PUBLICATIONS. Journal of Geophysical Research: Atmospheres

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1 PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Key Points: Both ceilometer and MPL define the cloud bases for the Arctic mixed-phase cloud The stronger the humidity inversion, the more Arctic mixed-phase cloud in winter When humidity inversion >0.9 g/kg, the mixed-phase cloud occurrence is ~40% Correspondence to: X. Dong, Citation: Qiu, S., X. Dong, B. Xi, and J.-L. F. Li (2015), Characterizing Arctic mixed-phase cloud structure and its relationship with humidity and temperature inversion using ARM NSA observations, J. Geophys. Res. Atmos., 120, , doi: / 2014JD Received 22 DEC 2014 Accepted 16 JUL 2015 Accepted article online 20 JUL 2015 Published online 8 AUG 2015 Characterizing Arctic mixed-phase cloud structure and its relationship with humidity and temperature inversion using ARM NSA observations Shaoyue Qiu 1, Xiquan Dong 1, Baike Xi 1, and J.-L. F. Li 2 1 Department of Atmospheric Sciences, University of North Dakota, Grand Forks, North Dakota, USA, 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Abstract In this study, the characteristics of the Arctic mixed-phase cloud (AMC) have been investigated using data collected at the Atmospheric Radiation Measurement North Slope Alaska site from October 2006 to September AMC has an annual occurrence frequency of 42.3%, which includes 18.7% of single-layered AMCs and 23.6% for multiple layers. Two cloud base heights (CBHs) are defined from ceilometer and micropulse lidar (MPL) measurements. For single-layered AMC, the ceilometer-derived CBH represents the base of the liquid-dominant layer near the cloud top, while MPL-derived CBH represents base of the lower ice-dominant layer. The annual mean CBHs from ceilometer and MPL measurements are 1.0 km and 0.6 km, respectively, with the largest difference (~1.0 km) occurring from December to March and the smallest difference in September. The humidity inversion occurrence decreases with increasing humidity inversion intensity (stronger in summer than in winter). During the winter months, AMC occurrences increase from 15% to 35% when the inversion intensity increases from 0.1 to 0.9 g/kg. On the contrary, despite a higher frequency of strong humidity inversion in summer, AMC occurrences are nearly invariant for different inversion intensities. On average, humidity and temperature inversion frequencies of occurrence above an AMC are 5 and 8 times, respectively, as high as those below an AMC. The strong inversion occurrences for both humidity and temperature above an AMC provide the moisture sources from above for the formation and maintenance of AMCs. This result helps to reconcile the persistency of AMCs even when the Arctic surface is covered by snow and ice American Geophysical Union. All Rights Reserved. 1. Introduction Recent studies show that the Arctic is the most sensitive region in terms of climate change and Arctic clouds have exerted significant influences on the radiative flux and surface energy budget in addition to playing a role in climate feedback mechanisms [e.g., Garrett and Zhao, 2006; Bennartz et al., 2013]. Representing Artic clouds and their climate feedback in global climate models (GCMs) remains a pressing challenge to reduce and quantify uncertainties associated with climate change projections [Xie et al., 2013; Barton et al., 2014; English et al., 2014]. Among all cloud types, the Arctic mixed-phase cloud (AMC) is the most frequently observed cloud type over the Arctic [Shupe, 2011]. AMC exhibits a large influence on both the radiative and energy fluxes [Shupe and Intrieri, 2004] and has complex interaction with the surrounding environment through entrainment, evaporation, precipitation, and local dynamics [Morrison et al., 2012]. Due to these complexities, the formation, persistence, and dissipation processes of AMC are not well understood. In order to correctly simulate AMC, it is necessary to characterize its vertical thermodynamic structure and to understand the physical processes associated with its formation mechanism. Previous studies [Dong and Mace, 2003; Wang et al., 2004; Shupe et al., 2006, 2008; de Boer et al., 2009] reveal the unique structure of AMC with a thin liquid-dominant layer at the top and ice particles developing into the lower layer. This unique feature can be clearly observed by radar reflectivity profile and lidar backscatter profiles. Owing to different cloud base retrieval algorithms among lidars, however, they may detect different layers within a single-layered AMC. In this study, we find that a standard ceilometer can detect the base of a liquid-dominant top layer within a single-layered AMC. A low-powered micropulse lidar (MPL) [Spinhirne, 1993], on the other hand, can detect the base of the ice-dominant bottom layer. With the combination of these two instruments, detailed AMC boundary and vertical structure can be characterized and used to validate both model simulations of AMC and the satellite observations. QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7737

