Influence of Upper-Troposphere Stratification and Cloud-Radiation Interaction on Convective Overshoots in the Tropical Tropopause Layer

Tropical Tropopause ◽

The Difference

AbstractIt is still debated whether radiative heating observed in the tropical tropopause layer (TTL) is balanced primarily by cooling from convective overshoots, as in an entrainment layer, or by adiabatic cooling from large-scale eddy-driven upwelling. In this study, three-dimensional cloud-resolving model simulations of radiative-convective equilibrium were carried out with three different cloud microphysics schemes and 1-km horizontal resolution. We demonstrate that overshooting cooling in the TTL can be strongly modulated by upper-troposphere stratification. Two of the schemes produce a hard-landing scenario in which convective overshoots reach the TTL with frequent large vertical velocity leading to strong overshooting cooling (~ −0.2 K day-1). The third scheme produces a soft-landing scenario in which convective overshoots rarely reach the TTL with large vertical velocity and produce little overshooting cooling (~ −0.03 K day-1). The difference between the two scenarios is attributed to changes in the upper-troposphere stratification related to different atmospheric cloud radiative effects (ACRE). The microphysics scheme that produces the soft-landing scenario has much stronger ACRE in the upper troposphere leading to a ~3K warmer and more stable layer which acts as a buffer zone to slow down the convective updrafts. The stratification mechanism suggests the possibility for the ozone variation or eddy-driven upwelling in the TTL to modulate convective overshoots. We further test the sensitivity of overshooting cooling to changes in model resolution by increasing the horizontal resolution to 100 m. The corresponding change of overshooting cooling is much smaller compared with the difference between the hard-landing and soft-landing scenarios.

The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)

10.5194/acp-15-6467-2015 ◽

2015 ◽

Vol 15 (11) ◽

pp. 6467-6486 ◽

Cited By ~ 21

Author(s):

W. Frey ◽

R. Schofield ◽

P. Hoor ◽

D. Kunkel ◽

F. Ravegnani ◽

...

Keyword(s):

Lower Stratosphere ◽

Deep Convection ◽

Horizontal Resolution ◽

Transport Processes ◽

Convective System ◽

Upper Troposphere ◽

Tropical Tropopause ◽

Transport And Mixing ◽

The Impact

Abstract. In this study we examine the simulated downward transport and mixing of stratospheric air into the upper tropical troposphere as observed on a research flight during the SCOUT-O3 campaign in connection with a deep convective system. We use the Advanced Research Weather and Research Forecasting (WRF-ARW) model with a horizontal resolution of 333 m to examine this downward transport. The simulation reproduces the deep convective system, its timing and overshooting altitudes reasonably well compared to radar and aircraft observations. Passive tracers initialised at pre-storm times indicate the downward transport of air from the stratosphere to the upper troposphere as well as upward transport from the boundary layer into the cloud anvils and overshooting tops. For example, a passive ozone tracer (i.e. a tracer not undergoing chemical processing) shows an enhancement in the upper troposphere of up to about 30 ppbv locally in the cloud, while the in situ measurements show an increase of 50 ppbv. However, the passive carbon monoxide tracer exhibits an increase, while the observations show a decrease of about 10 ppbv, indicative of an erroneous model representation of the transport processes in the tropical tropopause layer. Furthermore, it could point to insufficient entrainment and detrainment in the model. The simulation shows a general moistening of air in the lower stratosphere, but it also exhibits local dehydration features. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.

The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)

10.5194/acpd-15-1041-2015 ◽

2015 ◽

Vol 15 (1) ◽

pp. 1041-1091 ◽

Cited By ~ 1

Author(s):

W. Frey ◽

R. Schofield ◽

P. Hoor ◽

D. Kunkel ◽

F. Ravegnani ◽

...

