Variations in midlatitude atmospheric structure and winds at mesopause heights during winter 1979-1980
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Taubenheim, J.↗
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Behavior of neutral wind gradients at meteor heights over midlatitude stations
The variation of wind gradients in the altitude range of 80 to 100 km, which contributes information on propagational characteristics of wave phenomena prevailing at those altitudes, was studied. Diurnal and semidiurnal components of the zonal (EW) and meridional (NS) neutral wind data collected over Atlanta using the Georgia Tech Meteor Wind Radar Facility during the period of August 1974 through March of 1978 over the height range of 80 to 100 km are analyzed in detail to obtain information on height gradients in amplitude and phase of neutral wind components over height intervals of 80 to 90 and 90 to 100 km. The details of the data analysis, major results, and conclusions are presented.
Devara, P. C. S.↗
Aircraft measurements of aerosols in the upper troposphere at midlatitudes
Aircraft measurements of aerosols were made during the years 1982 to 1984 in the upper troposphere. In some cases, the influence of the local atmospheric pollution originating from the land surface was observed. The background concentration of the Mie particle was about 0.1/cu. cm. at an altitude of 6 to 8 km. An air mass of stratospheric origin was observed over the Japan sea in the winters of 1983 and 1984. The Mie particle concentration increased and the count ratio of two size ranges was found to be a large value in 1983, the value decreased to that of a background level in the stratosphere in 1984. The volcanic eruption of Mt. El Chichon seems to be responsible for the large aerosol count ratio of the measurements of 1983.
Morita, Y.↗
Long-term variations in midlatitude Southern Hemisphere mesospheric winds
The monthly mean zonal winds and semidiurnal tides at 80 and 90 km, in January and July, at Christchurch (44 S) for the period 1978 to 1986 are presented. There are significant trends but evidence for solar control of the mean zonal wind and the semidiurnal tide is not conclusive.
Fraser, G. J.↗
Gravity wave climatology at midlatitude from Rayleigh lidar data
Atmospheric sounding of the middle atmosphere by Rayleigh scattering has been performed in France for several years, from two stations with different orographic situations: one in the Alps, the Observatoire de Haute Provence, one on the Atlantic coast at Biscarosse. The vertical profiles of density and temperature are obtained with a temporal and spatial resolution of, respectively, 15 mn and 300 m between 30 and 80 km. A statistical study of the atmospheric fluctuations due to gravity waves was performed and the main results are presented: climatology of the gravity wave activity, distribution of energy versus vertical wave number and altitude, and comparison of the observations at the two sites. Conclusions are presented on the saturation of the wave field, the filtering by the mean wind, the transfer of energy and momentum into the atmosphere.
Wilson, R.↗
A midlatitude interhemispheric model of the O(+) (2P) airglow emission at 7320 A
The results are reported of the first interhemispheric model of the 7320 A airglow covering mid- and low latitudes, at all longitudes. A comprehensive model of the ionosphere and thermosphere is used to compute volume emission rates as a function of altitude, latitude, longitude, and local time. Selected results are shown here to illustrate the computational capability. In particular, the diurnal and seasonal variability and interhemispheric coupling of conjugate photoelectrons are discussed. The model is particularly well suited for airglow studies, and provides a valuable tool for the comparison, interpretation, and planning of spectroscopic observations made from orbiting platforms for all significant thermospheric emissions.
