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Abstract We divide the atmosphere into distinct spheres based on their physical, chemical, and dynamical traits. In deriving chemical budgets and climate trends, which differ across spheres, we need clearly defined boundaries. Our primary spheres are the troposphere and stratosphere (∼99.9% by mass), and the boundary between them is the tropopause. Every global climate‐weather model has one or more methods to calculate the lapse rate tropopause, but these involve subjective choices and are known to fail near the sub‐tropical jets and polar regions. Age‐of‐air tracers clock the effective time‐distance from the tropopause, allowing unambiguous separation of stratosphere from troposphere in the chaotic jet regions. We apply a global model with synthetic tracer e90 (90‐day e‐folding), focusing on ozone and temperature structures about the tropopause using ozone sonde and satellite observations. We calibrate an observation‐consistent tropopause for e90 using tropics‐plus‐midlatitudes and then apply it globally to calculate total tropospheric air‐mass and tropopause ozone values. The tropopause mixing barrier for the current UCI CTM is identified by a transition in the vertical transport gradient to stratospheric values of 15 days km−1, corresponding to an e90 tropopause at 81 ± 2 ppb with a global tropospheric air mass of 82.2 ± 0.3%. The best e90 tropopause based on sonde pressures is 70–80 ppb; but that for ozone is 80–90 ppb, implying that the CTM tropopause ozone values are too large. This approach of calibrating an age‐of‐air tropopause can be readily applied to other models and possibly used with observed age‐of‐air tracers like sulfur hexafluoride.
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Evaluating Stratospheric Tropical Width Using Tracer Concentrations
Abstract Quantifying the width of the tropics has important implications for understanding climate variability and the atmospheric response to anthropogenic forcing. Considerable effort has been placed on quantifying the width of the tropics at tropospheric levels, but substantially less effort has been placed on quantifying the width at stratospheric levels. Here we probe tropical width in the stratosphere using chemical tracers, which are accessible by direct measurement. Two new tracer‐based width metrics are developed, denoted here as the “1σ method” and the gradient weighted latitude (GWL) method. We evaluate widths from three tracers, CH4, N2O, and SF6. We demonstrate that unlike previously proposed stratospheric width methods using tracers, these metrics perform consistently throughout the depth of the stratosphere, at all times of year and on coarse temporal data. The GWL tracer‐based widths correlate well with the turnaround latitude and the critical level, where wave dissipation occurs, in the upper and midstratosphere during certain months of the year. In the lower stratosphere, the deseasonalized tracer‐based widths near the tropical tropopause correlate with the deseasonalized tropopause‐height based metrics. We also find that tracer‐tracer width correlations are strongest at pressure levels where their chemical lifetimes are similar. These metrics represent another useful way to estimate stratospheric tropical width and explore any changes under anthropogenic forcing.
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- PAR ID:
- 10454405
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 125
- Issue:
- 21
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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