bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of wintertime stratospheric transport in the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model
Authors: Eluszkiewicz Janusz , Plumb R. Alan , Nakamura Noboru
Published: February 2004

The kinematics of air motions in and around the polar vortices in the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model are investigated by means of a Lagrangian particle analysis. Particles initialized in the mesosphere and upper stratosphere rapidly descend to the middle stratosphere. This descent is unmixed in the sense that the isentropic mass transport into the vortex is less than 5% of the vortex mass per month. Transport out of the vortex is less than 10% of the vortex mass per month. The reversible component of the Lagrangian mean downward velocity (in isentropic coordinates) is approximated to better than 20% by the diabatic heating rate at the time mean location of the center of mass. The interplay between diabatic descent and horizontal mixing in causing the steepness of tracer contours (e.g., N2O) around the vortex edge is vividly illustrated in our trajectory experiments. These experiments also illustrate the limitations of the concept of “air parcel” within the midlatitude surf zone. The dynamics of polar descent are investigated by diagnosing the forcing of the residual circulation. In the upper stratosphere, diabatic descent inside polar vortices is driven by Eliassen‐Palm flux divergences associated with motions of period shorter than 2 days (presumably gravity waves) in the Antarctic, but of period greater than 3 days (presumably planetary waves) in the Arctic. In the lower stratosphere, long period processes, mainly from the 10 to 1 hPa region, produce descent near the vortex edge in both hemispheres.
On the polar stratospheric cloud formation potential of the northern stratosphere
Authors: Pawson Steven , Naujokat Barbara , Labitzke Karin
Published: February 2004

A 29‐year daily temperature record at 30 hPa in the northern hemisphere stratosphere has been used to estimate the potential of the atmosphere to support polar stratospheric cloud (PSC) formation. Under certain caveats, type 1 PSC formation is assumed to occur at temperatures of 192 K (195 K) or lower at 30 hPa (50 hPa). At 30 hPa in January at least one point in the northern hemisphere was colder than 192 K on 34% of the days in the record. The PSC “season” begins in late November and ends in middle March; the maximum likelihood is attained in January. The PSC formation potential is defined as the fractional area of the northern hemisphere cold enough (on the synoptic scale) to support PSC formation. In the mean it exceeds 1% between late December and early February at 30 hPa and is generally larger at 50 hPa. There is considerable interannual variability connected with the synoptic evolution of the stratospheric winter, so that up to 4% of the northern hemisphere is cold enough for PSC formation for shorter periods in individual years. Temperatures low enough for H2O ice cloud formation can occur in January and February; these are generally associated with adiabatic ascent in amplifying planetary waves. The location of the cold areas relative to the vortex center is examined and a measure introduced to quantify this.
Polar stratospheric clouds and volcanic aerosol during spring 1992 over McMurdo Station, Antarctica: Lidar and particle counter comparisons
Authors: Adriani A. , Deshler T. , Donfrancesco G. Di , Gobbi G. P.
Published: February 2004

Coordinated observations with lidar and balloon‐borne particle counters were used to characterize polar stratospheric clouds and to estimate a particle index of refraction. The index of refraction was estimated from comparisons of calculated and measured scattering ratios at a wavelength of 532 nm. The clouds, measured from McMurdo Station, Antarctica (78°S), were observed above 11 km at temperatures below 198 K and were divided into three classes based on their scattering properties and particle size. Clouds with a low scattering ratio, high depolarization, and significant fraction of particles with radii of >2.0 μm had a mean index of refraction of 1.42±0.04 and a mode of 1.43. Clouds with a moderate scattering ratio, low depolarization, and fewer particles of >2.0 μm, had a mean index of refraction of 1.39±0.03 and a mode of 1.37. Ice clouds, apparent from measurements of high scattering ratio, high depolarization, and high concentrations of particles of >1.0 μm, had a mean index of refraction of 1.32±0.02 and a mode of 1.31. Measurements in volcanic aerosol indicated a mean index of 1.43±0.04.
Validation of nitric oxide and nitrogen dioxide measurements made by the Halogen Occultation Experiment for UARS platform
Authors: Gordley L. L. , Russell J. M. , Mickley L. J. , Frederick J. E. , Park J. H. , Stone K. A. , Beaver G. M. , McInerney J. M. , Deaver L. E. , Toon G. C. , Murcray F. J. , Blatherwick R. D. , Gunson M. R. , Abbatt J. P. D. , Mauldin R. L. , Mount G. H. , Sen B. , Blavier J.‐F.
Published: February 2004

