bluuebunny/crossref_metadata_2025 huggingface dataset:
An analysis of HALOE observations in summer high latitudes using airmass trajectory and photochemical model calculations
Authors: Luo M. , Park J. H. , Lee K. M. , Russell J. M. , Bruehl C.
Published: February 2004

Stratospheric ozone, HCl, NO, and NO2 observed by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) in late summer at high latitudes show nonuniform distributions in their volume mixing ratios along a latitude circle, while the mixing ratios of the long‐lived tracers, such as CH4 and HF, are observed to be quite uniform. These anomalous distributions of the reactive gases are confined in approximately 20–35 km. It is found that in the summer high latitudes, the isosurfaces of long‐lived tracers nearly coincide with isentropes. On the basis of meridional distributions of these species, it appears that the HALOE‐observed features are the results of quasi‐isentropic transport across latitude lines with a timescale shorter than the chemical relaxation times of those HALOE observed ozone and other gases. A spectral analysis of the United Kingdom Meteorological Office meridional wind data indicates the existence of episodically amplified wave 1 that is quasi‐stationary in July and then slowly westward moving after mid‐August 1992. A quasi‐stationary wave 2 is also found to be important in late August. The isentropic trajectory calculations show that these planetary scale waves seem to pull air masses out of the polar region as far as 10°–15° latitude equatorward in a week. Midlatitude air is also pushed into the high summer latitude region. A photochemical box model is used to simulate the chemical evolution of HALOE‐observed species along air parcel trajectories which are initialized at HALOE positions. Reasonable agreement is found in comparisons between coincident HALOE measurements and the model results in later days. The characteristic structures in HALOE‐observed ozone and other gases in the low and middle stratosphere indicate that the chemical relaxation times taken for these species to relax to their new local mixing ratios under new sunlit conditions are comparable with or longer than the timescale of the meridional transport induced by waves in late summer.
Development and evaluation of the diffusion denuder‐sulfur chemiluminescence detector for atmospheric SO2 measurements
Authors: Benner Richard L. , Wu Jianguo , Nicks Dennis K.
Published: February 2004

The University of Alaska Fairbanks participated in the Gas‐Phase Sulfur Intercomparison Experiment (GASIE) to evaluate the sulfur chemiluminescence detector (SCD). The SCD is a relatively new sulfur selective detection technology (Benner and Stedman, 1989, 1990, 1994; Benner, 1991; Stedman and Benner, 1995). The system used during GASIE is referred to as the diffusion denuder/sulfur chemiluminescence detector (DD/SCD). We chose to use a diffusion denuder to separate SO2 from other sulfur species rather than a gas chromatograph because one of our long‐term goals is to develop the DD/SCD into a unique system capable of resolving low pptv levels of SO2 on timescales of 1 min or less. The principles of detection of the DD/SCD and how it was operated during GASIE are presented. Results from the GASIE program are presented in detail by Stecher et al. (this issue). In this paper, we briefly describe what was learned about the diffusion DD/SCD technique from GASIE. To address the problems revealed, the DD/SCD has been completely redesigned and rigorously tested in our laboratory. The modifications made, how these modification eliminate many of the problems found during GASIE, and results form subsequent laboratory testing are discussed.
N2O transport in a three‐dimensional model driven by U. K. Meteorological Office winds
Authors: Lou Guang Ping , Alyea Fred N. , Cunnold Derek M. , Kindler T. P.
Published: February 2004

