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Pressure anomalies from the January 2022 Hunga Tonga‐Hunga Ha'apai eruption

2022; Wiley; Volume: 77; Issue: 3 Linguagem: Inglês

10.1002/wea.4170

1477-8696

R. G. Harrison,

Tropical and Extratropical Cyclones Research

The major eruption of the Hunga Tonga-Hunga Ha'apai volcano at 0410 utc on 15 January 2022 following lesser activity in the previous days, was so substantial that it sent a shock wave propagating around the globe. As was the case for the eruption of Krakatoa in August 1883 (Symons, 1888), the initial pressure changes associated with the shock wave were so large that they were detected around the world with barometers in routine meteorological use, and are well within the capability of inexpensive digital sensors (Harrison, 2021) bringing the prospect of a considerable amount of high quality data. The Tonga pressure wave propagated outwards from the volcano, leading to its first appearance in the United Kingdom around 1800 utc on 15 January having passed over the North Pole, with a second appearance at about 0200 utc on 16 January via the South Pole route. Figure 1 shows a preliminary analysis of pressure anomalies over the UK from roadside monitoring sensors, as used previously for the 2015 eclipse (Gray and Harrison, 2016). It shows the arrival of the first pressure pulse from the north and propagating southwards, and the second pressure pulse arriving from the south and propagating northwards at a similar speed to the first pulse. Both caused transient pressure anomalies of 0.5 to 1hPa, which were initially positive and then negative, and made more observable by reasonably settled anticyclonic conditions. At Reading, the first sign of a pressure pulse was at about 1845 utc, 14.5 hours after the eruption. For the separation between Reading and Tonga of about 16 500km, this corresponds to a wave speed of 315ms−1, which is close to the speed of sound at standard conditions. Figures 2(a) and (b) show the first two pressure pulses considered in Figure 1 arriving at Reading, observed using a Druck DPI140 barometer with ±0.01hPa resolution operating on the vibrating drum principle (Harrison, 2014), sampled at 1s intervals. The pulse shapes are reminiscent of the initial pressure pulses observed following the Krakatoa eruption (Figures 2c and d), with a slow increase to a ragged maximum followed by a rapid descent to a minimum. With the greater resolution present in the modern data, the first pulse at Reading can be seen to be followed by high frequency variability lasting several hours. Compared with the period before the first pulse, greater background variability continued after the second pulse, implying that the atmosphere remained disturbed. As for Krakatoa, pairs of further pulses arrived at Reading with intervals of about 35 to 36 hours from the previous pair of pulses. Numbering the pulses individually, the first and subsequent odd-numbered pulses had initially taken the North Pole route, and the even-numbered pulses the longer South Pole route. Figure 3(a) shows a continuation of the pressure data from Figure 2(a), with the pressure pulses identified in Figure 3(b). For the later pulses, the amplitude decreased, making it difficult to identify the pulses uniquely. The approach adopted was a combination of expected timing and a comparison with the typical undisturbed variability – found as two standard deviations – in the pressure fluctuations before the first pulse. Subsequent maxima in pressure fluctuations which both (1) exceeded this variability, and (2) occurred in a time window estimated from the previous pulse, were regarded as contenders for having been generated by the returning pressure wave. On this basis, six pulses (P1 to P6) were reasonably straightforwardly identified. However, changed background variability through passage of a weak front gives less confidence in identifying pulses 7 and 8. Pulse 10 only marginally meets the criteria applied, the detailed application of which and the choices made in the detrending method used then become more important. Further analysis using pattern matching (as for the pressure pulse from the 2005 Buncefield explosion, Mather et al. (2007)), or spectral methods, may improve on this. Table 1 summarises the timing information obtained for the first six pulses, using the time of the pulse maximum in each case. The mean speeds of the odd pulses (North Pole route initially) and evenly numbered pulses are 309ms−1 and 314ms−1 respectively. A similar difference was found for pulse pairs in the Krakatoa event, which was attributed by Symons (1888) to an assisting or opposing contribution of the prevailing winds globally. The 5ms−1 difference observed is consistent with this. The passage of a large pressure disturbance might be expected to generate additional local atmospheric changes associated with fluid motion. Whilst the pressure was sampled with a rapid response instrument (Figure 4a), standard meteorological instruments respond less rapidly, so a straightforward comparison is difficult. The Vaisala CL31 laser ceilometer operating at Reading, however, provides one minute samples of cloud base height. During the passage of the first pressure pulse on 15 January, low stratus cloud was present, with a mean cloud base of 1200m between 1700 and 2300 utc. Removing slow changes in the mean cloud base using a spline method, the variability generated in the cloud base during the passage of the pressure wave becomes apparent (Figure 4b). The sudden rise in the cloud base height just before 2000 utc was also observed as a decrease in both the long-wave down flux and the surface potential gradient (which responds to charge at the cloud base changing its position), and an increase in the surface air temperature sensed by an aspirated thermometer able to respond within one minute. The Krakatoa pressure wave was a defining event in atmospheric science, due to its detection in the early international measurement network through the impressively thorough activities of the Krakatoa committee. The 2022 Hunga Tonga-Hunga Ha'apai pressure wave, which has been exquisitely sampled by modern instruments and satellite imagery in a way inconceivable in 1883, is likely to prove equally valuable. I am grateful to David Bullock of Vaisala for providing the roadside station data used for Figure 1. Data used are available at: https://doi.org/10.17864/1947.000354.