Lower Tropical Tropopause Layer Research Articles

Impacts of the SSTs over different regions on the troposphere-to-stratosphere transport over the tropical western Pacific in the boreal winter

The strong upward motion in the lower tropical tropopause layer (TTL) over the tropical western Pacific (TWP) and the large-scale tropical upwelling of the Brewer-Dobson circulation (BDC) in boreal winter form a critical pathway for tropospheric trace gases entering the stratosphere. An index, called the TSTI, is defined using the combination of the upward motion in the lower TTL and the tropical upwelling in the lower stratosphere to reflect the troposphere-to-stratosphere transport (TST) over the TWP in boreal winter (NDJFM). The TSTIs derived from JRA55 and CMIP6-AMIP historical simulations show significantly increasing trends in the past decades. The impacts of the changes in the SSTs over 9 different regions on the TSTI in NDJFM during 1958–2017 are further investigated with WACCM4 numeric simulations. The results show that the warmed sea surface temperatures (SSTs) over the TWP make a significant contribution to the increasing trends of the upward motion over the TWP and the tropical upwelling in the lower stratosphere, resulting in an enhanced TSTI over the TWP. The changes in SSTs over other regions may singly impact the upward motion in the lower TTL over the TWP or the tropical upwelling in the lower stratosphere, but their overall impacts on the TSTI over the TWP are weak. The WACCM4 simulations further suggest that the increased TST over the TWP could not lead to a wetter lower stratosphere since the enhanced deep convection accompanied by the increased TST over the TWP induces a decreased tropopause temperature and hence a drier lower stratosphere.

Aircraft observations of gravity wave activity and turbulence in the tropical tropopause layer: prevalence, influence on cirrus clouds, and comparison with global storm-resolving models

Abstract. The tropical tropopause layer (TTL) is a sea of vertical motions. Convectively generated gravity waves create vertical winds on scales of a few to thousands of kilometers as they propagate in a stable atmosphere. Turbulence from gravity wave breaking, radiatively driven convection, and Kelvin–Helmholtz instabilities stirs up the TTL on the kilometer scale. TTL cirrus clouds, which moderate the water vapor concentration in the TTL and stratosphere, form in the cold phases of large-scale (> 100 km) wave activity. It has been proposed in several modeling studies that small-scale (< 100 km) vertical motions control the ice crystal number concentration and the dehydration efficiency of TTL cirrus clouds. Here, we present the first observational evidence for this. High-rate vertical winds measured by aircraft are a valuable and underutilized tool for constraining small-scale TTL vertical wind variability, examining its impacts on TTL cirrus clouds, and evaluating atmospheric models. We use 20 Hz data from five National Aeronautics and Space Administration (NASA) campaigns to quantify small-scale vertical wind variability in the TTL and to see how it varies with ice water content, distance from deep convective cores, and height in the TTL. We find that 1 Hz vertical winds are well represented by a normal distribution, with a standard deviation of 0.2–0.4 m s−1. Consistent with a previous observational study that analyzed two out of the five aircraft campaigns that we analyze here, we find that turbulence is enhanced over the tropical west Pacific and within 100 km of convection and is most common in the lower TTL (14–15.5 km), closer to deep convection, and in the upper TTL (15.5–17 km), further from deep convection. An algorithm to classify turbulence and long-wavelength (5 km < λ < 100 km) and short-wavelength (λ < 5 km) gravity wave activity during level flight legs is applied to data from the Airborne Tropical TRopopause EXperiment (ATTREX). The most commonly sampled conditions are (1) a quiescent atmosphere with negligible small-scale vertical wind variability, (2) long-wavelength gravity wave activity (LW GWA), and (3) LW GWA with turbulence. Turbulence rarely occurs in the absence of gravity wave activity. Cirrus clouds with ice crystal number concentrations exceeding 20 L−1 and ice water content exceeding 1 mg m−3 are rare in a quiescent atmosphere but about 20 times more likely when there is gravity wave activity and 50 times more likely when there is also turbulence, confirming the results of the aforementioned modeling studies. Our observational analysis shows that small-scale gravity waves strongly influence the ice crystal number concentration and ice water content within TTL cirrus clouds. Global storm-resolving models have recently been run with horizontal grid spacing between 1 and 10 km, which is sufficient to resolve some small-scale gravity wave activity. We evaluate simulated vertical wind spectra (10–100 km) from four global storm-resolving simulations that have horizontal grid spacing of 3–5 km with aircraft observations from ATTREX. We find that all four models have too little resolved vertical wind at horizontal wavelengths between 10 and 100 km and thus too little small-scale gravity wave activity, although the bias is much less pronounced in global SAM than in the other models. We expect that deficient small-scale gravity wave activity significantly limits the realism of simulated ice microphysics in these models and that improved representation requires moving to finer horizontal and vertical grid spacing.

Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

Abstract. Tropical ice clouds have an important influence on the Earth's radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales. These processes are not yet fully understood and lead to large uncertainties in climate projections. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform convection-permitting simulations with the regional Consortium for Small-Scale Modelling isotope-enabled (COSMOiso) model for the period from June to July 2016. First, we evaluate our model simulations using space-borne observations of mid-tropospheric water vapour isotopes, monthly station data of precipitation isotopes, and satellite-based precipitation estimates. Next, we explore the isotope signatures of tropical deep convection in atmospheric water vapour and ice based on a case study of a mesoscale convective system (MCS) and a statistical analysis of a 1-month period. The following five key processes related to tropical ice clouds can be distinguished based on isotope information: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdraughts that enriches the environmental vapour, and (5) the freezing of liquid water just above the 0 ∘C isotherm in convective updraughts. Importantly, we note large variations in the isotopic composition of water vapour in the upper troposphere and lower tropical tropopause layer, ranging from below −800 ‰ to over −400 ‰, which are strongly related to vertical motion and the moist processes that take place in convective updraughts and downdraughts. In convective updraughts, the vapour is depleted by the preferential condensation and deposition of heavy isotopes, whereas the non-fractionating sublimation of ice in convective downdraughts enriches the environmental vapour. An opposite vapour isotope signature emerges in thin-cirrus cloud regions where the direct transport of enriched (depleted) vapour prevails in large-scale ascent (descent). Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the West African monsoon water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

Quantifying the Radiative Impact of Clouds on Tropopause Layer Cooling in Tropical Cyclones

AbstractA ubiquitous cold signal near the tropopause, here called “tropopause layer cooling” (TLC), has been documented in deep convective regions such as tropical cyclones (TCs). Temperature retrievals from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) reveal cooling of order 0.1–1 K day−1on spatial scales of order 1000 km above TCs. Data from the Cloud Profiling Radar (onboardCloudSat) and from the Cloud–Aerosol Lidar with Orthogonal Polarization [onboard theCloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations(CALIPSO)] are used to analyze cloud distributions associated with TCs. Evidence is found that convective clouds within TCs reach the upper part of the tropical tropopause layer (TTL) more frequently than do convective clouds outside TCs, raising the possibility that convective clouds within TCs and associated cirrus clouds modulate TLC. The contribution of clouds to radiative heating rates is then quantified using theCloudSatandCALIPSOdatasets: in the lower TTL (below the tropopause), clouds produce longwave cooling of order 0.1–1 K day−1inside the TC main convective region, and longwave warming of order 0.01–0.1 K day−1outside; in the upper TTL (near and above the tropopause), clouds produce longwave cooling of the same order as TLC inside the TC main convective region, and up to one order of magnitude smaller outside. Considering that clouds also produce shortwave warming, it is suggested that cloud radiative effects inside and outside TCs only explain modest amounts of TLC while other processes must provide the remaining cooling.

