Abstract
Abstract. Recent studies on the formation of a quasi-permanent 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.
Highlights
The sharpness of the tropopause in the extratropics has gained increased attention in recent years (e.g., Gettelman and Wang, 2015)
We focus mainly on the following questions: (1) how do non-conservative processes, i.e., diabatic processes, alter the tropopause inversion layer (TIL) evolution in baroclinic life cycles compared to the well-known evolution in the adiabatic and frictionless case? (2) What is the relative importance of individual processes that contribute to the formation the TIL during different stages of the life cycles?
We first analyzed the effect of individual diabatic processes, i.e., related to clouds, radiation, and mixing processes before we estimated the relative importance of each process
Summary
The sharpness of the tropopause in the extratropics has gained increased attention in recent years (e.g., Gettelman and Wang, 2015). In several studies it was shown that a TIL can form from balanced, adiabatic, and frictionless dynamics without explicit contributions from radiation in the extratropics These idealized model simulations span the range from local to global scales, with studies of the dynamics of upper-level anomalies of potential vorticity (further abbreviated with PV) (Wirth, 2003, 2004), of baroclinic life cycles (Erler and Wirth, 2011), and of the dynamical response to a forcing of a Held–Suarez test (Held and Suarez, 1994) in a dry general circulation model (Son and Polvani, 2007).
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