Torr, Marsha R.↗
Remote sensing estimates of cirrus particle size for tropical and midlatitude cirrus: Hexagonal crystals and ice spheres
A large discrepancy exists in current estimates of a mean cirrus particle size appropriate for calculations of the effects of these ice clouds on solar and thermal infrared radiative fluxes. For spheres with large size parameter (x = (2 pi r / lambda) is greater than 30, where r is particle radius), and moderate absorption (n(sup i) x less than 1, where n(sup i) is imaginary index of refraction for ice), the optimal effective particle radius is given by: r(sub e) = integral of r(exp 3)n(r)dr / integral of r(exp 2)n(r)dr. For the remote sensing of cirrus particle size at wavelengths of 0.83, 1.65, and 2.21 mu m, a 50 mu m ice sphere would have a size parameter of about 200, and values of n(sup i) x of 0, 0.045, and 0.06, satisfying the above conditions. However, while r(sub e) is a well-defined parameter for spheres, this cross-section area-weighted particle radius can only be extended to non-spherical particles by defining some equivalent sphere, typically an equivalent volume or equivalent cross-section area sphere. Using equivalent volume spheres, values of r(sub e) obtained over Lake Michigan on October 28, 1986, during FIRE phase I varied from 200 mu m (King Air 2D Imaging probes) to 60 mu m (Landsat reflectances at 0.83, 1.65, and 2.2 mu m), to 25 mu m (HIS spectrometer thermal emission between 8 and 12 mu m). Three major uncertainties were identified in this comparison: small ice particles missed by the 2D-C aircraft probes, uncertain ice refractive index, and uncertainties in the single scatter albedos and scattering phase functions used in the radiative calculations. Since the first FIRE cirrus results, advances have been made in all three areas. The present paper reports on improvements in the radiative modeling of ice particles at 0.83, 1.65, and 2.21 mu m wavelengths appropriate for comparisons to Landsat Thematic Mapper data. The paper also includes new results for Landsat observations of ice clouds in the eastern and western tropical Pacific.
Wielicki, Bruce A.↗
Sources and distribution of NO(x) in the upper troposphere at northern midlatitudes
A simple quasi 2-D model is used to study the zonal distribution of NO(x). The model includes vertical transport in form of eddy diffusion and deep convection, zonal transport by a vertically uniform wind, and a simplified chemistry of NO, NO2 and HNO3. The NO(x) sources considered are surface emissions (mostly from the combustion of fossil fuel), lightning, aircraft emissions, and downward transport from the stratosphere. The model is applied to the latitude band of 40 deg N to 50 deg N during the month of June; the contributions to the zonal NO(x) distribution from the individual sources and transport processes are investigated. The model predicted NO(x) concentration in the upper troposphere is dominated by air lofted from the polluted planetary boundary layer over the large industrial areas of Eastern North America and Europe. Aircraft emissions are also important and contribute on average 30 percent. Stratospheric input is minor about 10 percent, less even than that by lightning. The model provides a clear indication of intercontinental transport of NO(x) and HNO3 in the upper troposphere. Comparison of the modelled NO profiles over the Western Atlantic with those measured during STRATOZ 3 in 1984 shows good agreement at all altitudes.
Rohrer, Franz↗
Vegetation stress as a feedback mechanism in midlatitude drought
An atmospheric general circulation model with land surface properties represented by the Simplified Simple Biosphere Model is used to investigate the effect of soil moisture and vegetation stress on drought in the mid-latitudes. An idealized land-sea distribution with simple topography is used to remove as many external sources of climate variation as possible. The land consists of a single, flat, rectangular continent covered with prairie vegetation and centered on 44 deg N of an aqua planet. A control integration of 4 years is performed, and several sets of seasonal anomaly integrations are made to test the sensitivity of seasonal climate to low initial (1 April) soil moisture and dormant vegetation like what would occur during a severe drought. It is found that the inclusion of dormant vegetation during the spring and early summer greatly reduces evapotranspiration by eliminating transpiration. This affects local climate more strongly as summer progresses. Low initial soil moisture, combined with dormant vegetation, leads to a severe drought. The reduction in precipitation is much greater in magnitude than that due to low soil moisture alone, and greater than the sum of the effects computed separately. Although the short-term drought is more severe, the dormancy of the vegetation prevents further depletion of moisture in the root zone of the soil, so soil moisture begins to rebound toward the middle of summer.
