Stratosphere and troposphere are separated by the tropopause, which is conventionally defined by a change in the vertical temperature lapse rate (thermal tropopause) or by a certain value of potential vorticity (dynamical tropopause). As the tropopause is at higher potential temperatures in the tropics (about 380 K) than in the extratropics, some isentropes (surfaces of constant potential temperature) cross the tropopause. The region of the stratosphere below 380K potential temperature, where isentropes are not entirely in the stratosphere, is referred to as the LMS. Air masses observed above the 380K isentropic surface, must have passed the tropical tropopause, as the main stratospheric transport at extratropical latitudes is downward. The source region for all air masses entering the stratosphere is the TTL. It has been recognized that the level of maximum convective outflow in the tropics is mostly around 10 to 12 km altitude, and hardly ever above 14 km (Gettelman and Forster, 2002). Above this altitude, an increase in static stability of the atmosphere is observed, and vertical transport time scales are much longer than in the tropical troposphere below 14 km.
The lower and LMS is of high importance for the radiative energy budget of the atmosphere. It is therefore essential to understand the transport processes governing this region. This can be achieved by observations of trace gases, whose distributions are often determined by transport processes. The chemical composition and the transport pathways for air to enter the TTL, and to be transported upward in the TTL into the stratosphere, control the chemical composition of the stratosphere. In particular the freeze–drying at the tropical tropopause, which results in the extreme dryness of the stratosphere, can only be understood through investigations of the dynamics of the TTL. As observations in this area are very sparse, seasonalities and variabilities (e.g., longitudinal) in the TTL are poorly quantified.
Observations of water vapor and CO2 (Rosenlof et al., 1997, Strahan et al., 1998) in the lower stratosphere show that the seasonal cycles of these compounds, which originate in the tropics can be observed up to potential temperatures of about 450 K. The strength of the tropical influence seems to vary with season and also show inter–annual variability, as can be seen from the variability of long lived tracers in the lower and LMS. All air masses in the lower stratosphere must have passed the tropical tropopause. However, part of these air masses is transported horizontally on rather short time scales from the tropics to the mid-latitudes, other parts are transported downwards from the middle stratosphere (Hoor et al., 2005).
The composition of the air in the LMS is in addition affected by direct transport of extra–tropical tropospheric air over the extra–tropical tropopause. These air masses have complex chemical characteristics. Whereas descending air from the middle stratosphere has low water vapor and high ozone, the tropical fraction will have low water vapor and lower ozone. Tropospheric air is characterized by low ozone mixing ratios and high water vapor.
Mid-Latitude Weather Systems and Tropospheric Transport
The daily life in countries of the mid-latitudes is largely influenced by the continuous sequence of high and low pressure systems. Especially High Impact Weather (HIW) events, i.e., storms, floods, droughts, impact the society and economy. Although the predictability of weather has improved significantly in the past decades a number of forecast busts remain. Predictions of HIW still need to be improved. The use of a long range and high altitude research aircraft offers new possibilities to investigate various aspects of weather systems and allows creating new knowledge in related physical processes and transporting mechanisms.
A scientific goal is to use a combined observational and modeling strategy to elucidate the essential dynamical mechanisms that limit the skill of the numerical weather prediction of such systems (especially, the precise prediction of the heavy precipitation and storm events) and that determine the transport by those circulation systems. Weather systems cover a wide range of spatial scales as well as span forecast times from now-casting to medium range. With HALO, multi-scale observations of the decisive parameters, from mesoscale (convection, embedded convection, squall lines) to synoptic–scale systems (Mediterranean depression, Atlantic cyclogenesis, and tropical cyclones) become possible.
Throughout the last years research campaigns focused on the benefit of different observation types with the aim to improve the observing and data assimilation systems. The role of additional observations by novel instruments for the forecast of mid-latitude weather was investigated. Recently, the focus shifted to observations of dynamical processes affecting predictability.
One of the limiting factors for the predictability of mid-latitude weather systems in Numerical Weather Prediction (NWP) models is believed to be the incorrect representation of diabatic processes that can strongly influence the evolution and intensity of cyclones. The rapid growthof errors from small scales can modify upper level Rossby waves and the jet stream structure.
This in turn has substantial impact on the downstream weather evolution. The diabatic processes are associated with release of latent heat due to phase transitions of water, surface fluxes, or radiative effects. For Europe strong diabatic processes are connected with tropopause polar vortices, extratropical cyclones and their WBC outflows, and tropical cyclones transitioning into the extra tropics. Diabatic processes strongly depend on the moisture content in the boundary layer and in turn to transport of water vapor. Multi-scale observations of wind, water vapor, microphysical properties and radiation are needed to quantify the impact of diabatic processes on the development of weather systems, upper-level Rossby waves and the downstream weather. So far these systems that often start developing over the North Atlantic where not reachable and their spatial extent exceeded the range of established airborne platforms.
Dynamics at upper levels also depend on diabatic processes related to radiative processes at the tropopause. The distribution of moisture at the tropopause, which is also highly relevant for climate, is an important factor that depends on the transport of moisture to the upper troposphere and LMS by mid-latitude weather systems. Highly accurate observations at low moisture contents are needed to investigate the distribution of water vapor in the upper troposphere for weather and climate research.
The downstream triggered extreme weather events are responsible for damaging impacts due to strong wind, heavy precipitation and flooding. Such phenomena are neither easy to analyze nor to predict and there are many factors associated with their development, evolution, and predictability that are poorly understood. Consequences of missing information about their development might lead to economic and societal as well as ecological damage in the area affected by extreme weather events. Flow patterns as envisaged for investigation with HALO are lower tropospheric cyclones generated rapidly downstream of north Atlantic troughs and often causing severe weather with storm, heavy precipitation and subsequent flooding over the European continent.