scholarly journals Quantifying tracer transport in the tropical lower stratosphere using WACCM

2013 ◽  
Vol 13 (5) ◽  
pp. 13245-13283 ◽  
Author(s):  
M. Abalos ◽  
W. J. Randel ◽  
D. E. Kinnison ◽  
E. Serrano
Keyword(s):  
Lower Stratosphere ◽  
Climate Model ◽  
Latitudinal Gradients ◽  
Explicit Calculation ◽  
Boreal Summer ◽  
Tracer Transport ◽  
Annual Cycles ◽  
Tropical Tropopause ◽  
Eddy Transport ◽  
Isentropic Coordinates

Abstract. The zonal mean transport of ozone and carbon monoxide (CO) near the tropical tropopause is investigated using the Whole-Atmosphere Community Climate Model version 4 (WACCM4). The variability in temperature, ozone and CO in the model shows good agreement with satellite and balloon observations. Modeled temperature and tracers exhibit large and closely coupled annual cycles in the tropical lower stratosphere, as in the observations. The thermodynamic and tracer budgets in the model are analyzed based on the Transformed Eulerian Mean (TEM) framework on log-pressure coordinates and also using the isentropic formulation. Results show that the coupled seasonal cycles are mainly forced by tropical upwelling over altitudes with large vertical tracer gradients, in agreement with previous observational studies. The model also allows explicit calculation of eddy transport terms, which make an important contribution to ozone tendencies in the tropical lower stratosphere. The character of the eddy fluxes changes with altitude. At higher levels (~2 km above the cold point tropopause), isentropic eddy transport occurs during winter and spring in each hemisphere in the sub-tropics, associated with transient Rossby waves acting on strong background latitudinal gradients. At lower altitudes, close to the tropical tropopause, there is a maximum in horizontal eddy transport during boreal summer associated with the Asian monsoon anticyclone. Sub-seasonal variability in ozone and CO, tied to fluctuations in temperature, is primarily driven by transient tropical upwelling. In isentropic coordinates, the overall tracer budgets are similar to the log-pressure results, highlighting cross-isentropic mean advection as the main term in the balance. However, in isentropic coordinates the tracer variability is largely reduced on both seasonal and sub-seasonal timescales, because the tracer and temperature fluctuations are highly correlated (as a response to upwelling).

scholarly journals Quantifying tracer transport in the tropical lower stratosphere using WACCM

2013 ◽  
Vol 13 (21) ◽  
pp. 10591-10607 ◽  
Author(s):  
M. Abalos ◽  
W. J. Randel ◽  
D. E. Kinnison ◽  
E. Serrano
Keyword(s):  
Lower Stratosphere ◽  
Climate Model ◽  
Latitudinal Gradients ◽  
Explicit Calculation ◽  
Boreal Summer ◽  
Tracer Transport ◽  
Annual Cycles ◽  
Tropical Tropopause ◽  
Eddy Transport ◽  
Isentropic Coordinates

Abstract. The zonal mean transport of ozone and carbon monoxide (CO) near the tropical tropopause is investigated using the Whole-Atmosphere Community Climate Model version 4 (WACCM4). The variability in temperature, ozone and CO in the model shows good agreement with satellite and balloon observations. Modeled temperature and tracers exhibit large and closely coupled annual cycles in the tropical lower stratosphere, as in the observations. The thermodynamic and tracer budgets in the model are analyzed based on the Transformed Eulerian Mean (TEM) framework on log-pressure coordinates and also using the isentropic formulation. Results show that the coupled seasonal cycles are mainly forced by tropical upwelling over altitudes with large vertical tracer gradients, in agreement with previous observational studies. The model also allows explicit calculation of eddy transport terms, which make an important contribution to ozone tendencies in the tropical lower stratosphere. The character of the eddy fluxes changes with altitude. At higher levels (~2 km above the cold point tropopause), isentropic eddy transport occurs during winter and spring in each hemisphere in the sub-tropics, associated with transient Rossby waves acting on strong background latitudinal gradients. At lower altitudes, close to the tropical tropopause, there is a maximum in horizontal eddy transport during boreal summer associated with the Asian monsoon anticyclone. Sub-seasonal variability in ozone and CO, tied to fluctuations in temperature, is primarily driven by transient tropical upwelling. In isentropic coordinates, the overall tracer budgets are similar to the log-pressure results, highlighting cross-isentropic advection as the main term in the time-mean balance, with large seasonality above the tropopause. However, in isentropic coordinates the tracer variability is largely reduced on both seasonal and sub-seasonal timescales, because tracer fluctuations are highly correlated with temperature (as a response to upwelling).