A three‐dimensional spectral chemical transport model truncated at T21 is employed to simulate N2O transport. The wind and vertical motion fields are taken from the U.K. Meteorological Office four‐dimensional assimilation data set. UARS cryogenic limb array etalon spectrometer (CLAES) N2O measurements are used to initialize the model in late August 1992. Model results are shown to simulate the CLAES measurements quite well over the first few months: N2O variability is similar at extratropical latitudes in the southern hemisphere over the period September 2–17, 1992, at 4.6 and 10 mbar, and there is good agreement in the synoptic maps of minor warmings during this period. Prior to a large warming event on September 30, minor stratospheric warmings are shown to produce negligible changes in the vortex below 4.6 mbar, but considerable mixing of air from the vortex edge and subtropical air is indicated. This results in a steepening of the N2O gradient at the vortex edge. During a warming event when the vortex center moves away from the pole, downward transport by the residual circulation can be large. This is offset by eddy transport effects, but these terms reverse during the recovery from the warming. From September 2 to 17, there is evidence of continuous mixing at midlatitudes at 4.6 mbar in contrast to more discontinuous, warming‐associated mixing at 10 mbar. The breakup of the vortex is initiated by the September 30 warming, and a warming on October 13 has a strong influence on the breakup. The breakup propagates downward. The climatological distribution of N2O in the tropics follows the seasonal variation of the solar radiation with a maximum, which is determined by the strength of the upward residual motion, shifting towards the summer hemisphere by 10°–15° latitude. The surf zone in both the model and the observations at the middle latitudes is well defined, but the gradients of N2O at the edge of the tropics and at the edge of the vortex are smaller in the model than in the observations. This is probably being caused by excessive mixing in the model.
Reconstructing annual and seasonal climatic responses from volcanic events since A.D. 1270 as recorded in the deuterium signal from the Greenland Ice Sheet Project 2 ice core
Authors: White David E. , White James W. C. , Steig Eric J. , Barlow Lisa K.
Published: February 2004

A 200 m ice core at the GISP 2 site (Summit, Greenland) provides a 720 year record of volcanic eruptions, identified by pronounced increases in SO42− in relation to the background concentration. Deuterium/hydrogen isotope ratios (δD) measured in the same core provide a proxy climate record (temperature) used to examine the response of the climate system in Greenland to these eruptions. A group of 34 known volcanic events and a subgroup of the six largest volcanic events are isolated and examined. For both groups, significant negative (cooler) isotopic excursions occur beginning the year the volcanic event is detected in the ice core. Cooling is significant for 1 to 2 years. The maximum isotopic excursion averages 4‰ (∼0.7°C) for all 34 events and 10‰ (∼1.8°C) for the subgroup of largest events. For individual events the response in the isotope signal to volcanic events is highly variable but occurs at most 1 year before SO42− is detected in the core. Examination of subannual isotope data suggests that peak winter isotope values are lowered more than peak summer values. We hypothesize that this result is flawed by posted‐positional alteration of the isotopic profile by vapor diffusion. To test this hypothesis, we use numerical deconvolution to account for the smoothing effects of vapor diffusion in the firn. Analysis of the deconvolved data shows greater summer than winter cooling, a result more consistent with instrumental temperature data.
Turbulent eddy motion at the forest‐atmosphere interface
Authors: Katul Gabriel , Hsieh Cheng‐I , Kuhn Greg , Ellsworth David , Nie Dalin
Published: February 2004

Ejection and sweep eddy motions in the atmospheric surface layer (ASL) are widely accepted as being responsible for much of land surface evaporation, sensible heat flux, and momentum flux; however, less is known about this type of eddy motion within the canopy sublayer (CSL) of forested systems. The present study analyzed the ejection‐sweep properties at the canopy‐atmosphere interface of a 13 m tall, uniformly aged southern loblolly pine stand and a 33 m tall, unevenly aged hardwood stand using velocity and scalar (temperature, water vapor, and carbon dioxide) fluctuation measurements at the canopy‐atmosphere interface. It was found that the measured sweeps and ejections time fractions for scalars and momentum are comparable and are in good agreement with other laboratory and field experiments. This investigation demonstrates that the third‐order cumulant expansion method (CEM) reproduces the measured relative flux contribution of ejections and sweeps (ΔS0) and the difference between sweep and ejection time fractions for both momentum and scalars at the canopy‐atmosphere interface in contrast to findings from a previous ASL experiment. A linkage between ΔS0 and the scalar flux budget is derived and tested via the third‐order CEM at the canopy‐atmosphere interface for the pine and the hardwood stands. It is shown that ΔS0 can be related to the dimensionless scalar flux transport term whose gradient is central to the scalar variance budget. Also, the derived relationship is independent of canopy roughness or scalar sources and sinks. Hence this investigation establishes an analytical linkage between second‐order closure models, the ejection‐sweep cycle, and third‐order CEM at the canopy‐atmosphere interface. Dissimilarity between the ejection‐sweep cycle for scalar and momentum transport is considered via conditional probability distributions at both forest stands. In contrast to a laboratory heat dispersion experiment, it is shown that while the ejection‐sweep cycles for scalar and momentum transport are intimately linked, they are not identical. Therefore the results from momentum ejection‐sweeps investigations cannot be extrapolated to scalar transport. Comparisons with other laboratory experiments are also discussed, especially in relation to the scalar ejection and sweep time fractions.
Mass‐particle size distributions of atmospheric dust and the dry deposition of dust to the remote ocean
Authors: Arimoto R. , Ray B. J. , Lewis N. F. , Tomza U. , Duce R. A.
Published: February 2004

