# An Investigation of Optically Very Thin Ice Clouds from Ground-Based ARM Raman Lidars
## 6 Summary  
Optically very thin ice clouds are analyzed from ground-based Raman lidars (RLs) at the atmospheric radiation measurement (ARM) program Southern Great Plains (SGP) site for 8 years and at the ARM Tropical Western Pacific (TWP) site for 4 years. Also analyzed are optically very thin ice clouds from the space-borne CALIPSO for 10 years at the ARM SGP site, ARM TWP site and globally. The overall occurrence, nearby atmospheric environment, temporal structure, horizontal extent, vertical occurrence and the global cloudiness implications of optically very thin clouds (i.e., ice clouds with ice cloud column optical depth less than 0.01) are investigated. A case for a collocated section between the RL and CALIPSO was compared for the time (and space) evolution of ice cloud column optical depth and vertical profiles of all features detected. The RL temporal progression of the ice cloud column optical depth revealed a clear sky-cloud continuum that has been previously noted for aerosols and other cloud types [46; 47]. The case comparison emphasized the importance of only considering transparent profiles when comparing ground-based and space-based instruments. Additionally, the case study highlighted the importance for space-based lidars to observe ice clouds above liquid clouds as well as the importance of ground-based instruments with higher sensitivity to detect optically very thin ice clouds. CALIPSO was shown to detect significantly fewer ice clouds than the RL, particularly, optically very thin ice clouds, when considering transparent profiles. Transparent optically very thin ice clouds are separated into 3 categories based on the preceding and succeeding profiles: \"cloud edge\", \"in-between\" and \"isolated\". RL and CALIPSO observations both indicate that a majority of optically very thin ice clouds are directly adjacent to optically thicker ice clouds. CDFs of ice cloud optical depth for optically very thin ice clouds showed that the mean and distributions were lowest for \"isolated\", highest for \"in-between\" and closely followed by the total optically very thin ice cloud distribution for \"cloud edges\" from the RL. CALIPSO indicated little difference between the 3 categories in terms of means and distribution. The horizontal extent of the optically very thin ice clouds for both the RL and CALIPSO was calculated. The mean horizontal extent for all optically very thin ice cloud groups observed by the RL (CALIPSO) is 25 (14) km and 56 (21) km for SGP and TWP, respectively. The smaller horizontal extent for CALIPSO compared to the RL aligns with the higher sensitivity RL observing smaller optical depths. The vertical occurrence of optically very thin, thin (\(0.01\leq\tau_{i,c}<0.3\)) and relatively thick (\(\tau_{i,c}\geq 0.3\)) ice clouds was compared for collocated profiles where the transparent percentages were matched between the RL and CALIPSO. The optically thin and thick ice clouds reasonably agreed when the resolution and lidar ratio were also consistent. The optically very thin ice clouds disagreed near the height of maximum ice cloud occurrence of 10 km and 15 km for SGP and TWP, respectively. Additionally, disagreement was found around 1 km at SGP. Global cloudiness was estimated from the RL and CALIPSO observations. All clouds for all profiles considered indicate an average cloud fraction difference between the RL and CALIPSO at SGP and TWP of +0.13 and a CALIPSO global cloud fraction of 0.66. Ice clouds for transparent profiles estimate an average cloud fraction difference of +0.17 between the the RL and CALIPSO at SGP and TWP with a CALIPSO global cloud fraction of 0.36. This study shows that optically very thin ice clouds are prevalent and may increase the global cloud fraction estimate from those derived from satellite-based instruments. Higher-sensitivity ground-based instruments provide measurements of optically very thin ice clouds undetected by space-based instruments. Space-based instruments provide global spatial coverage unparalleled by ground-based instruments.