Showing papers on "Tropopause published in 2001"
TL;DR: In this article, the atmospheric distributions of the aerosol tracers Pb-210 and Be-7 are simulated with a global three-dimensional model driven by assimilated meteorological observations for 1991-1996 from the NASA Goddard Earth Observing System (GEOSl).
Abstract: The atmospheric distributions of the aerosol tracers Pb-210 and Be-7 are simulated with a global three-dimensional model driven by assimilated meteorological observations for 1991-1996 from the NASA Goddard Earth Observing System (GEOSl). The combination of terrigenic Pb-210 and cosmogenic Be-7 provides a sensitive test of wet deposition and vertical transport in the model. Our simulation of moist transport and removal includes scavenging in wet convective updrafts (40% scavenging efficiency per kilometer of updraft), midlevel entrainment and detrainment, first-order rainout and washout from both convective anvils and large-scale precipitation, and cirrus precipitation. Observations from surface sites in specific years are compared to model results for the corresponding meteorological years, and observations from aircraft missions over the Pacific are compared to model results for the days of the flights. Initial simulation of Be-7 showed that cross-tropopause transport in the GEOSl meteorological fields is too fast by a factor of 3-4. We adjusted the stratospheric Be-7 source to correct the tropospheric simulation. Including this correction, we find that the model gives a good simulation of observed Pb-210 and Be-7 concentrations and deposition fluxes at surface sites worldwide, with no significant global bias and with significant success in reproducing the observed latitudinal and seasonal distributions. We achieve several improvements over previous models; in particular, we reproduce the observed Be-7 minimum in the tropics and show that its simulation is sensitive to rainout from convective anvils. Comparisons with aircraft observations up to 12-km altitude suggest that cirrus precipitation could be important for explaining the low concentrations in the middle and upper troposphere.
648 citations
TL;DR: The first radio occultation measurements of the CHAMP (CHAllenging Minisatellite Payload) satellite using Global Positioning System (GPS) signals have been performed on February 11, 2001.
Abstract: The first radio occultation measurements of the CHAMP (CHAllenging Minisatellite Payload) satellite using Global Positioning System (GPS) signals have been performed on February 11, 2001. By the end of April 2001 more than 3000 occultations were recorded. Globally distributed vertical profiles of dry temperature and specific humidity are derived, of which a set of 438 vertical dry temperature profiles is compared with corresponding global weather analyses. The observed temperature bias is less than ∼1 K above the tropopause and even less than 0.5 K in the altitude interval from 12 to 20 km at latitudes >30°N. About 55% of the compared profiles reached the last kilometer above the Earth's surface. In spite of the activated anti-spoofing mode of the GPS system the state-of-the-art GPS flight receiver aboard CHAMP combined with favorable antenna characteristics allows for atmospheric sounding with high accuracy and vertical resolution.
467 citations
TL;DR: Horizontal transport through this cold trap region causes air parcels that reach the tropopause at other longitudes to be dehydrated to the very low saturation mixing ratios characteristic of the cold trap, and hence can explain why observed tropical stratospheric water vapor mixing ratios are often lower than the saturation mixing ratio at the mean Tropopause temperature as discussed by the authors.
Abstract: The coldest tropopause temperatures occur over the equatorial West Pacific during Northern Hemisphere winter. Horizontal transport through this “cold trap” region causes air parcels that reach the tropopause at other longitudes to be dehydrated to the very low saturation mixing ratios characteristic of the cold trap, and hence can explain why observed tropical stratospheric water vapor mixing ratios are often lower than the saturation mixing ratio at the mean tropopause temperature. Horizontal transport of water vapor can also explain how a persistent layer of subvisible cirrus can exist near the tropopause in the cold trap even though observations suggest that there is diabatic cooling and subsidence, rather than diabatic heating and rising through the tropopause in this region. Thus, horizontal transport in the tropical transition layer in the vicinity of the tropopause plays an important role in the water balance of the stratosphere.
384 citations
TL;DR: In this article, a short description of the various techniques that provided atmospheric ozone measurements valuable for long-term trend analysis is given, including satellite and ground-based measurements, with a focus on ozone depletion in the northern and southern midlatitudes.
