Smoke with Induced Rotation and Lofting (SWIRL) in the Stratosphere

Abstract

The Australian bushfires of 2019/20 produced an unusually large number of pyrocumulonimbus (pyroCb) that injected huge amounts of smoke into the lower stratosphere. The pyroCbs from 29 December - 4 January were particularly intense, producing hemispheric-wide aerosol that persisted for months. One plume from this so-called Australian New Year (ANY) event evolved into a stratospheric aerosol mass ~1000 km across and several km thick. This plume initially moved eastward towards South America in January, then reversed course and moved westward passing south of Australia in February and eventually reached South Africa in early March. The peculiar motion was related to the steady rise in plume potential temperature of ~8 K/day in January and ~6 K/day in February, due to local heating by smoke absorption of solar radiation. This heating resulted in a vertical temperature anomaly dipole, a positive potential vorticity (PV) anomaly, and anticyclonic circulation. We call this dynamical component of the smoke plume a SWIRL (Smoke With Induced Rotation and Lofting). This study uses Navy Global Environmental Model (NAVGEM) analyses to detail the SWIRL structure over two months. The main diagnostic tool is an anticyclone edge calculation based on the scalar Q diagnostic. This provides the framework for calculating the time-evolution of various SWIRL properties: PV anomaly, streamfunction, horizontal size, vertical thickness, flow speed, and tilt. In addition, we examine the temperature anomaly dipole, the SWIRL interaction with the large-scale wind shear, and the ozone anomaly associated with lofting of air from the lower to the middle stratosphere.

Reply on SC1

“…It is easy to generate overestimates of rising rates from a structure moving over a fixed observatory. For instance, Ohneiser et al (2020), and yourself in your comment claim an ascent rate of 1km/day when Koobor was near the tip of South America which is about three time the ascent rate found by Khaykin et al (2020) (corroborated by Allen et al (2020)) at the same time on the trajectory. The ascent of Koobor was basically smooth for three months, es-pecially in potential temperature, except during two episodes of vertical split and it rose from about 16 km to 35 km, which is a considerable way against the Brewer-Dobson circulation but it did not reach the ionosphere.…”

Reply on RC1

“…This section provides a helpful reference of other papers that have studied this event. The PV anomaly associated with smoke during the Canadian event was also examined in a recent paper by Allen et al (2020). While that paper focused primarily on the "Koobor" vortex, they also included a PV anomaly map for 28 August 2017 associated with what this paper calls "Vortex A" (see Figure 16 of the following reference).…”

“…• Line 335: The northwest-southeast tilt of the PV increments for the Koobor vortex, shown in Figure 10a, is interesting. In the recent analysis by Allen et al (2020), they examined how Koobor tilts with height and found a NW-SE tilt of the vortex in January. They used a dynamical argument to show how this tilt may develop from internal vortex dynamics in a shear flow.…”

“…The argument of Allen et al (2020) concerns the structure of the vortex and does not apply to the increments, although they are both related to the vertical shear. We now mention their explanation.…”

“…The ingredient of the beta-drift is the wrapping of planetary vorticity around the vortex (Sai-Lap Lam and Dritschel, 2001). This effect is visible in Fig.10 of Allen et al (2020) but the beta-drift is ignored by these authors who privilege the effect of the vertical tilt.…”