Dirmeyer, Paul A.↗
Estimating central pressures of oceanic midlatitude cyclones
A method of determining surface pressures in oceanic storm systems using Earth Remote-Sensing Satellite (ERS-1) scatterometer data is employed to determine the lowest pressure in 25 storms. This method uses the surface winds as a lower boundary condition on a planetary boundary layer model to determine gradient winds and, thereby, pressure gradients. An optimization scheme referenced to a pressure outside the storm provides a pressure field and an estimate of the low pressure. The values are compared to European Center for Medium-Range Weather Forecasts (ECMWF) analyses in each case; there is good agreement, with some expected differences.
Brown, R. A.↗
In situ observations in aircraft exhaust plumes in the lower stratosphere at midlatitudes
Instrumentation on the NASA ER-2 high-altitude aircraft has been used to observe engine exhaust from the same aircraft while operating in the lower stratosphere. Encounters with the exhaust plume occurred approximately 10 min after emission with spatial scales near 2 km and durations of up to 10 s. Measurements include total reactive nitrogen, NO(y), the component species NO and NO2, CO2, H2O, CO, N2O, condensation nuclei, and meteorological parameters. The integrated amounts of CO2 and H2O during the encounters are consistent with the stoichiometry of fuel combustion (1:1 molar). Emission indices (EI) for NO(x) (= NO + NO2), CO, and N2O are calculated using simultaneous measurements of CO2. EI values for NO(x) near 4 g/(kg fuel) are in good agreement with values scaled from limited ground-based tests of the ER-2 engine. Non-NO(x) species comprise less than about 20% of emitted reactive nitrogen, consistent with model evaluations. In addition to demonstrating the feasibility of aircraft plume detection, these results increase confidence in the projection of emissions from current and proposed supersonic aircraft fleets and hence in the assessment of potential long-term changes in the atmosphere.
Fahey, D. W.↗
Use of midlatitude soil moisture and meteorological observations to validate soil moisture simulations with biosphere and bucket models
Soil moisture observations in sites with natural vegetation were made for several decades in the former Soviet Union at hundreds of stations. In this paper, the authors use data from six of these stations from different climatic regimes, along with ancillary meteorological and actinometric data, to demonstrate a method to validate soil moisture simulations with biosphere and bucket models. Some early and current general circulation models (GCMs) use bucket models for soil hydrology calculations. More recently, the Simple Biosphere Model (SiB) was developed to incorporate the effects of vegetation on fluxes of moisture, momentum, and energy at the earth's surface into soil hydrology models. Until now, the bucket and SiB have been verified by comparison with actual soil moisture data only on a limited basis. In this study, a Simplified SiB (SSiB) soil hydrology model and a 15-cm bucket model are forced by observed meteorological and actinometric data every 3 h for 6-yr simulations at the six stations. The model calculations of soil moisture are compared to observations of soil moisture, literally 'ground truth,' snow cover, surface albedo, and net radiation, and with each other. For three of the stations, the SSiB and 15-cm bucket models produce good simulations of seasonal cycles and interannual variations of soil moisture. For the other three stations, there are large errors in the simulations by both models. Inconsistencies in specification of field capacity may be partly responsible. There is no evidence that the SSiB simulations are superior in simulating soil moisture variations. In fact, the models are quite similar since SSiB implicitly has a bucket embedded in it. One of the main differences between the models is in the treatment of runoff due to melting snow in the spring -- SSiB incorrectly puts all the snowmelt into runoff. While producing similar soil moisture simulations, the models produce very different surface latent and sensible heat fluxes, which would have large effects on GCM simulations.
