Abstract
Abstract. We report airborne remote-sensing observations of a tropopause fold during two crossings of the polar front jet over northern Italy on 12 January 2016. The GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) observations allowed for a simultaneous mapping of temperature, water vapour, and ozone. They revealed deep, dry, and ozone-rich intrusions into the troposphere. The mesoscale fine structures of dry filaments at the cyclonic shear side north of the jet and tongues of moist air entraining tropospheric air into the stratosphere along the anticyclonic shear side south of the jet were clearly resolved by GLORIA observations. Vertically propagating mountain waves with recorded temperature residuals exceeding ±3 K were detected above the Apennines. Their presence enhanced gradients of all variables locally in the vicinity of the tropopause. The combination of H2O−O3 correlations with potential temperature reveals an active mixing region and shows clear evidence of troposphere-to-stratosphere and stratosphere-to-troposphere exchange. High-resolution short-term deterministic forecasts of ECMWF's integrated forecast system (IFS) applying GLORIA's observational filter reproduce location, shape, and depth of the tropopause fold very well. The fine structure of the mixing region, however, cannot be reproduced even with the 9 km horizontal resolution of the IFS, used here. This case study demonstrates convincingly the capabilities of linear limb-imaging observations to resolve mesoscale fine structures in the upper troposphere and lower stratosphere, validates the high quality of the IFS data, and suggests that mountain wave perturbations have the potential to modulate exchange processes in the vicinity of tropopause folds.
Highlights
Tropopause folds are preferred regions of bidirectional stratosphere–troposphere exchange (STE) of mass and trace gases in the middle latitudes (e.g. Holton et al, 1995; Gettelman et al, 2011, and references therein)
Temperature observations across the tropopause fold confirm the expected reversal of the meridional temperature gradient from the lower troposphere to the upper troposphere, which is responsible for closing the jet stream in the lower stratosphere (Fig. 8b)
We refer to this layer as the extratropical transition layer (ExTL), Gettelmann et al (2011), as it marks the crossover from tropospheric to stratospheric air
Summary
Tropopause folds are preferred regions of bidirectional stratosphere–troposphere exchange (STE) of mass and trace gases in the middle latitudes (e.g. Holton et al, 1995; Gettelman et al, 2011, and references therein). Tropopause folds are preferred regions of bidirectional stratosphere–troposphere exchange (STE) of mass and trace gases in the middle latitudes Holton et al, 1995; Gettelman et al, 2011, and references therein) Their generation is related to cyclogenetically active regions Tropopause folds form preferentially along strong jet streams separating polar or Arctic air masses from those of subtropical origin. Upper-level jet and front systems are prone to the generation of clear-air turbulence, which is an important exchange mechanism for atmospheric constituents across the tropopause Dritschel and McIntyre, 2008; Martius et al, 2010) They provide a favourable medium for the vertical propagation of internal gravity waves excited in the troposphere (e.g. Preusse et al, 2006; Ern et al, 2018)
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