Transient ice ring observed during the 15 January 2022 Tonga eruption

The Hunga Tonga-Hunga Ha’apai (HTHH) eruption on 15 January 2022 was an exceptional event for many reasons; record breaking heights have been reported [1, 2], a significant amount of water was injected into the stratosphere [3] and wave disturbances from the surface to the ionosphere have been detected [4, 5]. Here we show that ∼2 h after the climactic eruption began, a transient ring of extremely small ice particles (∼2 μm) formed around the plume. We identify these parti- cles using the brightness temperature difference between two thermal infrared satellite measurements and find record positive values exceeding 50 K. We hypothesise that the ice ring was generated by the pas- sage of an atmospheric pressure wave (a Lamb wave) triggered by a violent explosion of the eruption. The passage of the Lamb wave pro- duced a rarefaction in the upper troposphere-lower stratosphere (UTLS), which in turn led to an adiabatic decrease in ambient temperature. This process reduced UTLS temperatures sufficiently for ambient water vapour to reach saturation with respect to ice, enabling the formation of ice particles in the wake of the radially propagating pressure wave. Our modelling suggests that a temperature drop of the order of 1 K below ambient temperatures and water vapour concentrations of ∼5– 6 ppmv near 100 hPa would be required to explain the formation of the small ice particles indicated by the satellite observations. This new phenomenon serves as an important natural experiment revealing the timescale upon which ice particles nucleate and grow via heterogeneous vapour deposition given an abrupt perturbation in ambient temperature.