A mass-weighted atmospheric isentropic coordinate for mapping chemical tracers and computing inventories

Abstract

Abstract. We introduce a transformed isentropic coordinate Mθe, defined as the dry air mass under a given equivalent potential temperature surface (θe) within a hemisphere. Like θe, the coordinate Mθe follows the synoptic distortions of the atmosphere, but unlike θe, has a nearly fixed relationship with latitude and altitude over the seasonal cycle. Calculation of Mθe is straightforward from meteorological fields. Using observations from the recent HIPPO and Atom airborne campaigns, we map the CO2 seasonal cycle as a function of pressure and Mθe, where Mθe is thereby effectively used as an alternative to latitude. We show that the CO2 cycles are more constant as a function of pressure using Mθe as the horizontal coordinate compared to latitude. Furthermore, short-term variability of CO2 relative to the mean seasonal cycle is also smaller when the data are organized by Mθe and pressure than when organized by latitude and pressure. We also present a method using Mθe to compute mass-weighted averages of CO2 on a hemispheric scale. Using this method with the same airborne data and applying corrections for limited coverage, we resolve the average CO2 seasonal cycle in the Northern Hemisphere (mass weighted tropospheric climatological average for 2009–2018), yielding an amplitude of 7.8 ± 0.14 ppm and a downward zero-crossing at Julian day 173 ± 6.1 (i.e., late June). Mθe may be similarly useful for mapping the distribution and computing inventories of any long-lived chemical tracer.

Evaluating Northern Hemisphere Growing Season Net Carbon Flux in Climate Models Using Aircraft Observations

“…Use of the transformed coordinate, M θ e , introduced by Jin et al. (2021) may further reduce uncertainty. M θ e is defined as the dry air mass under a given equivalent potential temperature surface within a hemisphere and its relationship with latitude and altitude is nearly fixed over the seasonal cycle.…”

“…When integrating to find the curtain average, Jin et al. (2021) show that using the transformed coordinate as an alternative to latitude reduces error in the curtain average due to sparse sampling and synoptic variability.…”

“…This corresponds to a more than 1% decrease contrary to the increase expected from Graven. However, this may not be accurate as there may not be enough data in either mission alone to fully constrain the seasonal cycle (Figure S3 in Supporting Information S1) and account for interannual variability in CO 2 fluxes, which were shown by Jin et al (2021) to be non-negligible.…”

“…M θe is defined as the dry air mass under a given equivalent potential temperature surface within a hemisphere and its relationship with latitude and altitude is nearly fixed over the seasonal cycle. When integrating to find the curtain average, Jin et al (2021) show that using the transformed coordinate as an alternative to latitude reduces error in the curtain average due to sparse sampling and synoptic variability.…”

“…Global-scale aircraft observations, such as those made during the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations project (HIPPO, 2009(HIPPO, -2011 and the Atmospheric Tomography Mission (ATom, 2016(ATom, -2018, are representative of large regions and capture the vertical profile of atmospheric CO 2 (Thompson et al, 2022;Wofsy, 2011). These campaigns measured the vertical structure of CO 2 in the atmosphere across a range of latitudes and over the full seasonal cycle and allowed for the analysis of seasonal changes in hemispheric-scale atmospheric CO 2 (e.g., Jin et al, 2021), which are dominated by land exchange. We use the seasonal cycle of atmospheric CO 2 concentrations measured during the HIPPO and ATom flight campaigns to develop a metric for evaluating the simulation of terrestrial CO 2 exchange in prognostic ESMs.…”