Transport and freeze‐drying in the tropical tropopause layer
TL;DR: In this paper, a Lagrangian, one-dimensional cloud model is used to simulate ice cloud formation and dehydration along trajectories in the tropical tropopause layer (TTL), and time-height curtains of temperature along the trajectory paths are extracted from meteorological analyses.
Abstract: [1] We use a Lagrangian, one-dimensional cloud model to simulate ice cloud formation and dehydration along trajectories in the tropical tropopause layer (TTL). Time-height curtains of temperature along the trajectory paths are extracted from meteorological analyses. The temperatures are adjusted near the tropopause such that the spatial average cold point temperature matches tropical radiosonde measurements. Temperature perturbations due to Kelvin waves, Rossby gravity waves, and high-frequency gravity waves are superimposed. The cloud model tracks the growth and sedimentation of individual ice crystals. Ice number densities in the cloud simulations without waves range from 1 cm−3) and smaller crystals (1–10 μm radius), resulting in less sedimentation but still effective dehydration overall. Inclusion of wave-driven temperature oscillations decreases the final TTL H2O mixing ratio somewhat primarily because the wave perturbations decrease the tropical average cold point tropopause temperature by ∼0.75 K. Ultimately, air rising through the TTL is effectively dehydrated with or without wave perturbations. In general, the model results suggest that the final water vapor mixing ratios are primarily controlled by the minimum temperatures encountered by parcels and that they are relatively insensitive to factors such as the wave-driven temperature variability, the supersaturation threshold for ice nucleation, and the rate of ascent through the tropopause layer. However, the frequency and geographical distribution of cloud formation is very sensitive to these parameters. On average, the clouds dehydrate the air along trajectories down to mixing ratios ∼10–40% higher than the temperature minimum saturation mixing ratio. The simulations predict efficient freeze-drying of air by cloud formation within the TTL: For the December–January 1995/1996 period simulated the average final H2O mixing ratios at the tropopause (370–380 K potential temperature) range from 2.5 to 3.2 ppmv. These values are somewhat lower than the estimates of the stratospheric water vapor entry value from satellite and in situ measurements; hence an additional source of water (such as injection by deep convection) may be required to explain the observed tropical tropopause humidity.
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TL;DR: The Tropical Tropopause Layer (TTL) as discussed by the authors is a 3D model of the troposphere, and it has been shown that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp "tropopause".
Abstract: [1] Observations of temperature, winds, and atmospheric trace gases suggest that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp “tropopause.” In the tropics, this layer is often called the “tropical tropopause layer” (TTL). We present an overview of observations in the TTL and discuss the radiative, dynamical, and chemical processes that lead to its time-varying, three-dimensional structure. We present a synthesis definition with a bottom at 150 hPa, 355 K, 14 km (pressure, potential temperature, and altitude) and a top at 70 hPa, 425 K, 18.5 km. Laterally, the TTL is bounded by the position of the subtropical jets. We highlight recent progress in understanding of the TTL but emphasize that a number of processes, notably deep, possibly overshooting convection, remain not well understood. The TTL acts in many ways as a “gate” to the stratosphere, and understanding all relevant processes is of great importance for reliable predictions of future stratospheric ozone and climate.
881 citations
Cites background from "Transport and freeze‐drying in the ..."
...…TTL [Wallace and Kousky, 1968] and affect tropopause height, temperature, cloud top height [Shimizu and Tsuda, 1997], cloud occurrence [Boehm and RG1004 Fueglistaler et al.: TROPICAL TROPOPAUSE LAYER 15 of 31 RG1004 Verlinde, 2000; Holton et al., 2001], and dehydration [Jensen and Pfister, 2004]....
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TL;DR: The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean as mentioned in this paper.
Abstract: The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO dat...
816 citations
TL;DR: In this paper, the authors present current knowledge and understanding on gravity waves near jets and fronts from observations, theory, and modeling, and discuss challenges for progress in coming years, including the need for improving parameterizations of nonorographic gravity waves in climate models that include a stratosphere.
