scholarly journals Age of stratospheric air in the ERA-Interim

2012 ◽  
Vol 12 (24) ◽  
pp. 12133-12154 ◽  
Author(s):  
M. Diallo ◽  
B. Legras ◽  
A. Chédin
Keyword(s):  
Annual Cycle ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Maximum Amplitude ◽  
Heating Rates ◽  
Annual Modulation ◽  
Model Driven ◽  
The Mean ◽  
Mean Circulation ◽  
The Tropics

Abstract. The Brewer-Dobson mean circulation and its variability are investigated in the ERA-Interim over the period 1989-2010 by using an off-line Lagrangian transport model driven by analysed winds and heating rates. At low and mid-latitudes, the mean age of air in the lower stratosphere is in good agreement with ages derived from aircraft, high altitude balloon and satellite observations of long-lived tracers. At high latitude and in the upper stratosphere, we find, however that the ERA-Interim ages exhibit an old bias, typically of one to two years. The age spectrum exhibits a long tail except in the low tropical stratosphere which is modulated by the annual cycle of the tropical upwelling. The distribution of ages and its variability is consistent with the existence of two separate branches, shallow and deep, of the Brewer-Dobson circulation. Both branches are modulated by the tropical upwelling and the shallow branch is also modulated by the subtropical barrier. The variability of the mean age is analysed through a decomposition in terms of annual cycle, QBO, ENSO and trend. The annual modulation is the dominating signal in the lower stratosphere and is maximum at latitudes greater than 50° in both hemispheres with oldest ages at the end of the winter. The phase of the annual modulation is also reversed between below and above 25 km. The maximum amplitude of the QBO modulation is of about 0.5 yr and is mostly concentrated within the tropics between 25 and 35 km. It lags the QBO wind at 30 hPa by about 8 months. The ENSO signal is small and limited to the lower northen stratosphere. The age trend over the 1989–2010 period, according to this ERA-Interim dataset, is significant and negative, of the order of −0.3 to −0.5 yr dec−1, within the lower stratosphere in the Southern Hemisphere and south of 40° N in the Northern Hemisphere below 25 km. The age trend is positive (of the order of 0.3 yr dec−1) in the mid stratosphere but there is no region of consistent significance. This suggests that the shallow and deep Brewer-Dobson circulations may evolve in opposite directions. Finally, we find that the long lasting influence of the Pinatubo eruption can be seen on the age of air from June 1991 until the end of 1993 and can bias the statistics encompassing this period.

scholarly journals Age of stratospheric air in the ERA-Interim

2012 ◽  
Vol 12 (7) ◽  
pp. 17087-17134 ◽  
Author(s):  
M. Diallo ◽  
B. Legras ◽  
A. Chedin
Keyword(s):  
High Altitude ◽  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Maximum Amplitude ◽  
Positive Trend ◽  
Heating Rates ◽  
Annual Modulation ◽  
The Mean

Abstract. The age of stratospheric air is calculated over 22 yr of the ERA-Interim reanalysis using an off-line Lagrangian transport model and heating rates. At low and mid-latitudes, the mean age of air is in good agreement with observed ages from aircraft flights, high altitude balloons and satellite observations of CO2 and SF6. The mid-latitude age spectrum in the lower stratosphere exhibits a long tail with a peak at 0.5 yr, which is maximum at the end of the winter, and a secondary flat maximum between 4 and 5 yr due to the combination of fast and slow branches of the Brewer-Dobson circulation and the reinforced barrier effect of the jet. At higher altitudes, the age spectrum exhibits the footprint of the annual modulation of the deep Brewer-Dobson circulation. The variability of the mean age is analysed through a decomposition in terms of annual cycle, QBO, ENSO and trend. The annual modulation is the dominating signal in the lower stratosphere and in the tropical pipe with amplitude up to one year. The phase of the oscillation is opposite in both hemisphere beyond 20° and is also reversed below and above 25 km with maximun arising in mid-March in the Northern Hemisphere and in mid-September in the Southern Hemisphere. The tropical pipe signal is in phase with the lower southern stratosphere and the mid northern stratosphere. The maximum amplitude of the QBO modulation is of about 0.5 yr and is mostly concentrated within the tropics between 25 and 35 km. It lags the QBO wind at 30 hPa by about 8 months. The ENSO signal is small and limited to the lower northen stratosphere. The trend is significant and negative, of the order of −0.3 to −0.5 yr dec−1, within the lower stratosphere in the Southern Hemisphere and under 40° N in the Northern Hemisphere below 25 km. It is positive (of the order of 0.3 yr dec−1) in the mid stratosphere but there is no region of consistent significance. This suggests that the shallow and deep Brewer-Dobson circulations may evolve in opposite directions. It is however difficult to estimate a reliable long-term trend from only 22 yr of data. For instance, a positive trend is found in the lower stratosphere if only the second half of the period is considered in agreement with MIPAS SF6 data excepted in the northern polar region and at high altitude. Finally, it is found that the long lasting influence of the Pinatubo eruption can be seen on the age of air from June 1991 until the end of 1993 and can bias the statistics encompassing this period. In our analysis, this eruption shifts the trend towards negative values by about 0.2 to 0.3 yr dec−1.

