Modeling of Early Ice Formation in Maritime Cumulus Clouds

The measurement of small ice particles has proven difficult, and the exact mechanisms behind ice nucleation often remain a fundamental uncertainty, particularly in maritime cumulus clouds where large ice concentrations have been observed at temperatures as warm as -5°C.

Here we present a unique comparison of a high-resolution 3D numerical cloud model representing 10 ice species, including 4 different small ice habits, with the locations and habits of ice less than 50 μm in diameter detected by the Small Ice Detector 2 HIAPER (SID-2H). The SID-2H data were collected in developing cumulus clouds during the Ice in Clouds Experiment – Tropical (ICE-T) field campaign, held in July of 2011 in the central Caribbean.

We use the model and observations to investigate the conditions necessary for the formation of early ice (defined here as ice forming within 500 m of ascending cloud top between -3° and -7°C), and its transport throughout the active cloud top region with time. “Virtual” cloud passes through the modeled fields are particularly useful in comparing with in situ data from aircraft cloud passes collected during ICE-T. Both the observations and the model demonstrate a propensity for the earliest ice to appear in the cloud edge downdrafts or in the updraft/downdraft interface. They are produced in the model without the addition of any special ice nucleation mechanisms and result from reduced temperatures due to evaporative cooling in these regions. The model also suggests that although graupel particles are often the first ice to appear in the observations, standard ice nucleation estimates, as given by the Fletcher parameterization, appear sufficient to explain the observed amount of graupel. Ice multiplication through rime splintering has been added to the model but does not appear to play a large role until later in the cloud simulation.