Abstract: For several decades, jets and fronts have been known from observations to be significant sources of internal gravity waves in the atmosphere. Motivations to investigate these waves have included their impact on tropospheric convection, their contribution to local mixing and turbulence in the upper troposphere, their vertical propagation into the middle atmosphere, and the forcing of its global circulation. While many different studies have consistently highlighted jet exit regions as a favored locus for intense gravity waves, the mechanisms responsible for their emission had long remained elusive: one reason is the complexity of the environment in which the waves appear; another reason is that the waves constitute small deviations from the balanced dynamics of the flow generating them; i.e., they arise beyond our fundamental understanding of jets and fronts based on approximations that filter out gravity waves. Over the past two decades, the pressing need for improving parameterizations of nonorographic gravity waves in climate models that include a stratosphere has stimulated renewed investigations. The purpose of this review is to present current knowledge and understanding on gravity waves near jets and fronts from observations, theory, and modeling, and to discuss challenges for progress in coming years.
327 citations
Cites background from "Transport and freeze‐drying in the ..."
...Gravity waves contribute to temperature fluctuations that will affect the freeze-drying process [Potter and Holton, 1995; Jensen et al., 1996], but it is possible that it does not modify significantly the final water vapor mixing ratios [Jensen and Pfister, 2004]....
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25 Oct 2005
TL;DR: In this paper, the authors examined the common properties of these extreme convective systems from a global perspective and found that 1.3% of tropical convection systems reach 14 km and 0.1% of them may even penetrate the 380 K potential temperature level.
Abstract: [1] Tropical deep convection with overshooting tops is identified by defining five different reference heights using a 5-year TRMM database. The common properties of these extreme convective systems are examined from a global perspective. It is found that 1.3% of tropical convection systems reach 14 km and 0.1% of them may even penetrate the 380 K potential temperature level. Overshooting convection is more frequent over land than over water, especially over central Africa, Indonesia and South America. The seasonal, diurnal and geodistribution patterns of overshooting deep convection show very little sensitivity to the definition of the reference level. The global distribution of overshooting area, volume and precipitating ice mass shows that central Africa makes a disproportionately large contribution to overshooting convection. A semiannual cycle of total overshooting area, volume and precipitating ice mass is found.
292 citations
TL;DR: In this article, a synthesis report concludes that transport and mixing in the tropopause region are closely linked with the Asian monsoon and other tropical circulation systems, with possible implications for the impacts of climate change on this important layer.
Abstract: Tropical climate and the composition of the global upper atmosphere are affected by the tropical tropopause layer. A synthesis report concludes that transport and mixing in the tropopause region are closely linked with the Asian monsoon and other tropical circulation systems, with possible implications for the impacts of climate change on this important layer. Tropical climate and the composition of the global upper atmosphere are affected by the tropical tropopause layer — the atmospheric transition zone between the well-mixed, convective troposphere (up to altitudes of 12–14 km) and the highly stratified stratosphere (above about 18 km). Featuring chemical and dynamical properties that are midway between those of the troposphere and stratosphere, the tropopause layer is maintained by a complex interplay between large- and small-scale circulation patterns, deep convection, clouds and radiation. Tropospheric air enters the stratosphere primarily in the tropics. Ozone- and aerosol-related constituents of the global stratosphere, as well as water vapour content, are therefore largely determined by the composition of the air near the tropical tropopause. Over the past years, it has emerged that both slow ascent and rapid deep convection contribute to the composition and thermal structure of the tropical tropopause layer. Ice formation processes at low temperatures affect the efficacy of freeze drying as air passes through the cold tropopause region. Transport and mixing in the tropopause region has been found to be closely linked with the Asian monsoon and other tropical circulation systems. Given these connections, climate change is expected to influence the tropopause layer, for example through enhanced large-scale upwelling of air and potential changes in tropical convection, air temperature, chemical composition and cirrus.
