Abstract
The sensitivity of the main characteristics of baroclinically unstable waves with respect to fundamental parameters of the atmosphere (the static stability parameterσ0and vertical shear of a zonal windΛ) is theoretically explored. Two types of waves are considered: synoptic scale waves and planetary scale (ultralong) waves based on an Eady-type model and model with vertically averaged primitive equations. Sensitivity functions are obtained that estimate the impact of variations inσ0andΛon the growth rate and other characteristics of unstable waves and demonstrate that waves belonging to the short-wave part of the spectrum of unstable waves are more sensitive to changes in the static stability parameter than waves belonging to the long-wave part of the spectrum. The obtained theoretical results show that the increase of the static stability and decrease of the meridional temperature gradient in midlatitude baroclinic zones in some areas of the southern hemisphere lead to a slowing of the growth rate of baroclinic unstable waves and an increasing wavelength of baroclinic unstable wave maximum growth rate, that is, a spectrum shift of unstable waves towards longer wavelengths. These might affect the favorable conditions for the development of baroclinic instability and, therefore, the intensity of cyclone generation activity.
Highlights
Static stability and the meridional temperature gradient (MTG) are among the most important fundamental parameters characterizing the state of the atmosphere and, in particular, midlatitude large-scale eddy dynamics [1, 2]
Taking into account the significant role of baroclinic instability in the development of midlatitude cyclones and the intensity changes of baroclinic instability in some areas of the southern hemisphere (SH) in recent decades, this paper examines the sensitivity of the main characteristics of baroclinically unstable waves to fundamental atmospheric parameters: the static stability parameter σ0 which is characterized by temperature lapse rate Γ = −∂T/∂z, and zonal wind vertical shear Λ which, by thermal wind balance, characterizes the meridional temperature gradient
[50, 51, 54]: an increase in static stability and a decrease of the MTG have occurred over the past few decades in some areas of the SH, which has led to a decrease in the growth rate of baroclinic unstable waves, a shift of the spectrum of unstable waves in the long wavelength part of spectrum, and a weakened intensity of cyclogenesis
Summary
Static stability and the meridional temperature gradient (MTG) are among the most important fundamental parameters characterizing the state of the atmosphere and, in particular, midlatitude large-scale eddy dynamics [1, 2]. Taking into account the significant role of baroclinic instability in the development of midlatitude cyclones and the intensity changes of baroclinic instability in some areas of the SH in recent decades, this paper examines the sensitivity of the main characteristics of baroclinically unstable waves (e.g., the growth rates of unstable waves as function of wavelength) to fundamental atmospheric parameters: the static stability parameter σ0 which is characterized by temperature lapse rate Γ = −∂T/∂z, and zonal wind vertical shear Λ which, by thermal wind balance, characterizes the meridional temperature gradient. To study synoptic scale waves, the Eady-type model is used with uniform zonal wind shear between upper and lower boundaries on an f-plane In this context parameters σ0 and Λ are considered to be variables that control the development of baroclinic instability in the atmosphere. Simplified models such as the Eady model of baroclinic instability and models with vertically averaged equations, despite their simplicity, allow solutions to be obtained that clearly illustrate real physical processes in the atmosphere
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