2 The surprising resilience of AMC has also been studied widely after Pinto [1998] pointed out this phenomenon, and different hypotheses have been made to explain the persistence [Morrison et al., 2012]. Solomon et al. [2011] found that the high occurrence of both temperature and humidity inversions over the Arctic, especially the coincidence of both inversions near the top of AMC, can serve as a moisture source for AMC formation through cloud top entrainment and could be one of the reasons for its persistent nature. Solomon et al. [2014] further investigated the influence of relative location of the humidity inversion on AMC from a case study. However, these conclusions were based on model simulations and a case study. It is thus necessary to validate their conclusions using long-term ground-based observations. In this study, the ground-based measurements at the Atmospheric Radiation Measurement (ARM) North Slope Alaska (NSA) have been collected and analyzed (detailed site information refer to Stamnes et al. [1999] and Stokes and Schwartz [1994]), providing a nearly continuous, long-term, and high-resolution observation for both cloud and environmental properties. We will first define the cloud boundaries for the AMC using on-site lidar data sets, study the characteristics of AMC as observed by these different instruments, and finally investigate the relationships between humidity/temperature inversions and AMC occurrence. Through an integrative analysis of 3 year observations at the ARM NSA site, we attempt to answer the following three scientific questions in this study: 1. What are the characteristics of AMC and what are the similarities and differences in describing AMC properties by different instruments? 2. What is the impact of humidity inversion intensity on the AMC? Does the AMC occurrence increase with stronger inversion? 3. What is the impact of the relative location of humidity and temperature inversions on the AMC? 2. Instruments and Methodology Measurements used in this study include the normalized backscatter (sr 1 km ) from the ceilometer and MPL [Campbell et al., 2002], depolarization ratio from the polarized MPL [Sassen, 1991; Intrieri et al., 2002], reflectivity from the millimeter wavelength cloud radar (MMCR) [Moran et al., 1998], cloud liquid water path (LWP) from the microwave radiometer [Liljegren et al., 2001], and temperature and humidity profiles from the ARM Merged Sounding value-added product [Troyan, 2012]. The Active Remote Sensing of Clouds (ARSCL) data [Clothiaux et al., 2000] at the NSA site are available from March 1998 to March 2011, and the polarized MPL data are available since 28 September However, the MPL algorithm for deriving the cloud base height (CBH) and cloud fraction (CF) in the ARSCL data over the NSA site has been changed after September 2009, and the monthly mean values for the MPL-derived CF are only half of the climate mean values. Therefore, this study will focus on the time period from October 2006 to September Both ceilometer and MPL measurements are commonly used for deriving CBH due to their sensitivity to hydrometer number concentration and the ability to distinguish precipitation below cloud base [Dong and Mace, 2003; Dong et al., 2005]. CBH retrievals from these two instruments depend on their retrieval algorithms as well as the corresponding definition of clouds. The ceilometer CBH used in ARSCL data retrievals is calculated by a built-in algorithm developed by the manufacturer [Flynn, 2004]. The CBH retrieval algorithm for the MPL data is based on Campbell et al. [1998] and Clothiaux et al. [1998]. The primary difference between these two algorithms is that the ceilometer uses local maxima in backscatter coefficient to define CBH while MPL uses a clear-sky and cloudy sky backscatter threshold difference (33 sr 1 km ). Therefore, ceilometer-defined CBH is frequently higher than the MPL CBH. Furthermore, the ceilometer algorithm has a more strict definition of clouds, in that they are particles reducing visibility less than 100 m to a pilot. Thus, MPL usually can detect more optically thin ice clouds [Turner, 1996; Clothiaux et al., 1998]. Since this study focuses primarily on mixed-phase clouds, the first step is classifying the cloud phase. Shupe [2007] classified Arctic mixed-phase clouds through an integrative analysis of MPL and ceilometer backscatter data, MPL depolarization ratio, and cloud LWP. The existence of liquid cloud droplets is identified by the following criteria: ceilometer/mpl backscatter data >300 sr 1 km and MPL depolarization ratio <0.1, or cloud LWPs >25 g m 2 [Shupe, 2007]. Different from the Shupe s [2007] study where the classification is applied to each cloud pixel, this study only applies the classification to each cloud layer. In addition, the definition of a mixed-phase cloud is the coexistence of both liquid cloud droplets and ice particles in the QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7738

3 same layer. The occurrences of AMC have been calculated in following two categories: single-layered AMC and AMC under multilayer condition when an ice cloud layer is above an AMC. The merged sounding data used in this study are a combination of radiosonde soundings, surface meteorological observation, and European Centre for Medium-Range Weather Forecasts model output. The data define the atmospheric thermodynamic state at a 1 min temporal resolution and 20 m vertical resolution below 3 km [Troyan, 2012]. The inversion layer is identified as being where the specific humidity and temperature increase with height. Inversion depth is the difference between the base and top of each inversion layer, and its intensity is the difference in specific humidity/temperature between the inversion top and base [Nygård et al., 2013]. After each cloud and inversion layer (both humidity and temperature) is identified, the relationship between the mixed-phase cloud and inversion is further classified into two groups: if the inversion base is within or above an AMC and the inversion top is 200 m within the top of an AMC, it is classified as an inversion above the mixed-phase cloud. Similarly, if the inversion base is below an AMC and the inversion top is 200 m within the top of an AMC, it is identified as an inversion below the mixed-phase cloud. 3. Results and Discussions 3.1. A Case Study To demonstrate the temporal and vertical structures of an AMC, an integrative analysis has been performed, combining the ARM ground-based observations from ceilometer, MPL, MMCR, and microwave radiometer on 4 5 March 2008 in Figure 1. As illustrated in Figure 1, a liquid-dominant layer was located at 2 3 km with a MPL depolarization ratio less than 0.1 (Figure 1a) [Sassen, 1991; Shupe, 2007], ceilometer and MPL backscatter data (Figures 1b and 1c) greater than 300 sr 1 km , and LWPs (Figure 1a) greater than 25 g m 2 [Shupe, 2007]. As shown in Figure 1b, the ceilometer-retrieved CBH is identified as the lower boundary of this liquid-dominant cloud layer despite the strong backscatter observed below it. However, the MPL algorithm identified the CBHs by a strong gradient of backscatter data at ~1 km (Figure 1c). The best estimated CBH from the ARSCL data [Clothiaux et al., 2000] (purple dots, Figure 1d) fluctuated between these two. This presents a question of which CBHs should be used to define the lower boundary of an AMC. As discussed in section 2, compared with radar reflectivity measurements (the sixth moment of the particle size distribution), the lidar backscatter profiles (MPL and ceilometer) are more sensitive to particle number concentration (the second moment) rather than particle size, so that we can conclude that the strong gradients of both MPL and ceilometer backscatter data around 1 km indicate the increase in particle number concentration. The MPL-defined CBH corresponds to this increase in particle number concentration. Within the region between the ceilometer and MPL CBHs, most of the MPL depolarization ratios are greater than 0.1 and the backscatter data from both platforms mostly fall between 80 and 300 sr 1 km as demonstrated in Figure 1. Additionally, the Doppler velocities in this region are generally smaller than 1 m/s (figure not shown). Furthermore, Figure 1 does show some of the MPL depolarization ratios