Abstract: Ozone plays a very important role in our atmosphere because it protects any living organisms at the Earth's surface against the harmful solar UVB and UVC radiation. In the stratosphere, ozone plays a critical role in the energy budget because it absorbs both solar UV and terrestrial IR radiation. Further, ozone in the tropopause acts as a strong greenhouse gas, and increasing ozone trends at these altitudes contribute to climate change. This review contains a short description of the various techniques that provided atmospheric ozone measurements valuable for long-term trend analysis. The anthropogenic emissions of substances that deplete ozone (chlorine- and bromine-containing volatile gases) have increased from the 1950s until the second half of the 1980s. The most severe consequence of the anthropogenic release of ozone-depleting substances is the “Antarctic ozone hole.” Long-term observations indicate that stratospheric ozone depletion in the southern winter-spring season over Antarctica started in the late 1970s, leading to a strong decrease in October total ozone means. Present values are only approximately half of those observed prior to 1970. In the Arctic, large ozone depletion was observed in winter and spring in some recent years. Satellite and ground-based measurements show no significant trends in the tropics but significant long-term decreasing trends in the northern and southern midlatitudes (of the order of 2–4% per decade in the period from 1970 to 1996 and an acceleration in trends in the 1980s). Ozone at northern midlatitudes decreased by −7.4±2% per decade at 40 km above mean sea level, while ozone loss was small at 30 km. Large trends were found in the lower stratosphere, −5.1±1.8% at 20 km and −7.3±4.6% at 15 km, where the bulk of the ozone resides. The possibility of a reduction in the observed trends has been discussed recently, but it is very hard to distinguish this from the natural variability. As a consequence of the Montreal Protocol process, the emissions of ozone-depleting substances have decreased since the late 1980s. Chlorine is no longer increasing in the stratosphere, although the total bromine amount is still increasing. Considering anthropogenic emissions of substances that deplete ozone, the turnaround in stratospheric ozone trends is expected to take place in the coming years. However, anthropogenic climate change could have a large influence on the future evolution of the Earth's ozone shield.
323 citations
TL;DR: A temporally and spatially comprehensive depiction of the tropical tropopause is presented in this article, based on radiosonde data from 83 stations, for three levels: the conventional lapse-rate Tropopause (LRT), the cold-point Tropophere (CPT), and the 100 hPa level.
Abstract: A temporally and spatially comprehensive depiction of the tropical tropopause is presented, based on radiosonde data from 83 stations. Climatological statistics for 1961- 1990 are computed for three levels: the conventional lapse-rate tropopause (LRT), the cold-point tropopause (CPT), and the 100 hPa level. Mean values and seasonal and interannual variations of temperature, pressure, height, potential temperature, and water vapor saturation mixing ratio at these levels are compared. The tropopause is higher, colder, and at lower pressure in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH) in NH winter. This pattern reverses in NH summer, except that the tropopause remains colder in the NH than in the SH. The climatological locations of minimum tropopause temperature differ from those of maximum height and minimum pressure: In NH winter the tropopause is coldest over the western tropical Pacific warm pool region, but it is highest and at lowest pressure over the western Atlantic. Correlations of interannual anomalies in zonal-mean characteristics reveal that the height of the tropopause reflects the temperature of the underlying troposphere. Tropopause temperature, on the other hand, shows little association with tropospheric characteristics but is significantly correlated with the temperature and pressure of the lower stratosphere. The 100 hPa level is a poor surrogate for the tropical tropopause. Changes in radiosonde instrumentation limit the potential for detecting tropopause trends. However, the following (nonmonotonic) trends in the tropopause in the deep tropics during 1978 -1997 seem robust: an increase in height of about 20 m decade 21 , a decrease in pressure of about 0.5 hPa decade 21 , a cooling of about 0.5 K decade 21 , little change in potential temperature, and a decrease in saturation volume mixing ratio of about 0.3 ppmv decade 21 .
307 citations
TL;DR: In this paper, it was shown that if tropical tropopause cirrus clouds lie above convective anvils with tops above about 13km, net radiative cooling from the cirrus can be produced that is large enough to offset significant subsidence heating, even at the lowest temperatures observed in the tropics.
Abstract: If tropical tropopause cirrus lie above convective anvils with tops above about 13km, then net radiative cooling from the cirrus can be produced that is large enough to offset significant subsidence heating, even at the lowest temperatures observed in the tropics. Cirrus clouds near the tropopause are strongly heated by radiation unless they lie above convective anvil clouds. Radiative relaxation in the tropical troposphere is slow above about 14km unless clouds are present. Radiative cooling of tropopause cirrus may be important in processes that dehydrate air before it enters the stratosphere.
262 citations
TL;DR: In this article, a model of convective and advective transport across the tropical tropopause is described, where overshooting convective turrets inject dehydrated tropospheric air into a tropical "tropopause layer" (TTL) bounded approximately by the 50- and 150-hPa surfaces, a layer similar to the "entrainment zone" at the top of the planetary boundary layer.
