Microphysics of deep tropical convective clouds over India, part-I: Aircraft observations

During summer 2009 a large set of unique in-situ measurements of deep convective clouds over India has been performed within the framework of the CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment) project of the Indian Institute of Tropical Meteorology in Pune, India. The measurements were carried out up to heights of 8 km msl. The clouds developed in extremely moist atmosphere over the flooded Indo-Gangetic plains and over the adjacent ocean. Cloud base heights were low with temperatures of about 25°C, making these clouds relevant to thermodynamically maritime clouds. Very heavy air pollution created CCN concentrations exceeding 5000 cm-3 at supersaturation of 0.6%, which delayed the onset of rain to heights above the freezing level. These polluted clouds produced mainly ice hydrometeors that were initiated as supercooled rain. Significant electrical activity was observed in these clouds. In the less polluted conditions the clouds produced rain well below the freezing level, leaving little cloud water to become supercooled, and respectively little cloud electrification. These two extremes are consistent with the conceptual model by which air pollution changes the vertical disposition of latent heating in convective clouds, in line with the conceptual model that postulates that dust and pollution aerosols weaken the intensity of tropical storms. The observations are the basis for simulations of the same clouds, presented in Part-II.