as being less than 0.1 (Figure 1a) with the ceilometer backscatter greater than 300 sr 1 km (Figure 1b) in this region. Therefore, we define this region as the ice-dominant cloud layer within an AMC with the base from the MPL CBH and the top from the ceilometer CBH, in addition to the liquid-dominant cloud layer above it. Below the MPL CBH is ice virga falling out of the AMC. The ice virga exhibits a relatively large particle size and low number concentration and is therefore sensitive enough to be detected by cloud radar but not easily detectable with the MPL algorithm. Figure 1d also shows that most of the best-estimated CBHs from ARSCL data (dark purple dots) are the same as those derived from the ceilometer and some of the best-estimated CBHs are the averages of the ceilometer and MPL CBHs. Thus, the boundary of AMC is not clearly defined in the ARSCL data. Based on the previous discussions, we decide to use both ceilometer- and MPL-derived CBHs to characterize an AMC vertical structure in this study. The seasonal variation of CBHs from the two instruments as well as seasonal variation of AMC structure will be discussed in the following section. To further analyze the environmental conditions and to demonstrate the thermodynamic structures for this case, temperature and humidity profiles are presented at two time periods for the single-layered AMC (22:00 UTC on 4 March 2008, as marked by black dashed line in Figure 1d) and AMC under multilayered QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7739

4 Figure 1. An integrative analysis of the Arctic mixed-phase cloud (AMC) on 4 and 5 March 2008 through the DOE ARM NSA ground-based observations. (a) Microwave radiometer-retrieved cloud liquid water path (LWP, ) and micropulse lidar (MPL) depolarization ratio profile (contour) (>0.1 representing ice particles occurrence), (b) the laser ceilometer backscatter profile and determined cloud base height (CBH, ), (c) the MPL backscatter profile and determined CBH ( ), and (d) the millimeter wavelength cloud radar (MMCR) reflectivity profile and CBHs derived from MPL and ceilometer measurements and ARM best estimated one. The black dashed lines in Figure 1d represent the time of soundings in Figure 2. condition (05:00 UTC on 5 March 2008), as shown in Figure 2. The pink dotted line in Figure 2 is the MPL-derived CBH, and the black dotted line is the ceilometer-derived CBH. On 4 March 2008, the surface temperature was ~245 K and a strong temperature inversion was represented below 1km, where atmospheric temperature increased from 245 K at the surface to ~260 K at ~1 km. Along with the temperature inversion, strong humidity inversion also developed and increased from ~0.2 g/kg to ~1.4 g/kg at the same altitude (~1 km) (Figure 2a). The intensity of the humidity inversion (1.2 g/kg near 1 km) is much stronger than the climate mean inversion intensity value in March (~0.15 g/kg; discussed and shown in Figure 6). Due to the sharp increase of water vapor, this layer saturated relative to ice. The MPL CBH effectively denotes this water vapor saturation level, which demonstrates that MPL CBH should be used as the effective cloud base for this AMC. Above the inversion peaks, both temperature and humidity decreased with height, and a slight humidity inversion with strength of ~0.2 g/kg was formed at cloud top. As the Arctic surface is usually covered by snow/ice from September to the following May (or June), there is minimum surface moisture contribution to the AMC, and this moist and warm layer provides the moisture source for the cloud. Figure 2b shows the temperature and specific humidity profiles for AMC under the multilayered condition, where an ice cloud layer developed above the AMC at 05:00 UTC on 5 March Temperature and humidity profiles in Figure 2b were similar to their counterparts in Figure 2a, except for the existence of a humidity inversion with an intensity of ~0.3 g/kg near the lower boundary (~4.5 km) of the upper ice cloud layer. For this case, both the humidity and temperature inversions exhibited below the AMC QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7740

