Showing papers on "Tropopause published in 1992"

The atmosphere of Neptune : an analysis of radio occultation data acquired with Voyager 2

Abstract: The vertical structure of Neptune's atmosphere is studied on the basis of recordings of the tracking signals received from Voyager 2 during its occultation by Neptune. The measurements began at a planetographic latitude of 62 deg north and ended near 45 deg south, and cover an altitude interval of about 5000 km. The 1-bar isobaric surface has equatorial and polar radii of 24,766 +/-15 km and 24,342 +/-30 km, respectively, and a corresponding oblateness of 0.0171 +/-0.0014. At this pressure, the temperature was 72 +/-2 K. The tropopause was detected approximately 40 km above the 1-bar level at a pressure of about 100 mbar. A comparison with IR observations indicates that the gas at the tropopause consists of 78-84-percent hydrogen by number density with the rest being mostly helium. The temperature in this region was 52 +/-2 K.

Tropical stratospheric circulation deduced from satellite aerosol data

Abstract: THE dispersal of volcanic material from large tropical eruptions provides insight into the circulation of the lower stratosphere. Here we infer stratospheric motions from zonal mean cross-sections of satellite observations of the aerosol layer taken during 1979–81 and 1984–91. By examining the aerosol distribution following volcanic eruptions in the tropics, we find that poleward transport occurs readily at altitudes within a few kilometres above the tropopause, whereas in the altitude range of 21–28 km, aerosols tend to remain within 20° of the Equator. We further deduce that the aerosol distribution in this upper regime is controlled by the phase of the quasibiennial oscillation. When the easterly shear is present, aerosols are lofted over the Equator, whereas when the westerly shear is present, descent relative to the mean stratospheric circulation occurs over the Equator. From the aerosol distributions, we suggest that the tropical stratosphere may be regarded as a temporary reservoir for trace constituents entering the stratosphere through the tropical tropopause.

A GCM Simulation of the Relationship between Tropical-Storm Formation and ENSO

Abstract: A low-resolution Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model has been integrated for 15 years. In the course of this experiment, the observed month-to-month sea surface temperature (SST) variations in the tropical Pacific Ocean were incorporated in the lower boundary condition. The output from this model run was used to investigate the influence of El Nin-Southern Oscillation (ENSO) events on the variability of tropical-storm formation. Criteria for detecting tropical cyclogenesis and tropical-storm formation were developed for the model. Tropical storms appearing in the model atmosphere exhibit many typhoonlike characteristics: strong cyclonic vorticity and convergence in the lower troposphere, strong anticyclonic vorticity and divergence near the tropopause, and intense precipitation. It is demonstrated that, despite its coarse resolution, the model is capable of reproducing the observed geographical distribution and seasonal variation of tropical-storm formation. The...

Propagation of Planetary Waves between the Troposphere and Stratosphere

Abstract: The propagation of planetary waves between the troposphere and stratosphere is investigated using a linear, time-dependent, primitive equation model. It is found that the tropopause acts like a valve for the propagation of planetary waves. The key parameters controlling the valve are the vertical gradient of buoyancy frequency and the vertical shear of the zonal winds at the tropopause. For a given wind profile smaller gradient of buoyancy frequency enhances the propagation of planetary waves. For a given profile of buoyancy frequency the larger the shear, the more the wave activity is trapped in the troposphere, and therefore, there is less left to propagate into the stratosphere. The transmission across the tropopause is, however, not sensitive to zonal winds in the upper stratosphere. The propagation of planetary waves is very sensitive to transience. More transient waves propagate more vertically within the troposphere, and for more transient waves, more wave activity is transferred into the stratosphere from the troposphere.