Keyword(s):

Lower Stratosphere ◽

Deep Convection ◽

Horizontal Resolution ◽

Transport Processes ◽

Convective System ◽

Upper Troposphere ◽

Tropical Tropopause ◽

Transport And Mixing ◽

The Impact

Abstract. In this study we examine the simulated downward transport and mixing of stratospheric air into the upper tropical troposphere as observed on a research flight during the SCOUT-O3 campaign in connection to a deep convective system. We use the Advanced Research Weather and Research Forecasting (WRF-ARW) model with a horizontal resolution of 333 m to examine this downward transport. The simulation reproduces the deep convective system, its timing and overshooting altitudes reasonably well compared to radar and aircraft observations. Passive tracers initialised at pre-storm times indicate the downward transport of air from the stratosphere to the upper troposphere as well as upward transport from the boundary layer into the cloud anvils and overshooting tops. For example, a passive ozone tracer (i.e. a tracer not undergoing chemical processing) shows an enhancement in the upper troposphere of up to about 30 ppbv locally in the cloud, while the in situ measurements show an increase of 50 ppbv. However, the passive carbon monoxide tracer exhibits an increase, while the observations show a decrease of about 10 ppbv, indicative of an erroneous model representation of the transport processes in the tropical tropopause layer. Furthermore, it could point to insufficient entrainment and detrainment in the model. The simulation shows a general moistening of air in the lower stratosphere but it also exhibits local dehydration features. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.

Overshooting of clean tropospheric air in the tropical lower stratosphere as seen by the CALIPSO lidar

10.5194/acp-11-9683-2011 ◽

2011 ◽

Vol 11 (18) ◽

pp. 9683-9696 ◽

Cited By ~ 32

Author(s):

J.-P. Vernier ◽

J.-P. Pommereau ◽

L. W. Thomason ◽

J. Pelon ◽

A. Garnier ◽

...

Keyword(s):

Biomass Burning ◽

Lower Stratosphere ◽

Thermal Structure ◽

Radiative Heating ◽

Major Contributor ◽

Upper Troposphere ◽

Volcanic Plumes ◽

Tropical Tropopause ◽

Organic Particles

Abstract. The evolution of aerosols in the tropical upper troposphere/lower stratosphere between June 2006 and October 2009 is examined using the observations of the space borne CALIOP lidar aboard the CALIPSO satellite. Superimposed on several volcanic plumes and soot from an extreme biomass-burning event in 2009, the measurements reveal the existence of fast-cleansing episodes in the lower stratosphere to altitudes as high as 20 km. The cleansing of the layer, which extends from 14 to 20 km, takes place within 1 to 4 months during the southern tropics convective season that transports aerosol-poor tropospheric air into the lower stratosphere. In contrast, the convective season of the Northern Hemisphere summer shows an increase in the particle load at the tropopause consistent with a lofting of air rich with aerosols. These aerosols can consist of surface-derived material such as mineral dust and soot as well as liquid sulfate and organic particles. The flux of tropospheric air during the Southern Hemisphere convective season derived from CALIOP observations is shown to be 5 times at 16 km and 20 times at 19 km larger, respectively, than that associated with flux caused by slow ascent through radiative heating. These results suggest that convective overshooting is a major contributor to troposphere-to-stratosphere transport with concomitant implications for the Tropical Tropopause Layer top height, the humidity, the photochemistry and the thermal structure of the layer.

A modelling case study of a large-scale cirrus in the tropical tropopause layer

10.5194/acp-16-3881-2016 ◽

2016 ◽

Vol 16 (6) ◽

pp. 3881-3902 ◽

Cited By ~ 4

Author(s):

Aurélien Podglajen ◽

Riwal Plougonven ◽

Albert Hertzog ◽

Bernard Legras

Keyword(s):

