Global Statistics of Ice Microphysical and Optical Properties at Tops of Optically Thick Ice Clouds

Abstract

The sizes and shapes of ice crystals in clouds affect fundamental microphysical processes, such as sedimentation and aggregation, as well as their optical properties. The evolution of ice crystal size and shape depends on temperature and supersaturation, as well as on other processes that may lead to various coexisting complex shapes. Here we present a global assessment of collocated size and shape characteristics and shortwave scattering properties of ice crystals at the tops of optically thick clouds inferred from space‐borne multiwavelength reflectance measurements and multiangle polarimetry. The results indicate systematic covariations of ice size, shape, and distortion, as well as variations with temperature that can be plausibly related to simplified ice crystal growth theory and in situ and laboratory data. This simplicity may be attributable to the temperature dependence of cloud top ice size and shapes commonly being dominated by vapor growth at conditions similar to those at cloud top. Such a conclusion may be viewed as somewhat surprising given the expectation that ice properties at cloud top will be an integral manifestation of processes occurring at lower levels within updrafts. We also find that, contrary to commonly used models, ice scattering asymmetry parameters decrease with increasing effective radius, reducing sensitivity of cloud reflectance to particle size.

A Lagrangian Perspective on Tropical Anvil Cloud Lifecycle in Present and Future Climate

“…The convective scheme provides this information in an arbitrary way, the detrained ice crystal effective radius is a tunable parameter, set to 12 μm in the model version used here. This is inconsistent with observational evidence, which shows that the ice particle size in convective cores decreases with altitude (Van Diedenhoven et al., 2016, 2020) and may therefore lead to an underestimation of the lifetime of the detrained ice crystals at the convective cloud tops and overestimation at lower levels. Nevertheless, despite the use of parametrized convection and its associated problems, we found E3SM to reproduce the observed albedo‐OLR histogram in the tracking region remarkably well and to simulate MCS in a reliable way compared to geostationary observations of tropical maritime convection.…”

The NASA MODIS-VIIRS Continuity Cloud Optical Properties Products

“…Moreover, the ice phase 2.x µm CER retrieval differences shown in Figures 11 and 15 suggest a deficiency in the ice cloud forward model, perhaps attributable to similar complex index of refraction assumptions for ice (e.g., [55] showed temperature sensitivities of the ice refractive index in the IR), though the agreement for the 1.6 and 3.7 µm CER retrievals for all three products shown suggests otherwise. Alternatively, the larger spectral mismatch in the 2.x µm channels (see Table 1), coupled with the fact that ice is substantially less absorbing in the VIIRS 2.25 µm channel versus the MODIS 2.13 µm channel (see Figure 1 above), may indicate that vertical heterogeneity arising from real microphysical processes, e.g., crystal growth and sedimentation that would tend towards particle sizes that decrease with height within the cloud [56], may be influencing the behavior of CER retrievals from spectral channels having different vertical sensitivities. This hypothesis is consistent with the fact that CER retrievals from the 1.6 and 2.x µm spectral regions are much larger than those from the 3.7 µm spectral region (see again Figures 11 and 15) where ice absorption is largest, as well as with the VIIRS 2.25 µm channel, where ice absorption is weakest, giving the largest CER retrievals.…”

“…In addition, the CLDPROP VIIRS and MODIS CER differences (center column) are quite large, particularly for the 2.x µm retrievals, for which VIIRS retrieves significantly larger CER than does MODIS, up to 6 µm or more at the monthly mean scale shown here. It is not clear if these differences for the 2.x µm channel retrievals are related to the bulk ice complex index of refraction assumption (e.g., [55] demonstrated sensitivities in the IR), as was the case for the liquid 2.x µm CER retrievals, to vertical heterogeneity arising from real microphysical process, e.g., crystal growth, sedimentation [56], or to some combination. As with the 3.7 µm liquid CER differences, further investigation into the 2.x µm ice CER differences is warranted.…”

Confronting the Challenge of Modeling Cloud and Precipitation Microphysics

“…Where satellite measurements generally struggle to determine robustly the thermodynamic phase of clouds with tops between the homogeneous freezing and melting temperatures, polarimetric detection of a cloudbow is a virtually unambiguous indication of liquid drops at the tops of clouds (Riedi et al, 2010, van Diedenhoven, Cairns, et al, 2012). For ice‐topped clouds, multiangle polarimetry allows inference of crystal shape (Baum et al, 2011; van Diedenhoven, Fridlind, et al, 2012; van Diedenhoven, 2018; van Diedenhoven et al, 2020), which may be especially valuable for evaluation of microphysical models predicting ice shape characteristics (e.g., Harrington et al, 2013; Hashino & Tripoli, 2007; Jensen et al, 2017). Furthermore, the inferred ice shape constrains the ice optical model used for retrievals of ice cloud optical thickness and effective radius from shortwave infrared measurements (van Diedenhoven et al, 2014; van Diedenhoven et al, 2020), reducing uncertainties.…”