A Potential-Vorticity Diagnosis of the importance of initial Structure and Condensational Heating in Observed Extratropical Cyclogenesis

Abstract: The dynamics of a cyclone development over the midwestern United States on 15 December 1987 are investigated with a focus on the relationship between cyclone structure and condensational heating. Low-level cyclogenesis is initiated by a large-amplitude tropopause perturbation that develops over western North America. Using potential-vorticity (PV) inversion diagnostics, we show how the near-surface winds associated with this upper disturbance create a localized, warm, thermal anomaly within a surface baroclinic zone. The distribution of precipitation and the diabatic generation of a positive low-level PV feature near the cyclone center are also controlled by the tropopause perturbation. Development culminates in a superposition of positive anomalies of tropopause PV, moisture-induc6d PV, and surface potential temperature θ, with contributions to the total low-level circulation being about 30%, 20%, and 50%, respectively. This case is compared with a different cyclogenesis event (4–5 February 1988...

Annual, quasi-biennial, and El Niño-Southern Oscillation (ENSO)time-scale variations in equatorial total ozone

Abstract: Equatorial total ozone variations with time scales of annual, quasi-biennial, and about 4-year periodicities are described by paying attention to their longitudinal structure. Analyses are made for 11 years from 1979 to 1989, using the global total ozone data derived from the total ozone mapping spectrometer on board the Nimbus 7 satellite. Over the equator an annual cycle in total ozone is conspicuous. Zonal mean values are maximum around September and minimum around January. The longitudinal structure shows a zonal wavenumber 1 pattern with minimum values around 140°E to the date line all year-round, indicating a close relationship to a region where the convective cloud activity is vigorous. By removing the climatological annual cycle from the original data, there appears the quasi-biennial oscillation in total ozone. This variation is characterized by zonally uniform phase changes and is strongly coupled with the quasi-biennial oscillation of the equatorial zonal wind in the lower stratosphere. Moreover, subtracting zonal mean values from the anomaly data mentioned above, we see an east-west seesaw variation with a nodal longitude around the date line. This east-west variation, having a characteristic time scale of about 4 years, is clearly related to the El Nino and the Southern Oscillation cycle. During El Nino events the longitudinal anomaly field in total ozone is positive in the western Pacific and negative in the eastern Pacific; the anomaly pattern is reversed during anti-El Nino events. Because the active region of convective clouds is located relatively in the eastern Pacific sector during El Nino events, it is suggested that the stronger upwelling and the higher tropopause associated with the convective cloud activity bring about less total ozone.

Summertime tropospheric observations related to N x O y distributions and partitioning over Alaska: Arctic Boundary Layer Expedition 3A

Abstract: Measurements of NO, NO2, PAN and NO(y) are presented for the summertime middle/lower troposphere over northern high latitudes. Chemical signatures from concurrent measurements of O3, CO, C2H2, C2H6, C3H8, C2Cl4, and H2O are used to characterize factors affecting the budget and distribution of N(x)O(y) in the Arctic and Sub-Arctic tropospheric air masses sampled over Alaska during the NASA Arctic Boundary Layer Expedition (ABLE 3A). The results implicate biomass burning in Siberia as the probable source of about one-third of the NO(y) abundance within the middle lower troposphere over Alaska and the downward transport of air from altitude in the vicinity of the tropopause as a major contributor to the abundance of NO(y) within the lower 6 km column over Alaska.

The potential effects of volcanic aerosols on cirrus cloud microphysics

Abstract: The potential impact of volcanic aerosols on nucleation of ice crystals in upper tropospheric cirrus clouds is examined from a microphysical perspective The sulfuric acid aerosols which form in the stratosphere are presumably transported into the troposphere by sedimentation and tropopause folding The tropospheric volcanic aerosol size distribution is estimated from 10-micron lidar backscatter and in situ measurements Microphysical simulations suggest that at temperatures below about -50 C the concentration of ice crystals which nucleate may be as much as a factor of 5 larger when volcanic aerosols are present The simulations suggest that the presence of volcanic aerosols may increase the net radiative forcing (surface warming) of certain types of cirrus near the tropopause by as much as 8 W/sq m Further observations are required to determine whether these effects actually occur, and their global impact

Gaseous sulfuric acid and sulfur dioxide measurements in the Arctic troposphere and lower stratosphere: Implications for hydroxyl radical abundances

