Additional global climate cooling by clouds due to ice crystal complexity

Abstract

Abstract. Ice crystal submicron structures have a large impact on the optical properties of cirrus clouds and consequently on their radiative effect. Although there is growing evidence that atmospheric ice crystals are rarely pristine, direct in situ observations of the degree of ice crystal complexity are largely missing. Here we show a comprehensive in situ data set of ice crystal complexity coupled with measurements of the cloud angular scattering functions collected during a number of observational airborne campaigns at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61 % and 81 %) of atmospheric ice crystals sampled in the different regions contain mesoscopic deformations and, as a consequence, a similar flat and featureless angular scattering function is observed. A comparison between the measurements and a database of optical particle properties showed that severely roughened hexagonal aggregates optimally represent the measurements in the observed angular range. Based on this optical model, a new parameterization of the cloud bulk asymmetry factor was introduced and its effects were tested in a global climate model. The modelling results suggest that, due to ice crystal complexity, ice-containing clouds can induce an additional short-wave cooling effect of −1.12 W m2 on the top-of-the-atmosphere radiative budget that has not yet been considered.

Lateral facet growth of ice and snow – Part 1: Observations and applications to secondary habits

“…Away from the tip, rounding of the side vertices has been attributed to SCR forming due to a decaying gradient in surface supersaturation (Nelson, 2001;Frank, 1974). In slower-growing tabular crystals, Keller et al (1980) observed disk crystals faceting as they became larger and thicker, as did Knight (2012). In our previous experiments (Nelson and Knight, 1996), we also saw small disk crystals develop facets as they grew.…”

“…Thus, the notch and plane must have a stacking fault, not a grain boundary, with the depth of the pockets suggesting that a region of faults may be present. Itoo (1953) called such crystals "twin prisms", and found them to be very common in light precipitation at −30 • C. Kobayashi and Ohtake (1974) observed a similar notch, suggesting a specific type of stacking fault. A more recent study found that extended regions of stacking disorder are common when small water droplets freeze near −40 • C (Malkin et al, 2012) but are unlikely to form during vapor growth (Hudait and Molinero, 2016).…”

Tropical Anvil Clouds: Radiative Driving Toward a Preferred State

“…Ice cloud treatment follows Fu (1996) and Fu et al (1998). Recent work has shown that cirrus cloud radiative forcing can be underestimated by radiative transfer algorithms that do not account for ice crystal surface complexity (Järvinen et al, 2018). Cirrus optical properties are also sensitive to assumptions about ice crystal habit (Wendisch et al, 2007).…”

Evaluation of cloud properties from reanalyses over East Asia with a radiance-based approach

“…With the advances in computation capability and the improvement in global observing systems, an increasing number of observed datasets are assimilated into reanalysis by more advanced data assimilation methods and systems, and the reanalysis is closer to depicting realistic atmospheres. A series of atmospheric reanalysis datasets have been produced, for example, the National Centers for Environmental Prediction (NCEP) 40-year reanalysis project (Kalnay et al, 1996), the ECMWF 40year reanalysis (Uppala et al, 2005), the Japanese 25-year reanalysis (Onogi et al, 2007), the Modern-Era Retrospective analysis for Research and Applications (Rienecker et al, 2011), the ECMWF ERA-Interim reanalysis (Dee et al, 2011), and the Japanese 55-year reanalysis (Kobayashi et al, 2015). Some schemes and systems that support the assimilation of cloud-affected satellite radiance have been developed (Chevallier et al, 2004;McNally, 2009).…”