Abstract: A model of convective and advective transport across the tropical tropopause is described. In this model overshooting convective turrets inject dehydrated tropospheric air into a tropical ‘‘tropopause layer’’ (TTL) bounded approximately by the 50- and 150-hPa surfaces, a layer similar to the ‘‘entrainment zone’’ at the top of the planetary boundary layer. The overshooting process occurs only in limited regions. In the TTL, mixtures of overshooting and ambient air undergo buoyancy-driven settling, then slowly loft through the TTL and eventually enter the main stratosphere throughout the Tropics. It is found that for reasonable parameter settings the combined action of convection, isentropic mixing, and advection by the large-scale circulation in the model can produce realistic water vapor and ozone profiles while balancing the energy budget. Some of the observed peculiarities that can be simulated are (i) the widespread absence of vapor saturation at the tropopause despite tropical mean upward motion, (ii) an ozone minimum below the mean tropopause, and (iii) the typical location of stratiform cloud tops below the mean tropopause. In contrast to inferences from typical ‘‘cold trap’’ models, the relative humidity of air crossing the tropopause is found to be sensitive to ice microphysics.
197 citations
TL;DR: The connection between the Arctic Oscillation and the stratosphere on intra-annual timescales is investigated in this paper, where both the National Centers for Environmental Prediction reanalysis data and a general circulation model simulation are used.
Abstract: The connection between the Arctic Oscillation and the stratosphere is investigated on intra-annual timescales. Both the National Centers for Environmental Prediction reanalysis data and a general circulation model simulation are used. In the winter half year November–April the dominant variability in the stratosphere in middle and high latitudes has the form of downward propagation of zonal mean zonal wind anomalies. The strength of the anomalies decays below 10 hPa, but often the anomalies reach the surface. The time for the propagation from 10 hPa to the surface is ∼15 days. When positive anomalies reach the surface, the phase of the Arctic Oscillation tends to be positive. The stratospheric variability and the downward propagation is found to be driven by the vertical component of the Eliassen-Palm flux. This flux propagates from the lower troposphere to the tropopause on a timescale of 5 days. Model and reanalysis compare well in the structure of the stratospheric variability and the connection between the stratosphere and troposphere. However, the strength of the stratospheric variability is ∼25% weaker in the model.
153 citations
TL;DR: The covariance model used in The Met. Office Global Three-Dimensional Variational (3D-Var) Data Assimilation system represents the variation of vertical covariances with latitude reasonably well, but the longer horizontal scales in the stratosphere are not currently reproduced.
Abstract: Previous studies and different methods of estimating short-range forecast errors are summarized. Zonally and temporally averaged statistics based on differences of one- and two-day forecasts valid at the same time are presented and an attempt is made to explain many of the features by reference to dynamical concepts.
Vertical correlation length-scale tends to increase with horizontal correlation scale but to be very short in the Tropics; horizontal scale is longest in the Tropics and in the stratosphere. The variations in vertical correlation are much more pronounced for largely balanced variables such as rotational wind and temperature than they are for divergent wind or humidity. The extratropics are dominated by an equivalent barotropic mode with the level of the maximum wind amplitude (and the zero crossing of the temperature correlation) being determined by the tropopause. Surface pressure is negatively correlated with low-level temperature as expected (except over the Antarctic plateau where it is positively correlated); it is also negatively correlated with temperatures near/above the tropopause in the extratropics.
The covariance model used in The Met. Office Global Three-Dimensional Variational (3D-Var) Data Assimilation system represents the variation of vertical covariances with latitude reasonably well, but the longer horizontal scales in the stratosphere are not currently reproduced. The implied covariances used operationally have been modified so that the correlation length-scales, both horizontal and vertical, are somewhat shorter than those direct from the forecast differences. Recent changes to the representation are briefly described, with an indication of their impact on the forecasts. The impacts are significant relative to other changes tested, and the covariance model has played a major role in the successful implementation and subsequent improvement of our 3D-Var system.
138 citations
TL;DR: In this paper, the authors describe aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the TOTE/VOTE in December 1995 and February 1996.
Abstract: This work describes aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) in December 1995 and February 1996. Thin cirrus clouds were found at the tropopause on each of the four flights which penetrated within 15° of the equator at 200–210 east longitude. South of 15°N, thin cirrus were detected above the aircraft about 65% of the time that data were available. The altitudes of these clouds exceeded 18 km at times. The cirrus observations could be divided into two basic types: thin quasi-laminar wisps and thicker, more textured structures. On the basis of trajectory analyses and temperature histories, these two types were usually formed respectively by (1) in situ cooling on both a synoptic scale and mesoscale and (2) recent (a few days) outflow from convection. There is evidence from one case that the thicker clouds can also be formed by in situ cooling. The actual presence or absence of thin cirrus clouds was also consistent with the temperature and convective histories derived from back trajectory calculations. Notably, at any given time, only a relatively small portion (at most 25%) of the west central tropical Pacific has been influenced by convection within the previous 10 days. The structures of some of the thin cirrus clouds formed in situ strongly resembled long-wavelength (500–1000 km) gravity waves observed nearly simultaneously by the ER-2 on one of the flights. Comparison with in situ water vapor profiles made by the NASA ER-2 aircraft provide some observational support for the hypothesis that thin cirrus clouds play an important role in dehydrating tropospheric air as it enters the stratosphere.