Size‐separated mineral aerosol samples were collected and analyzed to investigate the relationships between the mass‐particle size distributions (MSDs) of dust particles and the dust loadings in the atmosphere. The data also were used to assess the changes in the MSDs of dust in relation to transport processes and especially the associated effects on dry deposition. Atmospheric dust concentrations, as indicated by aluminum or scandium, in samples collected from three sites in the remote North Atlantic were higher than those in samples collected during a cruise in the North Pacific on board the R/V Moana Wave. However, the mass median diameters (MMDs) for the North Pacific samples were both larger on average (∼3 μm versus ∼2 μm aerodynamic equivalent diameter) and more variable than those from the North Atlantic; this difference was attributed to wet conditions and particle aggregation over the North Pacific. In addition, for the ensemble of all samples the geometric standard deviations of the mass‐particle size distributions, which are analogous to the sorting values used to characterize sedimentary materials, tended to vary inversely and nonlinearly with the mass median diameters. Model‐derived dry deposition velocities for the samples were at most weakly related to either the dust concentrations or the MMDs. However, the dry deposition velocities for two subsets of samples were correlated with the geometric standard deviations of the distributions; this is further evidence that the mass flux of dust via dry deposition can be controlled by a relatively small fraction of aerodynamically large particles.
Emissions from smoldering combustion of biomass measured by open‐path Fourier transform infrared spectroscopy
Authors: Yokelson Robert J. , Susott Ronald , Ward Darold E. , Reardon James , Griffith David W. T.
Published: February 2004

Biomass samples from a diverse range of ecosystems were burned in the Intermountain Fire Sciences Laboratory open combustion facility. Midinfrared spectra of the nascent emissions were acquired at several heights above the fires with a Fourier transform infrared spectrometer (FTIR) coupled to an open multipass cell. In this report, the results from smoldering combustion during 24 fires are presented including production of carbon dioxide, carbon monoxide, methane, ethene, ethyne, propene, formaldehyde, 2‐hydroxyethanal, methanol, phenol, acetic acid, formic acid, ammonia, hydrogen cyanide, and carbonyl sulfide. These were the dominant products observed, and many have significant influence on atmospheric chemistry at the local, regional, and global scale. Included in these results are the first optical, in situ measurements of smoke composition from fires in grasses, hardwoods, and organic soils. About one half of the detected organic emissions arose from fuel pyrolysis which produces white smoke rich in oxygenated organic compounds. These compounds deserve more attention in the assessment of fire impacts on the atmosphere. The compound 2‐hydroxyethanal is a significant component of the smoke, and it is reported here for the first time as a product of fires. Most of the observed alkane and ammonia production accompanied visible glowing combustion. NH3is normally the major nitrogen‐containing emission detected from smoldering combustion of biomass, but from some smoldering organic soils, HCN was dominant. Tar condensed on cool surfaces below the fires accounting for ∼1% of the biomass burned, but it was enriched in N by a factor of 6–7 over the parent material, and its possible role in postfire nutrient cycling should be further investigated.
Seasonal variation of turbulent energy dissipation rates at high latitudes as determined by in situ measurements of neutral density fluctuations
Authors: Lübken F.‐J.
Published: February 2004