scholarly journals Comparison of mean age of air in five reanalyses using the BASCOE transport model

2018 ◽  
Vol 18 (19) ◽  
pp. 14715-14735 ◽  
Author(s):  
Simon Chabrillat ◽  
Corinne Vigouroux ◽  
Yves Christophe ◽  
Andreas Engel ◽  
Quentin Errera ◽  
...  
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Height Distribution ◽  
Heating Rates ◽  
Worst Case ◽  
Quasi Biennial Oscillation ◽  
Model Driven ◽  
Dipole Structure ◽  
Linear Trends

Abstract. We present a consistent intercomparison of the mean age of air (AoA) according to five modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Japanese Meteorological Agency's Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) and the National Aeronautics and Space Administration's Modern Era Retrospective analysis for Research and Applications version 1 (MERRA) and version 2 (MERRA-2). The modeling tool is a kinematic transport model driven only by the surface pressure and wind fields. It is validated for ERA-I through a comparison with the AoA computed by another transport model. The five reanalyses deliver AoA which differs in the worst case by 1 year in the tropical lower stratosphere and more than 2 years in the upper stratosphere. At all latitudes and altitudes, MERRA-2 and MERRA provide the oldest values (∼5–6 years in midstratosphere at midlatitudes), while JRA-55 and CFSR provide the youngest values (∼4 years) and ERA-I delivers intermediate results. The spread of AoA at 50 hPa is as large as the spread obtained in a comparison of chemistry–climate models. The differences between tropical and midlatitude AoA are in better agreement except for MERRA-2. Compared with in situ observations, they indicate that the upwelling is too fast in the tropical lower stratosphere. The spread between the five simulations in the northern midlatitudes is as large as the observational uncertainties in a multidecadal time series of balloon observations, i.e., approximately 2 years. No global impact of the Pinatubo eruption can be found in our simulations of AoA, contrary to a recent study which used a diabatic transport model driven by ERA-I and JRA-55 winds and heating rates. The time variations are also analyzed through multiple linear regression analyses taking into account the seasonal cycles, the quasi-biennial oscillation and the linear trends over four time periods. The amplitudes of AoA seasonal variations in the lower stratosphere are significantly larger when using MERRA and MERRA-2 than with the other reanalyses. The linear trends of AoA using ERA-I confirm those found by earlier model studies, especially for the period 2002–2012, where the dipole structure of the latitude–height distribution (positive in the northern midstratosphere and negative in the southern midstratosphere) also matches trends derived from satellite observations of SF6. Yet the linear trends vary substantially depending on the considered period. Over 2002–2015, the ERA-I results still show a dipole structure with positive trends in the Northern Hemisphere reaching up to 0.3 yr dec−1. No reanalysis other than ERA-I finds any dipole structure of AoA trends. The signs of the trends depend strongly on the input reanalysis and on the considered period, with values above 10 hPa varying between approximately −0.4 and 0.4 yr dec−1. Using ERA-I and CFSR, the 2002–2015 trends are negative above 10 hPa, but using the three other reanalyses these trends are positive. Over the whole period (1989–2015) each reanalysis delivers opposite trends; i.e., AoA is mostly increasing with CFSR and ERA-I but mostly decreasing with MERRA, JRA-55 and MERRA-2. In view of this large disagreement, we urge great caution for studies aiming to assess AoA trends derived only from reanalysis winds. We briefly discuss some possible causes for the dependency of AoA on the input reanalysis and highlight the need for complementary intercomparisons using diabatic transport models.

scholarly journals Comparison of mean age of air in five reanalyses using the BASCOE transport model