137 citations
TL;DR: In this paper, both the thermal and the dynamical criteria are applied to each dataset and the results show that the annual cycle of the polar tropopause can be classified into three different patterns.
Abstract: The polar and subpolar tropopause in both hemispheres is investigated using the ECMWF Reanalysis (ERA) data from 1979 to 1993 and radiosonde data from 1989 to 1993. Both the thermal and the dynamical criteria are applied to each dataset. The tropopauses derived from the radiosonde data are used to validate the ERA-derived tropopauses and to investigate the sharpness of the tropopause. The validation reveals that the ERA data are well suited for the determination of the tropopause. A comparison between the thermal and the dynamical tropopause shows a very good agreement except for polar winter, and there is clear evidence that the dynamical criterion is more appropriate in winter. The results show that the annual cycle of the polar tropopause can be classified into three different patterns. A single wave with a tropopause pressure maximum in winter and a minimum in summer is typical for the subpolar parts of eastern Siberia and North America. A double wave with pressure maxima in spring and autumn...
TL;DR: In this paper, the authors used operational sounding data (1973-1998) to determine cold point tropopause (CPT) characteristics and found a cooling trend (−0.57±0.06 K/decade during 1973-1998), which is opposite to what has been hypothesized to explain the trend in stratospheric water vapor.
Abstract: Operational sounding data (1973–1998) were used to determine cold point tropopause (CPT) characteristics. A cooling trend (−0.57±0.06 K/Decade during 1973–1998) in tropical CPT temperatures has been found, which is opposite to what has been hypothesized to explain the trend in stratospheric water vapor. Given this trend, the annual averages of the CPT saturation mixing ratios (SMRs) inferred from the analysis of the 1994–1997 data in the work of Dessler [1998] are substantially smaller than those from data before the mid-1990s. This implies that while Newell and Gould-Stewart's “stratospheric fountain” might not have been necessary to explain stratospheric water vapor during the mid-1990s, it was necessary in most years before that. Changes in tropical convection occurrence frequency and/or strength are suggested to explain the cooling trend in the CPT temperatures. It is suggested that the observed positive trend in stratospheric water vapor is probably due to changes in the residual circulation.
TL;DR: This is a scalable system for high spatial resolution of vertical temperature profiles in the troposphere and lower stratosphere, even in the presence of aerosols.
Abstract: A high-spectral-resolution lidar can measure vertical profiles of atmospheric temperature, pressure, the aerosol backscatter ratio, and the aerosol extinction coefficient simultaneously. We describe a system with these characteristics. The transmitter is a narrow-band (FWHM of the order of 74 MHz), injection-seeded, pulsed, double YAG laser at 532 nm. Iodine-vapor filters in the detection system spectrally separate the molecular and aerosol scattering and greatly reduce the latter (−41 dB). Operating at a selected frequency to take advantage of two neighboring lines in vapor filters, one can obtain a sensitivity of the measured signal-to-air temperature ratio equal to 0.42%/K. Using a relatively modest size transmitter and receiver system (laser power times telescope aperture equals 0.19 Wm2), our measured temperature profiles (0.5–15 km) over 11 nights are in agreement with balloon soundings to within 2.0 K over an altitude range of 2–5 km. There is good agreement in the lapse rates, tropopause altitudes, and inversions. In principle, to invert the signal requires a known density at one altitude, but in practice it is convenient to also use a known temperature at that altitude. This is a scalable system for high spatial resolution of vertical temperature profiles in the troposphere and lower stratosphere, even in the presence of aerosols.
TL;DR: In this paper, the authors used a cloud model to simulate dehydration which occurs due to formation of optically thin, laminar cirrus as air rises slowly across the tropopause.
