Dependence of Polar Low Development on Baroclinicity and Physical Processes: An Idealized High-Resolution Numerical Experiment

Abstract

Polar low dynamics in an idealized atmosphere in which baroclinicity, stratification, and average temperature are varied in the typically observed range is investigated using a 5-km-resolution nonhydrostatic model. The baroclinicity is found to be the most important factor that strongly controls the polar low dynamics. When the baroclinicity is weak, a small, nearly axisymmetric vortex develops through a cooperative interaction between the vortex flow and cumulus convection. The surface friction promotes the vortex dynamics by transporting the sensible heat and moisture into the vortex center. The vortex development has a strong sensitivity to the initial perturbation. As the baroclinicity is increased, most of the characteristics of polar low dynamics change smoothly without showing any significant regime shift. The vortex for an intermediate baroclinicity, however, moves northward, which is a unique behavior. This is caused by vortex stretching on the northern side of the vortex where intense convection produces a stronger updraft. When the baroclinicity is strong, a larger vortex with a comma-shaped cloud pattern develops. The condensational heating, baroclinic conversion from the basic available potential energy, and conversion from the basic kinetic energy through the vertical shear all contribute to the vortex development, which depends little on the initial perturbation. The above relations between baroclinicity and vortex dynamics are proved to be robust in the typically observed range of stratification and average temperature.

Impact of different initial conditions on the growth of polar lows: idealised baroclinic channel simulations

“…The rest of the experiments focus on the relative influence of static stability N (N1), baroclinicity B (B1), and surface potential temperature θ s (θ 1) of the basic state on the formation and growth of a PL. Magnitudes of N, B and θ s prescribed for the CTL simulation are such that the conditions in the baroclinic channel are favourable for the formation of a PL (Sardie and Warner, 1983;Yanase and Niino, 2007). The reduction in the baroclinicity was achieved in the B1 simulation by weakening the vertical wind shear.…”

“…On the other hand, Linders et al (2011) found a relatively weak response in sea-level pressure to SST variations of about -0.6 hPa K −1 . Apart from the differences in the model set-up, these widely varying results can be linked to the fact that several processes may operate simultaneously during the life cycle of a PL (Yanase and Niino, 2007). In the real atmosphere, the sensitivity of a PL to various conditions may vary from case to case depending on, for example, the dominating physical mechanism, geographical location, proximity of sea-ice, and so on.…”

“…Craig (1995) found that radiative cooling is likely to increase the growth rate and maximum intensity of a convective PL. Using a three-dimensional non-hydrostatic model, Yanase and Niino (2007) investigated the way baroclinic and convective processes influence the development and energetics of PLs. The basic state for the study was characterised by an axisymmetric vortex at the surface with a finite radius and a uniform vertical shear of the zonal wind.…”

The role of horizontal resolution for polar low simulations

“…The heating produces a positive anomaly of potential vorticity (PV) at low levels which contributes to the development of a warm core cyclone. Idealized numerical experiments (Yanase and Niino, 2007) have indicated that both diabatic heating due to convection and baroclinic instability have important roles in the development of polar lows. Yanase and Niino performed simulations on an initial vortex with diabatic heating and sea surface fluxes turned on and off and with different degrees of baroclinicity in the zonal flow.…”

Toward a Multi-Decadal Climatology of North Pacific Polar Lows Employing Dynamical Downscaling

“…Since Polar Lows usually form in cold air masses moving from cold continents or ice-covered regions over open water, this pattern of formation reflects mostly the geographically different sea surface temperature conditions. In a number of case studies it was demonstrated that Polar Lows can be described by dynamical high-resolution models [for the Japan Sea, see Fu et al (2004a), Yanase et al (2004), Yanase and Niino (2007); the Bering Sea see Bresch et al (1997); the Gulf of Alaska see Businger (1987) and Blier (1996)]. In all cases the model results were consistent with satellite observations.…”