation budget, but characterizing cirrus clouds in climate models has also proven challenging over the years. Climatologies of cirrus clouds have been created based on their vertical and spatial distributions, relative occurrence through the year, microphysical properties, and dynamics driving their formation. However, little is known about how relative humidity (RH), specifically RH with respect to ice (RHI), varies in cirrus clouds under different synoptic regimes. This study attempts to bridge this gap in our understanding by incorporating a sophisticated synoptic classification method with collocated radiosonde and millimeter wavelength cloud radar (MMCR) data at the Atmospheric Radiation (ARM) program's Southern Great Plains (SGP) site. This study characterizes mid-latitude cirrus cloud-top RHI according to a variety of synoptic classifications. The classification methodology was developed by Marchand et al. (Journal of Climate; 2009) for the SGP site, and was updated in 2013 to include 21 classifications (grouped under five parent classifications). Cirrus clouds were identified from the MMCR dataset covering January 2004 through January 2011. Care was taken to ensure no liquid clouds were added to the dataset, and to ensure cirrus clouds formed a sufficient distance away from convection. Using this methodology, 964 cirrus cloud cases were identified. Key cirrus cloud top RHI results include variations of ice saturation according to parent classification and a higher frequency of cirrus cloud occurrence in regimes dominated by southerly flow. RHI distributions in cirrus cloud tops for summer classifications are found to be statistically different from the other parent classifications, suggesting synoptic-scale weather systems as opposed to mesoscale convective systems are more responsible for higher ice saturation in cirrus clouds.