Title: Global statistics of microphysical and optical properties of cloud-top ice crystals Presenter: Bastiaan van Diedenhoven Abstract: Ice crystals in clouds are highly complex. Their sizes, macroscale shape (i.e., habit), mesoscale shape (i.e., aspect ratio of components) and microscale shape (i.e., surface roughness/distortion) determine optical properties and affect physical properties such as fall speeds, growth rates and aggregation efficiency. Our current understanding on the formation and evolution of ice crystals under various conditions can be considered poor. Commonly, ice crystal size and shape are related to ambient temperature and humidity, but global observational statistics on the variation of ice crystal size and particularly shape have not been available. In this talk, I will show results of a project aiming to infer ice crystal size, shape and scattering properties from a combination of MODIS measurements and POLDER-PARASOL multi-angle polarimetry on a 6x6 km resolution. The shape retrieval procedure infers the mean aspect ratios of components of ice crystals and the mean crystal distortion levels, which are quantifiable parameters that mostly affect the scattering properties, in contrast to ÒhabitÓ. We present one year of global statistics and discuss the variation of ice effective radius, component aspect ratio, distortion and scattering asymmetry parameter as a function of cloud top temperature, latitude, location, season, etc. The results indicate systematic covariations of ice size, shape and distortion, as well as variations with temperature that can be plausibly related to ice crystal growth theory and in situ and laboratory data. Similar to observed and theorized ice growth rates, effective radii generally increase with decreasing cloud top temperature above the melting level, peaks at about 247K and decreases for further decreasing temperatures. Inferred aspect ratios of crystal components show a weak temperature dependence but a clear local minimum at around 260K, consistent with observed and predicted thin plates at such temperatures. Crystal distortion levels increase with effective radius, which is consistent with recent observations of a positive relation between growth rates and crystal distortion and complexity. Contrary to commonly used models, asymmetry parameters are found to decrease with increasing effective radius, reducing sensitivity of cloud reflectance to effective radius. A parameterization of crystal distortion as a function of size and temperature is suggested. The presented statistics can be used as observational targets for modeling efforts and to better constrain other satellite remote sensing applications and their uncertainties.