Observational characteristics of double tropopauses Randel, William J.; Seidel, Dian J.; Pan, Laura L.
Journal of Geophysical Research - Atmospheres,
16 April 2007, Letnik:
112, Številka:
D7
Journal Article
Recenzirano
Odprti dostop
Temperature profiles in the extratropics often exhibit multiple tropopauses (as defined using the lapse rate definition). In this work we study the observational characteristics of double tropopauses ...based on radiosondes, ERA40 reanalysis, and GPS radio occultation temperature profiles. Double tropopauses are associated with a characteristic break in the thermal tropopause near the subtropical jet, wherein the low latitude (tropical) tropopause extends to higher latitudes, overlying the lower tropopause; this behavior can extend to polar latitudes. Tropopause statistics derived from radiosondes and GPS data show good agreement, and GPS data allow mapping of double tropopause characteristics over the globe. The occurrence frequency shows a strong seasonal variation over NH midlatitudes, with ∼50–70% occurrence in profiles during winter, and a small fraction (∼10%) over most of the hemisphere during summer (with the exception of a localized maximum over the poleward flank of the Asian monsoon anticyclone). SH midlatitude statistics show a smaller seasonal variation, with occurrence frequencies of ∼30–50% over the year (maximum during winter). Over the extratropics, the occurrence frequency is substantially higher for cyclonic circulation systems. Few double tropopauses are observed in the tropics. Ozone measurements from balloons and satellites show that profiles with double tropopauses exhibit systematically less ozone in the lower stratosphere than those with a single tropopause. Together with the meteorological data, the ozone observations identify double tropopauses as regions of enhanced transport from the tropics to higher latitudes above the subtropical jet cores.
The Deep Convective Clouds and Chemistry (DC3) field experiment produced an exceptional dataset on thunderstorms, including their dynamical, physical, and electrical structures and their impact on ...the chemical composition of the troposphere. The field experiment gathered detailed information on the chemical composition of the inflow and outflow regions of midlatitude thunderstorms in northeast Colorado, west Texas to central Oklahoma, and northern Alabama. A unique aspect of the DC3 strategy was to locate and sample the convective outflow a day after active convection in order to measure the chemical transformations within the upper-tropospheric convective plume. These data are being analyzed to investigate transport and dynamics of the storms, scavenging of soluble trace gases and aerosols, production of nitrogen oxides by lightning, relationships between lightning flash rates and storm parameters, chemistry in the upper troposphere that is affected by the convection, and related source characterization of the three sampling regions. DC3 also documented biomass-burning plumes and the interactions of these plumes with deep convection.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A significant source of ozone in the troposphere is transport from the stratosphere. The stratospheric contribution has been estimated mainly using global models that attribute the transport process ...largely to the global‐scale Brewer‐Dobson circulation and synoptic‐scale dynamics associated with upper tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ozone‐rich stratospheric air downward into the upper troposphere by a leading‐line‐trailing‐stratiform mesoscale convective system with convection overshooting the tropopause altitude. The fine‐scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm‐scale dynamics. Thus, the upper tropospheric ozone budget simulated by global chemistry‐climate models where large‐scale dynamics and photochemical production from lightning‐produced NO are the controlling factors may require modification.
Key Points
Tropopause‐reaching MCSs entrain ozone‐rich stratospheric air into troposphereAirborne lidar measurement is key to revealing this transport mechanismA missing transport pathway for ozone budget in major global models
We present a method of identifying the tropical tropopause transition layer (TTL) using chemical tracer‐tracer relationships. Coincident ozone (O3) and water vapor (H2O) measurements over Alajuela, ...Costa Rica (~10°N), in July and August 2007 are used to demonstrate the concept. In the tracer‐tracer space, the O3 and H2O relationship helps to separate the transition layer air mass from the background troposphere and stratosphere. This tracer relationship‐based transition layer is found to span an approximately 40 K potential temperature range between 340 and 380 K and is largely confined between the level of minimum stability (LMS) and the cold point tropopause (CPT). This chemical composition‐based transition layer is, therefore, consistent with a definition of the TTL based on the thermal structure, for which the LMS and CPT are the lower and upper boundaries of TTL, respectively. We also examine the transition layer over the region of Asian summer monsoon (ASM) anticyclone using the measurements over Kunming, China (~25°N), and compare its behavior with the TTL structure in the deep tropics. The comparison shows that the transition layer over the ASM is similar to the TTL, although the data suggest the ASM transition layer lies at higher potential temperature levels and is potentially prone to the influence of extratropical processes.
