# An Investigation of Optically Very Thin Ice Clouds from Ground-Based ARM Raman Lidars
## 4 Results
### Optically Very Thin Ice Clouds Adjacent Profiles  
Figure 4 shows the percentage breakdown of transparent ice cloud profiles with ice cloud column optical depths below 0.01 separated into cloud edge, in-between clouds or isolated based on the immediately adjacent profiles for the RL and CALIPSO. The percentage breakdown is constructed for the number of columns in each category counted individually as well as the number of groups of continuous profiles of transparent ice cloud column optical depth below 0.01. For example, if 5 consecutive profiles of ice cloud column optical depths below 0.01 are preceded by a clear-sky profile and succeeded by an ice cloud column optical depth greater than 0.01, the column count for \begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{2}{c}{**SGP**} & \multicolumn{2}{c}{**TWP**} \\ \cline{2-4}  & **RL** & **CALIPSO** & **RL** & **CALIPSO** \\ \hline Total & 479,517 & 168,730 & 36,640 & 157,125 \\ Transparent & 334,675 & 100,640 & 25,958 & 100,652 \\ Transparent, \(\tau_{i,c}\) \textgreater{} 0 & 121,912 & 24,847 & 20,257 & 54,025 \\ Transparent, \(\tau_{i,c}\) \textless{} 0.01 & 28,394 (23\%) & 1000 (4\%) & 4471 (22\%) & 3681 (7\%) \\ Transparent, \(\tau_{i,c}\) \textless{} 0.005 & 21,630 (18\%) & 415 (2\%) & 3026 (15\%) & 1199 (2\%) \\ Transparent, \(\tau_{i,c}\) \textless{} 0.001 & 11,180 (9\%) & 45 (0\%) & 1053 (5\%) & 108 (0\%) \\ \hline \end{tabular}
\end{table} 
Table 1: Number of total available profiles, transparent profiles, and transparent ice cloud profiles (i.e., transparent profiles where ice clouds are detected) for the RL and CALIPSO at the SGP and TWP sites. For the transparent ice cloud profiles, the number of profiles with ice cloud column optical depths greater than 0 (i.e., all transparent ice cloud profiles), less than 0.01, less than 0.005 and less than 0.001 are given. The numbers in the parentheses are the fraction relative to the number of all transparent ice cloud profiles. Data for the RL at the SGP site are from August 2008 to August 2016 and at the TWP site from December 2010 to January 2015. Data for CALIPSO for both the SGP and TWP sites are from June 2006 to August 2016.  
edge would increase by 5 and the group count for edge would increase by 1. The motivation to separate the analysis into columns and groups is for continuous columns to be counted both individually and as a single group because the continuous columns stem from the same time and atmospheric state. For the RL, the cloud edge category has the largest percentage of transparent ice cloud profiles with ice cloud column optical depths below 0.01 over both SGP (-44%) and TWP (-47%) regardless of columns or groups. When combining together the percentages for cloud edge and in-between columns, 66% and 76% of the optically very thin ice clouds are adjacent to optically thicker ice clouds at SGP and TWP, respectively. For the RL, the breakdown percentage is similar in terms of columns and groups. The large percentage of optically very thin ice clouds nearby optically thicker ice clouds provides confidence that the small optical depths are real phenomena and not noise (i.e., false positives). For the isolated category, about 10% more optically very thin ice clouds are observed at SGP than at TWP relative to the total number of optically very thin ice clouds observed. The difference is possibly related to differences between mid-latitude (i.e., SGP) and tropical (i.e., TWP) ice clouds, such as in their formation or horizontal extent (Section 4.3). Figure 4: Percentage breakdown of the transparent ice cloud profiles with ice cloud column optical depths below 0.01 into “edge”, “in-between” and “isolated” for the RL (left) and CALIPSO (right) at the SGP (top two) and TWP (bottom two) sites. “Edge” refers to one of the adjacent profiles with an ice cloud column optical depth above 0.01 and where the other adjacent profile is clear sky. “In-between” refers to both of the adjacent profiles with an ice cloud column optical depth above 0.01.