Using POLDER-3 and MODIS space-based instruments, both part of the A-Train, we analyzed cloud microphysical and radiative properties, spatially and temporally colocated with meteorological parameters from ERA-Interim reanalyses. From cloud top temperature distributions of liquid and ice clouds, we estimated the temperature at which clouds freeze for different latitudes, regimes of liquid-droplet effective radius, and regimes of large-scale vertical velocity. Our results show that the freezing temperature increases by 20°C between a regime where the effective radius ranges from five to nine micrometers to a regime where the effective radius ranges from 21 to 25 μm. Updraft velocity and latitude have a smaller impact on the freezing temperature.
We also colocated concentrations of a passive tracer of aerosols in a tracer transport model with cloud properties and meteorological parameters to observe that, locally, pollution concentrations lower the cloud freezing temperature by approximately 4°C, while the impact on freezing temperatures through changes in effective radius is relatively small.