Recent developments in Imaging Lightning with LOFAR

The LOw Frequency ARray (LOFAR) radio telescope is a phased array of thousands of VHF dipoles centered in the Netherlands but spread throughout Europe. While originally mostly designed to observe astronomical phenomena, it is perfectly suited to image terrestrial lightning. Our group has been using LOFAR to image lightning in full 3D and time with nanosecond and sub-meter precision, which has revealed a myriad of new lightning phenomena that gives us new insight into both lightning plasma physics as well as Dutch thunderstorm structure. For example, almost every single lightning flash we image starts with a downward-growing negative leader (plasma channel). This is opposite lightning observed elsewhere, where most lightning starts with an upward growing negative leader. The conclusion is that Dutch thunderstorms are “bottom-heavy” relative to more traditionally studied thunderstorms. That is, where most thunderstorms have a main positive charge layer at higher altitude than the negative layer; the main positive layer in Dutch thunderstorms is below the negative layer. We have also studied small sparks at the top of very intense thunderstorms. These imply that, while most are too weak, the strongest Dutch storms have strong very localized turbulence at their tops when they interact with the ceiling of the troposphere. This local intense turbulence produces small complex charge pockets at higher altitude than the normal large charge layers. In this talk we will discuss recent results from imaging lightning with LOFAR, including extremely high precision images of lightning initiation, the discovery of small sparks that occur at the top of thunderstorms and before lightning initiation, as well as the realization that Dutch thunderstorms have upside-down charge polarity relative to thunderstorms in other places.