The roles of deep convection and extratropical mixing in the tropical tropopause layer: An in situ measurement perspective

AbstractThe majority of air which enters the stratosphere passes through the tropical tropopause layer (TTL), where the temperature profile and amount of deep convection set the entry level for stratospheric water vapor. We use simultaneous in situ measurements of ozone, carbon monoxide, carbon dioxide, water vapor, and heavy water vapor (HDO) from the 2006 Costa Rica Aura Validation Experiment and 2007 Tropical Composition, Cloud, and Climate Coupling campaigns in a one‐dimensional mixing model to investigate how different transport pathways in the TTL impact the amount of water vapor which enters the stratosphere. We focused on four main pathways: (1) slow upward ascent, (2) isentropic transport from the midlatitude lowermost stratosphere, (3) convective injection, and (4) descent within the TTL coupled to convection. Each pathway brings air into the TTL with a specific chemical signature, such that anomalies in simultaneously measured tracer concentrations can be used to identify the source of the air mass. We found that isentropic transport from the midlatitude lowermost stratosphere accounted for 20–60% of the air in the lower TTL and 20–40% of air above the tropopause, bringing elevated ozone concentrations which radiatively warm the TTL, impacting the amount of dehydration which takes place. Convective input accounted for on average ~30% of the air measured below 370 K potential temperature, and ~10% of air above the tropopause in both summer and winter, increasing water vapor concentrations above the tropopause by 0.3 ppmv on average and by up to 1.6 ppmv in some air masses.

Very short-lived bromomethanes measured by the CARIBIC observatory over the North Atlantic, Africa and Southeast Asia during 2009–2013

Abstract. Short-lived organic brominated compounds make up a significant part of the organic bromine budget in the atmosphere. Emissions of these compounds are highly variable and there are limited measurements, particularly in the extra-tropical upper troposphere/lower stratosphere and tropical troposphere. Measurements of five very short-lived bromomethanes (VSLB) were made in air samples collected on the CARIBIC project aircraft over three flight routes; Germany to Venezuela/Columbia during 2009–2011, Germany to South Africa during 2010 and 2011 and Germany to Thailand/Kuala Lumpur, Malaysia during 2012 and 2013. In the tropical troposphere, as the most important entrance region to the stratosphere, we observe a total mean organic bromine derived from these compounds across all flights at 10–12 km altitude of 3.4 ± 1.5 ppt. Individual mean tropical tropospheric mixing ratios across all flights were 0.43, 0.74, 0.14, 0.23 and 0.11 ppt for CHBr3, CH2Br2, CHBr2Cl, CHBrCl2 and CH2BrCl respectively. The highest levels of VSLB-derived bromine (4.20 ± 0.56 ppt) were observed in flights between Bangkok and Kuala Lumpur indicating that the South China Sea is an important source region for these compounds. Across all routes, CHBr3 and CH2Br2 accounted for 34% (4.7–71) and 48% (14–73) respectively of total bromine derived from the analysed VSLB in the tropical mid-upper troposphere totalling 82% (54–89). In samples collected between Germany and Venezuela/Columbia, we find decreasing mean mixing ratios with increasing potential temperature in the extra-tropics. Tropical mean mixing ratios are higher than extra-tropical values between 340–350 K indicating that rapid uplift is important in determining mixing ratios in the lower tropical tropopause layer in the West Atlantic tropics. O3 was used as a tracer for stratospherically influenced air and we detect rapidly decreasing mixing ratios for all VSLB above ∼100 ppb O3 corresponding to the extra-tropical tropopause layer.

Global-scale seasonally resolved black carbon vertical profiles over the Pacific.

[1] Black carbon (BC) aerosol loadings were measured during the High-performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations (HIPPO) campaign above the remote Pacific from 85°N to 67°S. Over 700 vertical profiles extending from near the surface to max ∼14 km altitude were obtained with a single-particle soot photometer between early 2009 and mid-2011. The data provides a climatology of BC in the remote regions that reveals gradients of BC concentration reflecting global-scale transport and removal of pollution. BC is identified as a sensitive tracer of extratropical mixing into the lower tropical tropopause layer and trends toward surprisingly uniform loadings in the lower stratosphere of ∼1 ng/kg. The climatology is compared to predictions from the AeroCom global model intercomparison initiative. The AeroCom model suite overestimates loads in the upper troposphere/lower stratosphere (∼10×) more severely than at lower altitudes (∼3×), with bias roughly independent of season or geographic location; these results indicate that it overestimates BC lifetime.

