The composition of the extratropical transition layer (ExTL), which is the transition zone between the stratosphere and the troposphere in the midlatitudes, largely depends on dynamical processes ...fostering the exchange of air masses. The Wave-driven ISentropic Exchange (WISE) field campaign in 2017 aimed for a better characterization of the ExTL in relation to the dynamic situation. This study investigates the potential of the first-ever collocated airborne lidar observations of ozone (O3) and water vapor (H2O) across the tropopause to depict the complex trace gas distributions and mixing in the ExTL. A case study of a perpendicular jet stream crossing with a coinciding strongly sloping tropopause is presented that was observed during a research flight over the North Atlantic on 1 October 2017.The collocated and range-resolved lidar data that are applied to established tracer–tracer (T–T) space diagnostics prove to be suitable to identify the ExTL and to reveal distinct mixing regimes that enabled a subdivision of mixed and tropospheric air. A back projection of this information to geometrical space shows remarkably coherent structures of these air mass classes along the cross section. This represents the first almost complete observation-based two-dimensional (2D) illustration of the shape and composition of the ExTL and a confirmation of established conceptual models. The trace gas distributions that represent typical H2O and O3 values for the season reveal tropospheric transport pathways from the tropics and extratropics that have influenced the ExTL. Although the combined view of T–T and geometrical space does not inform about the process, location and time of the mixing event, it gives insight into the formation and interpretation of mixing lines. A mixing factor diagnostic and a consideration of data subsets show that recent quasi-instantaneous isentropic mixing processes impacted the ExTL above and below the jet stream which is a confirmation of the well-established concept of turbulence-induced mixing in strong wind shear regions. At the level of maximum winds reduced mixing is reflected in jumps in T–T space that occurred over small horizontal distances along the cross section. For a better understanding of the dynamical and chemical discontinuities at the tropopause, the lidar data are illustrated in isentropic coordinates. The strongest gradients of H2O andO3 are found to be better represented by a potential vorticity-gradient-based tropopause compared to traditional dynamical tropopause definitions using constant potential vorticity values. The presented 2D lidar data are considered to be of relevance for the investigation of further meteorological situations leading to mixing across the tropopause and for future validation of chemistry and numerical weather prediction models.
The Aeolus satellite mission of the European Space Agency (ESA) has brought the first wind LiDAR to space to satisfy the long-existing need for global wind profile observations. Until the successful ...launch on 22 August 2018, pre-launch campaign activities supported the validation of the measurement principle, the instrument calibration, and the optimization of retrieval algorithms. Therefore, an airborne prototype instrument has been developed, the ALADIN Airborne Demonstrator (A2D), with ALADIN being the Atmospheric Laser Doppler Instrument of Aeolus. Two airborne campaigns were conducted over Greenland, Iceland and the Atlantic Ocean in September 2009 and May 2015, employing the A2D as the first worldwide airborne direct-detection Doppler Wind LiDAR (DWL) and a well-established coherent 2-µm wind LiDAR. Both wind LiDAR instruments were operated on the same aircraft measuring Mie backscatter from aerosols and clouds as well as Rayleigh backscatter from molecules in parallel. This paper particularly focuses on the instrument response calibration method of the A2D and its importance for accurate wind retrieval results. We provide a detailed description of the analysis of wind measurement data gathered during the two campaigns, introducing a dedicated aerial interpolation algorithm that takes into account the different resolution grids of the two LiDAR systems. A statistical comparison of line-of-sight (LOS) winds for the campaign in 2015 yielded estimations of the systematic and random (mean absolute deviation) errors of A2D observations of about 0.7 m/s and 2.1 m/s, respectively, for the Rayleigh, and 0.05 m/s and 2.3 m/s, respectively, for the Mie channel. In view of the launch of Aeolus, differences between the A2D and the satellite mission are highlighted along the way, identifying the particular assets and drawbacks.
Airborne lidar backscatter data is used to determine in- and out-of-cloud regions. Lidar measurements of water vapor together with model temperature fields are used to calculate relative humidity ...over ice (RH
i
). Based on temperature and RH
i
we identify different stages of cirrus evolution: homogeneous and heterogeneous freezing, depositional growth, ice sublimation and sedimentation. We will present our classification scheme and first applications on mid-latitude cirrus clouds.