291 citations
References
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TL;DR: This work recognizes the need for additional dissipation in any higher-order Godunov method of this type, and introduces it in such a way so as not to degrade the quality of the results.
3,892 citations
TL;DR: The ozone depletions in Antarctica were largely confined to the region from about 10 to 20 km, during the period from August to October as discussed by the authors, and the ozone changes were observed only in the spring season.
Abstract: Recent observations by Farman et al1 reveal remarkable depletions in the total atmospheric ozone content in Antarctica The observed total ozone decreased smoothly during the period from about 1975 to the present, but only in the spring season The observed ozone content at Halley Bay was ∼30% lower in the Antarctic spring seasons (October) of 1980–84 than in the springs of 1957–73 No such obvious perturbation is observable in other seasons, or at other than the very highest latitudes in the Southern Hemisphere, and the magnitude of the observed change there far exceeds climatological variability2 We present here balloonsonde ozone data3,4 which show that these ozone changes are largely confined to the region from about 10 to 20 km, during the period August to October We show that homogeneous (gas phase) chemistry as presently understood cannot explain these observed depletions On the other hand, a unique feature of the Antarctic lower stratosphere is its high frequency of polar stratospheric clouds5, providing a reaction site for heterogeneous reactions A heterogeneous reaction between HCl and ClONO2 is explored as a possible mechanism to explain the ozone observations This process produces changes in ozone that are consistent with the observations, and its implications for the behaviour of HNO3 and NO2 in the Antarctic stratosphere are consistent with observations of those species there, providing an important check on the proposed mechanism Similar ozone changes are obtained with another possible heterogeneous reaction, H2O + ClONO2
1,391 citations
TL;DR: In this article, the authors studied the vertical distribution of water vapour and helium in the lower stratosphere over southern England and found that the helium content of the air is remarkably constant up to 20 km but the water content is found to fall very rapidly just above the tropopause, and in the lowest 1 km of the stratosphere the humidity mixing ratio falls through a ratio of 10-1.
Abstract: Information is now available regarding the vertical distribution of water vapour and helium in the lower stratosphere over southern England. The helium content of the air is found to be remarkably constant up to 20 km but the water content is found to fall very rapidly just above the tropopause, and in the lowest 1 km of the stratosphere the humidity mixing ratio falls through a ratio of 10—1.
The helium distribution is not compatible with the view of a quiescent stratosphere free from turbulence or vertical motions. The water-vapour distribution is incompatible with a turbulent stratosphere unless some dynamic process maintains the dryness of the stratosphere. In view of the large wind shear which is normally found just above the tropopause it is unlikely that this region is free from turbulence.
The observed distributions can be explained by the existence of a circulation in which air enters the stratosphere at the equator, where it is dried by condensation, travels in the stratosphere to temperate and polar regions, and sinks into the troposphere. The sinking, however, will warm the air unless it is being cooled by radiation and the idea of a stratosphere in radiative equilibrium must be abandoned. The cooling rate must lie between about 0.1 and 1.1°C per day but a value near 0.5°C per day seems most probable. At the equator the ascending air must be subject to heating by radiation.
The circulation is quite reasonable on energy considerations. It is consistent with the existence of lower temperatures in the equatorial stratosphere than in polar and temperate regions, and if the flow can carry ozone from the equator to the poles then it gives a reasonable explanation of the high ozone values observed at high latitudes. The dynamic consequences of the circulation are not considered. It should however be noted that there is considerable difficulty to account for the smallness of the westerly winds in the stratosphere, as the rotation of the earth should convert the slow poleward movement into strong westerly winds.
1,205 citations
TL;DR: In this paper, a line-by-line radiance model FASCODE has been developed and applied to the calculation of cooling rates for atmospheric water vapor, which achieves computational accuracies for the longwave upwelling and downwelling fluxes of the order of 0.2%.