Robock, Alan↗
The mesoscale forcing of a midlatitude upper-tropospheric jet streak by a simulated convective system. 2: Kinetic energy and resolution analysis
A kinetic energy (KE) analysis of the forcing of a mesoscale upper-tropospheric jet streak by organized diabatic processes within the simulated convective system (SCS) that was discussed in Part 1 is presented in this study. The relative contributions of the ageostrophic components of motion to the generation of KE of the convectively generated jet streak are compared, along with the KE generation by the rotational (nondivergent) and irrotational (divergent) mass transport. The sensitivity of the numerical simulations of SCS development to resolution is also briefly examined. Analysis within isentropic coordinates provides for an explicit determination of the influence of the diabatic processes on the generation of KE. The upper-level production of specific KE is due predominatly to the inertial advective ageostrophic component (IAD), and as such represents the primary process through which the KE of the convectively generated jet streak is realized. A secondary contribution by the inertial diabatic (IDI) term is observed. Partitioning the KE generation into its rotational and irrotational components reveals that the latter, which is directly linked to the diabatic heating within the SCS through isentropic continuity requirements, is the ultimate source of KE generation as the global area integral of generation by the rotational component vanishes. Comparison with an identical dry simulation reveals that the net generation of KE must be attributed to latent heating. Both the IAD and IDI ageostrophic components play important roles in this regard. Examination of results from simulations conducted at several resolutions supports the previous findings in that the effects of diabatic processes and ageostrophic motion on KE generation remain consistent. Resolution does impact the location and timing of SCS development, a result that has important implications in forecasting the onset of convection that develops from evolution of the large-scale flow and moisture transport. Marked differences are observed in the momentum field aloft subsequent to the life cycle of the SCS in the 1 deg, 30-level base case (MP130) simulation discussed in Part 1 versus its 2 deg counterparts in that the MP130 simulation with higher spatial resolution contains from 14% to 30% more total KE.
Wolf, Bart J.↗
Nocturnal Observations of the Semidiurnal Tide at a Midlatitude Site
Fabry-Perot interferometer observations of the mesospheric hydroxyl emission and the lower thermospheric OI (5577A) emission have been conducted from an airglow observatory at a dark field site in southeastern Michigan for the past several years. The primary functions of the observatory are to provide a database for correlative observations with the UARS satellite and to provide a synoptic measurement program for the coupling energetics and dynamics of atmospheric regions effort, An intensive operational effort between May 1993 and July 1994 has resulted in a substantial data set from which neutral winds have been determined from the bifilter acquisition sequence. A 'best fit' analysis in the least squares sense of the simultaneous measurements of the neutral winds to a 12-hour periodicity has provided amplitude and phase parameters for the semidiurnal tide as well as a measure of the mean wind. The measured tidal amplitude is greater at the higher altitude, though the seasonal behavior at both altitudes is similar with greater amplitudes during August/September and April/May. Both meridional and zonal wind components are consistent with a semidiurnal tidal description during the entire observational sequence except for the May to July 1993 period. The mean winds show annual variation in the meridional flow, being equatorward from May to October and poleward during the winter. The zonal flow is primarily eastward during the entire observational window with higher speed flows during May/June at the higher attitude and June/July at the lower altitude. A comparison with a semidiurnal tidal model indicates that the measured tidal amplitudes are a factor of 2 times greater, while the phases show similar equinoctial transitions.
Niciejewski, R. J.↗
Rocket/Radar Investigation of Lower Ionospheric Electrodynamics Associated with Intense Midlatitude Sporadic-E Layers
Sporadic layers, which appear in the region from 100 km to 120 km are thought to be formed by convergent Pedersen drifts induced by altitude gradients in the zonal neutral wind. In this altitude region NO+ and 02+ are the major ions produced by photoionization and charge exchange of atmospheric and ionospheric species. The relative composition of atmospheric ions and meteoric ions in sporadic layers is important in determining their persistence, the time scales for formation, and the electrical conductivity of the layers. This rocket investigation will include a diagnosis of the neutral wind field and the electric field distribution. Coupled with ion composition measurements we will be able to expose the relevant formation mechanisms and the electrodynamic consequences of their existence. A rocket trajectory has been chosen to provide substantial horizontal sampling of the layer properties and knowledge of the horizontal gradients in composition and density are essential to determine the polarization electric fields that may be associated with ionospheric layers. The University of Texas at Dallas (UTD) is responsible for designing, building, and operating the ion mass spectrometers included on these rockets. The following provides a summary of the UTD accomplishments in the second year of the project as well as a description of the plans for the third year's activities. The UTD mass spectrometer acronym has been coined as PRIMS for Puerto Rico Ion Mass Spectrometer.
