A convolution of observational and model data to estimate age of air spectra in the northern hemispheric lower stratosphere

Abstract

Abstract. Derivation of mean age of air (AoA) and age spectra from atmospheric measurements remains a challenge and often requires output from atmospheric models. This study tries to minimize the direct influence of model output and presents an extension and application of a previously established inversion method to derive age spectra from mixing ratios of long- and short-lived trace gases. For a precise description of cross-tropopause transport processes, the inverse method is extended to incorporate air entrainment into the stratosphere across the tropical and extratropical tropopause. We first use simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) to provide a general proof of concept of the extended principle in a controllable and consistent environment, where the method is applied to an idealized set of 10 trace gases with predefined constant lifetimes and compared to reference model age spectra. In the second part of the study we apply the extended inverse method to atmospheric measurements of multiple long- and short-lived trace gases measured aboard the High Altitude and Long Range (HALO) research aircraft during the two research campaigns POLSTRACC–GW-LCYCLE–SALSA (PGS) and Wave-driven Isentropic Exchange (WISE). As some of the observed species undergo significant loss processes in the stratosphere, a Monte Carlo simulation is introduced to retrieve age spectra and chemical lifetimes in stepwise fashion and to account for the large uncertainties. Results show that in the idealized model scenario the inverse method retrieves age spectra robustly on annual and seasonal scales. The extension to multiple entry regions proves reasonable as our CLaMS simulations reveal that in the model between 50 % and 70 % of air in the lowermost stratosphere has entered through the extratropical tropopause (30–90∘ N and S) on annual average. When applied to observational data of PGS and WISE, the method derives age spectra and mean AoA with meaningful spatial distributions and quantitative range, yet large uncertainties. Results indicate that entrainment of fresh tropospheric air across both the extratropical and tropical tropopause peaked prior to both campaigns, but with lower mean AoA for WISE than PGS data. The ratio of moments for all retrieved age spectra for PGS and WISE is found to range between 0.52 and 2.81 years. We conclude that the method derives reasonable and consistent age spectra using observations of chemically active trace gases. Our findings might contribute to an improved assessment of transport with age spectra in future studies.

Asymmetry and pathways of inter-hemispheric transport in the upper troposphere and lower stratosphere

“…Comparing the left and middle column of Fig. 6, we find that the age of SH-extratropical-origin air in the NH is younger than the age of NH-extratropical-origin air in the SH, associated with the fast flushing of the NH lower stratosphere with young air in summer (Hegglin and Shepherd, 2007;Bönisch et al, 2009;Orbe et al, 2016;Konopka et al, 2017). The latitudinal mean AoA gradients in the tropical upper troposphere are weak because of the increased latitudinal transport caused by the upper branch of the Hadley circulation and isentropic mixing.…”

Asymmetry and pathways of inter-hemispheric transport in the upper troposphere and lower stratosphere

“…The age spectra with respect to NH extratropical tropopause in the high latitude lower stratosphere (Hauck et al, 2020, their Fig. 3) show a less distinct multimodal shape with much weaker seasonality compared to spectra with respect to the NH extratropical boundary layer which are strongly affected by the seasonal variation (Fig.…”

“…However, recent studies show substantial transport uncertainties depending on the used methods, models, and meteorological reanalyses (e.g. Krol et al, 2018;Ploeger et al, 2019 (Hauck et al, 2020, their Fig. 2).…”

“…Hauck et al (2020) estimated the age spectra and AMF also using simulations from CLaMS but with the pulse tracers released from the tropopause level over the NH extratropics (30-90 • N), SH extratropics (30-90 • S), and tropics (30 • S-30 • N). The first obvious difference to our results is that the NH extratropics and SH extratropics origin air pulsed in the boundary layer contributes much less to the lower stratosphere compared to air originating at tropopause level…”

Impact of Lagrangian transport on lower-stratospheric transport timescales in a climate model

“…The largest differences occur at the edges of the tropical pipe (around 25 • N/S) and in the summertime middle-and high-latitude stratosphere. The summer edge of the tropical pipe shows the larger differences than the winter edge, particularly around 600 K. This particular point has been identified as a local minimum in diffusive activity by both Haynes and Shuckburgh (2000) and Abalos et al (2016), suggesting that the large inter-scheme differences here (as well as the winter side of the tropical pipe) are due to weaker nonphysical diffusion in EMAC-CLaMS over EMAC-FFSL. Above 500 K in southern high latitudes, EMAC-CLaMS shows younger air than EMAC-FFSL.…”