The Halogen Occultation Experiment (HALOE) experiment on Upper Atmosphere Research Satellite (UARS) performs solar occultation (sunrise and sunset) measurements to infer the composition and structure of the stratosphere and mesosphere. Two of the HALOE channels, centered at 5.26 μm and 6.25 μm, are designed to infer concentrations of nitric oxide and nitrogen dioxide respectively. The NO measurements extend from the lower stratosphere up to 130 km, while the NO2 results typically range from the lower stratosphere to 50 km and higher near the winter terminator. Comparison with results from various instruments are presented, including satellite‐, balloon‐, and ground‐based measurements. Both NO and NO2 can show large percentage errors in the presence of heavy aerosol concentrations, confined to below 25 km and before 1993. The NO2 measurements show mean differences with correlative measurements of about 10 to 15% over the middle stratosphere. The NO2 precision is about 7.5 × 10−13 atm, degrading to 2 × 10−12 atm in the lower stratosphere. The NO differences are similar in the middle stratosphere but sometimes show a low bias (as much as 35%) between 30 and 60 km with some correlative measurements. NO precision when expressed in units of density is nearly constant at 1 × 10−12 atmospheres, or approximately 0.1 ppbv at 10.0 mb or 1.0 ppbv at 1.0 mb, and so forth when expressed in mixing ratio. Above 65 km, agreement in the mean with Atmospheric Trace Molecule Spectroscopy (ATMOS) NO results is very good, typically ±15%. Model comparisons are also presented, showing good agreement with both expected morphology and diurnal behavior for both NO2 and NO.
Laboratory studies of the formation of polar stratospheric clouds: Nitric acid condensation on thin sulfuric acid films
Authors: Iraci Laura T. , Middlebrook Ann M. , Tolbert Margaret A.
Published: February 2004

Thin sulfuric acid films were exposed to 5 × 10−8 − 8 × 10−7 torr HNO3 and 2 − 3 × 10−4 torr H2O and cooled to temperatures near the ice frost point. Fourier transform infrared (FTIR) spectroscopy was used to probe the condensed‐phase species during isothermal experiments, and gas pressures were monitored with mass spectrometry. Supercooled liquid sulfuric acid films exposed to HNO3 (6 ≤ SNAT ≤ 114) showed indications of HNO3 uptake to form ternary solutions of approximately 4 wt % HNO3, 38 wt % H2SO4, and 59 wt % H2O, followed by crystallization of nitric acid trihydrate (NAT). NAT crystallization did not initiate significant crystallization of the supercooled H2SO4, but the H2SO4 often crystallized to sulfuric acid tetrahydrate (SAT) upon warming. In contrast, when crystalline SAT films were exposed to HNO3 and water, NAT did not condense within several hours, even at HNO3 saturation ratios of 30 or higher. Calculations of the contact parameter from experimental data indicate that m <0.76 for NAT on SAT. Our film studies suggest that crystalline polar stratospheric cloud (PSC) growth is most easily accomplished when stratospheric sulfate aerosols (SSAs) remain liquid, absorb HNO3, and produce crystalline nitric acid trihydrate via heterogeneous nucleation. If SSAs crystallize to SAT at some point during the winter, nitric acid condensation is hindered, and PSC formation could become more difficult.
Walker circulation and tropical upper tropospheric water vapor
Authors: Newell Reginald E. , Zhu Yong , Browell Edward V. , Read William G. , Waters Joe W.
Published: February 2004

Maps of the boundary layer divergent wind component derived from velocity potential for two 35‐day periods in 1991 and 1994 exhibit a pattern of convergence in the lower levels in the tropical western Pacific, following the Sun between hemispheres, and divergence in the eastern Pacific, these two features agreeing with schematics of the Walker circulation. Vertical motion maps show rising motion in the western Pacific and sinking motion in the eastern Pacific; water vapor maps derived from the microwave limb sounder on the Upper Atmosphere Research Satellite show a similar picture with moist plumes evident at 215 hPa in rising regions and dryness in sinking regions. While these three indicators of vertical motion are internally consistent and fit with accepted views of the Walker circulation, it is shown that there is not equally clear evidence that the eastern and western regions are directly linked by horizontal motions, particularly in the upper troposphere. Mass flux studies based on the divergent winds show that the southeastern Pacific anticyclonic region is fed by upper tropospheric inflow from all sides; in October 1991 the inflow from the west, east, south, and north contributed almost equally, while in February 1994 the inflow from the west was the largest contribution, but flow across the eastern and southern boundaries was also important. The water vapor maps suggest that a substantial fraction of air in the upper troposphere to the wrest of South America originates over the continent. Lidar measurements of cirrus show good evidence of diverging flow into the northern hemisphere from the rising motion in the west Pacific near 10°S during February 1994. The situation in the southeastern Pacific cell in October 1991 is somewhat analogous to that in the northwestern Pacific cell in February 1994 in that flow from a nearby landmass is entrained into the circulation. From daily water vapor maps coupled with potential voracity maps and cross sections, it is suggested that direct exchange between the stratosphere and the troposphere in the form of almost vertical plumes may occur in subsiding regions in the tropics.
Radiative forcing of climate by hydrochlorofluorocarbons and hydrofluorocarbons
Authors: Pinnock Simon , Hurley Michael D. , Shine Keith P. , Wallington Timothy J. , Smyth Timothy J.
Published: February 2004

We measure infrared absorption spectra of 18 hydrochlorofluorocarbons and hydrofluorocarbons, seven of which do not yet appear in the literature. The spectra are used in a narrowband model of the terrestrial infrared radiation to calculate radiative forcing and global warming potentials. We investigate the sensitivity of the radiative forcing to the absorption spectrum temperature dependence, halocarbon vertical profile, stratospheric adjustment, cloudiness, spectral overlap, and latitude, and we make some recommendations for the reporting of radiative forcings that would help to resolve discrepancies between assessments. We investigate simple methods of estimating instantaneous radiative forcing directly from a molecule's absorption spectrum and we present a new method that agrees to within 0.3% with our narrowband model results.
Atmospheric impact of NOx emissions by subsonic aircraft: A three‐dimensional model study
Authors: Brasseur Guy P. , Müller Jean‐François , Granier Claire
Published: February 2004

