Analysis of the Thermodynamic Phase Transition of Tracked Convective Clouds Based on Geostationary Satellite Observations

摘要

Clouds are liquid at temperature greater than 0 degrees C and ice at temperature below -38 degrees 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 degrees 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 degrees 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. Plain Language Summary It is difficult to quantify the temperature at which clouds transition from liquid to ice. Indeed, between -38 degrees C and 0 degrees C, clouds can be composed of liquid cloud droplets, ice crystals, or mixture of both, but the theory cannot predict the observations. Satellites usually give a snapshot of microphysical properties of clouds at one time step during their lifetime. Therefore, statistical tools are needed to infer how clouds behave during their life cycle from a composite of several clouds. Here, we temporally track the properties of 796 convective clouds over Europe from a tracking algorithm based on the geostationary satellite SEVIRI. We are able to study the same clouds from their initiation to their dissipation including their transition from liquid to ice with a temporal resolution of 15 min. We find that, on average, clouds freeze at -21.6 degrees C and that the size of cloud droplets has a large impact on the temperature of glaciation: The larger the cloud droplets, the higher the temperature of glaciation. Our results aim to better understand how clouds transition from liquid to ice, and they can improve predictions of the impact of clouds in future climate.