scholarly journals Sensitivities of modelled water vapour in the lower stratosphere: temperature uncertainty, effects of horizontal transport and small-scale mixing

2018 ◽  
Vol 18 (12) ◽  
pp. 8505-8527 ◽  
Author(s):  
Liubov Poshyvailo ◽  
Rolf Müller ◽  
Paul Konopka ◽  
Gebhard Günther ◽  
Martin Riese ◽  
...  
Keyword(s):  
Water Vapour ◽  
Lower Stratosphere ◽  
Climate Model ◽  
Global Radiation ◽  
Future Climate Change ◽  
Boreal Summer ◽  
Small Scale ◽  
Strong Mixing ◽  
Horizontal Transport ◽  
Tropical Tropopause

Abstract. Water vapour (H2O) in the upper troposphere and lower stratosphere (UTLS) has a significant role for global radiation. A realistic representation of H2O is therefore critical for accurate climate model predictions of future climate change. In this paper we investigate the effects of current uncertainties in tropopause temperature, horizontal transport and small-scale mixing on simulated H2O in the lower stratosphere (LS). To assess the sensitivities of simulated H2O, we use the Chemical Lagrangian Model of the Stratosphere (CLaMS). First, we examine CLaMS, which is driven by two reanalyses, from the European Centre of Medium-Range Weather Forecasts (ECMWF) ERA-Interim and the Japanese 55-year Reanalysis (JRA-55), to investigate the robustness with respect to the meteorological dataset. Second, we carry out CLaMS simulations with transport barriers along latitude circles (at the Equator, 15 and 35∘ N/S) to assess the effects of horizontal transport. Third, we vary the strength of parametrized small-scale mixing in CLaMS. Our results show significant differences (about 0.5 ppmv) in simulated stratospheric H2O due to uncertainties in the tropical tropopause temperatures between the two reanalysis datasets, JRA-55 and ERA-Interim. The JRA-55 based simulation is significantly moister when compared to ERA-Interim, due to a warmer tropical tropopause (approximately 2 K). The transport barrier experiments demonstrate that the Northern Hemisphere (NH) subtropics have a strong moistening effect on global stratospheric H2O. The comparison of tropical entry H2O from the sensitivity 15∘ N/S barrier simulation and the reference case shows differences of up to around 1 ppmv. Interhemispheric exchange shows only a very weak effect on stratospheric H2O. Small-scale mixing mainly increases troposphere–stratosphere exchange, causing an enhancement of stratospheric H2O, particularly along the subtropical jets in the summer hemisphere and in the NH monsoon regions. In particular, the Asian and American monsoon systems during a boreal summer appear to be regions especially sensitive to changes in small-scale mixing, which appears crucial for controlling the moisture anomalies in the monsoon UTLS. For the sensitivity simulation with varied mixing strength, differences in tropical entry H2O between the weak and strong mixing cases amount to about 1 ppmv, with small-scale mixing enhancing H2O in the LS. The sensitivity studies presented here provide new insights into the leading processes that control stratospheric H2O, which are important for assessing and improving climate model projections.

scholarly journals Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

2016 ◽  
Author(s):  
Anne A. Glanville ◽  
Thomas Birner
Keyword(s):  
Transport Model ◽  
Vertical Mixing ◽  
Tape Recorder ◽  
Boreal Summer ◽  
Tracer Transport ◽  
Vertical Advection ◽  
Tropical Tropopause ◽  
Horizontal Mixing ◽  
Isentropic Coordinates

