Uncertainties in modelling the UTLS and its impact on weather and climate are in part due to insufficient understanding and/or parameterisation of relevant small-scale processes which numerical weather prediction models (NWP) and/or climate models do not resolve. (1) Microphysical processes of aerosols and cloud particles and their precursors, are far from being well parameterised. (2) Uncertainties in the description of species transport towards and through the UTLS are to a significant part due to small-scale dynamics. (3) Small-scale diabatic and non-conservative dynamical processes also influence the structure of the tropopause. Owing to their resolution and insufficient small-scale parameterisations, reanalysis data or coarse grid resolution models (such as chemistry climate models), e.g., account for these processes only in parts.
The tropopause structure itself, acting as an imperfect transport barrier, as well as tropopause microphysics, mixing, and transport processes determine the moisture, tracer, and aerosol compositions in the lower stratosphere that (1) are taken up by the Brewer-Dobson circulation (BDC), that (2) affect the radiation budget, and that (3) also affect cirrus clouds in the lower stratosphere with their potentially important radiative impact. Inaccuracies in the treatment of these processes, e.g., lead to a moist bias in global models above the extratropical tropopause, and hence a considerable cold bias there with potentially important consequences for the local dynamics and chemistry, or even for dynamics on larger scales.
Pure ice clouds, i.e. cirrus clouds with a non-negligible impact on the UTLS water-vapour budget, on radiative forcing, and on dynamics on various scales, and their precursors, so-called ice-supersaturated regions (ISSRs) frequently occur in the tropopause region, often in a close spatial connection to the thermal tropopause. Ground based cloud observations from mid latitude lidar stations show frequent cirrus events at and above the tropopause. Water vapour observations and meteorological analyses suggest a link between the isentropic transport of subtropical air masses at 350 K and cirrus formation at mid and high latitudes, associated with the isentropic poleward transport of subtropical air masses.
Stratosphere-troposphere exchange (STE) of species can be considerably affected both by the forcing of the upper branch of the BDC by gravity waves (GWs) and by the direct occurrence of GWs at the tropopause. GWs at the tropopause also potentially induce turbulence and mixing, thereby redistributing trace gases and aerosols in the tropopause region. Turbulence from GW breaking above deep convective systems has been identified as the major process which may lead to irreversible transport of trace gases and aerosols across the tropopause by deep convection. In regions of a strong jet the transient effect of GWs lead to a significant vertical shear of the horizontal wind, hence low Richardson numbers, and thus providing conditions favourable for turbulent STE. While the impact on planetary-wave propagation is resolved by present-day weather-forecast and climate models, at least in climate models this is at best insufficiently the case for GWs.
Turbulent mixing in the UTLS is usually parameterised with 1D-turbulence parameterisations designed for the atmospheric boundary layer with possible modification of exchange coefficient or source terms in the TKE equation. The impact of turbulence on the life cycle of cirrus clouds is not known yet. Since ice crystal formation and growth is crucially affected by local dynamics, a significant impact on ice particle growth might be possible.
The upper-tropospheric composition constitutes the chemical lower boundary condition for transport and mixing of gases and aerosols into the lower stratosphere, where most species have longer life time due to lower temperatures and the absence of loss processes (e.g. scavenging). Ozone, water vapour and aerosol in the upper troposphere are strongly affected by rapid vertical transport from the boundary layer to the upper troposphere. Short-lived precursors of ozone and aerosol precursors may affect ozone and aerosol abundance and composition in the upper troposphere and partially the lower stratosphere when transported across the tropopause. Most prominent is the warm conveyor belt (WCB) which occurs ahead of the eastward propagating surface cold front in the warm sector, leading to rapid uplift of moist air masses partly from the boundary layer to the upper troposphere. The rapid uplift of moisture leads to a significant release of latent heat and the formation of liquid or mixed phase clouds during uplift and ice clouds at the low temperatures in the upper troposphere. Hence, WCBs affect cirrus occurrence and properties, and in turn the distribution of water vapour in the UTLS. Ice and aerosol formation triggered by vertical uplift in mid-latitude frontal systems play an important role for cirrus occurrence in addition to in-situ formation associated with the ISSRs in ridges.
Research questions (B)
RQ-B1: What is the impact of small-scale microphysical processes on the structure of the tropopause region and what are the implications for cross tropopause exchange?
RQ-B2: How do gravity waves, instabilities and turbulence in the UTLS affect troposphere-stratosphere coupling, mixing and cloud life cycles at the tropopause and how does the UTLS affect the waves’ propagation?
RQ-B3: What is the effect of diabatic dynamics on the UTLS composition and cross tropopause exchange from the turbulent to the synoptic scale?