# An Investigation of Optically Very Thin Ice Clouds from Ground-Based ARM Raman Lidars
## 2 Data
### Case Study  
To understand how the optically very thin ice clouds manifest themselves for the RL and CALIPSO observations, we first show a collocated section case, which allows a comparison of the time (and space) evolution of ice cloud column optical depth and vertical profiles of features, including those other than transparent ice clouds. The case is for the collocated section on 18 January 2010 with the RL observations in Figure 1 and CALIPSO observations in Figure 2. CALIPSO passed near the SGP site at around 20:00 UTC. The ice cloud column optical depth for all profiles (including attenuated profiles) are plotted versus time for the RL and versus latitude/longitude for CALIPSO. All feature types detected, including aerosols and liquid clouds, are plotted by time (latitude/longitude) and height for the RL (CALIPSO). The corresponding backscatter is also plotted for all features detected in terms of particulate backscatter for the RL and layer-integrated attenuated backscatter for CALIPSO. The RL profiles reveal the detection of optically very thin ice clouds. The consecutive profiles of small optical depths are directly adjacent to profiles with ice cloud column optical depths exceeding 0.01 and ice cloud-free profiles; i.e., cloud edges. Around 20:30 UTC, the temporal structure of the optical depths indicates a general increase from around 0.001 to 0.1 and then a decrease back to near 0.001. The same structure is observed at around 23:00 UTC with increasing optical depth from below to above the threshold of 0.01. The temporal evolution provides evidence for the clear sky-cloud continuum [46; 47]. The continuum suggests that a transition period exists between clear sky and clouds (aerosols) such that typical binary approaches to clear versus cloudy skies do not adequately capture the atmosphere. The high sensitivity of the RL to detect optically very thin ice clouds provides the capability to observe the clear sky-ice cloud continuum. The CALIPSO-collocated profiles for this case study do not contain optically very thin ice clouds, although CALIPSO does observe optically very thin ice clouds (Figure 3). Nonetheless, there are ice clouds above lower liquid clouds for attenuated profiles, which may go undetected by a ground-based lidar. This highlights the importance of satellite-based lidars to observe high altitude ice clouds without having to pass through low altitude liquid clouds. Additionally, this highlights the importance of considering transparent profiles when comparing satellite-based and ground-based instruments. Figure 1: Raman lidar (RL) retrievals on 18 January 2010 at the Southern Great Plains (SGP) site for collocated profiles with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) (\(\pm\)4 h). (**a**) Ice cloud column optical depths for transparent profiles (red) and attenuated profiles (blue) versus time in UTC. The optically very thin ice cloud threshold (\(\tau_{i,c}<0.01\)) is denoted by the dot-dashed black line. The percentage of transparent ice cloud column optical depths below this threshold compared to all transparent ice cloud column optical depths for this collocated section is labeled to the right of the graph. The percentage of ice cloudy profiles that are transparent and attenuated are given above the graph to the right. (**b**) Feature classification as either liquid cloud (red), rain (purple), horizontally-oriented ice (HOI; brown), aerosol (gray), transparent ice clouds with \(\tau_{i,c}\geq 0.01\) (light blue), transparent ice clouds with \(\tau_{i,c}<0.01\) (dark blue) or attenuated ice clouds (black). (**c**) Particulate backscatter (\(\beta\), expressed as the log, units: km\({}^{-1}\)sr\({}^{-1}\)) for all features detected. Figure 2: CALIPSO retrievals on 18 January 2010 over the SGP site for collocated profiles with RL (within 5\({}^{\circ}\) latitude by 5\({}^{\circ}\) longitude box centered over SGP). (**a**) Ice cloud column optical depths for transparent profiles (red) and attenuated profiles (blue) versus latitude/longitude.