5 Figure 2. Merged sounding profile at Barrow, Alaska: (a) 22:00 UTC on 4 March 2008 and (b) 05:00 UTC on 5 March The red line is the temperature, and the blue line is the specific humidity; the gray shading area represents the cloud layer observed by ARM MMCR in Figure 1d; and the black and pink dashed lines are the cloud base heights defined by ceilometer and MPL, respectively. Figure 3. Monthly means of (a) AMC occurrence and (b) cloud base heights (CBHs) derived from the ceilometer (black) and MPL (pink) measurements at the ARM NSA site during the period October of 2006 September (inversion base below cloud base), and the humidity inversion with the AMC was much stronger than the climate mean value. To further explore the relationship between AMC occurrence with the humidity inversion intensity and their relative location, the 3 year ground-based measurements are analyzed and discussed in the sections Seasonal Variations of AMC Occurrence Frequency and CBH Figure 3 shows the monthly means of AMC frequency of occurrence derived from the ceilometer and MPL measurements, as well as their CBHs at the ARM NSA site. In general, AMC frequency of occurrence is very low during January March, with a minimum of 10% in March. It then increases significantly through May and peaks in October (72.2%). The annual average of AMC frequency of occurrence is 42.3% during the 3 year period, which is close to the result in Shupe [2011]. The seasonal variation of AMC frequency of occurrence follows closely with the QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7741

6 Figure 4. (a) Humidity inversion frequency of occurrences with a range of intensities from 0.1 to 0.9 g/kg for different months. (b) Trend of the occurrence of humidity inversion with increasing intensity for different seasons. seasonal variation of total cloud occurrence [Intrieri et al., 2002; Dong et al., 2010; Shupe et al., 2011; Shupe, 2011] with lower magnitudes. The monthly mean CBHs (Figure 3b) from both the ceilometer and MPL exhibit similar trends throughout the year, and on annual average the ceilometer-derived CBH is 0.4 km higher than MPL. The largest differences between the two CBHs happen during winter months (December to March), with the minimum differences in September. As discussed in the last section, the region between the two CBHs is defined as the ice-dominant cloud layer within an AMC. Therefore, the largest differences between the two CBHs indicate that the ice-dominant cloud layer within an AMC is deepest during the winter months. During the summer months, this layer is either shallow or does not exist Characteristics of Arctic Humidity Inversion and Its Relationship With AMC In this section, the seasonal variation of humidity inversion occurrence and intensity (specific humidity difference between the inversion top and base), as well as their relationships with AMC frequency of occurrence are investigated. Figure 4a illustrates the monthly means of the humidity inversion occurrences for different inversion intensity thresholds from 0.1 to 0.9 g/kg (y axis) from January to December (x axis). The colors in Figure 4a represent the monthly mean inversion frequency of occurrence for each corresponding month and intensity. As shown in Figure 4a, the humidity inversion occurred ~70 to 90% of time in the Arctic region with the lowest intensity threshold of >0.1 g/kg (bottom line of Figure 4a), similar to the findings of Nygård et al. [2013]. October is the only month with a humidity inversion occurrence below 50%. As expected, humidity inversion occurrence decreases with increasing humidity inversion intensity. For instance, in January, the humidity inversion occurrences decreased from 80 90% with the intensity threshold of >0.1 g/kg to below 10% with the threshold of >0.8 g/kg. Figure 4b is a brief summary of Figure 4a for all four seasons. From December to March (blue line in Figure 4b), more than 60% of the inversions were relatively weak, with intensities less than 0.3 g/kg, and occurrences decreased significantly with increasing intensity. In contrast to the cold months, occurrences of the humidity inversion decreased linearly with increasing intensity during the summer months (June to August; yellow line in Figure 4b). The different humidity inversion occurrences and intensities during the summer and winter seasons may be a result of different mechanisms, and thus, further study is warranted. Similar to Figure 4b, Figure 5 shows the AMC occurrences under different humidity inversion strengths for the four seasons. During the winter months, AMC occurrences increase from 15% to 35% when the inversion intensity increases from 0.1 g/kg to 0.9 g/kg (blue line in Figure 5). On the contrary, despite the higher frequency of strong humidity inversion in summer (as shown in Figure 4b), AMC occurrences are nearly invariant for different inversion intensities. Additionally, when the inversion intensity is stronger than 0.9 g/kg, AMC occurrences can reach ~40% for all seasons except for summer (June to August). QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7742

