# An Investigation of Optically Very Thin Ice Clouds from Ground-Based ARM Raman Lidars
## 1 Introduction  
Ice clouds are located globally and are recognized to be climatically important [1]. Ice clouds with small optical depths are prevalent spatially and temporally [2; 3; 4; 5; 6; 7; 8]. Optically thin ice clouds are important for accurate cloud net radiative forcing estimation [9; 10; 11; 6; 12]. Many recent investigations have expanded the knowledge of the ice cloud radiative forcing for ice clouds [13; 11; 14; 15; 16], with an increased emphasis on the importance of ice clouds with small optical depths [11]. In particular, [PERSON] et al. [14] found the very thinnest ice clouds to impart a net daytime cooling effect. [PERSON] et al. [17] showed that tropical thin cirrus clouds play an important role in modulating the temperature structure in the tropical tropopause layer [18]. Accurate global ice cloud observations in terms of occurrence and optical depth, therefore, prove crucial to understanding the impact of ice clouds on the climate.  
Satellite-based measurements provide the opportunity for a globally-comprehensive ice cloud understanding. The launch of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on-board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) has offered an unprecedented global representation of ice cloud occurrence and optical depth [19]. Many studies have leveraged CALIPSO's capability to characterize ice clouds, providing new insights into the global ice cloud observations [20; 21; 22; 23; 24; 25; 26; 27].  
The satellite capabilities afforded by CALIPSO's lidar include detecting tenuous clouds that present a challenge to radars [28] and passive sensors [29]. CALIPSO observations show optically thin clouds which were previously difficult to detect [13; 15; 30; 31]. At the lower end of optical depths are subvisual ice clouds (\(\tau\) < 0.03) [32], which CALIPSO observations reveal are distributed globally [20; 25].  
CALIPSO can detect ice clouds with optical depths of 0.01 or less with sufficient averaging [19]. However, optically thin features, including both aerosols and clouds, can still go undetected by CALIPSO [33; 34; 35]. [PERSON] et al. [34] and [PERSON] et al. [35] found that CALIPSO was unable to detect all radiatively significant aerosols. [PERSON] et al. [33] found when comparing observations from aircraft lidar measurements off the Pacific Coast of Central America to CALIPSO that the majority of cirrus with optical depths below 0.01 went undetected by CALIPSO.  
The atmospheric radiation measurement (ARM) program [36] operates ground-based Raman lidars (RL), which have been shown to perform well for ice cloud detection [8; 37; 38; 39]. In particular, the RL's high sensitivity is capable of detecting optically thin ice clouds [8]. Here, we assess ice clouds with an optical depth below 0.01, which are detected by the high-sensitivity RL.  
In this study, the ground-based ARM RLs are used to investigate optically very thin ice clouds (\(\tau<0.01\)) at the ARM Southern Great Plains (SGP) and Tropical Western Pacific (TWP) sites. The overall occurrence, nearby atmospheric environment, temporal structure, horizontal extent and vertical occurrence of optically very thin ice clouds are examined for the RL and CALIPSO. In addition, the potential implications for global cloudiness estimates are explored by leveraging the global CALIPSO cloud observations with high-sensitivity RL observations.  
The ground-based RL and satellite-based CALIPSO datasets are described in Section 2, including a case of a collocated section between the two instruments [8]. The methodology is presented in Section 3. Optically very thin ice clouds from the RL and CALIPSO are analyzed and compared in Section 4. A discussion of the uncertainties in the optical depths is presented in Section 5. The summary and concluding remarks are given in Section 6.