In the last 6 years a series of 22 sounding rockets were launched in order to investigate the dynamic state of the mesosphere and lower thermosphere. Of these flights, 19 were performed at high latitudes, either from the Andøya Rocket Range (17 flights) in northern Norway (69°N) or from Esrange (2 flights) in northern Sweden (68°N). An ionization gauge mounted on board these sounding rockets measured neutral density fluctuations down to very small spatial scales of a few meters. During several flights, small‐scale density fluctuations were found in layers of a few kilometers thickness. Subsequent analysis of these fluctuations indicates that they were caused by turbulent motions. The high resolution of these measurements makes it possible to unambiguously deduce turbulent energy dissipation rates ε from the spectra of the relative density fluctuations. The ε profiles and the corresponding heating rates obtained at high latitudes show a significant and systematic seasonal variation: Whereas in winter the turbulent heating rates are comparatively small (typically 0.1 K/d and 1–2 K/d below and above ∼75 km, respectively) in the entire mesosphere and lower thermosphere, much stronger mean values of ∼10–20 K/d are observed around the summer mesopause (∼90 km). During none of the seven summer flights have we detected any noticeable turbulence in the middle and lower mesosphere. Turbulence is confined to a relatively small height region of 78–97 km during summer but covers the entire mesosphere from 60 to 100 km during winter. From our measurements we arrive at the curious conclusion that turbulent heating in the mesosphere is strongest at the coldest part of the atmosphere, namely, at the polar mesopause in summer. Our observations imply that turbulent heating is an important contribution to the energy budget of the upper mesosphere in summer, whereas it is presumably negligible in the entire mesosphere in winter. Mean turbulent velocities wturb and mean turbulent diffusion coefficients K do not exhibit such a distinct seasonal variation. In the lower and upper mesosphere, typical values for wturb are 0.3 and 1–3 m/s, respectively, and typical values for K are 4 and 100 m2/s, respectively. The seasonal variation of mean profiles of turbulent parameters as obtained by high‐resolution in situ techniques puts a serious constraint on models dealing with the energy budget of the upper atmosphere, in particular the parameterization of subgrid process in terms of mean state quantities, e.g., used in gravity wave breaking scenarios.
Hemispheric asymmetries in water vapor and inferences about transport in the lower stratosphere
Authors: Rosenlof Karen H. , Tuck Adrian F. , Kelly Kenneth K. , Russell James M. , McCormick M. Patrick
Published: February 2004

Both satellite water vapor measurements and in situ aircraft measurements indicate that the southern hemisphere lower stratosphere is drier than that of the northern hemisphere in an annual average sense. This is the result of a combination of factors. At latitudes poleward of ∼50°S, dehydration in the Antarctic polar vortex lowers water vapor mixing ratios relative to those in the north during late winter and spring. Equatorward of ∼50°S, water vapor in the lower stratosphere is largely controlled by the tropical seasonal cycle in water vapor coupled with the seasonal cycle in extratropical descent. During the tropical moist period (June, July, and August), air ascending in the Indian monsoon region influences the northern hemisphere more than the southern hemisphere, resulting in a moister northern hemisphere lower stratosphere. This tropical influence is confined to levels beneath 60 mbar at low latitudes, and beneath 90 mbar at high latitudes. During the tropical dry period (December, January, and February), dry air spreads initially into both hemispheres. However, the stronger northern hemisphere wintertime descent that exists relative to that of southern hemisphere summer transports the dry air out of the northern hemisphere lower stratosphere more quickly than in the south. This same hemispheric asymmetry in winter descent (greater descent rates during northern hemisphere winter than during southern hemisphere winter) brings down a greater quantity of “older” higher water vapor content air in the north, which also acts to moisten the northern hemisphere lower stratosphere relative to the southern hemisphere. These factors all act together to produce a drier southern hemisphere lower stratosphere as compared to that in the north. The overall picture that comes from this study in regards to transport characteristics is that the stratosphere can be divided into three regions. These are (1) the “overworld” where mass transport is controlled by nonlocal dynamical processes, (2) the “tropically controlled transition region” made up of relatively young air that has passed through (and been dehydrated by) the cold tropical tropopause, and (3) the stratospheric part of the “middleworld” or “lowermost stratosphere”, where troposphere‐stratosphere exchange can occur adiabatically. Satellite water vapor measurements indicate that the base of the “overworld” is near 60 mbar in the tropics, or near the 450 K isentropic surface.
A statistical method for testing a general circulation model with spectrally resolved satellite data
Authors: Haskins Robert D. , Goody Richard M. , Chen Luke
Published: February 2004