Key Points
The TTL is identified using the ozone and water vapor relationship
The tracer TTL is consistent with the CPT and LMS as upper and lower boundaries
The transition layer in the Asian monsoon region is compared to the TTL
We present a case study on the formation and structure of the Extratropical Transition Layer (ExTL) using in situ observations and a Lagrangian chemical transport model. The results show that the ...model with mixing parameterized from the large‐scale flow deformations well reconstructs the observed asymmetric structure of the ExTL with a deeper transition layer on the cyclonic side of the jet stream. Information from the model and observations are integrated using tracer‐tracer correlations between ozone (O3) and carbon monoxide (CO). Transport of chemical tracers from the stratospheric or tropospheric background to the ExTL through mixing is identified by the change of the CO‐O3correlation in the CO‐O3 space. The ExTL formation process simulated by the model, therefore, provides a scenario to connect the mixed air parcels to the history of mixing. An estimate of timescales of ExTL formation is made using model experiments. The results show that the fastest formation of the ExTL occurs on the isentropic levels below the subtropical jet core, e.g. around 3 weeks for 310 K, whereas at 360 K level (jet core) the formation of the ExTL needs around 3 months. Overall, this result demonstrates the important role of mixing in transport of trace gases across the tropopause.
Key Points
Physical mixing successfully parameterized by the large‐scale flow deformations
Model with mixing well reconstructs the observed structure of the ExTL
Mixing time scales below the jet are much shorter compared with those above
Deep convection in the Asian summer monsoon is a significant transport process for lifting pollutants from the planetary boundary layer to the tropopause level. This process enables efficient ...injection into the stratosphere of reactive species such as chlorinated very-short-lived substances (Cl-VSLSs) that deplete ozone. Past studies of convective transport associated with the Asian summer monsoon have focused mostly on the south Asian summer monsoon. Airborne observations reported in this work identify the East Asian summer monsoon convection as an effective transport pathway that carried record-breaking levels of ozone-depleting Cl-VSLSs (mean organic chlorine from these VSLSs ~500 ppt) to the base of the stratosphere. These unique observations show total organic chlorine from VSLSs in the lower stratosphere over the Asian monsoon tropopause to be more than twice that previously reported over the tropical tropopause. Considering the recently observed increase in Cl-VSLS emissions and the ongoing strengthening of the East Asian summer monsoon under global warming, our results highlight that a reevaluation of the contribution of Cl-VSLS injection via the Asian monsoon to the total stratospheric chlorine budget is warranted.
The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was an aircraft‐based field campaign conducted from Guam (14°N, 145°E) during January–February 2014. Aircraft ...measurements included over 80 vertical profiles from the boundary layer to the upper troposphere (~15 km). A large fraction of these profiles revealed layered structures with very low water vapor (relative humidity <20%) and enhanced ozone, primarily in the lower‐middle troposphere (~3–9 km). Comparing CONTRAST water vapor measurements with co‐located profiles from National Centers for Environmental Prediction Global Forecast System (GFS) analyses, we find good agreement for dry layers, including profile‐by‐profile comparisons and statistical behavior. We then utilize GFS data to evaluate the frequency of occurrence and 3‐D structure of dry layers for the CONTRAST period to provide perspective to the campaign measurements and evaluate the global climatological behavior based on a longer record. GFS data show that dry layers occur ~50–80% of the time in the subtropical troposphere, maximizing on the equatorward side of the subtropical jets in the winter hemisphere. Subtropical dry layers occur most frequently over isentropic levels ~320–340 K, which extend into the extratropical upper troposphere‐lower stratosphere (UTLS). Similar statistical behavior of dry, ozone‐rich layers is found in long‐term balloon measurements from Reunion Island (21°S, 56°E). The climatologically frequent occurrence of dry, ozone‐rich layers, plus their vertical and spatial structures linked to the subtropical jets, all suggest that dry layers are linked to quasi‐isentropic transport from the extratropical UTLS and suggest a ubiquitous UTLS influence on the subtropical middle troposphere.