Three‐dimensional model calculations suggest that the world's fleet of subsonic aircraft has enhanced the abundance of nitrogen oxides in the upper troposphere by up to 20–35% and has produced a significant increase in the ozone concentration in this region of the atmosphere (4% in summer and 1% in winter). In year 2050, on the basis of current scenarios for growth in aviation, the concentration of NOx at 10 km could increase by 30–60% at midlatitudes, and the concentration of ozone could be enhanced by 7% and 2% in summer and winter, respectively (relative to a situation without aircraft effects). The perturbation is not limited to the flight corridors but affects the entire northern hemisphere. The magnitude (and even the sign) of the ozone change depends on the level of background atmospheric NOx and hence on NOx sources (lightning, intrusion from the stratosphere, and convective transport from the polluted boundary layer) and sinks which are poorly quantified in this region of the atmosphere. On the basis of our model estimates, 20% of the NOx found at 10 km (midlatitudes) is produced by aircraft engines, 25% originates from the surface (combustion and soils), and approximately 50% is produced by lightning. For a lightning source enhanced in the model by a factor of 2, the increase in NOx and ozone at 10 km due to aircraft emissions, is reduced by a factor of 2. The magnitude of aircraft perturbations in NOx is considerably smaller than the uncertainties in other NOx sources.
Correlated high‐speed video and radio interferometric observations of a cloud‐to‐ground lightning flash
Authors: Mazur Vladislav , Krehbiel Paul R. , Shao Xuan‐Min
Published: February 2004

A six‐stroke cloud‐to‐ground lightning flash has been studied using observations from a high‐speed video camera (1000 frames s−1) and a VHF radio interferometer (1‐μs time resolution), as well as additional electric, magnetic, and optical measurements. The flash produced strokes along two channels to ground and a long (550 ms) continuing current. The video observations provided time‐resolved pictures of stepped and dart leaders, short and long continuing currents, and M components during the continuing currents, and complemented and confirmed the interferometer observations of flash structure and development. The M components were initiated by fast negative streamers inside the cloud which propagated into the conducting channel of the continuing current and subsequently brightened the channel to ground. We call this sequence an M event. Dart leaders and the fast streamers of M and K events were found to be significantly brighter than stepped leaders and continuing currents to ground. A number of streamers did not initiate M events that were identical to in‐cloud K streamers. Analysis of the M and K event occurrences indicated that the conducting channels of the continuing current both expanded and contracted with time. An M‐type event was also observed during a dart leader. It is proposed that the channel multiplicity within the flash resulted from cutoff of the channel to ground while charge continued to flow down the channel from the stroke source region, stranding negative charge along the channel. Dart‐stepped leaders (such as occurred during the third stroke) are similarly explained. Because the stranded charge is observed to be greatest for initial strokes, new channels to ground of stepped and dart‐stepped leaders are expected to follow initial strokes, as is usually observed. The video and electric field observations indicate that all return strokes have at least a short continuing current, of the order of one or a few milliseconds. The results also reinforce the well‐known observation that long continuing currents tend to follow relatively weak return strokes.
Calculation of exhaust plume structure and emissions of the ER 2 aircraft in the stratosphere
Authors: Anderson M. R. , Miake‐Lye R. C. , Brown R. C. , Kolb C. E.
Published: February 2004

Calculations are presented for the NASA ER 2 aircraft at high altitude to simulate measurements taken of its own emissions during a wake‐crossing event. Results are presented for a Mach = 0.71 case in the lower stratosphere with an engine NOx emission index of 4.6 corresponding to the measured value. A series of codes was used in the analysis to calculate the flow field and chemical kinetics, from the engine combustor out to a distance of about 20.2 km (97 s). Initial plume properties were calculated with a two‐dimensional computational fluid dynamics (CFD) code with finite rate chemistry. The results of the plume code initialized a three‐dimensional parabolized Navier‐Stokes (PNS) reacting flow solution, where the plume dynamics interacting with the aircraft wake were calculated out to the region of plume breakup. Results show that the early shape and mixing rate of the engine exhaust plume are dominated by the presence of the aircraft vortex wake. Model results for NOY emissions compare well to in situ measurements taken in the field. Calculated exhaust species evolutions predict several species ratios in good agreement with field data. The mixing rate of the engine plume was also predicted to be consistent with dilution measured in the field.
Vertical sampling and analysis of nonmethane hydrocarbons for ozone control in urban North Carolina
Authors: Lawrimore Jay H. , Das Mita , Aneja Viney P.
Published: February 2004