2018 ◽  
Author(s):  
Simon Chabrillat ◽  
Corinne Vigouroux ◽  
Yves Christophe ◽  
Andreas Engel ◽  
Quentin Errera ◽  
...  
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Height Distribution ◽  
Heating Rates ◽  
Worst Case ◽  
Model Driven ◽  
Model Studies ◽  
Dipole Structure ◽  
Linear Trends

Abstract. We present a consistent intercomparison of the mean Age of Air (AoA) according to five modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Japanese Meteorological Agency’s Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) and the National Aeronautics and Space Administration’s Modern Era Retrospective-analysis for Research Applications version 1 (MERRA) and version 2 (MERRA-2). The modeling tool is a kinematic transport model driven only by the surface pressure and wind fields. It is validated for ERA-I through a comparison with the AoA computed by another transport model. The five reanalyses deliver AoA which differ in the worst case by one year in the tropical lower stratosphere and more than two years in the upper stratosphere. At all latitudes and altitudes, MERRA-2 and MERRA provide the oldest values (~ 5–6 years in mid-stratosphere at mid-latitudes) while JRA-55 and CFSR provide the youngest values (~ 4 years) and ERA-I delivers intermediate results. The spread of AoA at 50 hPa is as large as the spread obtained in a comparison of Chemistry-Climate Models. The differences between tropical and mid-latitudes AoA are in better agreement except for MERRA-2. Compared with in-situ observations, they indicate that the upwelling is too fast in the tropical lower stratosphere. The general hierarchy of reanalyses delivering older AoA (MERRA, MERRA-2) and younger AoA (JRA-55, CFSR) holds during the whole 1989–2015 period, with AoA derived from ERA-I keeping intermediate values. The spread between the five simulations in the northern mid-latitudes is as large as the observational uncertainties in a multidecadal time series of balloon observations, i.e., approximately two years. No global impact of the Pinatubo eruption can be found in our simulations of AoA, contrarily to a recent study which used a diabatic transport model driven by ERA-I and JRA-55 winds and heating rates. The time variations are also analyzed through multiple linear regression analyses taking into account the seasonal cycles, the Quasi-Biennal Oscillation and the linear trends over four time periods. The amplitudes of AoA seasonal variations in the lower stratosphere are significantly larger using MERRA and MERRA-2 than with the other reanalyses (up to twice as large at the 50 hPa pressure level). The linear trends of AoA using ERA-I confirm those found by earlier model studies, especially for the period 2002–2012 where the dipole structure of the latitude-height distribution (positive in the northern mid-stratosphere and negative in the southern mid-stratosphere) also matches trends derived from satellite observations of SF6. Yet the linear trends vary considerably depending on the considered period. Over 2002–2015 the ERA-I results still show a dipole structure but it is much less pronounced, with positive trends in the northern hemisphere remaining significant only in the polar lower stratosphere (where they reach 0.2 years per decade). No reanalysis other than ERA-I finds any dipole structure of AoA trends. The signs of the trends depend strongly on the input reanalysis and on the considered period, with values above 10 hPa varying between approximately −0.4 and 0.4 years per decade. Using ERA-I and CFSR, the 2002–2015 trends are negative above 10 hPa but using the three other reanalyses these trends are positive. Over the whole period 1989–2015 each reanalysis delivers opposite trends, i.e., AoA is mostly increasing with CFSR and ERA-I but mostly decreasing with MERRA, JRA-55 and MERRA-2. In view of these large disagreements, we urge great caution for studies aiming to assess AoA trends derived only from reanalysis winds. We briefly discuss some possible causes for the dependency of AoA on the input reanalysis and highlight the need for complementary intercomparisons using diabatic transport models.

scholarly journals Analysis of a rapid increase of stratospheric ozone during late austral summer 2008 over Kerguelen (49.4° S, 70.3° E)

2011 ◽  
Vol 11 (1) ◽  
pp. 363-373 ◽  
Author(s):  
H. Bencherif ◽  
L. El Amraoui ◽  
G. Kirgis ◽  
J. Leclair De Bellevue ◽  
A. Hauchecorne ◽  
...  
Keyword(s):  
Potential Vorticity ◽  
Large Scale ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Stratospheric Ozone ◽  
Three Dimensional ◽  
Austral Summer ◽  
Air Masses ◽  
Global Mapping ◽  
The Tropics

