Examining the dynamics of a Borneo vortex using a balance approximation tool

Hardy, Sam; Methven, John; Schwendike, Juliane; Harvey, Ben; Cullen, M. J. P.

doi:10.5194/egusphere-2023-1312

Cited by 2 publications

(1 citation statement)

References 36 publications

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“…The SGT inversion tool has been previously applied to quantify the importance of diabatic processes in model error growth over the North Atlantic (Sánchez et al, 2020). It can also be applied in the tropics using the weak-temperature gradient, as done in Hardy et al (2023) to explore the dynamics of synoptic vortices over the South China Sea.…”

Section: Semi-geotriptic Inversion Toolmentioning

confidence: 99%

How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos

Sanchez,

Gray,

Volonte

et al. 2023

Preprint

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Abstract. Medicane Ianos in September 2020 was one of the strongest medicanes observed in the last 25 years. It was, like other medicanes, a very intense cyclone evolving from a baroclinic mid-latitude low into a tropical-like cyclone. The dynamical elements necessary to improve the predictability of Ianos are explored with the use of simulations with the Met Office Unified Model (MetUM) at 2.2 km grid spacing for five different initialisation times, from four to two days before Ianos's landfall. Simulations are also performed with the Sea Surface Temperature (SST) uniformly increased and decreased by 2 K from analysis to explore the impact of enhanced and reduced sea-surface surface fluxes on Ianos's evolution. All the simulations with +2 K SST are able to simulate medicane Ianos, albeit too intensely. The simulations with control SST initialised at the two earliest times fail to capture intense preceding precipitation events at the right locations, and the subsequent development of Ianos. Amongst the simulations with -2 K SST, only the one initialised at the latest time develops the medicane. Links between sea-surface fluxes and upper-level baroclinic processes are investigated. We find (i) a bubble of low-valued potential vorticity (PV) formed within a trough above where Ianos developed, diabatic processes associated with a preceding precipitation event triggered a balanced divergent flow in the upper-levels which contributed to the creation and maintenance this low-PV bubble, as shown by results from a semi-geostrophic inversion tool, (ii) Upper-level geostrophic vorticity advection associated with the low-PV bubble forced quasi-geostrophic ascent during Ianos's cyclogenesis, and (iii) diabatic processes dominated by deep convection formed a vertical PV tower in Ianos and continued to produce diabatically-induced divergent outflow aloft, thus sustaining Ianos's development. Simulations missing any of these three elements do not develop medicane Ianos. Our results imply that preceding convection was essential for the subsequent development of Ianos, highlighting the importance of the interactions between near-surface small-scale diabatic processes and the upper-level quasi-geostrophic flow. A warmer SST strengthens the processes and thus enables Ianos to be predicted in simulations initiated at the earlier times that failed to generate the medicane with control SSTs.

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Section: Semi-geotriptic Inversion Toolmentioning

confidence: 99%

How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos

Sanchez,

Gray,

Volonte

et al. 2023

Preprint

View full text Add to dashboard Cite

show abstract

Examining the dynamics of a Borneo vortex using a balance approximation tool

Hardy,

Methven,

Schwendike

et al. 2023

Weather Clim. Dynam.

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Abstract. Cyclonic vortices that are weaker than tropical storm category can bring heavy precipitation as they propagate across the South China Sea and surrounding countries. Here we investigate the structure and dynamics responsible for the intensification of a Borneo vortex that moved from the north of Borneo across the South China Sea and impacted Vietnam and Thailand in late October 2018. This case study is examined using Met Office Unified Model (MetUM) simulations and a semi-geotriptic (SGT) balance approximation tool. Satellite observations and a MetUM simulation with 4.4 km grid initialised at 12:00 UTC on 21 October 2018 show that the westward-moving vortex is characterised by a coherent maximum in total column water and by a comma-shaped precipitation structure with the heaviest rainfall to the northwest of the circulation centre. The Borneo vortex comprises a low-level cyclonic circulation and a mid-level wave embedded in the background easterly shear flow, which strengthens with height up to around 7 km. Despite being in the tropics at 6∘ N, the low-level vortex and mid-level wave are well represented by SGT balance dynamics. The mid-level wave propagates along a vertical gradient in moist stability, i.e. the product between the specific humidity and the static stability, at 4.5 to 5 km and is characterised by a coherent signature in the potential vorticity, meridional wind, and balanced vertical velocity fields. The vertical motion is dominated by coupling with diabatic heating and is shifted relative to the potential vorticity so that the diabatic wave propagates westwards, relative to the flow, at a rate consistent with prediction from moist semi-geostrophic theory. Initial vortex development at low levels is consistent with baroclinic growth initiated by the mid-level diabatic Rossby wave, which propagates on baroclinic shear flow on the southern flank of a large-scale cold surge.

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Examining the dynamics of a Borneo vortex using a balance approximation tool

Cited by 2 publications

References 36 publications

How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos

How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos

Examining the dynamics of a Borneo vortex using a balance approximation tool

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