Abstract: Measurements of gaseous sulfuric acid and sulfur dioxide were made in the winter arctic troposphere and lower stratosphere, using aircraft-borne mass spectrometers. The measurements, covering altitudes between 3.5 and 11.4 km, took place on 14 and 18 February, 1987 in Northern Scandinavia. The abundance of H2SO4 was around 0.01 to 0.06 pptv (parts per trillion by volume) and the measured SO2 volume mixing ratios are around 50 to 400 pptv in the troposphere and decrease steeply above the tropopause to about 10 pptv. This decrease above the tropopause is in contrast to previous measurements showing constant or even increasing SO2 mixing ratios. The combined H2SO4 and SO2 measurements offer an interesting opportunity to infer hydroxyl radical concentrations. Inferred OH concentrations are around 0.1–1.0 × 105 cm−3 being roughly consistent with model predictions for winter time high latitudes.

A zonal average model of the stratospheric contributions to the tropospheric ozone budget

Abstract: A zonally averaged transport and chemistry model for atmospheric trace species is developed, based on an existing statistical-dynamical climate model. The modeled region is a single hemisphere from 1000 mbar to 240 mbar. The model is applied to a study of the budget of tropospheric ozone, in particular to an estimation of the relative importance of the stratospheric and in situ photochemical sources. A parameterization of cross-tropopause exchange is developed based on the concept of tropopause folding. It is used to provide transfers of air and ozone across the upper boundary. A basic tropospheric chemistry scheme is also incorporated, featuring methane oxidation and subsequent reactions, carbon monoxide oxidation and oxygen-hydrogen reactions. Nitrogen oxide re-actions are reduced to a minimum by specification of NO[sub x] distribution in space and time. Nonmethane hydrocarbon reactions are omitted. Transport and surface deposition of relevant species are performed. The model produces ozone distributions which are in general agreement with observations and other modeling studies. These are analyzed by considering, X[sub x], the mixing ratio of ozone of stratospheric origin with respect to the complete' ozone mixing ratio. The sources and sinks of X[sub x] are parameterized in terms of the relevant processes, and it canmore » be treated as a quasi-conservative variable with respect to transports. Zonally averaged fields of this mixing ratio are produced and show the influence of ozone of stratospheric origin to be very small (5 % at most) close to the surface, in this zonally averaged analysis. At 300 mbar. in middle to high latitudes, the influence is more significant (up to about 50 % in high-latitude winter). These results are in accord with the very limited observational evidence. 32 refs., 12 figs., 1 tab.« less

Observations of record cold cloud-top temperatures in Tropical Cyclone Hilda (1990)

Abstract: A detailed analysis is made of the development of a region of cold cloud-top temperatures in Tropical Cyclone Hilda (1990) in the Coral Sea off eastern Australia. Observed temperatures of approximately 173 K (−100°C) from two independent satellite sources indicate that the convective turrets penetrated well into the stratosphere to an estimated height of around 19.2 km. The analytical parcel model of Schlesinger is used, together with available observations from the cyclone vicinity, to estimate the convective updrafts required to produce the observed stratosphere penetration. Under realistic assumptions of entrainment and hydrometeor drag, an updraft speed of between 15 and 38 m s−1 at tropopause level is required to provide the observed stratospheric penetration. Independent calculations using observed anvil expansion and environmental CAPE (convective available potential energy) support these updraft findings.

On the Cross-tropopause Exchange of Air.

Abstract: By consideration of the changing atmospheric distribution and source strength of chlorofluorocarbon 11(CFCl3, CFC-11), the annual exchange of air between troposphere and stratosphere is estimated to be 5 (±2) × 1017 kg globally.