Abstract: A model for the accelerated calculation of clear sky fluxes based on the line-by-line radiance model FASCODE has been developed and applied to the calculation of cooling rates for atmospheric water vapor. The model achieves computational accuracies for the longwave upwelling and downwelling fluxes of the order of 0.2%, an accuracy well within current limitations imposed by uncertainties in the spectral parameters, the line shape, and the associated continua. For the same treatment of line shape, the Voigt profile with a 10 cm−1 cutoff and no continuum, the results from the present model are in acceptable agreement with those from two other line-by-line models reported as part of the intercomparison of radiation codes used in climate models (ICRCCM). For this line profile and the mid-latitude summer atmosphere, the largest difference between the results from our model and the Goddard Laboratory for Atmospheres (GLA) model occurs for the downwelling flux at the surface, with the present model providing a value greater than that from GLA. The differences are generally consistent with greater atmospheric opacity from the present model, attributable to the inclusion of a self-broadening component for the half width for water and to finer spectral sampling in the lower-pressure regime. Utilization of the line shape and associated continuum model included in FASCODE gives results that are significantly different from those provided by the other two models. This radiance model, including contributions from the foreign continuum as well as from a modified self-continuum, has received extensive validation against measured radiance spectra, an example of which is provided. For the mid-latitude summer atmosphere the principal contribution from the foreign continuum occurs in the upper troposphere in the 250–350 cm−1 spectral region, whereas the contribution from the self-continuum, dependent on the square of the water vapor density, is greatest in the lower troposphere. For the mid-latitude summer atmosphere the foreign continuum contributes 0.4 Kd−1 or 20% to the cooling in the upper troposphere and the self-continuum contributes 1.9 K d−1 to the cooling rate at the surface due to water vapor. The latter is 0.17 K d−1 less than the cooling rate from the GLA model which is principally due to a modification of the self-continuum. A significant result that has developed from the present work is the insight into atmospheric radiative processes provided by spectral profiles of the cooling rate. In the spectral domain there exists a mapping between the altitude and the molecular absorption strength as weighted by the Planck function. The extremely high correlation between the outgoing spectral radiance at the top of the atmosphere and the spectral cooling rate profile suggests that measurement of the outgoing spectral radiance can provide important information about atmospheric state that is not available from spectrally integrated quantities. Our results also indicate the critical importance of the spectral region from 100 to 600 cm−1 for the radiative transfer associated with atmospheric water vapor.
710 citations
TL;DR: Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported, suggesting a predominant potential impact of these nuclei on cirrus formed by slow, large-scale lifting or small cooling rates, including subvisual cirrus.
Abstract: This article addresses the need for new data on indirect effects of natural and anthropogenic aerosol particles on atmospheric ice clouds. Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported. Measurements support that cirrus formation occurs both by heterogeneous nucleation by insoluble particles and homogeneous (spontaneous) freezing of particles containing solutions. Heterogeneous ice nuclei concentrations in the cirrus regime depend on temperature, relative humidity, and the concentrations and physical and chemical properties of aerosol particles. The cirrus-active concentrations of heterogeneous nuclei measured in November over the western U.S. were <0.03 cm–3. Considering previous modeling studies, this result suggests a predominant potential impact of these nuclei on cirrus formed by slow, large-scale lifting or small cooling rates, including subvisual cirrus. The most common heterogeneous ice nuclei were identified as relatively pure mineral dusts and metallic particles, some of which may have origin through anthropogenic processes. Homogeneous freezing of large numbers of particles was detected above a critical relative humidity along with a simultaneous transition in nuclei composition toward that of the sulfate-dominated total aerosol population. The temperature and humidity conditions of the homogeneous nucleation transition were reasonably consistent with expectations based on previous theoretical and laboratory studies but were highly variable. The strong presence of certain organic pollutants was particularly noted to be associated with impedance of homogeneous freezing.
534 citations