Abstract: In this study, we use a cloud model to simulate dehydration which occurs due to formation of optically thin, laminar cirrus as air rises slowly across the tropopause. The slow ascent and adiabatic cooling, which balances the radiative heating near the tropopause, drives nucleation of a very small number of ice crystals (<1 L−1). These crystals grow rapidly and sediment out within a few hours. The clouds never become optically thick enough to be visible from the ground. The ice crystal nucleation and growth prevents the relative humidity with respect to ice (RHI) from rising more than a few percent above the threshold for ice nucleation (RHInuc ≃ 110–160%, depending upon the aerosol composition); hence, laminar cirrus can limit the mixing ratio of water vapor entering the stratosphere. However, the ice number densities are too low and their sedimentation is too rapid to allow dehydration of the air from RHInuc down to saturation (RHI = 100%). The net result is that air crosses the tropopause with water vapor mixing ratios about 1.1 to 1.6 times the ice saturation mixing ratio corresponding to the tropopause temperature, depending on the threshold of ice nucleation on aerosols in the tropopause region. If the cross-tropopause ascent rate is larger than that calculated to balance radiative heating (0.2 cm s−1), then larger ice crystal number densities are generated, and more effective dehydration is possible (assuming a fixed temperature). The water vapor mixing ratio entering the stratosphere decreases with increasing ascent rate (approaching the tropopause ice saturation mixing ratio) until the vertical wind speed exceeds the ice crystal terminal velocity (about 10 cm s−1). More effective dehydration can also be provided by temperature oscillations associated with wave motions. The water vapor mixing ratio entering the stratosphere is essentially controlled by the tropopause temperature at the coldest point in the wave. Hence, the efficiency of dehydration at the tropopause depends upon both the effectiveness of upper tropospheric aerosols as ice nuclei and the occurrence of wave motions in the tropopause region. In situ humidity observations from tropical aircraft campaigns and balloon launches over the past several years have provided a few examples of ice-supersaturated air near the tropopause. However, given the scarcity of data and the uncertainties in water vapor measurements, we lack definitive evidence that air entering the stratosphere is supersaturated with respect to ice.
TL;DR: In this article, a new quasi-biennial oscillation (QBO) indexing method is presented, which is based on the zonal mean zonal wind shear anomaly at the equator and is compared to the Singapore index.
Abstract: Global quasi-biennial variation in the lower stratosphere and tropopause region is studied using 41 years (1958–1998) of reanalyses from the National Centers for Environmental Prediction (NCEP). Horizontal wind, temperature, geopotential height, tropopause temperature and pressure fields are used. A new quasi-biennial oscillation (QBO) indexing method is presented, which is based on the zonal mean zonal wind shear anomaly at the equator and is compared to the Singapore index. A phase difference compositing technique provides “snapshots” of the QBO meridional-vertical structure as it descends, and “composite phases” provide a look at its time progression. Via binning large amounts of data, the first observation-based estimate of the QBO meridional circulation is obtained. High-latitude QBO variability supports previous studies that invoke planetary wave-mean flow interaction as an explanation. The meridional distribution of the range in QBO zonal wind is compared with the stratospheric annual cycle, with the annual cycle dominating poleward of ∼12° latitude but still being significant in the deep tropics. The issues of temporal shear zone asymmetries and phase locking with the annual cycle are critically examined. Subtracting the time mean and annual cycle removes ∼2/3 of the asymmetry in wind (and wind shear) zone descent rate. The NCEP data validate previous findings that both the easterly and westerly QBO anomalous wind regimes in the lower stratosphere change sign preferentially during northern summer. It is noteworthy that the NCEP QBO amplitude and the relationships among the reanalyzed zonal wind, temperature, and meridional circulation undergo a substantial change around 1978.
TL;DR: In this paper, the origin of the Northern Hemisphere summer subtropical anticyclones was investigated using a linear quasigeostrophic model, and it was found that the broad features in the model solutions forced by realistic heating fields acting on observed zonal flows agree well with those in the observations.
Abstract: The origin of the Northern Hemisphere summer subtropical anticyclones is investigated using a linear quasigeostrophic model. It is found that the broad features in the model solutions forced by realistic heating fields acting on observed zonal flows agree well with those in the observations. The realistic features of the model solutions include the subtropical continental lows and oceanic highs in the lower troposphere, and continental ridges and midoceanic troughs in the upper troposphere. The forced responses are largest near the surface and the tropopause with a vertical node around 500 hPa as observed. The results indicate that the model stationary waves owe their existence largely to the Asian heat source. The authors thus propose that the observed low-level subtropical anticyclones over the North Pacific and North Atlantic Oceans be interpreted as a remote response of Rossby waves forced by the large-scale heat sources over Asia. The results support the existing theory that the observed low...
TL;DR: The coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM is presented in this article which enables a simultaneous treatment of meteorology and atmospheric chemistry and their feedbacks.