Radar observations of an inertia-gravity wave in the troposphere and lower stratosphere

Abstract: Radar measurements at Aberystwyth (524°N, 41°W) during 25-27 June 1990 have been used to examine the characteristics of an inertia-gravity wave Above the tropopause, hodographs of horizontal perturbation velocity derived for 24-h time intervals show clockwise rotation with increasing height, corresponding to upwards propagation of energy, and vertical wavelengths of 15-26 km Below the tropopause, the results are not as distinct but the majority of hodographs show anticlockwise rotation A complementary rotary spectral analysis carried out for the full 2-day observation period shows that clockwise circular components dominate over anticlockwise components above the tropopause for vertical wavelengths in the range 1-7 km This analysis also shows the presence of saturation in this height range for vertical wavelengths up to about 15 km Again, the results for heights below the tropopause are not as clear From the spectral results, together with information on the direction of wave propagation inferred from the hodographs and from estimates of the vertical flux of horizontal momentum, the source of the wave is tentatively identified as a jet stream to the north

Vertical wind disturbances in the afternoon of mid‐summer revealed by the MU radar

Abstract: This paper reports a kind of vertical wind disturbances which appear daily in the afternoon of mid-summer in the troposphere A detailed analysis was made of wind data monitored by the MU radar (35°N, 136°E, Shigaraki, Shiga, Japan) for about five days in July 1987 as a typical example The disturbances propagate upward up to (sometimes over) the tropopause level with a speed of 04–18 m s−1 as a packet The timing of generation and propagation suggests that they are convective motions of cumulus clouds generated by solar heating on the ground A map of cloud-top height with high horizontal resolution of about 1 km provided by a satellite of NOAA-9 supports this inference The structures near the tropopause and spectral characteristics are shown

Genesis of Mobile Troughs in the Upper Westerlies

Abstract: Stability calculations on basic-state velocity profiles representative of the preferred regions for the development of the upper-level disturbances active in Type B cyclogenesis show that conditions in these regions (weak low-level baroclinicity, large low-level static stability, and large surface roughness) are favorable for the growth of baroclinic waves with maximum amplitude near the tropopause. The structure of these waves compares favorably with observations of developing short-wavelength upper-level troughs in the atmosphere. Basic states characteristic of the storm track regions (strong low-level baroclinicity and small surface roughness) favor the development of baroclinic waves with maximum amplitude at the surface. The dynamics of both the surface-trapped and the upper-tropospheric waves can be interpreted concisely using concepts of potential vorticity. Based on these results, a possible mechanism for Type B cyclogenesis in the storm track regions is proposed that involves the propagation and structural modification of baroclinic wave packets in a zonally varying basic flow.

Airborne remote sensing of tropospheric water vapor using a near infrared DIAL system

Abstract: Summarized here are the results of airborne water vapor measurements in the lower middle and upper troposphere using the Differential Absorption Lidar (DIAL) technique in the near infrared. The measurements were performed in July 1990 in Southern Bavaria between Allersberg and Straubing from 20 to 23 UTC taking advantage of night time conditions. The tropospheric H2O profiles were range investigated both horizontally and vertically. With the DIAL system that was used, water vapor measurements in the upper troposphere have been carried out for the first time. To calibrate the H2O-retrievals, effective absorption cross sections of selected H2O lines in terms of altitude around 724 nm were calculated using line parameter data from the literature (B. E. Grossmann et al). The frequency of the on-line measurements was adjusted by the spectra of a Polyacenic Semiconductor (PAS) cell filled with H2O. We found that the calibration error ranged between 0.005 and 0.015 cm(exp -1). The systematic errors of the H2O as a function of altitude were estimated below 7 km and 12 percent accuracy in the upper troposphere. The vertical H2O profile agrees well with in situ measurements in the investigated range between the top of the planetary boundary layer (PBL) up to near the tropopause. Horizontal and vertical H2O profiles are calculated by means of averaging single lidar returns. Typical horizontal resolutions range from 4 km in the lower to 11 km in the upper troposphere with vertical resolutions varying from 0.3 km up to 1 km, respectively, in order to satisfy a 5 - 10 percent accuracy in the statistical error. The measurement sensibility of the water vapor mixing ration in the upper troposphere is 0.01 g/kg.