Abstract. This paper reports on an increase of ozone event observed over Kerguelen (49.4° S, 70.3° E) in relationship with large-scale isentropic transport. This is evidenced by ground-based observations (co-localised radiosonde and SAOZ experiments) together with satellite global observations (Aura/MLS) assimilated into MOCAGE, a Méteo-France model. The study is based on the analyses of the first ozonesonde experiment never recorded at the Kerguelen site within the framework of a French campaign called ROCK that took place from April to August 2008. Comparisons and interpretations of the observed event are supported by co-localised SAOZ observations, by global mapping of tracers (O3, N2O and columns of O3) from Aura/MLS and Aura/OMI experiments, and by model simulations of Ertel Potential Vorticity initialised by the ECMWF (European Centre for Medium-Range Weather Forecasts) data reanalyses. Satellite and ground-based observational data revealed a consistent increase of ozone in the local stratosphere by mid-April 2008. Additionally, Ozone (O3) and nitrous oxide (N2O) profiles obtained during January–May 2008 using the Microwave Limb Sounder (MLS) aboard the Aura satellite are assimilated into MOCAGE (MOdèle de Chimie Atmosphérique à Grande Echelle), a global three-dimensional chemistry transport model of Météo-France. The assimilated total O3 values are consistent with SAOZ ground observations (within ±5%), and isentropic distributions of O3 match well with maps of advected potential vorticity (APV) derived from the MIMOSA model, a high-resolution advection transport model, and from the ECMWF reanalysis. The event studied seems to be related to the isentropic transport of air masses that took place simultaneously in the lower- and middle-stratosphere, respectively from the polar region and from the tropics to the mid-latitudes. In fact, the ozone increase observed by mid April 2008 resulted simultaneously: (1) from an equator-ward departure of polar air masses characterised with a high-ozone layer in the lower stratosphere (near the 475 K isentropic level), and (2) from a reverse isentropic transport from the tropics to mid- and high-latitudes in the upper stratosphere (nearby the 700 K level). The increase of ozone observed over Kerguelen from the 16-April ozonesonde profile is thus attributed to a concomitant isentropic transport of ozone in two stratospheric layers: the tropical air moving southward and reaching over Kerguelen in the upper stratosphere, and the polar air passing over the same area but in the lower stratosphere.

scholarly journals Evaluation of near-tropopause ozone distributions in the Global Modeling Initiative combined stratosphere/troposphere model with ozonesonde data

2008 ◽  
Vol 8 (1) ◽  
pp. 1589-1634 ◽  
Author(s):  
D. B. Considine ◽  
J. A. Logan ◽  
M. A. Olsen
Keyword(s):  
Annual Cycle ◽  
Transport Model ◽  
Model Simulation ◽  
Atmospheric Composition ◽  
Vertical Resolution ◽  
Residual Circulation ◽  
Vertical Diffusion ◽  
Global Modeling ◽  
Upper Troposphere ◽  
The Tropics

Abstract. The NASA Global Modeling Initiative has developed a combined stratosphere/troposphere chemistry and transport model which fully represents the processes governing atmospheric composition near the tropopause. We evaluate model ozone distributions near the tropopause, using two high vertical resolution monthly mean ozone profile climatologies constructed with ozonesonde data, one by averaging on pressure levels and the other relative to the thermal tropopause. Model ozone is high-biased at the SH tropical and NH midlatitude tropopause by ~45% in a 4° latitude × 5° longitude model simulation. Increasing the resolution to 2°×2.5&amp;deg increases the NH tropopause high bias to ~60%, but decreases the tropical tropopause bias to ~30%, an effect of a better-resolved residual circulation. The tropopause ozone biases appear not to be due to an overly vigorous residual circulation or excessive stratosphere/troposphere exchange, but are more likely due to insufficient vertical resolution or excessive vertical diffusion near the tropopause. In the upper troposphere and lower stratosphere, model/measurement intercomparisons are strongly affected by the averaging technique. NH and tropical mean model lower stratospheric biases are <20%. In the upper troposphere, the 2°×2.5&amp;deg simulation exhibits mean high biases of ~20% and~35% during April in the tropics and NH midlatitudes, respectively, compared to the pressure-averaged climatology. However, relative-to-tropopause averaging produces upper troposphere high biases of ~30% and 70% in the tropics and NH midlatitudes. This is because relative-to-tropopause averaging better preserves large cross-tropopause O3 gradients, which are seen in the daily sonde data, but not in daily model profiles. The relative annual cycle of ozone near the tropopause is reproduced very well in the model Northern Hemisphere midlatitudes. In the tropics, the model amplitude of the near-tropopause annual cycle is weak. This is likely due to the annual amplitude of mean vertical upwelling near the tropopause, which analysis suggests is ~30% weaker than in the real atmosphere.