As part of an effort by the state of North Carolina to develop a State Implementation Plan for ozone control in the Raleigh Metropolitan Statistical Area (MSA), vertical measurements of C2‐C10 hydrocarbons were made, in and above the surface inversion layer (SIL), as inputs to the urban airshed model (UAM). Three‐hour integrated ambient air samples were collected during August 1993 from 0500–0800 eastern daylight time (EDT). Additional samples were collected from 1200–1500 and 1700–2000 EDT on selected days. Vertical sampling was achieved from a 610‐m television tower located approximately 15 km southeast of the downtown area. Boundary layer wind and temperature profiles were determined by balloon soundings. For some compounds, e.g., propane, the average concentration was lower above the inversion layer (9.59 ppbC in the SIL and 2.08 ppbC above the SIL); however, other species such as 2‐methylpentane had higher concentrations above the inversion layer (1.28 ppbC in and 1.40 ppbC above the SIL). In addition, the vertical distributions of hydrocarbons within the convective boundary layer were compared to the vertical distribution estimated from calculations based on surface concentration, species reactivity, and eddy diffusivity. Compounds such as isoprene and N‐butane decreased as predicted by the equation, while others such as propane and benzene showed unexpected profiles. Calculations of propylene‐equivalent concentrations were used to estimate the effect of reactivity on the relative importance of individual hydrocarbons. The contribution of isoprene to the local hydrocarbon budget was analyzed and surface measurements were compared with data collected in Atlanta, Georgia, during the 1992 Southern Oxidants Study. Isoprene comprised more than 70% of the total propylene‐equivalent concentration in the afternoon in Raleigh but only 40% of the total in Atlanta.
Impact of soil water property parameterization on atmospheric boundary layer simulation
Authors: Cuenca Richard H. , Ek Michael , Mahrt Larry
Published: February 2004

Both the form of functional relationships applied for soil water properties and the natural field‐scale variability of such properties can significantly impact simulation of the soil‐plant‐atmosphere system on a diurnal timescale. Various input parameters for soil water properties including effective saturation, residual water content, anerobiosis point, field capacity, and permanent wilting point are incorporated into functions describing soil water retention, hydraulic conductivity, diffusivity, sorptivity, and the plant sink function. The perception of the meaning of these values and their variation within a natural environment often differs from the perspective of the soil physicist, plant physiologist, and atmospheric scientist. This article investigates the sensitivity of energy balance and boundary layer simulation to different soil water property functions using the Oregon State University coupled atmosphere‐plant‐soil (CAPS) simulation model under bare soil conditions. The soil parameterizations tested in the CAPS model include those of Clapp and Hornberger [1978], van Genuchten [1980], and Cosby et al. [1984] using initial atmospheric conditions from June 16, 1986 in Hydrologic Atmospheric Pilot Experiment‐Modélisation du Bilan Hydrique (HAPEX‐MOBILHY). For the bare soil case these results demonstrate unexpected model sensitivity to soil water property parameterization in partitioning all components of the diurnal energy balance and corresponding boundary layer development.
Sensitivity of a general circulation model to global changes in leaf area index
Authors: Chase Thomas N. , Pielke Roger A. , Kittel Timothy G. F. , Nemani Ramakrishna , Running Steven W.
Published: February 2004

Methods have recently become available for estimating the amount of leaf area at the surface of the Earth using satellite data. Also available are modeled estimates of what global leaf area patterns would look like should the vegetation be in equilibrium with current local climatic and soil conditions. The differences between the actual vegetation distribution and the potential vegetation distribution may reflect the impact of human activity on the Earth's surface. To examine model sensitivity to changes in leaf area index (LAI), global distributions of maximum LAI were used as surface boundary conditions in the National Center for Atmospheric Research community climate model (NCAR CCM2) coupled with the biosphere atmosphere transfer scheme (BATS). Results from 10‐year ensemble averages for the months of January and July indicate that the largest effects of the decreased LAI in the actual LAI simulation occur in the northern hemisphere winter at high latitudes despite the fact that direct LAI forcing is negligible in these regions at this time of year. This is possibly a result of LAI forcing in the tropics which has long‐ranging effects in the winter of both hemispheres. An assessment of the Asian monsoon region for the month of July shows decreased latent heat flux from the surface, increased surface temperature, and decreased precipitation with the actual LAI distribution. While the statistical significance of the results has not been unambiguously established in these simulations, we suspect that an effect on modeled general circulation dynamics has occurred due to changes of maximum LAI suggesting that further attention needs to be paid to the accurate designation of vegetation parameters. The incorporation of concomitant changes in albedo, vegetation fractional coverage, and roughness length is suggested for further research.
The 13C, 14C, and 18O isotopic composition of CO, CH4, and CO2 in the higher southern latitudes lower stratosphere
Authors: Brenninkmeijer C. A. M. , Lowe D. C. , Manning M. R. , Sparks R. J. , van Velthoven P. F. J.
Published: February 2004

Large air samples were collected in the lower stratosphere (10–12 km) from 43° to 85°S in June 1993, using a special compressor system. For the important trace gases CO, CH4 and CO2, concentration and isotopic analyses were carried out and significant correlations were discovered. The 14CO isotope is considerably in excess of tropospheric levels with very high values from 40 to 120 14CO molecules/cm3 STP (corresponding to 12,500 percent modern carbon, at 30 ppbv), and is negatively correlated with CO. The linear relationship is used to estimate OH to be 2.9×105 cm−3. The 18O/16O ratios for CO are the lowest ever measured and reflect the inverse kinetic isotope effect in the oxidation of CO by OH. The 13C/12C ratios for CO are not much different from tropospheric values and confirm that fractionation is small but also that the in situ contribution from CH4 oxidation is minor. For CH4 a correlation between δ13C and concentration exists from which a fractionation factor for the sink reaction (k12/k13) of about 1.012 is calculated, well in excess of results from laboratory experiments for OH +CH4. The most plausible explanation presently is the removal of approximately 9% of CH4 by Cl atoms, which, as laboratory experiments have just confirmed, induces a very large fractionation. We also reveal a linear correlation between 14CO and 14CO2, precursor and product. Finally, an analysis of potential vorticity shows a structure that seems to give an overall agreement with the trace gas variations.
The leakage problem of orthonormal wavelet transforms when applied to atmospheric turbulence
Authors: Qiu Jie , Paw U Kyaw Tha , Shaw Roger H.
Published: February 2004