Dehydration in the tropical tropopause layer estimated from the water vapor match

Abstract. We apply the match technique, whereby the same air mass is observed more than once and such cases are termed a "match", to study the dehydration process associated with horizontal advection in the tropical tropopause layer (TTL) over the western Pacific. The matches are obtained from profile data taken by the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaign network observations using isentropic trajectories calculated from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. For the matches identified, extensive screening procedures are performed to verify the representativeness of the air parcel and the validity of the isentropic treatment, and to check for possible water injection by deep convection, consistency between the sonde data and analysis field referring to the ozone conservation. Among the matches that passed the screening tests, we identified some cases corresponding to the first quantitative value of dehydration associated with horizontal advection in the TTL. The statistical features of dehydration for the air parcels advected in the lower TTL are derived from the matches. The threshold of nucleation is estimated to be 146 ± 1% (1σ) in relative humidity with respect to ice (RHice), while dehydration seems to continue until RHice reaches about 75 ± 23% (1σ) in the altitude region from 350 to 360 K. The efficiency of dehydration expressed by the relaxation time required for the supersaturated air parcel to approach saturation is empirically determined from the matches. A relaxation time of approximately one hour reproduces the second water vapor observation reasonably well, given the first observed water vapor amount and the history of the saturation mixing ratio during advection in the lower TTL.

Transport of short-lived species into the Tropical Tropopause Layer

Abstract. We use NAME, a trajectory model, to investigate the routes and timescales over which air parcels reach the tropical tropopause layer (TTL). Our aim is to assist the planning of aircraft campaigns focussed on improving knowledge of such transport. We focus on Southeast Asia and the Western Pacific which appears to be a particularly important source of air that enters the TTL. We first study the TTL above Borneo in November 2008, under neutral El Niño/Southern Oscillation (ENSO) conditions. Air parcels (trajectories) arriving in the lower TTL (below ~15 km) are most likely to have travelled from the boundary layer (BL; <1 km) above the West Pacific. Few air parcels found above ~16 km travelled from the BL in the previous 15 days. We then perform similar calculations for moderate El Niño (2006) and La Niña (2007) conditions and find year-to-year variability consistent with the phase of ENSO. Under El Niño conditions fewer air parcels travel from the BL to the TTL above Borneo. During the La Niña year, more air parcels travel from the BL to the mid and upper TTL (above ~15 km) than in the ENSO-neutral year, and again they do so from the BL above the West Pacific. We also find intra-month variability in all years, with day-to-day differences of up to an order of magnitude in the fraction of an idealised short-lived tracer travelling from the BL to the TTL above Borneo. These calculations were performed as a prelude to the SHIVA field campaign, which took place in Borneo during November 2011. So finally, to validate our approach, we consider measurements made in two previous campaigns. The features of vertical profiles of short-lived species observed in the TTL during CR-AVE and TC4 are in broad agreement with calculated vertical profiles of idealised short-lived tracers. It will require large numbers of observations to fully describe the statistical distribution of short-lived species in the TTL. This modelling approach should prove valuable in planning flights for the long-duration aircraft now capable of making such measurements.

Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon

Abstract. Trace gas and aerosol data collected in the tropical tropopause layer (TTL) between 12–18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA campaign over West Africa during the summer monsoon in August 2006 have been analysed in terms of their air mass origins. Analysis of domain filling back trajectories arriving over West Africa, and in the specific region of the flights, showed that the M55 flights were generally representative of air masses arriving over West Africa during the first 2 weeks of August, 2006. Air originating from the mid-latitude lower stratosphere was under-sampled (in the mid-upper TTL) whilst air masses uplifted from central Africa (into the lower TTL) were over-sampled in the latter part of the campaign. Signatures of recent (previous 10 days) origins were superimposed on the large-scale westward flow over West Africa. In the lower TTL, air masses were impacted by recent local deep convection over Africa at the level of main convective outflow (350 K, 200 hPa) and on certain days up to 370 K (100 hPa). Estimates of the fraction of air masses influenced by local convection vary from 10 to 50% depending on the method applied and from day to day during the campaign. The analysis shows that flights on 7, 8 and 11 August were more influenced by local convection than on 4 and 13 August allowing separation of trace gas and aerosol measurements into "convective" and "non-convective" flights. Strong signatures, particularly in species with short lifetimes (relative to CO2) like CO, NO and fine-mode aerosols were seen during flights most influenced by convection up to 350–365 K. Observed profiles were also constantly perturbed by uplift (as high as 39%) of air masses from the mid to lower troposphere over Asia, India, and oceanic regions resulting in import of clean oceanic (e.g. O3-poor) or polluted air masses from Asia (high O3, CO, CO2) into West Africa. Thus, recent uplift of CO2 over Asia may contribute to the observed positive CO2 gradients in the TTL over West Africa. This suggests a more significant fraction of younger air masses in the TTL and needs to taken into consideration in derivations of mean age of air. Transport of air masses from the mid-latitude lower stratosphere had an impact from the mid-TTL upwards (20–40% above 370 K) during the campaign period importing air masses with high O3 and NOy. Ozone profiles show a less pronounced lower TTL minimum than observed previously by regular ozonesondes at other tropical locations. Concentrations are less than 100 ppbv in the lower TTL and vertical gradients less steep than in the upper TTL. The air mass origin analysis and simulations of in-situ net photochemical O3 production, initialised with observations, suggest that the lower TTL is significantly impacted by uplift of O3 precursors (over Africa and Asia) leading to positive production rates (up to 2 ppbv per day) in the lower and mid TTL even at moderate NOx levels. Photochemical O3 production increases with higher NOx and H2O in air masses with O3 less than 150 ppbv.

Composition of the TTL over Darwin: local mixing or long-range transport?

Abstract. We present ozone and carbon monoxide measurements taken in Darwin, Australia, during the wet season of 2005/2006, to examine whether the composition of the Tropical Tropopause Layer (TTL) reflects that of the local boundary layer or is influenced more by advection from distant sources. We find that the latter predominates in the upper TTL, and is also the major influence in the lower TTL, except during an active monsoon phase. The day-to-day variability of ozone in the TTL is far greater than that in the lower troposphere, and correlates closely with air mass origin deduced from trajectory calculations based on standard ECMWF wind analyses. Although clear evidence of recent local uplift was found in carbon monoxide tracer measurements recorded inside convective anvils, no such signal was found out-of-cloud in the background TTL, where the measured variability correlated well with air mass origin deduced from back-trajectories. This study suggests that the composition of the TTL is ultimately determined by vertical mixing in certain "hot spot" regions of the tropics, with advection from these regions dominating the composition elsewhere.

The CO<sub>2</sub> tracer clock for the Tropical Tropopause Layer

Abstract. Observations of CO2 were made in the upper troposphere and lower stratosphere in the deep tropics in order to determine the patterns of large-scale vertical transport and age of air in the Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft over Central America and adjacent ocean areas took place in January and February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm Pool International Cloud Experiment, TWP-ICE). The data demonstrate that the TTL is composed of two layers with distinctive features: (1) the lower TTL, 350–360 K (potential temperature(θ); approximately 12–14 km), is subject to inputs of convective outflows, as indicated by layers of variable CO2 concentrations, with air parcels of zero age distributed throughout the layer; (2) the upper TTL, from θ=~360 K to ~390 K (14–18 km), ascends slowly and ages uniformly, as shown by a linear decline in CO2 mixing ratio tightly correlated with altitude, associated with increasing age. This division is confirmed by ensemble trajectory analysis. The CO2 concentration at the level of 360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air in the upper TTL monotonically ages while ascending. In combination with the winter slope of the CO2 seasonal cycle (+10.8±0.4 ppmv/yr), the vertical gradient of –0.78 (±0.09) ppmv gives a mean age of 26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s−1. The TTL near 360 K in the Southern Hemisphere over Australia is very close in CO2 composition to the TTL in the Northern Hemisphere over Costa Rica, with strong contrasts emerging at lower altitudes (<360 K). Both Pre-AVE and CR-AVE CO2 observed unexpected input from deep convection over Amazônia deep into the TTL. The CO2 data confirm the operation of a highly accurate tracer clock in the TTL that provides a direct measure of the ascent rate of the TTL and of the age of air entering the stratosphere.