Multi-thermals and High Concentrations of Secondary Ice: A Modeling Study of Convective Clouds During the ICE-D Campaign

The field campaign of the Ice in Clouds Experiment – Dust (ICE-D) took place in August 2015 in the Cape Verde region in order to investigate the impact of mineral dust on convective clouds. Aircraft, ground-based instruments and radar were deployed to measure aerosol, cloud and meteorological properties. The clouds in this study occurred on 21 August. Observational studies have showed the first ice particles appearing at approximately -3 ºC and high concentrations of ice in the Hallett-Mossop temperature zone. The concentrations were more than 40 per liter in one cloud, and 280 per liter in another. We used the Cloud Model One (CM1) to simulate the clouds. A simulation with the Morrison microphysics scheme produced secondary ice about 7 per gram. If the starting freezing temperature was relaxed from -8 to -3 ºC, the ice concentration was doubled to 15 per gram. When the Cooper parameterization was enhanced by 10, the ice concentration increased to 22 per gram. With both relaxation and enhancement in the Cooper parameterization, the concentration reached to 51 per gram, close to the observations of one cloud. The simulations implicate that two conditions should be met to get a similar amount of secondary ice production: higher starting temperature of freezing and higher concentration of ice nucleating particles. We conducted sensitivity experiments by varying the freezing schemes, but no simulations could produce secondary ice above 100 per gram. However, the concentration of ice exceeded 200 per gram in a simulation with two thermals using the original Morrison scheme without enhancement or relaxation, which suggest the possible role of multi-thermal in producing high secondary ice production.