scholarly journals Modeling the transport of very short-lived substances into the tropical upper troposphere and lower stratosphere

2009 ◽  
Vol 9 (5) ◽  
pp. 18511-18543 ◽  
Author(s):  
J. Aschmann ◽  
B. M. Sinnhuber ◽  
E. L. Atlas ◽  
S. M. Schauffler
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Three Dimensional ◽  
Chemical Transport Model ◽  
Heating Rates ◽  
Model Calculations ◽  
Upper Troposphere ◽  
Mass Fluxes ◽  
Tropical Upper Troposphere

Abstract. The transport of very short-lived substances into the tropical upper troposphere and lower stratosphere is investigated by a three-dimensional chemical transport model using archived convective updraft mass fluxes (or detrainment rates) from the European Centre for Medium-Range Weather Forecast's ERA-Interim reanalysis. Large-scale vertical velocities are calculated from diabatic heating rates. With this approach we explicitly model the large scale subsidence in the tropical troposphere with convection taking place in fast and isolated updraft events. The model calculations agree generally well with observations of bromoform and methyl iodide from aircraft campaigns and with ozone and water vapor from sonde and satellite observations. Using a simplified treatment of dehydration and bromine product gas washout we give a range of 1.6 to 3 ppt for the contribution of bromoform to stratospheric bromine, assuming a uniform source in the boundary layer of 1 ppt. We show that the most effective region for VSLS transport into the stratosphere is the West Pacific, accounting for about 55% of the bromine from bromoform transported into the stratosphere under the supposition of a uniformly distributed source.

scholarly journals The stratospheric Brewer–Dobson circulation inferred from age of air in the ERA5 reanalysis

2021 ◽  
Author(s):  
Felix Ploeger ◽  
Mohamadou Diallo ◽  
Edward Charlesworth ◽  
Paul Konopka ◽  
Bernard Legras ◽  
...  
Keyword(s):  
Diabatic Heating ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Residual Circulation ◽  
Heating Rates ◽  
Weather Forecasts ◽  
Trend Pattern ◽  
Transit Times ◽  
Medium Range

Abstract. This paper investigates the global stratospheric Brewer–Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS, driven by winds and total diabatic heating rates from the reanalysis. ERA5-based results are compared to those of the preceding ERA–Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA–Interim, manifesting in weaker diabatic heating rates and larger age of air. In the tropical lower stratosphere, heating rates are 30–40 % weaker in ERA5, likely correcting a known bias in ERA–Interim. Above, ERA5 age of air appears slightly high-biased and the BDC slightly slow compared to tracer observations. The age trend in ERA5 over 1989–2018 is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear over the period but exhibits steplike changes which could be caused by muti-annual variability or changes in the assimilation system. Over the 2002–2012 period, ERA5 age shows a similar hemispheric dipole trend pattern as ERA–Interim, with age increasing in the NH and decreasing in the SH. Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates similarly in both reanalyses while the deep branch accelerates in ERA5 and decelerates in ERA–Interim.

scholarly journals Quantifying pollution transport from the Asian monsoon anticyclone into the lower stratosphere

2017 ◽  
Vol 17 (11) ◽  
pp. 7055-7066 ◽  
Author(s):  
Felix Ploeger ◽  
Paul Konopka ◽  
Kaley Walker ◽  
Martin Riese
Keyword(s):  
Asian Monsoon ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Monsoon Circulation ◽  
Pollution Transport ◽  
Chemistry Transport Model ◽  
Transport Pathways ◽  
Air Mass ◽  
Mass Fractions ◽  
The Tropics

Abstract. Pollution transport from the surface to the stratosphere within the Asian monsoon circulation may cause harmful effects on stratospheric chemistry and climate. Here, we investigate air mass transport from the monsoon anticyclone into the stratosphere using a Lagrangian chemistry transport model. We show how two main transport pathways from the anticyclone emerge: (i) into the tropical stratosphere (tropical pipe), and (ii) into the Northern Hemisphere (NH) extratropical lower stratosphere. Maximum anticyclone air mass fractions reach around 5 % in the tropical pipe and 15 % in the extratropical lowermost stratosphere over the course of a year. The anticyclone air mass fraction correlates well with satellite hydrogen cyanide (HCN) and carbon monoxide (CO) observations, confirming that pollution is transported deep into the tropical stratosphere from the Asian monsoon anticyclone. Cross-tropopause transport occurs in a vertical chimney, but with the pollutants transported quasi-horizontally along isentropes above the tropopause into the tropics and NH.

