An evaluation of proxies of polar stratospheric cloud presence based on CALIPSO observations

Due to the relative scarcity of polar stratospheric cloud (PSC) observations throughout either polar vortex, studies of the impact of PSCs on ozone chemistry are often based on proxies for PSC formation derived from models and/or other observations. The most common of these is to approximate the total volume of the vortex occupied by PSCs (VPSC) using the volume of the vortex (VNAT) in which the observed temperature is less than the nitric acid trihydrate (NAT) formation temperature (TNAT). The latter is computed from either assumed (climatological) or measured nitric acid and water vapor mixing ratios. The scope of CALIPSO observations of PSCs allows an unprecedented ability to obtain direct estimates of VPSC throughout the polar winter and spring. Herein, we compare VPSC estimated from CALIPSO data and VNAT computed using Microwave Limb Sounder (MLS) nitric acid and water vapor data. Our results show a highly non-linear relationship wherein VNAT significantly overestimates VPSC. We will discuss uncertainties in our findings and potential alternatives for TNAT as a proxy for PSC occurrence.