Orthonormal wavelet transforms are becoming common in the study of turbulence phenomena. Although they are powerful tools in representing a signal, their use as tools to study the characteristics of turbulent structures can create appreciable errors in interpretation. It is shown here that although the orthonormal wavelet transform is computationally economical by taking advantage of multiresolution analysis, it has insufficient resolution in both scale and location to resolve detailed information of turbulence structures. Lacking in resolution, the energy at a particular frequency (or wavelength) may leak into neighboring frequencies and may pass down to smaller scales to produce an artificial “cascade” of energy (with a slope close to −2/3). The choice of wavelet basis function is important to the wavelet spectrum, especially in the study of turbulence flows dominated by coherent structures, since the method most accurately senses energy contained in pulses that have a similar pattern to the wavelet function. To use the method as a filter can be problematic owing to the low resolution of the orthonormal wavelet transform; nonorthonormal wavelet analysis should be employed when high resolution is important. When orthonormal wavelet transforms have to be used for signal analysis, segmented averaging should be employed.
Aerosol optical properties retrieved from solar aureole measurements over southern Sardinia
Authors: Dalu Giuseppe , Rao Ruizhong , Pompei Alberto , Boi Paolo , Tonna Glauco , Olivieri Bruno
Published: February 2004

Combined experiments on the aerosol properties were carried out with a solar aureolemeter, a Sun photometer, three aerosol cascade impactors, and the satellite advanced very high resolution radiometer data, in Cagliari and Carloforte, southern Sardinia, in May and June 1994. The experimental data of the aureolemeter, which operates in the wavelength range from the UV to the near‐IR, are analyzed in this paper. The equivalent refractive index of the aerosol particles and the equivalent ground albedo are derived with the inversion library method. With the equivalent refractive index and the equivalent ground albedo, the optical properties of the aerosols, such as the size distribution, the optical thickness, and the single‐scattering phase function are retrieved by an updated code, which employs a radiative intensity transfer algorithm and a nonlinear iterative inversion procedure. The columnar aerosols detected in this region have a trimodal size distribution with the volume mode radii located at about 0.15, 0.5, and 2.0 μm they present the typical maritime aerosol characteristics. The behavior of the aerosol optical thickness is described through its variations in 2 entire days. From this experiment we found that for the measurement of the diffuse solar radiation, three operational wavelengths in the UV, the visible, and the near‐IR are sufficient to obtain satisfactory information on the aerosol optical properties in the spectral range covered by these wavelengths.
Lagrangian evolution of an aerosol column during the Atlantic Stratocumulus Transition Experiment
Authors: Clarke Antony D. , Uehara Tomoe , Porter John N.
Published: February 2004

Two Lagrangian experiments were carried out during the Atlantic Stratocumulus Transition Experiment (ASTEX) with the intent of looking at aerosol evolution in the marine boundary layer (MBL). The second Lagrangian (L2) took place below broken stratus clouds and was more successful since little precipitation reached the surface. Aerosol below the inversion was primarily an aged pollution aerosol from central Europe. Vertical variability in aerosol concentrations was generally characterized by highest concentrations in the moist surface and transition layers. Concentrations were a factor of 2 or more lower in the dry subcloud layer and cloud layer and dropped to less than one tenth of surface values in the free troposphere above cloud. This behavior reflected the decoupled boundary layer below the main inversion and complicated the assessment of Lagrangian aerosol evolution. No evidence for new particle formation was observed during L2, and aerosol evolution proceeded only by mixing, coagulation, and removal mechanisms. An entrainment rate of about 0.6 cm s−1 from the free troposphere into the MBL was a key parameter affecting aerosol evolution and resulted in about a 35% column dilution during the 34‐hour L2 measurement period. Aerosol evolution in the decoupled subcloud and surface marine layer is consistent with an entrainment rate of about 0.45 cm s−1 into the surface layer and about 0.25 cm s−1 out of the layer. The ability to resolve the effects of separate processes influencing both gas and aerosol during Lagrangian evolution will depend upon (1) an adequate assessment of the variability in the air mass, (2) the ability to characterize this variability relative to the uncertainties in resampling the air mass, and (3) the extent to which the substantial changes due to entrainment alone can be reliably determined.
Vertical fine‐scale atmospheric structure measured from NASA DC‐8 during PEM‐West A
Authors: Newell R. E. , Wu Z.‐X. , Zhu Y. , Hu W. , Browell E. V. , Gregory G. L. , Sachse G. W. , Collins J. E. , Kelly K. K. , Liu S. C.
Published: February 2004