scholarly journals Quantifying pollution transport from the Asian monsoon anticyclone into the lower stratosphere

2017 ◽  
Author(s):  
Felix Ploeger ◽  
Paul Konopka ◽  
Kaley Walker ◽  
Martin Riese
Keyword(s):  
Asian Monsoon ◽  
Lower Stratosphere ◽  
Transport Model ◽  
Monsoon Circulation ◽  
Pollution Transport ◽  
Chemistry Transport Model ◽  
Transport Pathways ◽  
Air Mass ◽  
Mass Fractions ◽  
The Tropics

Abstract. Pollution transport from the surface to the stratosphere within the Asian monsoon circulation may cause harmful effects on stratospheric chemistry and climate. Here, we investigate air mass transport from the monsoon anticyclone into the stratosphere using a Lagrangian chemistry transport model. We show how two main transport pathways from the anticyclone emerge: (i) into the tropical stratosphere (tropical pipe), and (ii) into the Northern hemisphere (NH) extra-tropical lower stratosphere. Maximum anticyclone air mass fractions reach around 5 % in the tropical pipe and 15 % in the extra-tropical lowermost stratosphere over the course of a year. The anticyclone air mass fraction correlates well with satellite hydrogen cyanide (HCN) and carbon monoxide (CO) observations, corroborating that pollution is transported deep into the tropical stratosphere from the Asian monsoon anticyclone. Cross-tropopause transport occurs in a vertical chimney, but with the emissions transported quasi-horizontally along isentropes above the tropopause into the tropics and NH.

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

2007 ◽  
Vol 7 (21) ◽  
pp. 5639-5657 ◽  
Author(s):  
P. Ricaud ◽  
B. Barret ◽  
J.-L. Attié ◽  
E. Motte ◽  
E. Le Flochmoën ◽  
...  
Keyword(s):  
Lower Stratosphere ◽  
Transport Model ◽  
Longwave Radiation ◽  
Tropical Rainfall Measuring Mission ◽  
Trace Gas ◽  
Satellite Measurements ◽  
Minimum Concentration ◽  
Trace Species ◽  
Longitudinal Variations ◽  
The Tropics

Abstract. The mechanism of troposphere-stratosphere exchange in the tropics was investigated from space-borne observations of the horizontal distributions of tropospheric-origin long-lived species, nitrous oxide (N2O), methane (CH4) and carbon monoxide (CO), from 150 to 70 hPa in March-April-May by the ODIN/Sub-Millimeter Radiometer (SMR), the Upper Atmosphere Research Satellite (UARS)/Halogen Occultation Experiment (HALOE) and the TERRA/Measurements Of Pollution In The Troposphere (MOPITT) instruments in 2002–2004, completed by recent observations of the AURA/Microwave Limb Sounder (MLS) instrument during the same season in 2005. The vertical resolution of the satellite measurements ranges from 2 to 4 km. The analysis has been performed on isentropic surfaces: 400 K (lower stratosphere) for all the species and 360 K (upper troposphere) only for CO. At 400 K (and 360 K for CO), all gases show significant longitudinal variations with peak-to-trough values of ~5–11 ppbv for N2O, 0.07–0.13 ppmv for CH4, and ~10 ppbv for CO (~40 ppbv at 360 K). The maximum amounts are primarily located over Africa and, depending on the species, secondary more or less pronounced maxima are reported above northern South America and South-East Asia. The lower stratosphere over the Western Pacific deep convective region where the outgoing longwave radiation is the lowest, the tropopause the highest and the coldest, appears as a region of minimum concentration of tropospheric trace species. The possible impact on trace gas concentration at the tropopause of the inhomogeneous distribution and intensity of the sources, mostly continental, of the horizontal and vertical transports in the troposphere, and of cross-tropopause transport was explored 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, the CH4 (8–10 year lifetime) and N2O (130-year lifetime) longitudinal variations 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 large longitudinal contrast of N2O and CH4 concentrations reported by the space-borne instruments at the tropopause and in the lower stratosphere not captured by the model thus requires another explanation. The suggestion is 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 the same season. Compared to observations, the MOCAGE model forced by ECMWF analyses is found to ignore these fast local uplifts, but to overestimate the average uniform vertical transport in the UTLS at all longitudes in the tropics.

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