Abstract: . The coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM is presented which enables a simultaneous treatment of meteorology and atmospheric chemistry and their feedbacks. This is the first model which interactively combines a general circulation model with a chemical model, employing most of the important reactions and species necessary to describe the stratospheric and upper tropospheric ozone chemistry, and which is computationally fast enough to allow long-term integrations with currently available computer resources. This is possible as the model time-step used for the chemistry can be chosen as large as the integration time-step for the dynamics. Vertically the atmosphere is discretized by 39 levels from the surface up to the top layer which is centred at 10 hPa, with a relatively high vertical resolution of approximately 700 m near the extra-tropical tropopause. We present the results of a control simulation representing recent conditions (1990) and compare it to available observations. The focus is on investigations of stratospheric dynamics and chemistry relevant to describe the stratospheric ozone layer. ECHAM4.L39(DLR)/CHEM reproduces main features of stratospheric dynamics in the arctic vortex region, including stratospheric warming events. This constitutes a major improvement compared to earlier model versions. However, apparent shortcomings in Antarctic circulation and temperatures persist. The seasonal and interannual variability of the ozone layer is simulated in accordance with observations. Activation and deactivation of chlorine in the polar stratospheric vortices and their inter-hemispheric differences are reproduced. Considering methane oxidation as part of the dynamic-chemistry feedback results in an improved representation of the spatial distribution of stratospheric water vapour concentrations. The current model constitutes a powerful tool to investigate, for instance, the combined direct and indirect effects of anthropogenic trace gas emissions. Key words. Atmospheric composition and structure (middle atmosphere – composition and chemistry) – Meteorology and atmospheric dynamics (general circulation; middle atmosphere dynamics)
TL;DR: In this paper, the 7Be activity concentrations measured from 1996 to 1998 at four high-altitude stations, Jungfraujoch, Switzerland, Zugspitze, Germany, Sonnblick, Austria and Mt. Cimone, Italy, were analyzed in combination with a set of, meteorological and atmospheric parameters such as the tropopause height, relative and specific humidity and also in conjunction with 3D back-trajectories in order to investigate the climatological features of 7Be.
TL;DR: In this article, the authors investigated the transient behavior of the Asian monsoon anticyclone in the summertime upper troposphere for the four northern summers 1987-90 and found that the potential vorticity near the tropopause shows the development of westward migrating anticyclones breaking off from the main Anticyclone a few times each summer.
Abstract: The authors investigate the transient behavior of the Asian monsoon anticyclone in the summertime upper troposphere for the four northern summers 1987–90. The evolution of potential vorticity near the tropopause shows the development of westward migrating anticyclones breaking off from the main anticyclone a few times each summer. These disturbances are relatively shallow, being confined to the upper troposphere.
TL;DR: In this article, the authors used the Radiosonde and National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data to consider aspects of large-scale variability in tropopause height, temperature and pressure.
Abstract: Radiosonde and National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data are utilized to consider aspects of large-scale variability in tropopause height, temperature and pressure. This variability is related to coherent dynamical fluctuations in the troposphere and lower stratosphere through the use of linear correlation and regression. On interannual time-scales, significant global-scale tropopause fluctuations are tied to variability in sea surface temperature (SST) associated with the El Niňo/Southern Oscillation phenomenon. When SST is anomalously high in the central tropical Pacific, tropopause height (pressure) is high (low) throughout the Tropics, with largest perturbation amplitudes in the subtropical Pacific. At the same time, the tropopause is cold over the tropical and subtropical Pacific sector but warm elsewhere in the Tropics. Over the extratropics, wave-like perturbations in the tropopause are seen, with anomalous cyclonic flow corresponding to a lower tropopause height and higher tropopause temperature and pressure, and vice versa. The sign of the temperature anomalies in the lower stratosphere tends to match that at the tropopause over much of the globe, with opposite-signed anomalies in the upper troposphere. The vertical structure of these perturbations is consistent with the expected potential-vorticity anomalies induced by quasi-stationary Rossby waves and vertically propagating gravity waves forced by displacements of tropical convection. Similar relationships are associated with the eastward propprotion of tropical convection due to the Madden-Julian Oscillation on intraseasonal time-scales.
TL;DR: In this paper, the tropical cold point tropopause (CPT) characteristics have been calculated using ECMWF reanalyses and high resolution radiosonde soundings obtained in TOGA COARE as well as operational sounding data.
Abstract: Tropical cold point tropopause (CPT) characteristics have been calculated using ECMWF reanalyses and high resolution radiosonde soundings obtained in TOGA COARE as well as operational sounding data. It is found that ECMWF reanalyses are suitable for investigating the morphology of the tropical CPT and the variabilities of the tropopause over the entire Tropics, despite an almost constant one-side warm bias. The daily locations of the tropical coldest CPT are clustered over the western Pacific warm pool region in January and spread out longitudinally as the year progresses. During the Indian summer monsoon, the locations of the coldest CPT are deflected northward. The influences of the quasi-biennial oscillation (QBO) and the El Nino–Southern Oscillation (ENSO) on the tropical CPT have been separated using bivariate regression. The stratospheric zonal wind shear at 50 mb leads the variation in the tropical CPT temperatures by about 6 months. The QBO signature in the tropical CPT is mainly zonally ...
TL;DR: In this paper, the interannual variability of the tropical tropopause region between 14 and 18 km is examined using ob-servations of convection, winds, and tropopsause temperatures from reanalyses and water vapor from satellites.