Measurements of a suite of atmospheric trace constituents made from the NASA DC‐8 aircraft, while it was making vertical profiles during the Pacific Exploratory Mission A (PEM‐West A) to the western Pacific in September–October 1991 have revealed a layered structure in much of the region. Ozone, water vapor, carbon monoxide, and methane were available continuously and are the primary constituents used to define the layers; nonmethane hydrocarbons, carbon dioxide, hydrogen peroxide and methylhydroperoxide were available less frequently but were also used. From 105 vertical profiles, sampled at vertical speeds of about 6 m s−1, over 500 layers were identified using O3and H2O; their mean thickness was about 400 m. When these layers could be identified from lidar along‐track cross sections of O3made from the aircraft, they were found to extend over distances of 100–200 km or more. Combinations of several constituent deviations from vertical running means were used to identify possible layer sources; examples are high O3, low H2O, CO, and CH4, corresponding to the extrusion of stratospheric air into the troposphere; low O3and high H2O, originating from convection from the tropical boundary layer; high O3, CO and CH4, low H2O, coming from convection from the continental boundary layer and transport in the upper troposphere, sometimes from distant sources. The layers show no clear relationship with wind velocity profiles measured simultaneously on the aircraft; there is some relationship with temperature and potential temperature, with some layers which show a stratospheric signature lying on isentropes that are in the lower stratosphere at higher latitudes. Potential vorticity (PV) is used as an additional tracer to check the conclusions about the different types of layers. While PV is often used as an indicator of stratospheric air, its use as an indicator of convection is also illustrated here.
Potential climatic impacts of vegetation change: A regional modeling study
Authors: Copeland Jeffrey H. , Pielke Roger A. , Kittel Timothy G. F.
Published: February 2004

The human species has been modifying the landscape long before the development of modern agrarian techniques. Much of the land area of the conterminous United States is currently used for agricultural production. In certain regions this change in vegetative cover from its natural state may have led to local climatic change. A regional climate version of the Colorado State University Regional Atmospheric Modeling System was used to assess the impact of a natural versus current vegetation distribution on the weather and climate of July 1989. The results indicate that coherent regions of substantial changes, of both positive and negative sign, in screen height temperature, humidity, wind speed, and precipitation are a possible consequence of land use change throughout the United States. The simulated changes in the screen height quantities were closely related to changes in the vegetation parameters of albedo, roughness length, leaf area index, and fractional coverage.
Validation of Halogen Occultation Experiment CH4 measurements from the UARS
Authors: Park J. H. , Russell J. M. , Gordley L. L. , Drayson S. R. , Benner D. C. , McInerney J. M. , Gunson M. R. , Toon G. C. , Sen B. , Blavier J‐F. , Webster C. R. , Zipf E. C. , Erdman P. , Schmidt U. , Schiller C.
Published: February 2004

Global distributions of CH4 in the mesosphere and stratosphere have been measured continuously since October 11, 1991, by the Halogen Occultation Experiment (HALOE) onboard the UARS. CH4 mixing ratio is obtained using the gas filter correlation technique operating in the 3.3‐μm region. Since measurements are made during solar occultation in the 57° inclination orbit, data are collected 15 times daily for both sunrises and sunsets. This provides coverage of one hemisphere in a month period. One complete hemispheric sweep (from equator to ∼80° latitude) is made during the spring and summer seasons of two hemispheres, and a partial sweep (from equator to around 50° latitude) is made during the fall and winter seasons of two hemispheres. HALOE CH4 measurements are validated using direct comparisons with correlative data and internal consistency checks using other HALOE‐measured tracers, HF, and aerosols. It is estimated for the 0.3‐ to 50‐mbar region that the total error, including systematic and random components, is less than 15% and that the precision is better than 7%. The CH4 gas filter channel does not depend significantly on the Pinatubo aerosol extinction. An experimentally accurate measurement of CH4 is very important because CH4 is a primary interfering gas in the HALOE HCl channel and, subsequently, can cause HCl measurement error. Simultaneous measurements of CH4 and other HALOE species (O3, H2O, NO, NO2, HCl, HF, and aerosol extinction coefficients) provide important information on atmospheric dynamic and chemical processes, since CH4 can be used as a tracer and an indicator of atmospheric transport processes. Several new pieces of information on previously unreported HALOE‐observed features are also presented.
Impact of in situ deforestation in Amazonia on the regional climate: General circulation model simulation study
Authors: Sud Y. C. , Yang R. , Walker G. K.
Published: February 2004