Heelis, R. A.↗
Large Amplitude IMF Fluctuations in Corotating Interaction Regions: Ulysses at Midlatitudes
Corotating Interaction Regions (CIRs), formed by high-speed corotating streams interacting with slow speed streams, have been examined from -20 deg to -36 deg heliolatitudes. The high-speed streams emanate from a polar coronal hole that Ulysses eventually becomes fully embedded in as it travels towards the south pole. We find that the trailing portion of the CIR, from the interface surface (IF) to the reverse shock (RS), contains both large amplitude transverse fluctuations and magnitude fluctuations. Similar fluctuations have been previously noted to exist within CIRs detected in the ecliptic plane, but their existence has not been explained. The normalized magnetic field component variances within this portion of the CIR and in the trailing high-speed stream are approximately the same, indicating that the fluctuations in the CIR are compressed Alfven waves. Mirror mode structures with lower intensities are also observed in the trailing portion of the CIR, presumably generated from a local instability driven by free energy associated with compression of the high-speed solar wind plasma. The mixture of these two modes (compressed Alfven waves and mirror modes) plus other modes generated by three wave processes (wave-shock interactions) lead to a lower Alfvenicity within the trailing portion of the CfR than in the high-speed stream proper. The results presented in this paper suggest a mechanism for generation of large amplitude B(sub z) fluctuations within CIRS. Such phenomena have been noted to be responsible for the generation of moderate geomagnetic storms during the declining phase of the solar cycle.
Tsurutani, Bruce T.↗
Evidence of Convection as a Dominant Source of Condensation Nuclei in the Northern Midlatitude Upper Troposphere
We examine concurrent upper tropospheric measurements of CN (diameter greater than 4 nm). NO, and NO(Y) during the SONEX Experiment over the North Atlantic (Oct.-Nov., 1997). Elevated CN and NO(Y) concentrations observed in the upper troposphere are attributed largely to enhancements in convective outflows. We estimate that less than 7% of observed high-CN plumes (greater than 10000 /cc) may be attributed to aircraft emissions. Dilution of high-CN convective and aircraft plumes appears to be much more rapid than losses of NO(X) and CN by oxidation and coagulation, respectively, and accounts for much of observed CN concentrations. When taking into account of different time scales against dilution for observable aircraft and convective high-CN plumes (estimated to be 1:4), the contribution by aircraft emissions to CN concentrations is significant, about 20% of the convective source. We find no evidence that particle formation in convective plumes is limited by OH oxidation of SO2.
Wang, Y.↗
Impact of Aircraft Emissions on NO(x) in the Lowermost Stratosphere at Northern Midlatitudes
Airborne measurements of NO(x) total reactive nitrogen (NO(y)), O3 and condensation nuclei (CN) were made within air traffic corridors over the U.S. and North Atlantic regions (35-60 deg N) in the fall of 1997. NO(x) and NO(y) data obtained in the lowermost stratosphere (LS) were examined using the calculated increase in NO(y) ((delta)NO(y)) along five-day back trajectories as a parameter to identify possible effects of aircraft on reactive nitrogen. It is very likely that aircraft emissions had a significant impact on the NO(x) levels in the LS inasmuch as the NO(s), mixing ratios at 8.5-12 km were significantly correlated with the independent parameters of aircraft emissions, i.e., (delta)NO(y) levels and CN values. In order to estimate quantitatively the impact of aircraft emissions on NO(x), and CN, the background levels of CN and NO(x) at O3 = 100-200 ppbv were derived from the correlations of these quantities with (delta)NO(y)). On average, the aircraft emissions are estimated to have increased the NO(x) and CN values by 130 pptv and 400 STP,cc, respectively, which corresponds to 70 -/+ 30 % and 30 -/+ 20 % of the observed median values.
Kondo, Y.↗