This case study of a warm conveyor belt (WCB) event that was probed on a research flight during the THORPEX (The Observing‐System Research and Predictability Experiment) Pacific Asian Regional ...Campaign (T‐PARC) field experiment in 2008, investigates the sensitivity of the forecast of the WCB, the associated cyclone and the downstream waveguide to the moisture content in the inflow region of the WCB. By assimilating water vapour profiles of a differential absorption lidar (DIAL) into the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS), the inflow moisture in the analysis fields is adjusted and humidity is reduced in a broad region around the flight track. The initial reduction of moisture in the WCB inflow affects the latent heat release along the WCB, as well as the potential vorticity (PV) production at lower levels. This change led to a substantially lower outflow height of the forecasted WCB. Further, the height of the tropopause was reduced up to 20 hPa, which caused a change in the jet stream wind speeds of up to 15% downstream. Although the impact on the developing surface cyclone was small, improvements of the PV structure as well as of the kinetic energy could be identified.
The composition of the extratropical transition layer (ExTL), which is the transition zone between
the stratosphere and the troposphere in the midlatitudes, largely depends on dynamical processes
...fostering the exchange of air masses. The Wave-driven ISentropic Exchange (WISE) field campaign in
2017 aimed for a better characterization of the ExTL in relation to the dynamic situation. This
study investigates the potential of the first-ever collocated airborne lidar observations of ozone
(O3) and water vapor (H2O) across the tropopause to depict the complex trace
gas distributions and mixing in the ExTL. A case study of a perpendicular jet stream crossing with
a coinciding strongly sloping tropopause is presented that was observed during a research flight
over the North Atlantic on 1 October 2017. The collocated and range-resolved lidar data that are applied to established tracer–tracer (T–T) space
diagnostics prove to be suitable to identify the ExTL and to reveal distinct mixing regimes
that enabled a subdivision of mixed and tropospheric air. A back projection of this information to
geometrical space shows remarkably coherent structures of these air mass classes along the
cross section. This represents the first almost complete observation-based two-dimensional (2D)
illustration of the shape and composition of the ExTL and a confirmation of established conceptual
models. The trace gas distributions that represent typical H2O and O3 values
for the season reveal tropospheric transport pathways from the tropics and extratropics that have
influenced the ExTL. Although the combined view of T–T and geometrical space does not inform about
the process, location and time of the mixing event, it gives insight into the formation and
interpretation of mixing lines. A mixing factor diagnostic and a consideration of data subsets
show that recent quasi-instantaneous isentropic mixing processes impacted the ExTL above and below
the jet stream which is a confirmation of the well-established concept of turbulence-induced
mixing in strong wind shear regions. At the level of maximum winds reduced mixing is reflected in
jumps in T–T space that occurred over small horizontal distances along the cross section. For a
better understanding of the dynamical and chemical discontinuities at the tropopause, the lidar
data are illustrated in isentropic coordinates. The strongest gradients of H2O and
O3 are found to be better represented by a potential vorticity-gradient-based tropopause compared to
traditional dynamical tropopause definitions using constant potential vorticity values. The presented 2D lidar data
are considered to be of relevance for the investigation of further meteorological situations
leading to mixing across the tropopause and for future validation of chemistry and numerical
weather prediction models.
During a research flight of the Wave-driven ISentropic
Exchange (WISE) campaign, which was conducted over the eastern North
Atlantic on 1 October 2017, the composition of the upper troposphere and
...lower stratosphere (UTLS) across the North Atlantic jet stream was observed
by airborne, range-resolved differential absorption lidar (DIAL) profiles.
We investigate how the high variability in the paired H2O and O3 distribution along the two-dimensional lidar cross section is affected by
synoptic-scale weather systems, as revealed by the Lagrangian history of the
observed air masses. To this aim, the lidar observations are combined with
10 d backward trajectories along which meteorological parameters and
derived turbulence diagnostics are traced. The transport and mixing
characteristics are then projected to the vertical cross sections of the
lidar measurements and to the H2O–O3 phase space to explore
linkages with the evolution of synoptic-scale weather systems and their
interaction. Tropical, midlatitude, and arctic weather systems in the region
of the jet stream and the related transport and mixing explain the complex
H2O and O3 distribution to a large extent: O3-rich
stratospheric air from the high Arctic interacts with midlatitude air from
the North Pacific in a northward-deflected jet stream associated with an
anticyclone over the US and forms a filament extending into the tropopause
fold beneath the jet stream. In the troposphere, lifting related to
convection in the intertropical convergence zone (ITCZ) and two tropical
cyclones that continuously injected H2O into dry descending air from
the tropical Atlantic and Pacific form filamentary H2O structures. One
tropical cyclone that transitioned into a midlatitude cyclone lifted moist
boundary layer air, explaining the highest tropospheric H2O values.