Large Scale ◽

Cirrus Clouds ◽

Fair Agreement ◽

Radiative Heating ◽

Cloud Formation ◽

Vertical Position ◽

Microphysics Scheme ◽

Tropical Tropopause ◽

Equatorial Wave

Abstract. We use the Weather Research and Forecast (WRF) model to simulate a large-scale tropical tropopause layer (TTL) cirrus in order to understand the formation and life cycle of the cloud. This cirrus event has been previously described through satellite observations by Taylor et al. (2011). Comparisons of the simulated and observed cirrus show a fair agreement and validate the reference simulation regarding cloud extension, location and life time. The validated simulation is used to understand the causes of cloud formation. It is shown that several cirrus clouds successively form in the region due to adiabatic cooling and large-scale uplift rather than from convective anvils. The structure of the uplift is tied to the equatorial response (equatorial wave excitation) to a potential vorticity intrusion from the midlatitudes. Sensitivity tests are then performed to assess the relative importance of the choice of the microphysics parameterization and of the initial and boundary conditions. The initial dynamical conditions (wind and temperature) essentially control the horizontal location and area of the cloud. However, the choice of the microphysics scheme influences the ice water content and the cloud vertical position. Last, the fair agreement with the observations allows to estimate the cloud impact in the TTL in the simulations. The cirrus clouds have a small but not negligible impact on the radiative budget of the local TTL. However, for this particular case, the cloud radiative heating does not significantly influence the simulated dynamics. This result is due to (1) the lifetime of air parcels in the cloud system, which is too short to significantly influence the dynamics, and (2) the fact that induced vertical motions would be comparable to or smaller than the typical mesoscale motions present. Finally, the simulation also provides an estimate of the vertical redistribution of water by the cloud and the results emphasize the importance in our case of both rehydration and dehydration in the vicinity of the cirrus.

A Lagrangian view of convective sources for transport of air across the Tropical Tropopause Layer: distribution, times and the radiative influence of clouds

10.5194/acpd-11-18161-2011 ◽

2011 ◽

Vol 11 (6) ◽

pp. 18161-18209

Author(s):

A. Tzella ◽

B. Legras

Keyword(s):

Large Scale ◽

Significant Proportion ◽

Vertical Transport ◽

Radiative Heating ◽

Small Scale ◽

Back Trajectory ◽

Clear Sky ◽

Tropical Tropopause ◽

Wide Range

Abstract. The Tropical Tropopause Layer (TTL) is a key region controlling transport between the troposphere and the stratosphere. The efficiency of transport across the TTL depends on the continuous interaction between the large-scale advection and the small-scale intermittent convection that reaches the Level of Zero radiative Heating (LZH). The wide range of scales involved presents a significant challenge to determine the sources of convection and quantify transport across the TTL. Here, we use a simple Lagrangian model, termed TTL detrainment model, that combines a large ensemble of 200-day back trajectory calculations with high-resolution fields of brightness temperatures (provided by the CLAUS dataset) in order to determine the ensemble of trajectories that are detrained from convective sources. The trajectories are calculated using the ECMWF ERA-Interim winds and radiative heating rates, derived both under all-sky and clear-sky conditions, so that the radiative influence of clouds is established. We show that most trajectories are detrained near the mean LZH with the horizontal distributions of convective sources being highly-localized, even within the space defined by deep convection. As well as modifying the degree of source localization, the radiative heating from clouds facilitates the rapid upwelling of air across the TTL. However, large-scale motion near the fluctuating LZH can lead a significant proportion of trajectories to alternating clear-sky and cloudy regions, thus generating a large dispersion in the vertical transport times. The distributions of vertical transport times are wide and skewed and are largely insensitive to a bias of about ±1 km (∓5 K) in the altitude of cloud top heights (the main sensitivity appearing in the times to escape the immediate neighbourhood of the LZH) while seasonal and regional transport characteristics are only apparent at small time-scales. The strong horizontal mixing that characterizes the TTL ensures that most air of convective origin is well-mixed within the tropical and eventually within the extra-tropical lower-stratosphere.

Overshooting of clean tropospheric air in the tropical lower stratosphere as seen by the CALIPSO lidar

10.5194/acpd-11-163-2011 ◽

2011 ◽

Vol 11 (1) ◽

pp. 163-192 ◽

Cited By ~ 2

Author(s):

J. P. Vernier ◽

J. P. Pommereau ◽

L. W. Thomason ◽

J. Pelon ◽

A. Garnier ◽

...

