Atmospheric data used for calibrating the tropopause in global chemistry-climate or chemistry-transport models

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.  Focusing on atmospheric mass and greenhouse gases, our primary spheres are the troposphere and stratosphere (~99.9 % by mass), and the boundary between them is the tropopause. 

The standard method of locating the tropopause is based on the World Meteorological Organization’s lapse rate tropopause (LRT) defined from the vertical temperature gradient as observed by radiosonde balloons.  Every global climate-weather model has one or more methods to calculate the LRT. Both involve subjective choices: expert judgment given meter-scale variability of sonde temperature profiles; methods for calculating gradients from km-thick model layers. Further, LRT and similar methods are consistent only in regions where gradients are primarily vertical (core tropics and midlatitudes) and fail in others (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.  Such calibration can identify weak tropospheric mixing rates.  The concept of calibrating an age-of-air tropopause can be readily applied to other models and even to observed age-of-air tracers like sulfur hexafluoride.

Methods
This dataset was collected from several sources as outlined in the table below

The CTM 4-D data is published in https://doi.org/10.5061/dryad.qbzkh18qq as netcdf files (CTRL data)

Table 1.  Observational data sets

type

source

files

profiles

dates

ozone sondes

Wallops (WAL, 38°N), SHADOZV06

Ascension (ASC, 8°S) SHADOZV06

Lauder (LAU, 45°S) NIWA

1,477

831

1,973

1,477/tot

831/tot

1,973/tot

1995-2020

1998-2022

1986-2021

downloaded:  .dat's 2023-10-03 (WAL & ASC); csv's 2023-01-08 (LAU)

processing:  see Table 3.

OMPS

OMPS-NPP_LP-L2-O3-DAILY_v2.6

3,587

~1,160/day

2014-2023

downloaded: 2023-04-01 via wget,

https://data.gesdisc.earthdata.nasa.gov/data/SNPP_OMPS_Level2/...

accepted profiles: 'O3Convergence' <10; ' O3Status' =2:7; ' O3Quality' =0;

'QMV' =0; 'ASI_PMCFlag' =0.

processing:  'TropopauseAltitude' reported using GMAO FP-IT T profiles

ACE-FTS

ACEFTS_L2_v4p0_O3.nc (includes O3, p, T)

1

94,675/tot

2004-2020

downloaded: 2023-12-17 from doi:10.20383/101.0291

            https://databace.scisat.ca/level2/

processing:  grid is 1 km altitude; drop all O3 <0 and below 6 km (k=1:6).  For tropopause, find lowest troposphere level (LR > 2 K/km) with 2 stratosphere levels (LR ≤2) above it.

The data was processed to identify the tropopause values of O3, T, potential temperature using several tropopause algorithms described in the table below

Table 2.  CTM and the different tropopause definitions

CTM grid cells

The Chemistry-Transport Model here has a regular latitude-by-longitude 160x320 grid of resolution ~1.1º and 57 eta-coordinate levels defining the pressure edges of each layer based on the T159L60N80 grid of the ECMWF IFS model. The lowest 5 IFS L60 layers are combined into 2 CTM near-surface layers.  The vertical resolution of model layers In the upper-troposphere/lower-stratosphere increases regularly from 0.8 km near 10 km altitude to 1.1 km at 17 km altitude. The CTM has layer numbers increasing upward.

e90 tracer

Synthetic chemical tracer, emitted uniformly everywhere from the surface, decaying with a 90-day e-fold time.  Calculated in the CTM for every cell and every time step. Emissions are set to give an atmospheric mean abundance of 100 ppb. This techniques has been used to study O3 seasonality at the tpp (Prather et al., 2011)

e/90ppbU

Highest altitude model layer with e90 > 90 ppb is designated the uppermost tropospheric layer, and its upper boundary is the upper tropopause. The tropopause value for O3 and T is the mean value of the uppermost tropospheric layer.  No interpolation is done because of the large change in dO3/dz.

e/90ppbL

Lowest altitude model layer with e90 ≤ 90 ppb is designated the lowest stratospheric layer, and its lower boundary is the lower tropopause.  Often the same as e/90ppbU.

e/80ppbU

Highest altitude model layer with e90 > 80 ppb, as above

e/80ppbL

Lowest altitude model layer with e90 ≤ 80 ppb, as above

e/70ppbU

Highest altitude model layer with e90 > 70 ppb, as above

e/70ppbL

Lowest altitude model layer with e90 ≤ 70 ppb, as above

Lapse Rate

Lapse Rate (LR = -dT/dz, K/km) is calculated between 2 vertically aligned layers using the mean temperatures of each layer and the mid-layer altitude. The LR values and thresholds (LR <2 K/km = strat) apply to the upper boundary of the lower box used to calculate it. 

WMO-LRTU

WMO LR Tropopause (Upper).  Here we try to match the WMO definition as closely while following the logic of the PTG algorithm.  The Lapse Rate (LR = -dT/dz, K/km) is calculated between 2 adjacent layers using the mean temperatures of each layer and the mid-layer altitude.  Two LRs for layer k are calculated:  LR1 = -[T(k+1)-T(k)]/[z(k+1)-z(k)]; and LR2 = -[T(k+2)-T(k)]/[z(k+2)-z(k)]. Thus, LR1 spans ~1 km above the mid-point of layer k and LR2 spans ~2 km above.  All layers below 4.4 km are set as tropospheric (trop).  All layers above 31 km are stratospheric (strat).

