Convective invigoration within monsoon—evidence of from CAIPEEX observations and numerical simulations
Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) was conducted during the 2009-2011 over the Indian subcontinent. These observations are first of its kind during the monsoon season. The aerosol and cloud microphysics observations were conducted with the help of an instrumented aircraft and the cloud dropsize distribution in deep convective clouds were observed at several vertical levels, upto -15 oC to -20 oC. The main focus of the experiment was to investigate the rain formation under different aerosol conditions in deep convective clouds. There have been several instances where mixed phase was observed in these clouds with hydrometers of bigger sizes, such as aggregates and graupel. An overview of the experiment and the mixed phase ice microphysics observations will be illustrated.
While warm rain is suppressed in the polluted clouds, there is increase in supercooled liquid, which is closely associated with the increase in subcloud CCN concentrations. The impact on intensification of updrafts is also evident in the observations. First ice is formed at warmer temperatures in a clean monsoon cloud compared to that in a polluted monsoon cloud. It is noted that there is more graupel and aggregates at the tops of these convective clouds when subcloud aerosol number concentrations are high and pristine conditions exist at elevated layers. The presence of supercooled liquid enhances riming in the upper levels. It is hypothesized that the convective clouds are invigorated under high aerosol conditions within monsoon.
The microphysical mechanism linked with dynamics is illustrated for these events with observations and is also supported by the bin microphysical simulations. Hebrew University Cloud Model (HUCM) was used to investigate the mixed phase microphysics in response to aerosol pollution. HUCM was calibrated with CAIPEEX thermodynamics and aerosol observations. Multiple aerosol and cloud probes during CAIPEEX 2011 enabled the investigation of cloud processing of aerosol to investigate relationship between the aerosol spectrum (ASD) and cloud droplet spectrum (DSD). The simulated drop size distribution at different levels in the deep convective cloud are compared with observations from CAIPEEX, which illustrate aggregates at the tops of these convective clouds. The implication of these deep convection due to aerosol effects in the monsoon environment is also illustrated with the bin and bulk simulations.