The role of giant and ultragiant nuclei in the formation of early radar echoes in warm cumulus clouds

Observations of the formation of the first radar echoes in small cumulus clouds are compared with results of a stochastic coalescence model run in the framework of a closed parcel. The observations were made with an instrumented aircraft and a high-powered dual-wavelength radar during the Small Cumulus Microphysics Study (SCMS) in Florida. The principal conclusion is that coalescence growth on giant and ultragiant nuclei may be sufficient to explain observations. The concentration of cloud droplets varied from under 300 cm^-3 when surface winds were from the ocean. to over 1000 cm^-3 when the wind direction was from the mainland. Although there is a slight tendency for the altitude of the first O-dBZ echo to be lower on average in maritime than in continental clouds there were several cases where it was higher. The model results suggest that the lack of correlation is consistent with drops forming on giant and ultragiant nuclei. The first O-dBZ echo was observed to form at higher altitudes in clouds with stronger updrafts.