Define strat layers as meeting the criteria LR1 <2 K/km. If the first (lowermost) strat layer has 2 trop layers immediately above, then relabel it trop and keep going. Having found the first strat layer that passes this test, we check that either LR2 from the current layer or LR1 from the layer above meets the <2 K/km criterion.  If these conditions are met, we have the first strat layer and the tpp is the lower boundary of that layer.

Now look for a 2nd tpp above:  If there is an LR1 ≥ 2 K/km above (i.e., trop layer) then define the 2nd tpp (WMO-LRTU) as the lower boundary of the next strat layer.  Otherwise, the 2nd tpp is the same as the 1st.   

WMO-LRTL

WMO LR Tropopause (Lower). This is just the 1st tpp from WMO-LRTU.

WMO-altU

Alternate WMO Upper tpp.  Similar to WMO-LRT but easier to implement with CTM grid.  Identify trop layers from LR > 2 K/km.  Set layers below 6 km altitude to trop. Set layers with θ > 500 K to strat.

From the bottom, find the first trop layer with a gap of 2 strat layers (~2 km thick region) above it.  This is the 2nd (Upper) tpp. Repeat looking for a trop layer with a gap of at least 1 strat layer above.  This is the 1st (Lower) tpp. Both tpp can, and often are, the same, but strat-trop folds resolved by the CTM are easily identified.

WMO-altL

Alternate WMO Lower tpp. The 1st (Lower) tpp from WMO-altU search above.

Potential Temperature Gradient

The PTG (dθ/dz, K/km) is calculated like the LR between 2 vertically aligned layers using the mean θ of each layer and the mid-layer altitude. The PTG values and thresholds (PTG ≥ 10 K/km) apply to the upper boundary of the lower box used to calculate it.  

PTG-tppU

PTG Tropopause (Upper).  The algorithm here is similar to the original paper (Tinney et al., 2022), adapted to work efficiently with the CTM diagnostics. The PTG (dθ/dz, K/km) is calculated between 2 adjacent layers using the mean θ of each layer and the mid-layer altitude.  Two PTGs for layer k are calculated:  PTG1 = [θ(k+1)-θ(k)]/[z(k+1)-z(k)]; and PTG2 = [θ(k+2)-θ(k)]/[z(k+2)-z(k)]. Thus, PTG1 spans ~1 km above the mid-point of layer k and PTG2 spans ~2 km above.  All layers below 4.4 km are set as trop.  All layers above 31 km are strat.

Define strat layers as meeting the criteria PTG ≥ 10 K/km. If the first (lowermost) strat layer has 2 trop layers immediately above, then relabel it trop and keep going. Having found the first strat layer that passes this test, we check that either PTG2 from the current layer or PTG1 from the layer above meets the ≥10 K/km criterion.  If these conditions are met, we have the first strat layer and the 1st tpp is the lower boundary of that layer.

Now look for a 2nd tpp:  If we find a trop layer above the strat layer above the 1st tpp, then we search for the first strat layer above it using a revised threshold, PTG1 ≥ 15 K/km.  This identifies the 2nd tpp (PTG-tppU) as the lower boundary of the next strat layer.  Otherwise, the 2nd tpp is the same as the 1st.   

PTG-tppL

PTG Tropopause (Lower). This is just the 1st tpp from the PTG-tppU search above.

The ozonesonde data were preprocessed to obtain stable and consisten profiles of O3 and T vs. P as described in Table 3 

Table 3.  Ozonesonde processing and defining the tropopause

find O3 NaNs

Flagged (e.g., -999); negative; too large (>20 ppm); pressure (< 1 hPa)

find T NaNs

Flagged; T < 160 K

collapse profile

Remove: O3 NaN pts; yoyo (down-up) sections; descent part of profile; all p < 10 hPa.

drop whole profiles

With <25 O3 points; with top p > 65 hPa = ASC(44), LAU(24), WAL(58).

reduce # pts

Many profiles have 6000-8000 pts & resolution <10 m; create 4-pt (ASC & WAL) and 6-pt (LAU) averages to get 30-50 m spacing for p = 70-500 hPa.

calculate 100-m gradients

Use centered 2nd-order finite difference to get 100-m values for LR = -dT/dz and PTG = dθ/dz at each point.  These are too noisy to identify tropopause, see Fig. 2

calculate stable 1-km gradients

Average z, T, O3 for 20 points (~1 km) below/above each point; difference these to get a smoothed 1-km-average LR and PTG, see Figure 2.  Due to the smoothed 1-km resolution profile, we do not apply the WMO criterion that gradients must be sustained above the threshold point.

lower tropopause

LRT:  point below lowest LR ≤ 2 K/km, limited to 500 > p > 80 hPa

PTG:  point below lowest dθ/dz ≥ 10 K/km, ibid

upper tropopause

LRT:  highest LR > 2 K/km, limited to 500 > p > 80 hPa;

PTG:  highest dθ/dz < 10 K/km, ibid;

Thresholds are not changed for the upper LRT as in Tinney et al., 2022.