Abstract: The interannual variability of the tropical tropopause region between 14 and 18 km is examined using ob- servations of convection, winds, and tropopause temperatures from reanalyses and water vapor from satellites. This variability is compared to a simulation using the Community Climate Model version 3 (CCM3) general circulation model forced by observed sea surface temperatures. A coherent picture of the effect of the El Nino- Southern Oscillation (ENSO) on the tropopause region is presented in the NCEP-NCAR reanalyses and CCM3. ENSO modifies convection in the Tropics, and the temperature and circulation of the tropical tropopause region, in agreement with idealized models of tropical heating. CCM3 reproduces most details of these changes, but not the zonal mean temperature variations present in the analysis fields, which are not related to ENSO. ENSO also forces significant changes in observed and simulated water vapor fields. In the upper troposphere water vapor is at maximum near convection, while in the tropopause region water vapor is at minimum in the regions of convection and surrounding it. Convection, cirrus clouds, temperatures, and transport are all linked to describe the water vapor distribution and highlight the role of transport in the tropopause region.
TL;DR: The influence of wind shear on the life cycle of extratropical cyclones and their upper-level potential vorticity (PV) waves was investigated in this paper.
Abstract: The El Nino Southern Oscillation (ENSO) strongly influences the interannual and seasonal atmospheric circulation over the North Pacific. The present study shows that the meridional displacement of the time-mean upper-level jet associated with ENSO modulates the time-mean barotropic (meridional) wind shear over the central and eastern North Pacific storm track. Earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper-level potential vorticity (PV) waves. The present study suggests that differences in the time-mean flow associated with the 1997-99 ENSO cycle had a similar impact on tropopause PV structure, meridional eddy fluxes of momentum and temperature, and predictability.
TL;DR: In this article, the authors show that equatorial Kelvin waves around the tropopause act as a dehydration pump for the stratosphere and that wave breaking contributes to the irreversible transport of ozone across the Tropopause.
Abstract: Soundings of frost-point hygrometers, ozonesondes, and radiosondes at San Cristobal Island (0.9°S, 89.6°W) in September 1998 provide an observational evidence that equatorial Kelvin waves around the tropopause act as a dehydration pump for the stratosphere. During the downward-displacement phase of a Kelvin wave, dry and ozone-rich stratospheric air is transported into the upper troposphere. During the upward-displacement phase, on the other hand, higher specific-humidity air moves up in the tropopause region, but at the same time, this upward motion causes cooling of the air that limits the water vapor amount entering the stratosphere. Also, wave breaking contributes to the irreversible transport of ozone across the tropopause. Considering their omnipresence at the equatorial tropopause, we suggest that Kelvin waves may be one of the important agents for maintaining the dryness of the tropical lower stratosphere.
TL;DR: In this paper, the authors show that a realistic extratropical tropopause structure, marked by a strong change both in meteorological variables such as lapse rate and in transport characteristics, may be achieved solely through the stirring effects of baroclinic eddies acting against a smooth thermal relaxation.
Abstract: Numerical simulations in a simple atmospheric model show that a realistic extratropical tropopause structure, marked by a strong change both in meteorological variables such as lapse rate and in transport characteristics, may be achieved solely through the stirring effects of baroclinic eddies acting against a smooth thermal relaxation. The flow self-organizes to give a transport structure that has a sharp transition in the vertical. Below the transition level the eddies stir across entire isentropic surfaces. The transition level corresponds to the extratropical tropopause height. Above the transition level there is a midlatitude transport barrier, corresponding to the subtropical transport barrier in the real atmosphere, with eddy stirring on poleward and equatorward sides. Similar self-organizing mechanisms are likely to be relevant in the stratosphere and in oceanic flows.
TL;DR: In this article, the experimental data of the biological effective solar ultraviolet irradiance as a function of height, throughout the troposphere, showed an average increase of about 7.2±1.2% km−1, which is in close agreement with the theoretically derived one, by using an appropriate algorithm, providing the biological effectiveness solar UV irradiance at various altitudes from the Earth's surface.
Abstract: Observations of the biological effective solar ultraviolet irradiance, made by instrumentation flown on a Falcon aircraft over the entire Greek area, from the sea up to the tropopause level, obtained in the frame of the Radiation Field in the Troposphere-Scientific Training and Access to Aircraft for Atmospheric Research Throughout Europe project, are presented to discuss among others its altitude dependence. The experimental data of the biological effective solar UV irradiance as a function of height, throughout the troposphere, showed an average increase of about 7.2±1.2% km−1, which is in close agreement with the theoretically derived one, by using an appropriate algorithm, providing the biological effective solar UV irradiance at various altitudes from the Earth's surface. This increase rate has also been compared with the burden ozone content at each height level as it was derived from ozone concentration measurements obtained by both balloon ascents and the aircraft's instrumentation. This comparison showed a strong anticorrelation (correlation coefficient r = −0.98) between the biological effective solar UV irradiance and the integrated ozone content above each height level corresponding to 0.25% increase in UV per 1 Dobson Unit (1DU = 0.001 atm cm) decrease in total ozone column, approximately.