Two sets of 5‐year long integrations were performed with the GLA GCM with a simple biosphere model (SiB) to delineate the influence of Amazonian deforestation on the local as well as basin wide circulation and rainfall. The control case invoked a normally forested Amazonia, while the deforestation case distinguished the partially deforested Amazonia (until the year 1988) as savanna. Except for this one difference, all other initial and the prescribed boundary conditions were identical in both integrations. A differential analysis of these two simulations revealed the following. (1) There was a large decrease in evapotranspiration (1.15 mm d−1, roughly 33%) in the deforested region, while the basinwide decrease was only 0.24 mm d−1 (roughly 8%). (2) The associated decrease of precipitation in the deforested (basinwide) regions was about 0.30 (0.168) mm d−1. A precipitation decrease that is smaller than evapotranspiration decrease reflects a mitigating influence of increased moisture convergence, particularly within the boundary layer that gets warmed by the increased surface fluxes of sensible heat and longwave radiation. This implies that precipitation decrease in response to the 1988‐level of Amazonian deforestation could be reversed, i.e., reforestation is likely to emerge if the anthropogenic pressures were eliminated and deforestation were forestalled. (3) The changes in the surface wind and rainfall patterns together with increased runoff and surface temperature indicate potential hydrological as well as ecological consequences of deforestation. In our simulations, only small regions of the Amazon basin produced precipitation and circulation changes at 90% or better statistical significance level. This is a consequence of a more realistic deforestation in Amazonia as compared to the total deforestation scenario used in previous studies.
Heterogeneous atmospheric bromine chemistry
Authors: Lary D. J. , Chipperfield M. P. , Toumi R. , Lenton T.
Published: February 2004

This paper considers the effect of heterogeneous bromine reactions on stratospheric photochemistry. We have considered reactions on both sulfate aerosols and on polar stratospheric clouds (PSCs). It is shown that the hydrolysis of BrONO2 on sulfate aerosols enhances the HOBr concentration, which in turn enhances the OH and HO2 concentrations, thereby reducing the HCl lifetime and concentration. The hydrolysis of BrONO2 leads to a nighttime production of HOBr, making HOBr a major nighttime bromine reservoir. The photolysis of HOBr gives a rapid increase in the OH and HO2 concentration at dawn, as was recently observed by Salawitch et al. [1994]. The increase in the OH and HO2 concentration, and the decrease in the HCl concentration, leads to additional ozone depletion at all latitudes and for all season. At temperatures below 210 K the bulk phase reaction of HOBr with HCl in sulfate aerosols becomes important. The most important heterogeneous bromine reactions on polar stratospheric clouds are the mixed halogen reactions of HCl with HOBr and BrONO2 and of HBr with HOCl and ClONO2.
Radiative smoothing in fractal clouds
Authors: Marshak Alexander , Davis Anthony , Wiscombe Warren , Cahalan Robert
Published: February 2004

Spectral and structure function analyses are used to study the smoothness properties of the radiation fields for stratiform clouds whose horizontally fluctuating extinction fields are modeled with multiplicative cascades. Models of this type are “scale invariant,” meaning that their two‐point statistics obey power laws in the scale parameter. The independent pixel approximation (IPA) treats each pixel as a plane‐parallel layer and yields scale‐invariant albedo and radiance fields with the same exponents as the associated optical depth field. This is not the case with exact Monte Carlo (MC) results for which we confirm the existence of a characteristic “radiative smoothing” scale η. For scales larger than η, IPA and MC reflectance fields fluctuate together, and the IPA can be invoked to infer optical depths from measured radiances. We use a multifractal characterization of structure functions to assess the performance of such retrievals. For scales smaller than η, MC fields are much smoother than their IPA counterparts, and IPA‐based retrievals of the underlying optical depth field are unreliable. The scale break location η has been found to be closely related to the characteristic size 〈ρ〉 of the “spot” of multiply scattered light excited by illumination with a narrow beam, the random variable ρ being the horizontal distance between photon entry and exit points. New analytical arguments are presented for thick homogeneous media showing that 〈ρ〉 ≈ h[(1‐g)τ]−½, given the cloud's optical (τ) and geometrical (h) thicknesses (g is the asymmetry factor); this result is shown to hold numerically for fractal cloud models too. An improved “nonlocal” IPA is defined as the convolution product of the IPA field with a gamma‐type smoothing kernel dependent on 〈ρ〉.
Direct shortwave forcing of climate by the anthropogenic sulfate aerosol: Sensitivity to particle size, composition, and relative humidity
Authors: Nemesure Seth , Wagener Richard , Schwartz Stephen E.
Published: February 2004

Recent estimates of global or hemispheric average forcing of climate by anthropogenic sulfate aerosol caused by scattering of shortwave radiation (“direct” effect) are uncertain by somewhat more than a factor of 2. The principal sources of this uncertainty are atmospheric chemistry properties (yield, residence time), and microphysical properties (scattering efficiency, upscatter fraction, and the dependence of these properties on particle size, composition, and relative humidity, (RH)). This paper examines the sensitivity of forcing to these microphysical properties to identify and improve understanding of the properties required to reduce the uncertainty in the forcing. The relations between aerosol loading and forcing developed here are suitable for comparing modeled and measured aerosol forcing at specific locations and for use in climate models, provided aerosol composition and microphysical properties are known, calculated, or assumed. Results are presented showing the dependence of scattering efficiency, upscatter fraction, and normalized forcing (W m−2/g(SO24−) m−2 or W g(SO24−)−1) on dry particle size (expressed as mole(sulfate) per particle), composition ((NH4)2SO4, NH4HSO4, H2SO4), solar zenith angle, latitude, and season. Forcing is strongly dependent on dry particle size and RH but is relatively insensitive to composition. The normalized forcing can be integrated over a known or assumed size distribution to evaluate the sulfate aerosol forcing. Global and annual average values of the normalized forcing are evaluated as a function of particle size and RH. Depending on values of these variables, normalized forcing may be less than, intermediate to, or greater than the range of previous estimates of sulfate aerosol forcing.
Submicrosecond fields radiated during the onset of first return strokes in cloud‐to‐ground lightning
Authors: Krider E. Philip , Leteinturier Christiane , Willett John C.
Published: February 2004