The motivation for this paper is to understand better the means available for testing climate models. Statistics of observed, outgoing, thermal spectra are compared with those predicted from a climate model, on the basis of data collected over a period of approximately 1 year. This is a powerful approach to testing a model with respect to processes internal to the atmosphere. These processes, which have characteristic timescales of less than a year, define the atmosphere's response to external forcing. Second‐moment statistics are particularly important for testing model variability, which is key to predicting the results of forcing the atmosphere, for example, by ocean surface temperature changes, increase of greenhouse gases, etc. Comparisons are presented between statistical data from the infrared interferometer spectrometer (IRIS), an orbiting fourier transform spectrometer, and spectra calculated using the medium‐resolution spectral code, MODTRAN, applied to the temperature and humidity profiles from a well‐known climate model. Ten months of IRIS data are available, and we have compared means, standard deviations, skew, and kurtosis of its spectrally resolved brightness temperature in three tropical regions for individual months and for a range of timescales. Also presented are comparisons of covariances using Empirical Orthogonal Functions (EOFs) calculated in frequency space. All data that are presented are based on radiance differences from two like spectra, which eliminates many of the errors generated by the use of MODTRAN and most of the errors due to calibration uncertainties in IRIS. Important differences (i.e., residuals) between the IRIS and the GCM statistics are found in comparisons, demonstrating that the spectral data can provide a severe test of many aspects of the variability of a general circulation model. We discuss some of the residuals and how they may be used to improve model performance in the context of an adjoint formalism. In the long run the only way to have confidence in the performance of a model is to subject it to as many discriminating comparisons with data as are practicable, and we present a good candidate.
Köhler equation for finite systems: A simple estimation of possible condensation mechanisms in aircraft contrails
Authors: Konopka P. , Vogelsberger W.
Published: February 2004

A generalization of the Köhler equation for systems with a limited amount of available water vapor is presented. Following the classical nucleation theory, a condensation process on soluble particles is discussed under isothermal‐isochoric conditions. The resulting generalization of the Köhler equation deviates from the classical approach for droplets with radius larger than the critical radius. An additional minimum in the Köhler curve can be determined describing the effect of water limitation. The presented theory is applied for the estimation of possible water condensation processes during the formation of aircraft contrails. In particular, three different kinds of condensation centers (soot, soot covered with sulfuric acid, and pure H2O/H2SO4 droplets) are compared with respect to the critical conditions which are necessary for their activation and with respect to their suitability for water uptake during condensation processes in the plume. Based on such investigations, soot particles with or without sulfuric acid can be easier activated than small H2O/H2SO4 droplets for atmospheric situations where jets get supersaturated with respect to water and, consequently, may lead to sufficiently large frozen particles. If the plume is undersaturated with respect to water, there is no difference, at least in view of the water condensation processes, between the highly concentrated H2O/H2SO4 droplets and soot particles with or without sulfuric acid.
A two‐dimensional model of sulfur species and aerosols
Authors: Weisenstein Debra K. , Yue Glenn K. , Ko Malcolm K. W. , Sze Nien‐Dak , Rodriguez Jose M. , Scott Courtney J.
Published: February 2004

A two‐dimensional model of sulfate aerosols has been developed. The model includes the sulfate precursor species H2S, CS2, DMS, OCS, and SO2. Microphysical processes simulated are homogeneous nucleation, condensation and evaporation, coagulation, and sedimentation. Tropospheric aerosols are removed by washout processes and by surface deposition. We assume that all aerosols are strictly binary water‐sulfuric acid solutions without solid cores. The main source of condensation nuclei for the stratosphere is new particle formation by homogeneous nucleation in the upper tropical troposphere. A signficant finding is that the stratospheric aerosol mass may be strongly influenced by deep convection in the troposphere. This process, which could transport gas‐phase sulfate precursors into the upper troposphere and lead to elevated levels of SO2 there, could potentially double the stratospheric aerosol mass relative to that due to OCS photooxidation alone. Our model is successful at reproducing the magnitude of stratospheric aerosol loading following the Mount Pinatubo eruption, but the calculated rate of decay of aerosols from the stratosphere is faster than that derived from observations.
Lagrangian approach for deriving cloud characteristics from satellite observations and its implications to cloud parameterization
Authors: Boer E. R. , Ramanathan V.
Published: February 2004

