Abstract
AbstractClouds are liquid at temperature greater than 0°C and ice at temperature below −38°C. Between these two thresholds, the temperature of the cloud thermodynamic phase transition from liquid to ice is difficult to predict and the theory and numerical models do not agree: Microphysical, dynamical, and meteorological parameters influence the glaciation temperature. We temporally track optical and microphysical properties of 796 clouds over Europe from 2004 to 2015 with the space‐based instrument Spinning Enhanced Visible and Infrared Imager on board the geostationary METEOSAT second generation satellites. We define the glaciation temperature as the mean between the cloud top temperature of those consecutive images for which a thermodynamic phase change in at least one pixel is observed for a given cloud object. We find that, on average, isolated convective clouds over Europe freeze at −21.6°C. Furthermore, we analyze the temporal evolution of a set of cloud properties and we retrieve glaciation temperatures binned by meteorological and microphysical regimes: For example, the glaciation temperature increases up to 11°C when cloud droplets are large, in line with previous studies. Moreover, the correlations between the parameters characterizing the glaciation temperature are compared and analyzed and a statistical study based on principal component analysis shows that after the cloud top height, the cloud droplet size is the most important parameter to determine the glaciation temperature.
Highlights
The cloud thermodynamic phase impacts cloud radiative properties, cloud lifetime, and dynamical properties
We divide our result section in three main parts: First, we analyze the temporal evolution of cloud properties, we study the difference in Tg for different meteorological and microphysical regimes, and we present results from a statistical analysis to discuss the correlations that the different parameters have with each others and how they impact Tg
Our analysis is based on the temporal evolution of cloud microphysical properties of isolated convective clouds during summer over Europe from 2004 and 2015
Summary
The cloud thermodynamic phase impacts cloud radiative properties, cloud lifetime, and dynamical properties. −38◦C, clouds are in the ice phase and above 0◦C, clouds are in the liquid phase. Between −38◦C and 0◦C, clouds can be either liquid, ice or a mixture of the two phases and different parameters influence the phase transition: For example, aerosols can act as ice nuclei, while large cloud droplets are more prone to secondary ice production (Rosenfeld et al, 2011); both examples potentially increase the glaciation COOPMAN ET AL. Journal of Geophysical Research: Atmospheres temperature (Tg). Parameters, such as the cloud droplet size and aerosol concentration, can be correlated or anti-correlated with each other, and it is difficult to disentangle their respective effect on Tg (Coopman et al, 2018; Gryspeerdt et al, 2016)
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