Global scale analysis of relations between cloud top phase and thermodynamic parameters

Phase of condensed atmospheric water and its relations with main thermodynamic parameters play a critical role in the formation and evolution of clouds. Cloud particle microphysics is a first order parameter for computation of radiative exchange. It also governs strongly clouds lifetime through precipitation and sedimentation processes linked with particle size and shape and therefore phase. Not surprisingly, the representation of condensed water thermodynamic phase in models is a major source of uncertainty for the evaluation of clouds and water vapor feedbacks in an evolving climate.

Therefore, there is a critical need to better describe the distribution of condensed water between ice and liquid phase in clouds, and the relations existing with main thermodynamic parameters that governs this partitioning. As with many other problems, satellite observations are of outermost importance for the establishment and validation of relationship that exist between cloud phase and other atmospheric parameters.

The existing methods for determination of cloud phase from passive spaceborne observations however, are subject to biases that can be strongly correlated to cloud microphysics, vertical structure and temperature. Establishing robust and unbiased relations between cloud phase and other thermodynamic parameters thus requires an extremely accurate phase determination independently of other atmospheric parameters. This can be achieved for instance by combining information from various sources in order to increase the confidence in cloud phase retrievals.

A high confidence joint product has been derived from the 5 years of POLDER/Parasol and MODIS/Aqua coincident observational period. Quality of this dataset has been assessed against independent active observations from CALIOP and can be used as a benchmark for the evaluation of other cloud climatologies, for the assessment of cloud phase representation in models and the development of better cloud phase parametrization in GCMs.

We analyze in this study the frequency of occurrence of ice and liquid phase at cloud top as a function of temperature and other thermodynamic parameters. The study confirms at global scale the existence of liquid phase at very low temperature down to below 240 Kelvins as already observed in previous studies. More interestingly, this global high confidence dataset enable to evaluate the relation between phase and temperature for different thermodynamic conditions (including vertical velocity and humidity). Systematic differences in liquid phase subsistence at low negative temperature are found depending on considered geographical location or seasons. Different hypothesis for these differences will be exposed and discussed. Finally, perspectives will be drawn from these results for the evaluation of cloud cover representation in global circulation models.