An experiment to measure the electric field E and dE/dt signatures that are radiated by the first return stroke in cloud‐to‐ground lightning was conducted on the eastern tip of Cape Canaveral, Florida, during the summer of 1984. At this site, there was minimal distortion in the fields due to ground wave propagation when the lightning struck within a few tens of kilometers to the east over the Atlantic Ocean. Biases that are introduced by a finite threshold in the triggered recording system were kept to a minimum by triggering this system on the output of a wideband RF receiver tuned to 5 MHz. Values of the peak dE/dt during the initial onset of 63 first strokes were found to be normally distributed with a mean and standard deviation of 39 ± 11 V m−1 μs−1 after they were normalized to a range of 100 km using an inverse distance relation. Values of the full width at half maximum (FWHM) of the initial half‐cycle of dE/dt in 61 first strokes had a mean and standard deviation of 100 ± 20 ns and were approximately Gaussian. When these results are interpreted using the simple transmission line model, after correcting for the effects of propagation over 35 km of seawater, the average value of the maximum current derivative, (dI/dt)p, and its standard deviation are inferred to be 115 ± 32 kA μs−1, with a systematic uncertainty of about 30%. The FWHM after correction for propagation is about 75 ± 15 ns. The inferred values of (dI/dt)p are significantly higher than most previous measurements of natural first strokes during direct strikes to instrumented towers but are in good agreement with direct measurements of dI/dt during subsequent return strokes in rocket‐triggered discharges in Florida.
What constitutes a valid intercomparison of satellite and in situ stratospheric H2O measurements?
Authors: Schwab James J. , Pan Rong‐Juinn , Zhang Jian
Published: February 2004

We present a comparison of in situ stratospheric water vapor measurements with selected water vapor measurements from the stratospheric aerosol and gas II satellite instrument. The criteria for a valid intercomparison typically involve both measurements falling within specified space and time windows. While such an approach is easy to implement, it may be in some cases too restrictive, or even too permissive. We examine a variety of methods for choosing appropriate data sets for comparison. One approach is to use calculated isentropic back trajectories to determine which satellite measurement or measurements are best compared to specific balloon in situ soundings. This selection of intercomparison events does improve the agreement between satellite and in situ measurements. Another approach takes advantage of the long lifetime of water in the midlatitude stratosphere and looks at narrowly constrained zonal mean profiles constructed from the satellite measurements. Both of these approaches show general agreement between the data sets, but the uncertainty in the water vapor measurements largely precludes detailed comparison and examination of observed differences.
Estimates of regional natural volatile organic compound fluxes from enclosure and ambient measurements
Authors: Guenther Alex , Zimmerman Patrick , Klinger Lee , Greenberg Jim , Ennis Chris , Davis Kenneth , Pollock Walt , Westberg Hal , Allwine Gene , Geron Chris
Published: February 2004

Natural volatile organic compound (VOC) emissions were investigated at two forested sites in the southeastern United States. A variety of VOC compounds including methanol, 2‐methyl‐3‐buten‐2‐ol, 6‐methyl‐5‐hepten‐2‐one, isoprene and 15 monoterpenes were emitted from vegetation at these sites. Diurnal variations in VOC emissions were observed and related to light and temperature. Variations in isoprene emission from individual branches are well correlated with light intensity and leaf temperature while variations in monoterpene emissions can be explained by variations in leaf temperature alone. Isoprene emission rates for individual leaves tend to be about 75% higher than branch average emission rates due to shading on the lower leaves of a branch. Average daytime mixing ratios of 13.8 and 6.6 ppbv C isoprene and 5.0 and 4.5 ppbv C monoterpenes were observed at heights between 40 m and 1 km above ground level the two sites. Isoprene and monoterpenes account for 30% to 40% of the total carbon in the ambient non‐methane VOC quantified in the mixed layer at these sites and over 90% of the VOC reactivity with OH. Ambient mixing ratios were used to estimate isoprene and monoterpene fluxes by applying box model and mixed‐layer gradient techniques. Although the two techniques estimate fluxes averaged over different spatial scales, the average fluxes calculated by the two techniques agree within a factor of two. The ambient mixing ratios were used to evaluate a biogenic VOC emission model that uses field measurements of plant species composition, remotely sensed vegetation distributions, leaf level emission potentials determined from vegetation enclosures, and light and temperature dependent emission activity factors. Emissions estimated for a temperature of 30°C and above canopy photosynthetically active radiation flux of 1000 μmol m−2 s−1 are around 4 mg C m−2 h−1 of isoprene and 0.7 mg C m−2 h−1 of monoterpenes at the ROSE site in western Alabama and 3 mg C m−2 h−1 of isoprene and 0.5 mg C m−2 h−1 of monoterpenes at the SOS‐M site in eastern Georgia. Isoprene and monoterpene emissions based on land characteristics data and emission enclosure measurements are within a factor of two of estimates based on ambient measurements in most cases. This represents reasonable agreement due to the large uncertainties associated with these models and because the observed differences are at least partially due to differences in the size and location of the source region (“flux footprint”) associated with each flux estimate.