7 Figure 5. Same as Figure 4b but for the occurrence of the mixed-phase cloud with increasing humidity inversion intensities for different seasons T and q Inversion Location Relative to Single-Layered AMCs and AMCs Under Multilayered Condition As previously discussed, the inversion intensity plays a primary role in AMC occurrence. In this section, influence of the relative location (above or below an AMC) and intensity of both humidity and temperature inversions on AMC are investigated. In this study, AMCs are classified into single-layered and multiple-layered cases, and the inversions are classified into above and below an AMC. Inversion occurrence is calculated by accounting all inversions within 200 m above/below an AMC (regardless of the intensity), as shown in Figures 6 and 7. The inversion intensity is the mean intensity for all the inversion layers that are within 200 m above/below an AMC. As shown in Figures 6a and 6b, respectively, the seasonal variation of single-layered and multilayered AMC occurrences is similar to AMC Figure 6. Monthly means of humidity inversion occurrence and intensity at the ARM NSA site during the period of October 2006 September 2009 for (a and c) single-layered mixed-phase clouds and (b and d) mixed-phase clouds under multiplelayered condition. The red lines are for inversion above the cloud, and the blue lines are for below the cloud. The black lines in Figures 6a and 6b are the occurrences of single-layered and multilayered mixed-phase clouds, respectively. The black lines in Figures 6c and 6d are the annual mean humidity inversion intensity calculated for all-sky condition. P inv_ab and P inv_be represent the inversion occurrences above and below a mixed-phase cloud layer, respectively, and q inv_ab and q inv_be are the inversion intensities above and below a cloud layer. QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7743

8 Figure 7. (a d) Same as Figure 6 but for temperature inversion occurrence and intensity. occurrence overall (Figure 3a), with the maximum in October and the minimum in March (black line in Figures 6a and 6b). The annual mean occurrences for single-layered AMCs and AMCs under multilayered condition are 18.7% and 23.6%, respectively. AMC occurrence under multilayered condition is higher than single-layered AMC through the year except for September November. On average, the humidity inversion frequency of occurrence above an AMC is 5 times as high as that below an AMC (red lines compare with blue lines in Figures 6a and 6b). Seasonal variation in temperature inversion occurrence is very similar to its humidity inversion counterparts, and the temperature inversion frequency of occurrence above an AMC is about 8 times as high as that below an AMC (Figures 7a and 7b). This result agrees well with previous studies, where specific humidity inversions occurred coincidently with temperature inversions over the Arctic [Sedlar and Tjernström, 2009]. The strong inversion occurrences for both humidity and temperature above an AMC provide the moisture sources from above for the formation and maintenance of AMCs. This result helps to reconcile the persistency of AMCs even when the Arctic surface is covered by snow and ice. The annual mean of humidity inversion intensity (black line, Figures 6c and 6d), calculated for all-sky condition during the studying period, is 0.24 g/kg, with the strongest intensity during June July (~0.4 g/kg) and the weakest during October May (~0.2 g/kg). This seasonal variation strongly correlates with the snow-ice free (from early June to middle of September) and covered periods at the ARM NSA site. As shown in Figures 6c and 6d, although the humidity inversion frequency of occurrence above an AMC was much higher than that below an AMC, humidity inversion intensities above and below AMC layers were close in magnitude to each other. Nevertheless, the humidity inversion intensities with AMC layers were still higher than the climate mean value (0.24 g/kg), especially for AMCs under multilayered condition. Similar results were found for temperature inversion intensities in Figures 7c and 7d. Based on the results in Figures 6 and 7, we conclude that the inversion intensities for AMCs under multilayered condition are stronger than those of the single-layered AMCs. QIU ET AL. ARCTIC MIXED-PHASE CLOUD STRUCTURE 7744