Gaseous Sulfuric Acid and Sulfur Dioxide Measurements in the Arctic Troposphere and Lower Stratosphere: Implications for Hydroxyl Radical Abundances

Abstract: Measurements of gaseous sulfuric acid and sulfur dioxide were made in the winter arctic troposphere and lower stratosphere, using aircraft-borne mass spectrometers. The measurements, covering altitudes between 3.5 and 11.4 km, took place on 14 and 18 February, 1987 in Northern Scandinavia. Our mean measured H2SO4-concentrations are around 1.0–3.0 · 105 cm−3 and the measured SO2-volume mixing ratios are around 50–400 pptV in the troposphere and decrease steeply above the tropopause to about 10 pptV. The combined H2SO4- and SO2-measurements offer an interesting opportunity to infer hydroxyl radical concentrations. Inferred OH-concentrations are around 0.1–1.0 · 105 cm−3, roughly consistent with model predictions.

In Situ Observations of Temperature Microstructure above and below the Tropopause

Abstract: The small-scale (less than 1 km) structure of temperature near the tropopause, as indicated by ozone concentration, is examined using data recorded during GALE from the NCAR Sabreliner. A systematic difference is observed in the temperature microstructure of the troposphere and stratosphere in qualitative agreement with evidence from VHF radar observations. Spectral distribution of the temperature variations also differs across the tropopause.

Measurements of unusual aerosol layers in the upper troposphere over Laramie, Wyoming in the spring of 1991: Evidence for long range transport from the oil fires in Kuwait

Abstract: Unusual aerosol layers in the upper troposphere were observed on seven consecutive balloon soundings between 7 March and 17 June 1991, at Laramie, Wyoming (41°N, 106°W). The layers varied in thickness from 1 to 4 km and the top of the layer was always at or near the tropopause. Aerosol concentrations for the size range 0.15 to 1.0 μm radius were increased by factors of 5 to 10 over what is usual at those altitudes. Compared with the previous four year record, these seven soundings represent the highest sustained concentrations observed between 8 and 11 km, with the sounding on March 25 representing the highest concentrations observed. Although the origin of these unusual aerosol layers is not definite, trajectory calculations and impactor measurements indicate that, in the absence of an alternate explanation, the oil fires in Kuwait are the likely source.

A Preliminary Report on Radiosonde Observations of the Equatorial Atmosphere Dynamics over Indonesia

Abstract: Observations of the equatorial atmosphere dynamics by means of radiosondes were conducted, as a collaborative project between Japan and Indonesia, at the Watukosek observatory in East Jawa, Indonesia, for 25 days from 27 February to 22 March, 1990. Profiles of the wind velocity, temperature, pressure and humidity were obtained at 0-35km altitude with time and height resolutions of 5-7 hours and 150m, respectively. This paper gives an outline of the experiment and the background conditions of the atmosphere during the campaign period. The structure of the zonal mean winds at 10-25km was basically explained by QBO, showing westward winds of about 15m/s, while the meridional winds were fairly weak in the entire height range. The mean temperature profile indicated the tropopause to be at 17-18km altitude, with a minimum temperature of about 190K. Both wind velocity and temperature fluctuations were large indicating the manifestation of various wave activities. The relative humidity was fairly high in the lower troposphere, showing periodic fluctuations with time scales of 3-5 days in addition to the diurnal cycle.

Satellite-based Microwave Observations of Tropopause-Level Thermal Anomalies: Qualitative Applications in Extratropical Cyclone Events

Abstract: The evolution of upper-tropospheric thermal patterns associated with extratropical cyclone events is often not well represented by the conventional observational network, especially in marine situations. In this paper, a potential tool for qualitatively analyzing tropopause-level thermal structure and variations based on remotely sensed passive microwave data from satellites is examined. Specifically, warm anomalies associated with tropopause undulations in upper-tropospheric waves are captured in imagery from the 54.96-GHz channel of the Microwave Sounding Unit (MSU) onboard the current series of NOAA polar-orbiting satellites. Examples of this imagery during selected western North Atlantic cyclone events are presented, and the potential usefulness of these observations in analysis and forecasting is discussed.

