# An Investigation of Optically Very Thin Ice Clouds from Ground-Based ARM Raman Lidars
## 4 Results
### Optically Very Thin Ice Cloud Vertical Occurrence  
In addition to how often optically very thin ice clouds occur, information on the vertical distribution in the atmosphere is another key component to determine the ice cloud radiative effects [49]. Figure 7 shows the ice cloud occurrence with height for the RL with CALIPSO-like extinction resolution and CALIPSO, separated into optically very thin (\(\tau_{i,c}<0.01\)), thin (\(0.01\leq\tau_{i,c}<0.3\)) and relatively thick (\(\tau_{i,c}\geq 0.3\)) ice clouds. Collocated profiles where the transparent percentages are matched between the RL and CALIPSO are used (Section 2.3). The vertical ice cloud occurrence separated into optical depth ranges are also plotted for the RL and CALIPSO using a lidar ratio from the CALIPSO temperature-dependent effective lidar ratio parameterization [45] divided by a constant multiple scattering factor of 0.8 for SGP and 0.7 for TWP. This is the case that leads to a lidar ratio used for both the RL and CALIPSO, which is consistent with the RL's directly retrieved lidar ratios and CALIPSO's constrained retrievals of effective lidar ratios [8]. These factors (i.e., matching resolution, lidar ratio, and transparent percentages) are considered to ensure a fair comparison, which are among the factors that were used to reconcile ice cloud column optical depths between the RL and CALIPSO [8]. The only factor that is not considered is the higher sensitivity of the RL (i.e., larger cloud fraction), which is largely isolated in this comparison. The differences caused by the RL's higher sensitivity are examined by looking at where the optically very thin ice clouds vertically occur by comparing the solid blue and dashed-blue lines in Figure 7.  
Figure 7: Ice cloud occurrence profiles from CALIPSO (solid red) and the RL with CALIPSO-like extinction resolution (i.e., 8 min/60 m at SGP and 24 min/60 m at TWP) using the best-estimate lidar ratio (solid black) for \(\tau_{i,c}<0.01\) (top), \(0.01\leq\tau_{i,c}<0.3\) (middle), and \(\tau_{i,c}\geq 0.3\) (bottom) over SGP (left) and TWP (right). Also plotted are ice cloud occurrence profiles with lidar ratios determined by the CALIPSO’s Version 4 temperature-dependent effective lidar ratio parameterization divided by a constant multiple scattering factor of 0.8 for SGP and 0.7 for TWP for CALIPSO (blue) and the RL (dashed blue). Only considered are transparent collocated profiles by matching the percentage of transparent profiles from the RL and CALIPSO. Percentage of the distribution represented in each optical depth range are given for the RL, RL with matching lidar ratio between the RL and CALIPSO, CALIPSO and CALIPSO with matching lidar ratio between the RL and CALIPSO. Data are for times collocated between the RL and CALIPSO at the SGP site from August 2008 to August 2016 and the TWP site from December 2010 to January 2015.  
For the thin and relatively thick ice clouds, the RL and CALIPSO largely agree, except over SGP for \(\tau_{i,c}\geq 0.3\). The RL observes slightly more thick ice clouds than CALIPSO. As noted in Figure 3, the RL observes more optically very thin ice clouds at both SGP and TWP. These ice clouds occur at high altitudes, near the height for maximum occurrence of around 10 km and 15 km at SGP and TWP, respectively. Optically very thin ice clouds also occur around 1 km at SGP. Of all optically very thin ice clouds below 2 km at SGP, nearly all (\(\sim\)99.7%) occur between November and March. The wintertime occurrence suggest these optically very thin ice clouds may be precipitating ice such as snow. Overall, the vertical occurrences of optically very thin ice clouds agree with those for tropical (TWP) and mid-latitude (SGP) subvisual ice clouds as presented in Hong and Liu (2015).