A Lagrangian view is adopted for establishing the spatiotemporal cloud statistics and the scale dependent radiative properties using satellite data. Individual clouds are identified using a newly developed scheme. We sort all clouds by cloud type, cloud area, and number of clouds in each area bin, as well as their radiative properties. For seven different cloud types our analyses provide radiative properties, such as albedo and cloud top temperature, as a function of the cloud spatial scale. All clouds are marked by local time, and large clouds are tracked over time. These analyses provide diurnal variability, lifetimes, and evolution of cloud systems as a function of their spatial scales. These scale dependent cloud properties can be objectively used in guiding the development and evaluation of cloud parameterization in global climate models (GCMs). Particularly, we show how our Lagrangian approach can be used to establish the relative importance of resolvable and fully parameterized clouds to the total cloudy area and to the total amount of reflected visible irradiance. Focus in this 1 month satellite study is on the convective‐stratiform cloud systems over the western and central tropical Pacific Ocean, including the so‐called warm pool. We adopt the hourly Japanese geostationary satellite (GMS) window channel radiances in the visible and IR window region for cloud classification and characterization. To study the radiative contributions of different clouds in the area, we computed the bidirectional model (BDM) for the Visible and Infrared Spin Scan Radiometer instrument aboard GMS, which we show to agree well with the BDM of the Earth Radiation Budget instrument aboard the Nimbus 7 satellite. An iterative two‐stage cloud detection scheme was developed to identify individual clouds. Furthermore, a tracking algorithm was developed to study the time evolution of mesoscale convective systems (MCS). It operates on area and orientationally equivalent ellipsoidal representations of these MCS. We show that the temporal statistics of these convective anvil clouds show good agreement with those reported in the literature. Our data indicate that for the convective‐stratiform systems in the tropical Pacific, 95% of the radiatively important clouds (containing a core with an effective brightness temperature <219 K) are of scales resolvable by a GCM of about 50 km × 50 km. On the other hand, a GCM of 250 km × 250 km will only be able to resolve 50% of the radiatively important clouds. This, however, does not mean that the processes responsible for the formation and maintenance of these systems are also resolvable. The low clouds that are unattached to convective‐stratiform systems are mostly unresolvable by available GCMs.
Ground‐based observations of stratospheric NO2 at high and midlatitudes in Europe after the Mount Pinatubo eruption
Authors: Van Roozendael M. , De Mazière M. , Hermans C. , Simon P. C. , Pommereau J.‐P. , Goutail F. , Tie X. X. , Brasseur G. , Granier C.
Published: February 2004

Nitrogen dioxide has been monitored at the International Scientific Station at the Jungfraujoch (46°N, 8°E;) since June 1990 and at Sodankyla (67°N, 27°E) since January 1990. NO2 vertical column abundances are measured during the morning and evening twilights by application of the differential absorption method using the sunlight scattered at zenith in the visible range. The available time series shows a significant reduction of NO2 starting in winter 1992, after the eruption of the Mount Pinatubo volcano. A maximum decrease of about 35% is observed in January 1992 at both stations. The continued series of observations shows the recovery of the NO2 column until August 1995. These results are compared with two‐dimensional chemical model calculations, including the effect of heterogeneous reactions on observed Pinatubo aerosols. In general, the modeled NO2 columns agree qualitatively with the observations although the amplitude of the seasonal variation is underestimated, possibly due to internal limitations of the model which, for example, does not include diurnal changes. The observed and calculated NO2 percent changes are in good agreement, which confirms quantitatively the impact of the heterogeneous chemistry on stratospheric NO2.
Estimation of boundary layer humidity fluxes and statistics from airborne differential absorption lidar (DIAL)
Authors: Kiemle Christoph , Ehret Gerhard , Giez Andreas , Davis Kenneth J. , Lenschow Donald H. , Oncley Steven P.
Published: February 2004

