The tropopause inversion layer in baroclinic life-cycle experiments: the role of diabatic processes

Abstract

Abstract. Recent studies on the formation of a quasipermanent layer of enhanced static stability above the thermal tropopause revealed the contributions of dynamical and radiative processes. Dry dynamics leads to the evolution of a tropopause inversion layer (TIL), which is, however, too weak compared to observations and thus diabatic contributions are required. In this study we aim to assess the importance of diabatic processes in the understanding of TIL formation at midlatitudes. The non-hydrostatic model COSMO (COnsortium for Small-scale MOdelling) is applied in an idealized midlatitude channel configuration to simulate baroclinic life cycles. The effect of individual diabatic processes related to humidity, radiation, and turbulence is studied first to estimate the contribution of each of these processes to the TIL formation in addition to dry dynamics. In a second step these processes are stepwise included in the model to increase the complexity and finally estimate the relative importance of each process. The results suggest that including turbulence leads to a weaker TIL than in a dry reference simulation. In contrast, the TIL evolves stronger when radiation is included but the temporal evolution is still comparable to the reference. Using various cloud schemes in the model shows that latent heat release and consecutive increased vertical motions foster an earlier and stronger appearance of the TIL than in all other life cycles. Furthermore, updrafts moisten the upper troposphere and as such increase the radiative effect from water vapor. Particularly, this process becomes more relevant for maintaining the TIL during later stages of the life cycles. Increased convergence of the vertical wind induced by updrafts and by propagating inertia-gravity waves, which potentially dissipate, further contributes to the enhanced stability of the lower stratosphere. Finally, radiative feedback of ice clouds reaching up to the tropopause is identified to potentially further affect the strength of the TIL in the region of the clouds.

Wave modulation of the extratropical tropopause inversion layer

“…Regarding radiation, recent modeling studies highlighted the importance of including diabatic processes related to water vapor and clouds, apart from dynamics, in order to explain the midlatitude TIL (Ferreira et al, 2016;Kunkel et al, 2016). Particularly, the baroclinic life cycle experiments by Kunkel et al (2016) that included diabatic effects from water vapor and clouds showed a faster deepening of the wave and strengthening of the TIL, along with an increase in the crosstropopause gradient of water vapor which would in turn increase radiative cooling of the tropopause.…”

“…Particularly, the baroclinic life cycle experiments by Kunkel et al (2016) that included diabatic effects from water vapor and clouds showed a faster deepening of the wave and strengthening of the TIL, along with an increase in the crosstropopause gradient of water vapor which would in turn increase radiative cooling of the tropopause.…”

“…3) and the absence of a TIL in Figs. 5b and A2b after removing the wave signals, while also fitting with the results by Ferreira et al (2016) and Kunkel et al (2016). Radiation would then be of importance for enhancing baroclinic wave growth, among other factors that trigger the wave and its growth near the tropopause.…”

“…Other modeling studies suggest that radiation and diabatic processes related to water vapor and clouds could share importance with dynamics in strengthening the midlatitude TIL (Ferreira et al, 2016;Kunkel et al, 2016). …”

Processes Maintaining Tropopause Sharpness in Numerical Models

“…Chagnon et al () also argued that the transport of moisture by the WCB would enhance the effects of long‐wave radiation. Kunkel et al () showed similar results for the TIL: long‐wave radiation strengthened the TIL and transport of moisture to the tropopause results in a more rapid formation of the TIL. However, these results are limited to case studies (Chagnon & Gray, ; Chagnon et al, ) and idealized simulations (Kunkel et al, ).…”