Influence of Diabatic Effects and Tropopause Structure on Baroclinic Development

Abstract

Midlatitude cyclones play an important role in midlatitude weather and global climate. While baroclinic instability is the dominant mechanism for their intensification, midlatitude cyclone development is also influenced significantly by diabatic processes and tropopause structure. Of special importance is the relatively well-established role of latent heating associated with cloud condensation, which has been shown to enhance the intensification and reduce the horizontal scale of midlatitude cyclones. Less clear, however, is the role of other diabatic processes, such as latent cooling associated with evaporation of rain and sublimation or melting of snow as well as surface sensible and latent heat fluxes transferring heat and moisture between the ocean and the overlying cyclone. On top of these diabatic effects, baroclinic development depends on the structure of wind shear and stratification around the tropopause, which is essential for the coupling of the surface cyclone with the upper levels. However, the sensitivity of the development to various modifications of tropopause structure relative to diabatic processes remains unclear.

In this thesis, consisting of three papers, we examine the contribution of these less studied diabatic processes as well as the influence of tropopause structure on baroclinic development in a numerical extension of the linear quasi-geostrophic Eady model including effects of latent heating, latent cooling, surface sensible heat fluxes, and vertical modifications of wind shear and stratification across the tropopause. The linear approach focuses on effects of first order importance and restricts the study to the incipient stage of cyclone development, which is typically nearly linear.

In the first paper, we include a layer of latent cooling below a mid-tropospheric layer of latent heating in the ascent region of the cyclone. We find that low-level cooling weakens the vertical circulation and hence latent heating. With the intensifying effect of latent heating being reduced, latent cooling weakens baroclinic growth.

In the second paper, we find that baroclinic growth in the presence of latent heating also decreases when including surface sensible heat fluxes that are downward in the warm sector and upward in the cold sector, which is in line with reduced local horizontal temperature gradients and hence less low-level baroclinicity. Our findings remain similar when parametrising the surface sensible heat fluxes with respect to meridional temperature advection or vertical motion instead of temperature. In contrast, our findings are sensitive to the horizontal shift of surface sensible heat fluxes. Furthermore, estimates of increased latent heating related to moisture supply from surface latent heat fluxes indicate that the role of latent heat fluxes overcompensates for the detrimental effects from sensible heat fluxes, leaving the net effect of surface heat fluxes beneficial for cyclone intensification.

In the third paper, cyclone intensification is investigated further by modifying the wind shear and stratification around the tropopause in experiments with and without mid-tropospheric latent heating. We find that baroclinic growth is not very sensitive to the sharpness and the altitude of the tropopause, but rather sensitive to modifications of the low-stratospheric wind shear and stratification. However, for reasonable modifications of wind shear, stratification, and latent heating intensity, we show that baroclinic growth is more sensitive to mid-tropospheric latent heating than to tropopause structure.