Key Points
Airborne observed dry layers in the tropical troposphere are well represented in GFS analyses
GFS global climatology shows strong connection between dry layers and subtropical jet
Ubiquitous dry, ozone‐rich air is a signature of extratropical UTLS influence in subtropics
The subseasonal scale dynamics of the Asian summer monsoon (ASM) upper troposphere (UT) anticyclone has been identified as a primary mechanism for convectively lofted Asian boundary layer air to ...leave the confinement of the anticyclone and impact the global UT and lower stratosphere (UTLS). This work quantifies eastward eddy shedding associated with the subseasonal scale oscillation of the anticyclone and associated chemical transport. Using reanalysis data together with satellite trace gas data, we examine the correlation between enhanced tropospheric trace gas species and the presence of a secondary anticyclone over the western Pacific Ocean. To diagnose the role of the western Pacific anticyclone (WPA) in the transport of ASM air into the global UTLS, transport pathways and transit times of air found within the WPA are analyzed using Lagrangian kinematic trajectories. Results show that about two thirds of the air within the WPA originates from, or is influenced by, the greater region of the Tibetan anticyclone. After leaving the WPA, air favors two pathways forward: eastward following the subtropical jet and southwestward following UTLS anticyclonic flow. Approximately 60% of the air parcels cross the tropopause into the lower stratosphere within 30 days, though most cross within a few days. These results highlight opportunities for investigating the impact of the ASM on global UTLS chemical and aerosol composition over the western Pacific via research aircraft.
Plain Language Summary
The Asian summer monsoon system includes a large‐scale anticyclonic circulation in the upper troposphere spanning much of Asia and northern Africa. Within this anticyclone, high concentrations of surface pollutants and aerosols are reported from satellite observations. These chemical signatures highlight the role of monsoon convection in transporting regional emissions into the upper troposphere and lower stratosphere. In situ measurements of monsoon‐transported air masses are important for understanding the climate impacts of the system. In this work, we present a set of analyses to show that the subseasonal scale dynamics of the Asian monsoon anticyclone produces eastward shedding of monsoon‐lofted Asian boundary layer air over the western Pacific in a 10‐ to 20‐day time scale. The result of this transport analysis provides basic information for designing airborne situ measurement studies over the western Pacific Ocean.
Key Points
The Asian summer monsoon upper tropospheric anticyclone centers form a statistical trimodal distribution
The western Pacific mode is a key pathway for the convectively lofted Asian boundary layer air to enter the global upper troposphere
More than half of the upper tropospheric air mass in the western Pacific mode move into the stratosphere within a week
Simulations of observed convective systems with the Advanced Research Weather Research and Forecasting (ARW‐WRF) model are used to test the influence of the large‐scale lower stratosphere stability ...environment on the vertical extent of convective overshooting and transport above the extratropical tropopause. Three unique environments are identified (double tropopause, stratospheric intrusion, and single tropopause), and representative cases with comparable magnitudes of convective available potential energy are selected for simulation. Convective injection into the extratropical lower stratosphere is found to be deepest for the double‐tropopause case (up to 4 km above the lapse‐rate tropopause) and at comparable altitudes for the remaining cases (up to 2 km above the lapse‐rate tropopause). All simulations show evidence of gravity wave breaking near the overshooting convective top, consistent with the identification of its role as a transport mechanism in previous studies. Simulations for the double‐tropopause case, however, also show evidence of direct mixing of the overshooting top into the lower stratosphere, which is responsible for the highest levels of injection in that case. In addition, the choice of bulk microphysical parameterization for ARW‐WRF simulations is found to have little impact on the transport characteristics for each case.
Key Points
Convective overshooting and transport are sensitive to stratospheric stability
ARW‐WRF model is capable of simulating representative convective depths
Choice of model microphysics scheme has negligible impact
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