Impact of land convection on troposphere-stratosphere exchange in the tropics

[摘要] The mechanism of troposphere-stratosphere exchange in the tropics wasinvestigated from space-borne observations of the horizontal distributionsof tropospheric-origin long-lived species, nitrous oxide (N₂O), methane(CH₄) and carbon monoxide (CO), from 150 to 70 hPa in March-April-Mayby the ODIN/Sub-Millimeter Radiometer (SMR), the Upper Atmosphere ResearchSatellite (UARS)/Halogen Occultation Experiment (HALOE) and theTERRA/Measurements Of Pollution In The Troposphere (MOPITT) instruments in2002–2004, completed by recent observations of the AURA/Microwave LimbSounder (MLS) instrument during the same season in 2005. The verticalresolution of the satellite measurements ranges from 2 to 4 km. The analysishas been performed on isentropic surfaces: 400 K (lower stratosphere) forall the species and 360 K (upper troposphere) only for CO. At 400 K (and 360 Kfor CO), all gases show significant longitudinal variations withpeak-to-trough values of ~5–11 ppbv for N₂O, 0.07–0.13 ppmv forCH₄, and ~10 ppbv for CO (~40 ppbv at 360 K). The maximumamounts are primarily located over Africa and, depending on the species,secondary more or less pronounced maxima are reported above northern SouthAmerica and South-East Asia. The lower stratosphere over the Western Pacificdeep convective region where the outgoing longwave radiation is the lowest,the tropopause the highest and the coldest, appears as a region of minimumconcentration of tropospheric trace species. The possible impact on tracegas concentration at the tropopause of the inhomogeneous distribution andintensity of the sources, mostly continental, of the horizontal and verticaltransports in the troposphere, and of cross-tropopause transport wasexplored with the MOCAGE Chemistry Transport Model. In the simulations,significant longitudinal variations were found on the medium-lived CO(2-month lifetime) with peak-to-trough value of ~20 ppbv at 360 K and~10 ppbv at 400 K, slightly weaker than observations. However, theCH₄ (8–10 year lifetime) and N₂O (130-year lifetime) longitudinalvariations are significantly weaker than observed: peak-to-trough values of~0.02 ppmv for CH₄ and 1–2 ppbv for N₂O at 400 K. The largelongitudinal contrast of N₂O and CH₄ concentrations reported bythe space-borne instruments at the tropopause and in the lower stratospherenot captured by the model thus requires another explanation. The suggestionis of strong overshooting over land convective regions, particularly Africa,very consistent with the space-borne Tropical Rainfall Measuring Mission(TRMM) radar maximum overshooting features over the same region during thesame season. Compared to observations, the MOCAGE model forced by ECMWFanalyses is found to ignore these fast local uplifts, but to overestimatethe average uniform vertical transport in the UTLS at all longitudes in thetropics.

[学科分类] 大气科学