Tropical entrainment time scales inferred from stratospheric N₂O and CH₄ observations

Abstract

Abstract. Simultaneous in situ measurements of N20 and CH4 were made with a tunable diode laser spectrometer (ALIAS II) aboard the Observations from the Middle Stratosphere (OMS) balloon platform from New Mexico, Alaska, and Brazil during 1996 and 1997. We find different compact relationships of CH4 with N20 in the tropics and extra-tropics because mixing is slow between these regions. Transport into the extra-tropics from the tropics or the polar vortex leads to deviations from the normal compact relationship. We use measured N20 and CH4 and a simple model to quantify entrainment of mid-latitude stratospheric air into the tropics. The entrainment time scale is estimated to be 16 (+ 17, -8) months for altitudes between 20 and 28 km. The fraction of tropical air entrained from the extratropical stratosphere is 50% (+18%, -30%) at 20 km, increasing to 78% (+11%, -19%) at 28 km.

Assessment of the interannual variability and influence of the QBO and upwelling on tracer–tracer distributions of N2O and O3 in the tropical lower stratosphere

“…These values are also much higher than the lower stratospheric measurements by in situ instruments on aircraft or balloons (e.g. Strahan et al, 1999;Herman et al, 1998;Proffitt et al, 2003;Moore et al, 2003). Further, these values are even higher than the highly accurate ground-based measurements of N 2 O, of 319 ppbv in 2005 (Forster et al, 2007).…”

Retrieval of tropospheric column-averaged CH<sub>4</sub> mole fraction by solar absorption FTIR-spectrometry using N<sub>2</sub>O as a proxy

“…Species whose local lifetimes are longer than quasihorizontal transport time scales share surfaces of constant mixing ratio, and a scatter plot of the mixing ratio of one versus that results in a compact curve. These correlations have been demonstrated by data sets from both chemical transport models and in situ measurements, such as observations taken from an aircraft platform (Avallone et al, 1997), Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) observations and balloon observations (Herman et al, 1998). This is true in the case of CH 4 and N 2 O in the stratosphere.…”

Carbon and hydrogen isotopic compositions of stratospheric methane: 1. High‐precision observations from the NASA ER‐2 aircraft

“…Although Figure 2 graphically illustrates the general trend of decreasing CH 4 and increasingly enriched carbon and hydrogen isotopic compositions as altitude and latitude increase, these 78 samples were collected over four years during different seasons. Due to (1) the small number of samples, (2) known seasonal variations in CH 4 mixing ratios as a function of latitude and altitude [e.g., Michelsen et al , 1998], and (3) the fact that high spatial resolution aircraft measurements can resolve the existence of narrow filaments of air representative of a different region (e.g., polar or tropical filaments encountered during midlatitude flights [e.g., Boering et al , 1996; Herman et al , 1998]), the contours in Figure 2 should be considered a qualitative representation of the observed isotopic compositions only. Details of the contour shapes are not meaningful.…”

“…For specific application to the stratosphere, note that this reduction in α app from α KIE is true regardless of the type of mixing occurring. The example outlined above might be considered conservative mixing between two air parcels, such as occurs in the lower stratosphere when midlatitude and polar vortex air mix upon break‐up of the polar vortex in spring [ Waugh et al , 1997; Herman et al , 1998; Michelsen et al , 1999] or due to continuous weak isentropic mixing across the polar vortex edge during the time of maximum descent (and therefore during the time of maximum tracer gradients) [ Plumb et al , 2000]. If the stratosphere was a well‐mixed box (which, of course, is not the case), the constant input of CH 4 from the troposphere would result in lighter isotope:CH 4 mixing ratio relationships at steady‐state than predicted by Rayleigh fractionation, and therefore a smaller α app than that predicted from chemical KIEs alone for the same reason—i.e., the mixing of “high” CH 4 with “low” CH 4 air reduces the fractionation predicted from α KIE for a given CH 4 mixing ratio.…”