Abstract. Nearly all air enters the stratosphere through the tropical tropopause layer (TTL). The TTL therefore exerts a control on stratospheric chemistry and climate. The hemispheric meridional overturning (Brewer-Dobson) circulation spreads this TTL influence upward and poleward. Stratospheric water vapor concentrations are set near the tropical tropopause and are nearly conserved in the lowermost stratosphere. The resulting upward propagating tracer transport signal of seasonally varying entry concentrations is known as the tape recorder signal. Here, we study the roles of vertical and horizontal mixing in shaping the tape recorder signal in the tropical lowermost stratosphere. We analyze the tape recorder signal using data from satellite observations, a reanalysis, and a chemistry-climate model (CCM). Modifying past methods, we are able to capture the seasonal cycle of effective vertical transport velocity in the tropical lowermost stratosphere, which is found to be multiple times stronger than residual vertical velocities for the reanalysis and the CCM. We also study the tape recorder signal in an idealized one-dimensional transport model. By performing a parameter-sweep we test a range of different strengths of transport contributions by vertical advection, vertical mixing, and horizontal mixing. Introducing seasonality in the transport strengths we find that the most successful simulation of the observed tape recorder signal requires quadrupled vertical mixing in the lowermost tropical stratosphere compared to previous estimates in the literature. Vertical mixing is especially important during boreal summer when vertical advection is weak. The reanalysis requires excessive amounts of vertical mixing compared to observations but also to the CCM, which hints at the role of spurious dispersion due to data assimilation. Contrasting the results between pressure and isentropic coordinates allows further insights into quasi-adiabatic vertical mixing, e.g. associated with breaking gravity waves. Horizontal mixing, which takes place primarily along isentropes due to Rossby wave breaking, is captured more consistently in isentropic coordinates. Overall our study emphasizes the role of vertical mixing in lowermost tropical stratospheric transport, which appears to be as important as vertical advection by the residual mass circulation. This questions the perception of the "tape recorder" as a manifestation of slow upward transport as opposed to a phenomenon influenced by quick and intense transport through mixing, at least near the tape head.

Impact of tropical tropopause layer cooling trend on extreme deep convection in Asian-African sector

2021 ◽  
Author(s):  
Kunihiko Kodera ◽  
Nawo Eguchi ◽  
Rei Ueyama ◽  
Beatriz Funatsu ◽  
Marco Gaetani ◽  
...  
Keyword(s):  
Lower Stratosphere ◽  
Essential Feature ◽  
Deep Convection ◽  
Boreal Summer ◽  
Surface Precipitation ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Extreme Precipitation Events ◽  
Cooling Trend ◽  
Sahel Region

<p>Previous studies have suggested that the recent increase in tropical extreme deep convection, in particular over Asia and Africa during the boreal summer, has occurred in association with a cooling in the tropical lower stratosphere. The present study is focused on the Sahel region of West Africa, where an increased occurrence of extreme precipitation events has been reported over recent decades. The results show that the changes since the 1980s involve a cooling trend in the tropical lower stratosphere and tropopause layer, combined with a warming in the troposphere. This feature is similar to that which might result from increased greenhouse gas levels. It is suggested that the decrease in the vertical temperature gradient in the tropical tropopause region enhances extreme deep convection where penetrating convection is frequent, whereas tropospheric warming suppresses the shallower convection. The essential feature of the recent changes over the tropics is therefore the depth of convection, rather than the total amount of surface precipitation. This could enhance cooling in the lower stratosphere through decrease in ozone concentration.</p><p> </p>

scholarly journals Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

2017 ◽  
Vol 17 (6) ◽  
pp. 4337-4353 ◽  
Author(s):  
Anne A. Glanville ◽  
Thomas Birner
Keyword(s):  
Transport Model ◽  
Vertical Mixing ◽  
Vertical Transport ◽  
Tape Recorder ◽  
Boreal Summer ◽  
Vertical Advection ◽  
Tropical Tropopause ◽  
Horizontal Mixing ◽  
Isentropic Coordinates