During the two days before the observations, the air with mixed tropospheric
and stratospheric characteristics experienced frequent turbulence along the
North Atlantic jet stream, indicating a strong influence of turbulence on the
formation of the extratropical transition layer (ExTL). This investigation
highlights the complexity of stirring and mixing processes and their close
connection to interacting tropospheric weather systems from the tropics to the
polar regions, which strongly influenced the observed fine-scale H2O
and O3 distributions. The identified non-local character of
mixing should be kept in mind when interpreting mixing lines in
tracer–tracer phase space diagrams.
Localised regions of negative potential vorticity (PV) are frequently seen on the equatorward flank of the upper‐tropospheric jet streams in analysis and forecast products. Their positioning, on the ...anticyclonic side of the jet and often close to the jet core, suggest they are associated with an enhancement of jet stream maximum winds. Given that PV is generally positive in the northern hemisphere and is materially conserved under adiabatic conditions, the presence of negative PV is indicative of recent diabatic activity. However, little is understood on the mechanisms for its generation and subsequent impacts. In this paper, aircraft measurements from a recent field campaign are used to provide direct observational evidence for the presence of negative PV on the anticyclonic side of an upper‐tropospheric jet. Theory is then developed to understand the process by which PV can turn negative. The key ingredient is diabatic heating in the presence of vertical wind shear, and the resulting PV anomalies are shown to always result from a flux of PV directed “down the isentropic slope”. This explains why, for the typical situation of heating in a warm conveyor belt, negative PV values appear on the equatorward side of the upper‐tropospheric jet stream close to the jet core. These ideas are illustrated with a semi‐geostrophic model and the processes responsible for the observed negative PV are explored using an operational forecast model with online PV tracer diagnostics. The diabatic influence on jet stream winds and shear is of interest because it is pertinent to the predictability of extreme jet stream events and associated flight‐level turbulence, and is crucial to the propagation of Rossby waves at tropopause level, development of midlatitude weather systems and their subsequent impacts at the surface.
The process by which diabatic heating in warm conveyor belts (WCBs) can act to change the sign of potential vorticity (PV) is studied in observations, theory and models. In the Northern Hemisphere PV is typically positive, and the occurrence of localised regions of negative PV are evidence of recent diabatic activity. The schematic shows the typical structure of negative PV features associated with latent heating in WCBs.
A large-scale comparison of water-vapour vertical-sounding instruments took place over central Europe on 17 October 2008, during a rather homogeneous deep stratospheric intrusion event (LUAMI, ...Lindenberg Upper-Air Methods Intercomparison). The measurements were carried out at four observational sites: Payerne (Switzerland), Bilthoven (the Netherlands), Lindenberg (north-eastern Germany), and the Zugspitze mountain (Garmisch-Partenkichen, German Alps), and by an airborne water-vapour lidar system creating a transect of humidity profiles between all four stations. A high data quality was verified that strongly underlines the scientific findings. The intrusion layer was very dry with a minimum mixing ratios of 0 to 35 ppm on its lower west side, but did not drop below 120 ppm on the higher-lying east side (Lindenberg). The dryness hardens the findings of a preceding study (“Part 1”, Trickl et al., 2014) that, e.g., 73 % of deep intrusions reaching the German Alps and travelling 6 days or less exhibit minimum mixing ratios of 50 ppm and less. These low values reflect values found in the lowermost stratosphere and indicate very slow mixing with tropospheric air during the downward transport to the lower troposphere. The peak ozone values were around 70 ppb, confirming the idea that intrusion layers depart from the lowermost edge of the stratosphere. The data suggest an increase of ozone from the lower to the higher edge of the intrusion layer. This behaviour is also confirmed by stratospheric aerosol caught in the layer. Both observations are in agreement with the idea that sections of the vertical distributions of these constituents in the source region were transferred to central Europe without major change. LAGRANTO trajectory calculations demonstrated a rather shallow outflow from the stratosphere just above the dynamical tropopause, for the first time confirming the conclusions in “Part 1” from the Zugspitze CO observations. The trajectories qualitatively explain the temporal evolution of the intrusion layers above the four stations participating in the campaign.
PDF
