Microphysical evolution of thunderstorms in tropical environments associated with gigantic jet production using CM1

Low-light cameras installed in different locations in Colombia have recorded a total of 70 Gigantic Jets (GJ), mainly between 2016 and 2022. These are spectacular cloud-to-ionosphere lightning discharges reaching 90 km altitude and are mostly a tropical phenomenon. However, there are many more thunderstorms in the tropics that do not produce them.

We summarize the results of a statistical study of ERA5 profiles of 48 gigantic jet nights and 83 null cases in northern and western Colombia in a work published in Atmospheric Research (van der Velde et al. 2022). The main conclusion was that the environment of gigantic jet producing storms is characterized by greater warm cloud depth (to -10°C), weak low-level shear, weak lapse rates and reduced updraft and downdraft buoyancies, while no evidence was found for cloud top shear effects. It was hypothesized that warm cloud processes and secondary ice processes may result in a temporary low rime accretion rate, which could reverse the charging polarity and temporarily cause a highly negatively charged balance in the storm.

Mean vertical profiles have been constructed for null events and GJ events. They have minimal CIN (-10 J/kg) and were not modified. These are used to initiate Cloud Model 1 simulations at 0.25 km grid spacing or better and the NSSL microphysics scheme (other models and schemes may be considered). A sensitivity study was done to see the effect of initial lifting mechanism on the storm evolution. With default settings a very fragile storm was simulated. Here, we compare against satellite evolution of GJ producing storms, which tend to be rather strong. A setting of updraft nudging was selected that reproduces the storm well, while not causing unrealistically strong converging winds at the surface in response. The effect of cloud condensation nuclei is also tested.

The purpose of the simulations is to find differences to the microphysical particle distributions and densities, between GJ and null and in GJ case ERA5 profiles, which can affect the electrical charging efficiency and polarity. In a preliminary study with CM1+NSSL microphysics, we confirmed that there are indeed differences e.g. in graupel – ice/snow density overlap, but effects on charging and typical timing of the gigantic jets relative to the storm evolution are in need of further investigation.