Abstract. Nearly all air enters the stratosphere through the tropical tropopause layer (TTL). The TTL therefore exerts a control on stratospheric chemistry and climate. The hemispheric meridional overturning (Brewer–Dobson) circulation spreads this TTL influence upward and poleward. Stratospheric water vapor concentrations are set near the tropical tropopause and are nearly conserved in the lowermost stratosphere. The resulting upward propagating tracer transport signal of seasonally varying entry concentrations is known as the tape recorder signal. Here, we study the roles of vertical and horizontal mixing in shaping the tape recorder signal in the tropical lowermost stratosphere, focusing on the 80 hPa level. We analyze the tape recorder signal using data from satellite observations, a reanalysis, and a chemistry–climate model (CCM). By modifying past methods, we are able to capture the seasonal cycle of effective vertical transport velocity in the tropical lowermost stratosphere. Effective vertical transport velocities are found to be multiple times stronger than residual vertical velocities for the reanalysis and the CCM. We also study the tape recorder signal in an idealized 1-D transport model. By performing a parameter sweep, we test a range of different strengths of transport contributions by vertical advection, vertical mixing, and horizontal mixing. By introducing seasonality into the transport strengths, we find that the most successful simulation of the observed tape recorder signal requires vertical mixing at 80 hPa that is multiple times stronger compared to previous estimates in the literature. Vertical mixing is especially important during boreal summer when vertical advection is weak. Simulating the reanalysis tape recorder requires excessive amounts of vertical mixing compared to observations but also to the CCM, which hints at the role of spurious dispersion due to data assimilation. Contrasting the results between pressure and isentropic coordinates allows for further insights into quasi-adiabatic vertical mixing, e.g., associated with overshooting convection or breaking gravity waves. Horizontal mixing, which takes place primarily along isentropes due to Rossby wave breaking, is captured more consistently in isentropic coordinates. Overall, our study emphasizes the role of vertical mixing in lowermost tropical stratospheric transport, which appears to be as important as vertical advection by the residual mass circulation. This questions the perception of the tape recorder as a manifestation of slow upward transport as opposed to a phenomenon influenced by quick and intense transport through mixing, at least near the tape head. However, due to the limitations of the observational dataset used and the simplicity of the applied transport model, further work is required to more clearly specify the role of vertical mixing in lowermost stratospheric transport in the tropics.

scholarly journals Impact of convectively lofted ice on the seasonal cycle of tropical lower stratospheric water vapor

2019 ◽  
Author(s):  
Xun Wang ◽  
Andrew E. Dessler ◽  
Mark R. Schoeberl ◽  
Wandi Yu ◽  
Tao Wang
Keyword(s):  
Water Vapor ◽  
Seasonal Cycle ◽  
Climate Model ◽  
Boreal Summer ◽  
Small Contribution ◽  
Asian Monsoon Region ◽  
Stratospheric Water Vapor ◽  
Trajectory Model ◽  
Tropical Tropopause ◽  
Important Region

Abstract. We use a forward Lagrangian trajectory model to diagnose mechanisms that produce the tropical lower stratospheric (LS) water vapor seasonal cycle observed by the Microwave Limb Sounder (MLS) and reproduced by the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM) in the tropical tropopause layer (TTL). We confirm in both the MLS and GEOSCCM that the seasonal cycle of water vapor is primarily determined by the seasonal cycle of TTL temperatures. However, we find that the seasonal cycle of temperature predicts a smaller seasonal cycle of LS water vapor between 10° N–40° N than observed by MLS. We show that including evaporation of convectively lofted ice in the trajectory model increases the simulated maximum value in the 10° N–40° N water vapor seasonal cycle by 1.9 ppmv (47 %) and increases the seasonal amplitude by 1.26 ppmv (123 %), which improves the prediction of LS water vapor annual cycle. We conclude that the moistening effect from convective ice evaporation in the TTL plays a key role regulating and maintaining the tropical LS water vapor seasonal cycle. Most of the convective moistening in the 10° N–40° N range comes from convective ice evaporation occurring at the same latitudes. A small contribution to the moistening comes from convective ice evaporation occurring between 10° S–10° N. Within 10° N–40° N, the Asian monsoon region is the most important region for convective ice evaporation and convective moistening during boreal summer and autumn.