Diurnal variation in vertical motion over the central equatorial Pacific from VHF wind‐profiling Doppler radar observations at Christmas Island (2° N, 157° W)

Abstract: Wind-profiling Doppler radars measure vertical motions along a vertically-directed beam. By averaging over long periods it is possible to measure very small mean vertical motions. Long-term measurements at Christmas Island have been analyzed to determine the diurnal variation in the vertical motion field. The outstanding feature that emerges from the analysis is a pronounced diurnal cycle in the subsiding motion of the main troposphere (4–10 km) and an equally pronounced diurnal cycle in upward motion centered around the altitude of the tropopause. The possibility of radiative forcing of the observed diurnal cycle is considered in the context of the diurnal variation of atmospheric diabatic heating and cooling rates.

An Isentropic Vertical Coordinate Model: Design and Application to Atmospheric Frontogenesis Studies

Abstract: Summary The isentropic vertical coordinate model developed at UCLA is briefly reviewed. The review includes an outline of the approach used to overcome technical difficulties in handling model layers with small mass. The model's performance is demonstrated by simulating the evolution of a middle-latitude baroclinic disturbance. During the evolution of the disturbance, sharp frontal zones are generated in the upper and middle troposphere with realistic tropopause folding. The extent to which different dynamical processes contribute to frontogenesis is analyzed. While the model successfully simulates frontogenesis in the upper and middle troposphere, it has a difficulty in simulating surface fronts. The difficulty arises due to the lack of degrees of freedom in surface temperatures since an isentropic vertical coordinate model requires a large number of vertical layers to obtain a high horizontal resolution at the lower boundary. This suggests the potential of a hybrid vertical coordinate, which approaches 0 at upper levels and cr at lower levels.

An investigation into the causes of stratospheric ozone loss in the southern Australasian region

Abstract: Measurements of total ozone at Macquarie Island (55 deg S, 159 deg E) reveal statistically significant reductions of approximately twelve percent during July to September when comparing the mean levels for 1987-90 with those in the seventies. In order to investigate the possibility that these ozone changes may not be a result of dynamic variability of the stratosphere, a simple linear model of ozone was created from statistical analysis of tropopause height and isentropic transient eddy heat flux, which were assumed representative of the dominant dynamic influences. Comparison of measured and modeled ozone indicates that the recent downward trend in ozone at Macquarie Island is not related to stratospheric dynamic variability and therefore suggests another mechanism, possibly changes in photochemical destruction of ozone.

Underestimates of O3 9.6 μm heating rates as a cause of the cold bias in the low tropical stratosphere of GCMs

Abstract: General circulation models (GCMs) which include the middle atmosphere show a cold bias in the low tropical stratosphere; the zonal-mean temperature minimum is around 10 K lower than the observed climatology, and the tropopause is too high. Evidence is presented that this could arise because of inadequate energy absorption by O 3 at 9.6 μm. This is suggested by the results of modifying a GCM, which led to a decrease in the upwelling radiative flux at the tropopause. It is supported by the calculated changes in radiative heating rates when the energy input into the low stratosphere is reduced, and by the response of a fixed dynamical heating model to this flux change. It is proposed that an important topic of research is to develop accurate models of the 9.6 μm bands in the lower stratosphere

Ozone and temperature trends in the lower stratosphere

Abstract: It is shown that the pattern of temperature trend with height in the upper troposphere and lower stratosphere is consistent with that calculated from a model incorporating the observed ozone changes. The magnitude of the observed temperature decrease is less than that determined from the numerical model. It is found that the obverse ozone-temperature relationship does not hold. If the temperature at 20 km decreases, this by itself would result in an increase in the ozone values through the inverse temperature sensitivity of the chemical reactions. In the lower stratosphere a positive correlation between ozone and temperature tends to exist through the dynamics. Ozone-rich air moving downward undergoes an increase in temperature through adiabatic compression and vice-versa. That the temperature trends indicate no apparent variation with latitude suggests a nondynamic cause.