Keyword(s):

Transport Process ◽

Lower Stratosphere ◽

Thermal Structure ◽

Radiative Heating ◽

Convective Activity ◽

Upper Troposphere ◽

Volcanic Plumes ◽

Tropical Tropopause ◽

Hemisphere Winter

Abstract. The evolution of aerosols in the tropical upper troposphere/lower stratosphere between June 2006 and October 2009 is examined using the observations of the space borne CALIOP lidar aboard the CALIPSO satellite. Superimposed on several volcanic plumes and soot from an extreme biomass-burning event in 2009, the measurements reveal the existence of fast cleansing episodes of the lower stratosphere to altitudes as high as 20 km. The cleansing of the full 14–20 km layer takes place within 1–4 months. Its coincidence with the maximum of convective activity in the southern tropics, suggests that the cleansing is the result of a large number of overshooting towers, injecting aerosol-poor tropospheric air into the lower stratosphere. The enhancements of aerosols at the tropopause level during the NH summer may be due to the same transport process but associated with intense sources of aerosols at the surface. Since, the tropospheric air flux derived from CALIOP observations during North Hemisphere winter is 5–20 times larger than the slow ascent by radiative heating usually assumed, the observations suggest that convective overshooting is a major contributor to troposphere-to-stratosphere transport with concommitant implications to the Tropical Tropopause Layer top height, chemistry and thermal structure.

A Lagrangian view of convective sources for transport of air across the Tropical Tropopause Layer: distribution, times and the radiative influence of clouds

10.5194/acp-11-12517-2011 ◽

2011 ◽

Vol 11 (23) ◽

pp. 12517-12534 ◽

Cited By ~ 31

Author(s):

A. Tzella ◽

B. Legras

Keyword(s):

Large Scale ◽

Significant Proportion ◽

Vertical Transport ◽

Radiative Heating ◽

Small Scale ◽

Back Trajectory ◽

Clear Sky ◽

Tropical Tropopause ◽

Wide Range

Abstract. The tropical tropopause layer (TTL) is a key region controlling transport between the troposphere and the stratosphere. The efficiency of transport across the TTL depends on the continuous interaction between the large-scale advection and the small-scale intermittent convection that reaches the Level of Zero radiative Heating (LZH). The wide range of scales involved presents a significant challenge to determine the sources of convection and quantify transport across the TTL. Here, we use a simple Lagrangian model, termed TTL detrainment model, that combines a large ensemble of 200-day back trajectory calculations with high-resolution fields of brightness temperatures (provided by the CLAUS dataset) in order to determine the ensemble of trajectories that are detrained from convective sources. The trajectories are calculated using the ECMWF ERA-Interim winds and radiative heating rates, and in order to establish the radiative influence of clouds, the latter rates are derived both under all-sky and clear-sky conditions. We show that most trajectories are detrained near the mean LZH with the horizontal distributions of convective sources being highly-localized, even within the space defined by deep convection. As well as modifying the degree of source localization, the radiative heating from clouds facilitates the rapid upwelling of air across the TTL. However, large-scale motion near the fluctuating LZH can lead a significant proportion of trajectories to alternating clear-sky and cloudy regions, thus generating a large dispersion in the vertical transport times. The distributions of vertical transport times are wide and skewed and are largely insensitive to a bias of about ±1 km (∓5 K) in the altitude of cloud top heights (the main sensitivity appearing in the times to escape the immediate neighbourhood of the LZH) while some seasonal and regional transport characteristics are apparent for times up to 60 days. The strong horizontal mixing that characterizes the TTL ensures that most air of convective origin is well-mixed within the tropical and eventually within the extra-tropical lower-stratosphere.

Vertical transport rates and concentrations of OH and Cl radicals in the Tropical Tropopause Layer from Observations of CO<sub>2</sub> and halocarbons: implications for distributions of long- and short-lived chemical species

10.5194/acpd-10-6059-2010 ◽

2010 ◽

Vol 10 (3) ◽

pp. 6059-6095

Author(s):

S. Park ◽

E. L. Atlas ◽

R. Jiménez ◽

B. C. Daube ◽

E. W. Gottlieb ◽

...