scholarly journals Impact of convectively lofted ice on the seasonal cycle of water vapor in the tropical tropopause layer

2019 ◽  
Vol 19 (23) ◽  
pp. 14621-14636 ◽  
Author(s):  
Xun Wang ◽  
Andrew E. Dessler ◽  
Mark R. Schoeberl ◽  
Wandi Yu ◽  
Tao Wang
Keyword(s):  
Water Vapor ◽  
Seasonal Cycle ◽  
Climate Model ◽  
Boreal Summer ◽  
Small Contribution ◽  
Asian Monsoon Region ◽  
Tropical Tropopause Layer ◽  
Trajectory Model ◽  
Tropical Tropopause ◽  
Important Region

Abstract. We use a forward Lagrangian trajectory model to diagnose mechanisms that produce the water vapor seasonal cycle observed by the Microwave Limb Sounder (MLS) and reproduced by the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) in the tropical tropopause layer (TTL). We confirm in both the MLS and GEOSCCM that the seasonal cycle of water vapor entering the stratosphere is primarily determined by the seasonal cycle of TTL temperatures. However, we find that the seasonal cycle of temperature predicts a smaller seasonal cycle of TTL water vapor between 10 and 40∘ N than observed by MLS or simulated by the GEOSCCM. Our analysis of the GEOSCCM shows that including evaporation of convective ice in the trajectory model increases both the simulated maximum value of the 100 hPa 10–40∘ N water vapor seasonal cycle and the seasonal-cycle amplitude. We conclude that the moistening effect from convective ice evaporation in the TTL plays a key role in regulating and maintaining the seasonal cycle of water vapor in the TTL. Most of the convective moistening in the 10–40∘ N range comes from convective ice evaporation occurring at the same latitudes. A small contribution to the moistening comes from convective ice evaporation occurring between 10∘ S and 10∘ N. Within the 10–40∘ N band, the Asian monsoon region is the most important region for convective moistening by ice evaporation during boreal summer and autumn.

scholarly journals Variability in upwelling across the tropical tropopause and correlations with tracers in the lower stratosphere

2012 ◽  
Vol 12 (23) ◽  
pp. 11505-11517 ◽  
Author(s):  
M. Abalos ◽  
W. J. Randel ◽  
E. Serrano
Keyword(s):  
Time Series ◽  
Temporal Variability ◽  
Seasonal Cycle ◽  
Strong Influence ◽  
Lower Stratosphere ◽  
Momentum Balance ◽  
Annual Cycles ◽  
Tropical Tropopause ◽  
Vertical Gradients ◽  
Good Agreement

Abstract. Temporal variability of the upwelling near the tropical tropopause on daily to annual timescales is investigated using three different estimates computed from the ERA-Interim reanalysis. These include upwelling archived by the reanalysis, plus estimates derived from thermodynamic and momentum balance calculations. Substantial variability in upwelling is observed on both seasonal and sub-seasonal timescales, and the three estimates show reasonably good agreement. Tropical upwelling should exert strong influence on temperatures and on tracers with large vertical gradients in the lower stratosphere. We test this behavior by comparing the calculated upwelling estimates with observed temperatures in the tropical lower stratosphere, and with measurements of ozone and carbon monoxide (CO) from the Aura Microwave Limb Sounder (MLS) satellite instrument. Time series of temperature, ozone and CO are well correlated in the tropical lower stratosphere, and we quantify the influence of tropical upwelling on this joint variability. Strong coherent annual cycles observed in each quantity are found to reflect the seasonal cycle in upwelling. Statistically significant correlations between upwelling, temperatures and tracers are also found for sub-seasonal timescales, demonstrating the importance of upwelling in forcing transient variability in the lower tropical stratosphere.

