Jovian Thunderstorms as Observed by the Juno Mission

Abstract

<p>On November 29<sup>th</sup>, 2021, the Juno Spacecraft completed its 38<sup>th</sup> perijove as part of its Extended Mission. Three of the spacecraft’s instruments, JunoCam, JIRAM, and MWR, imaged a thunderstorm in the North Equatorial Belt (NEB) at approximately 9<sup>o</sup>N planetocentric latitude.  JunoCam and the MWR captured data from an altitude of a few thousand kilometers, following JIRAM’s images of the storm four hours before. Ground-based observers tracked this storm over a period of a few days, providing a planetary-scale perspective to Juno’s observations.  This region of the planet has been quite active throughout 2021 and 2022. </p><p> </p><p>The morphology of the storm as shown in JunoCam’s RGB filters (observations with the methane filter were not conducted), and from ground-based observers, is highly suggestive of a moist-convective thunderstorm complex with clouds reaching the upper troposphere. Furthermore, JunoCam images suggest that the storm is shaped by vertical shear as the presumed anvil is offset from a thicker region of white clouds. On Earth, vertical shear is necessary for non-tropical cyclone thunderstorm systems to persist for prolonged periods.  JunoCam imaging also suggests a previous anvil top located to the west of the optically thick clouds, which may indicate a temporarily-varying nature to the convection, which is consistent with ground-based observations showing upwelling at this location for several days before the Juno images. JIRAM’s observations show a cold spot at 4.78 µm near the region of the thickest white clouds, which would be expected from optically thick clouds blocking heat transport to space. Spectroscopic retrievals show a slight enhancement of H<sub>2</sub>0 and PH<sub>3</sub> compared to the surrounding region, which is expected from upwelling from the interior. The microwave radiometer (MWR) instrument detected numerous lightning flashes at 0.6 GHz (Channel 1) and several flashes at 1.2 and 2.4 GHz (Channels 2 and 3, respectively), which are correlated with JunoCam and JIRAM’s observations of optically thick clouds.  However, the brightness temperature signals of the storm in the MWR observations appears to be confined to the upper most 9 bars of the atmosphere ("weather layer") indicating that a deep-seated convective plume originating from beneath the water-cloud base is probably not responsible for this storm system.  Instead, a humidity or temperature front contained within the weather layer may be a likely source.</p><p> </p><p>These observations may ultimately shed light on the mechanisms that form, sustain, and characterize moist convective storms in hydrogen-dominated atmospheres.  Here we summarize our observations to date and compare the PJ38 storm to other storms in Jupiter's NEB and those on Saturn. </p>