The spatial heterogeneity and inhomogeneity of cirrus microphysical properties evaluated globally using in situ measurements

Cirrus clouds having a high degree of spatially heterogeneous/inhomogeneous cloud properties have been shown to correspond with increased wave activity (e.g., Podglajen et al., 2018) and increased uncertainty in remote sensing retrievals (Fauchez et al., 2015, 2018); and incorporating cirrus cloud spatial heterogeneity/inhomogeneity into climate models has been shown to improve simulated output (e.g., Gu and Liou, 2006). These findings highlight the importance of evaluating spatial heterogeneity/inhomogeneity globally, which may provide information of common evolutionary pathways of cirrus clouds. However, relatively few studies have evaluated the spatial heterogeneity and inhomogeneity (often used interchangeably) of cirrus properties, which have primarily been case studies derived from relatively small datasets. Further, such studies often evaluate macro-scale properties such as optical thickness or cloud fraction rather than leveraging high resolution, airborne in situ measurements. 

We evaluate the spatial heterogeneity and inhomogeneity of cirrus bulk microphysical properties using ~65 hours of in situ measurements from eight field campaigns taking place in different regions globally. The spatial heterogeneity of ice concentration (ice water content) increases with decreasing ice concentration (ice water content), revealing more tenuous cirrus are more spatially heterogenous. This is suspected to be due to a greater competition amongst ice crystals for available water vapor within thin, in-situ formed cirrus (i.e., cirrus formed directly at temperatures below ~-38°C) compared with thicker, liquid-origin cirrus (i.e., cirrus formed via freezing of rising liquid or mixed phase clouds) which have an abundant amount of available water vapor. This is also a positive finding, since previous modeling work has shown that retrieval uncertainties associated with cirrus heterogeneity are greatest for optically thick cirrus (Fauchez et al., 2015).

Cirrus clouds often contain sets of ice concentration samples whose distributions are heavily skewed. These “heavily skewed” (i.e., more inhomogeneous) clouds also tend to possess higher spatial heterogeneity than “weakly/non-skewed” (i.e., less inhomogeneous) clouds. This skewness results from the absence of small ice crystals (diameter<~50 µm) within localized regions of the cirrus clouds. Clouds having heavily skewed distributions are observed ~20%–40% of the time in each flight campaign, suggesting the ubiquity of temperature perturbations preferentially removing small ice and/or the kelvin effect within cirrus clouds globally. 

 

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