Twenty-First Century Trends in Mixing Barriers and Eddy Transport in the Lower Stratosphere

2021 ◽  
Author(s):  
Marta Abalos ◽  
Alvaro de la Cámara
Keyword(s):  
Lower Stratosphere ◽  
Climate Model ◽  
Substantial Reduction ◽  
Polar Vortex ◽  
Effective Diffusivity ◽  
Wave Drag ◽  
First Century ◽  
Austral Spring ◽  
Eddy Transport ◽  
Climate Model Simulations

<p>Future trends in isentropic mixing in the lower stratosphere remain largely unexplored, in contrast with the advective component of the Brewer-Dobson circulation. This study examines trends in effective diffusivity (κ<sub>eff</sub> ), a measure of the potential of the flow to produce isentropic mixing, in recent chemistry-climate model simulations. The results highlight substantial reduction of κ<sub>eff</sub>  in the upper flanks of the subtropical jets from fall to spring, which are strengthened in response to greenhouse gas increases. This contrasts with stronger eddy transport, associated with increased wave drag in the region, peaking in summer near the critical lines. The projected ozone recovery leads to enhanced κ<sub>eff</sub> in polar austral spring and summer, associated with a weaker and shorter-lived austral polar vortex by the end of the 21st century. </p>

scholarly journals Sensitivities of modelled water vapour in the lower stratosphere: temperature uncertainty, effects of horizontal transport and small-scale mixing

2017 ◽  
Author(s):  
Liubov Poshyvailo ◽  
Rolf Müller ◽  
Paul Konopka ◽  
Gebhard Günther ◽  
Martin Riese ◽  
...  
Keyword(s):  
Water Vapour ◽  
Lower Stratosphere ◽  
Climate Model ◽  
Global Radiation ◽  
Lagrangian Model ◽  
Future Climate Change ◽  
Small Scale ◽  
Asian Monsoon Region ◽  
Horizontal Transport ◽  
Tropical Tropopause

Abstract. Water vapour (H2O) in the upper troposphere and lower stratosphere (UTLS) is a key player for global radiation. A realistic representation of H2O is critical for climate model predictions of future climate change. Here, we investigate the effects of current uncertainties in tropopause temperature, horizontal transport and small-scale mixing on simulated H2O in the lower stratosphere (LS). To assess the sensitivities of simulated H2O, we use the Chemical Lagrangian Model of the Stratosphere (CLaMS). First, we examine CLaMS driven by two different reanalysis, ERA-Interim and Japanese 55-year (JRA-55) reanalysis, to investigate the robustness with respect to the meteorological dataset. Second, we carry out CLaMS simulations with transport barriers along latitude circles (at the equator, 15° N/S and 35° N/S) to assess the effects of horizontal transport. Third, we vary the strength of parametrized small-scale mixing in CLaMS. Our results show significant differences (about 0.5 ppmv) in simulated stratospheric H2O due to uncertainties in the tropical tropopause temperatures between current reanalysis datasets. The JRA-55 based simulation is significantly moister when compared to ERA-Interim, due to a warmer tropical tropopause in JRA-55 reanalysis. The transport barrier experiments demonstrate that the Northern Hemisphere (NH) subtropics have a strong moistening effect on global stratospheric H2O. Interhemispheric exchange shows only a very weak effect on stratospheric H2O. Small-scale mixing mainly increases troposphere–stratosphere exchange, causing an enhancement of stratospheric H2O, particularly along the subtropical jets and in the Asian monsoon region. The sensitivity studies presented here provide new insights into the leading processes that control stratospheric H2O, important for assessing and improving climate model projections.

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