Keyword(s):

Large Scale ◽

Chemical Species ◽

Vertical Transport ◽

Upward Movement ◽

Upper Troposphere ◽

Tropical Tropopause ◽

Validation Experiment ◽

In Situ Observations

Abstract. Rates for large-scale vertical transport of air in the Tropical Tropopause Layer (TTL) were determined using high-resolution, in situ observations of CO2 concentrations in the tropical upper troposphere and lower stratosphere during the NASA Tropical Composition, Cloud and Climate Coupling (TC4) campaign in August 2007. Upward movement of trace gases in the deep tropics was notably slower in TC4 than during the Costa Rica AURA Validation Experiment (CR-AVE), in January 2006. Transport rates in the TTL were combined with in situ measurements of chlorinated and brominated organic compounds from whole air samples to determine chemical loss rates for reactive chemical species, providing empirical vertical profiles for 24-h mean concentrations of hydroxyl radicals (OH) and chlorine atoms in the TTL. The analysis shows that important short-lived species such as CHCl3, CH2Cl2, and CH2Br2 have longer chemical lifetimes than the time for transit of the TTL, implying that these species, which are not included in most models, could readily reach the stratosphere and make significant contributions of chlorine and/or bromine to stratospheric loading.

Vertical transport rates and concentrations of OH and Cl radicals in the Tropical Tropopause Layer from observations of CO<sub>2</sub> and halocarbons: implications for distributions of long- and short-lived chemical species

10.5194/acp-10-6669-2010 ◽

2010 ◽

Vol 10 (14) ◽

pp. 6669-6684 ◽

Cited By ~ 17

Author(s):

S. Park ◽

E. L. Atlas ◽

R. Jiménez ◽

B. C. Daube ◽

E. W. Gottlieb ◽

...

Keyword(s):

Large Scale ◽

Chemical Species ◽

Vertical Transport ◽

Upward Movement ◽

Upper Troposphere ◽

Tropical Tropopause ◽

Validation Experiment ◽

In Situ Observations

A modelling case study of a large-scale cirrus in the tropical tropopause layer

10.5194/acpd-15-31089-2015 ◽

2015 ◽

Vol 15 (21) ◽

pp. 31089-31131 ◽

Cited By ~ 1

Author(s):

A. Podglajen ◽

R. Plougonven ◽

A. Hertzog ◽

B. Legras

Keyword(s):

Large Scale ◽

Cirrus Clouds ◽

Fair Agreement ◽

Radiative Heating ◽

Cloud Formation ◽

Vertical Position ◽

Microphysics Scheme ◽

Tropical Tropopause ◽

Equatorial Wave

Abstract. We use the Weather Research and Forecast (WRF) model to simulate a large-scale tropical tropopause layer (TTL) cirrus, in order to understand the formation and life cycle of the cloud. This cirrus event has been previously described through satellite observations by Taylor et al. (2011). Comparisons of the simulated and observed cirrus show a fair agreement, and validate the reference simulation regarding cloud extension, location and life time. The validated simulation is used to understand the causes of cloud formation. It is shown that several cirrus clouds successively form in the region due to adiabatic cooling and large-scale uplift rather than from ice lofting from convective anvils. The equatorial response (equatorial wave excitation) to a midlatitude potential vorticity (PV) intrusion structures the uplift. Sensitivity tests are then performed to assess the relative importance of the choice of the microphysics parametrisation and of the initial and boundary conditions. The initial dynamical conditions (wind and temperature) essentially control the horizontal location and area of the cloud. On the other hand, the choice of the microphysics scheme influences the ice water content and the cloud vertical position. Last, the fair agreement with the observations allows to estimate the cloud impact in the TTL in the simulations. The cirrus clouds have a small but not negligible impact on the radiative budget of the local TTL. However, the cloud radiative heating does not significantly influence the simulated dynamics. The simulation also provides an estimate of the vertical redistribution of water by the cloud and the results emphasize the importance in our case of both re and dehydration in the vicinity of the cirrus.