TL;DR: In this paper, the authors investigated the climate response to a set of idealized ozone perturbations by integrations with a coupled atmosphere-ocean model and found that the model is more sensitive to ozone than to an equivalent CO2 perturbation.
Abstract: The climate response to a set of idealized ozone perturbations is investigated by integrations with a coupled atmosphere-ocean model. Although all perturbations, including a homogeneous CO2 increase, induce the same stratosphere adjusted, tropopause radiative forcing, the climate response is quite variable within the set of experiments. Except for an upper tropospheric ozone increase, our model is more sensitive to ozone perturbations than to an equivalent CO2 perturbation. This applies in particular to a lower stratospheric ozone increase. The accompanying changes in the stratospheric water vapor (SWV) distribution are found to impose additional forcings on climate that may well exceed the forcings due to the original perturbations. Without SWV feedback on radiation the climate sensitivity to a lower stratospheric ozone increase draws remarkably near the respective value for equivalent CO2. This emphasizes the crucial role SWV may have in the forcing-response relationship.
TL;DR: In this paper, the authors used a genetic algorithm constrained by in situ observations of CO2 and N2O in the lower and middle stratosphere to estimate the mean ages of air in the tropical tropopause and a point in the stratosphere.
Abstract: Empirical age spectra for the northern midlatitude lower stratosphere were derived using a genetic algorithm constrained by in situ observations of CO2 and N2O. An age spectrum is the probability distribution function describing the ensemble of transit times between the tropical tropopause and a point in the stratosphere. Age spectra with two distinct maxima fit observed CO2 mixing ratios significantly better than age spectra consisting of a single peak. The mean transit time (mean age) of the younger peak increased with altitude from the tropopause to ∼1 year at 19.5 km, while the mean age of the older peak was 5.4±0.4 years throughout this altitude region. Near the tropopause, ∼10% of the air was represented by the older peak, increasing to ∼50% at 19.5 km. We argue that the younger peak represents rapid quasihorizontal transport of air from the tropics, while the older peak represents downwelling of air from higher altitudes in the mean meridional circulation. Mean ages for the younger and older peaks provide estimates of the residence times for air in the lower and middle stratosphere, respectively. The clear separation of the peaks implies a significant altitude region where there is little meridional exchange between midlatitudes and the tropics.
TL;DR: This article showed that the correlated k-distribution treatment with sufficient resolution is capable of simulating accurately cooling by stratospheric water vapor, and obtained equilibrium cooling of about 0.3K that extends from 20 km to the top of the atmosphere, and adjusted radiative forcing of 0.12 Wm−2.
Abstract: The stratosphere has been cooling by about 2K/decade at 30–60 km over the past several decades and by lesser amounts toward the tropopause. Climate model calculations suggest that stratospheric water vapor is an important contributor to the observed stratospheric cooling, but there are large differences among recent GCM simulations for prescribed changes in stratospheric water vapor, which point to problems with the current GCM treatment of the absorption and emission by stratospheric water vapor. We show that the correlated k-distribution treatment with sufficient resolution is capable of simulating accurately cooling by stratospheric water vapor. We obtain equilibrium cooling of about 0.3K that extends from 20 km to the top of the atmosphere, and adjusted radiative forcing of 0.12 Wm−2, for a stratospheric water vapor increase of 0.7 ppmv which has been estimated for the period 1979–1997.
TL;DR: In this paper, a new technical procedure is introduced to determine the stratosphere adjusted radiative forcing at the tropopause in the framework of the 3-D climate model ECHAM4.
Abstract: A new technical procedure is introduced to determine the stratosphere adjusted radiative forcing at the tropopause in the framework of the 3-D climate model ECHAM4. However, the procedure appears to be appropriate for other GCMs as well. It allows to study in a straightforward way the problem of the general usefulness of radiative forcing as a reliable predictor of climate change. Some examples are given for illustration. It is, once again, confirmed that the climate sensitivity is practically equal for experiments with increased solar insolation and increased CO2 concentration. However, a higher climate sensitivity is obtained for ozone perturbations with a horizontally or vertically inhomogeneous distribution. The latter finding is in qualitative agreement with respective results reported in other studies, whereas the value of the climate sensitivity is exceptionally high in our model. The physical reasons for the unique model behaviour in the ozone experiments are currently not understood.