Comparing Modeled Cirrus Radiative Forcing Estimates from Constrained Lidar Extinction Retrievals to Direct Coincident Radiometer Measurements

Cirrus are the most common cloud type in our atmosphere (Stubenrauch et al. 2013), and play a crucial role in modulating Earth’s radiative energy balance. Due to their complexity, cirrus are poorly represented in global circulation models (GCM), and are a significant source of their uncertainty. They are the only cloud type that can have a positive net daytime cloud radiative forcing (CRF) at the top of the atmosphere (TOA), however, depending on their microphysical properties can also have a net cooling affect (Zhang et al. 1999). Lidar instruments provide valuable vertical profiles of cirrus layer extinction and ice water content (IWC), which are incorporated into radiative transfer models (RTM). The NASA Cloud Physics Lidar (CPL) flew in the REThinC (Radiative Effects of Thin Cirrus) field campaign out of Houston, TX in August 2017 and 2018 where it viewed thousands of cirrus cloud profiles collocated with both in-situ microphysical properties and radiometer measurements. The libRadtran (library of radiative transfer, Mayer and Kaylling, 2005) RTM was tasked with performing the radiative transfer simulations initialized with ice cloud parameterizations based on the in-situ crystal habit information, and the CPL retrieved cloud optical depth and IWC to best represent each cirrus layer. Here, the CRF model estimates are analyzed and compared with the coincident radiometer measurements taken during the REThinC science flights. These comparisons provide valuable insight into the deviation of modeled CRF estimates from direct radiometer measurements along with the impact and importance of correctly parameterizing cirrus microphysical and optical properties.