9 4. Summary and Conclusions In this study, ground-based measurements at the Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) site during the period of October 2006 to September 2009 have been collected and analyzed to study the characteristics of Arctic mixed-phase clouds (AMC). The seasonal occurrence and intensity of humidity and temperature inversions, as well as their relationships with AMC occurrence, have been investigated. Through an integrative analysis of 3 year observations at the ARM NSA site, we attempt to answer the three scientific questions posed at the beginning: 1. The annual mean of AMC occurrence is 42.3% with the highest occurrence in October (72%) and the lowest occurrence in March (10%). Due to the complicated structure of AMC, two cloud base heights (CBHs) are defined by the ceilometer and MPL measurements in this study. The ceilometer-retrieved CBH typically represents the base of the liquid-dominant layer, while the MPL-retrieved CBH represents the base of the ice-dominant cloud layer within an AMC. The annual mean CBHs from ceilometer and MPL measurements are 1.0 km and 0.6 km, respectively, with the largest difference (~1.0 km) occurring during winter and the smallest difference during summer and autumn (~0.1 km). The largest difference between the two CBHs indicates that the ice-dominant cloud layer within an AMC is deepest during the winter months, while it is either shallow or does not exist during the summer months. 2. Humidity inversion occurrence decreases with increasing humidity inversion intensity. During winter months, AMC occurrences increase from 15% to 35% when the inversion intensity threshold increases from 0.1 g/kg to 0.9 g/kg. On the contrary, despite the higher frequency of strong humidity inversion in summer, AMC occurrences are nearly invariant for different inversion intensities. Additionally, when the humidity inversion intensity is strong than 0.9 g/kg, the occurrence of AMC reaches ~40% for all seasons except for June to August. 3. The annual mean occurrences for single-layered AMCs and AMCs under multilayered condition are 18.7% and 23.6%, respectively. AMC occurrence under multilayered condition is higher than singlelayered AMC throughout the year except for September November, and the inversion intensities for AMCs under multilayered conditions are stronger than those from the single-layered AMCs. On average, the humidity and temperature inversion frequencies of occurrence above an AMC are 5 8 times, respectively, as high as those below an AMC. The high inversion occurrences for both humidity and temperature above an AMC provide the moisture sources from above for the formation and maintenance of AMCs. This result helps to reconcile the persistency of AMCs even when the Arctic surface is covered by snow and ice. The influence of temperature inversion intensity on AMC has also been investigated in this study, and no relation was found between AMC occurrence and temperature inversion intensity. The results presented in this study are only based on ground-based measurement over the ARM NSA site. Due to the limitation of point measurements, the results and conclusions from this study only represent the local environmental effect on AMC near the ARM NSA site. Future investigation related to the impact of large-scale synoptic patterns, the advection of moisture and temperature, and their influence on the radiative properties and lifetime of AMCs are needed, which will further clarify the formation and persistency of AMC as well as their interactions and feedback with the environment. Acknowledgments The data were obtained from the Atmospheric Radiation Measurement (ARM) Program sponsored by the U.S. Department of Energy (DOE) Office of Energy Research, Office of Health and Environmental Research, Environmental Sciences Division. The data can be downloaded from arm.gov/. This study was primarily supported by the DOE ASR project at University of North Dakota with award DE-SC and the NASA CERES project at University of North Dakota project under grant NNX14AP84G. References Barton, N. P., A. K. Stephen, and S. B. James (2014), On the contribution of longwave radiation to global climate model biases in Arctic lower tropospheric stability, J. 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