The water vapor differential absorption lidar (DIAL) of the German Aerospace Research Establishment (DLR) was flown aboard the National Center for Atmospheric Research (NCAR) Electra research aircraft during the Boreal Ecosystem‐Atmosphere Study (BOREAS). The downward looking lidar system measured two‐dimensional fields of aerosol backscatter and water vapor mixing ratio in the convective boundary layer (CBL) and across the CBL top (zt). We show a case study of DIAL observations of vertical profiles of mean water vapor, water vapor variance, skewness, and integral scale in the CBL. In the entrainment zone (EZ) and down to about 0.3 zi the DIAL observations agree with in situ observations and mixed‐layer similarity theory. Below, the water vapor optical depth becomes large and the DIAL signal‐to‐noise ratio degrades. Knowing the water vapor surface flux and the convective velocity scale w* from in situ aircraft measurements, we derive entrainment fluxes by applying the mixed‐layer gradient (MLG) and mixed‐layer variance (MLV) methods to DIAL mixing ratio gradient and variance profiles. Entrainment flux estimates are sensitive to our estimate of zt. They are shown to be rather insensitive to the input surface flux and to the DIAL data spatial resolution within the investigated range. The estimates break down above about 0.9 zt as the flux‐gradient and flux‐variance relationships were developed to describe the large‐scale mixing in the mid‐CBL. The agreement with in situ entrainment flux estimations is within 30% for the MLV method. On a flight leg with significant mesoscale variability the entrainment flux turns out to be 70% higher than the in situ value. This is in good agreement with the fact that large‐eddy simulations (LES) of mean water vapor profiles and variances, upon which the MLG and MLV methods are based, do not include mesoscale variability. The additional water vapor variance from mesoscales may then lead to the overestimate of the flux. Deviations from the in situ observations may also be due to poor LES resolution of small‐scale mixing in the EZ, similarly coarse resolution of the DIAL data, or a capping inversion in the LES model (8 K) which is significantly stronger than the observed inversion (3–4 K).
Inhomogeneity effects on cloud shortwave absorption measurements: Two‐aircraft simulations
Authors: Marshak Alexander , Davis Anthony , Wiscombe Warren , Cahalan Robert
Published: February 2004

We set up an Observation System Simulation in order to quantify the idiosyncratic uncertainties caused by cloud inhomogeneity in two‐aircraft measurements. An independently validated fractal model is used to simulate the horizontal variability of optical depth for boundary layer stratus, with and without gaps. A spectral band between 0.9 and 1.0 μm, with a combination of strong and weak water vapor absorption and negligible weak liquid water absorption, is selected for a detailed study of column absorption. Measurements of up welling and downwelling radiative fluxes are simulated by the Monte Carlo method at two altitudes in a realistic cloudy atmosphere. We consider three methods of estimating column absorption: cloud forcing ratio, reflectance (R) versus transmittance (T) regression, and Ackerman‐Cox correction. Emphasis is on the second method, R versus T, which is shown to be biased in the direction of enhanced absorption as long as the R and T data points are affected by horizontal fluxes. In other words, to reduce the bias to an acceptable level, (R, T) measurements must be at a scale well within the regime where the independent pixel approximation is accurate. Under completely cloudy skies, radiative smoothing processes dominate the bias in the absorption estimate at small scales; hence spatial averages over at least 30 times the radiative smoothing scale, which is commensurate with cloud thickness, are required. If there are substantial gaps in the cloud cover, geometrical shadowing and cloud side‐illumination effects overwhelm the radiative smoothing. As a result, the bias in R versus T methodology is worsened, and aircraft positioning becomes more critical. We also examine the effect of Sun angle and instrumental parameters such as vertical separation and horizontal offset between aircraft. The Ackerman‐Cox correction offers an alternative to spatial averaging; a modification of this intrinsically small‐scale approach that can virtually eliminate horizontal flux contributions in the data is presented.
Variational data assimilation for tropospheric chemistry modeling
Authors: Elbern Hendrik , Schmidt Hauke , Ebel Adolf
Published: February 2004

The method of variational adjoint data assimilation has been applied to assimilate chemistry observations into a comprehensive tropospheric gas phase model. The rationale of this method is to find the correct initial values for a subsequent atmospheric chemistry model run when observations scattered in time are available. The variational adjoint technique is esteemed to be a promising tool for future advanced meteorological forecasting. The stimulating experience gained with the application of four‐dimensional variational data assimilation in this research area has motivated the attempt to apply the technique to air quality modeling and analysis of the chemical state of the atmosphere. The present study describes the development and application of the adjoint of the second‐generation regional acid deposition model gas phase mechanism, which is used in the European air pollution dispersion model system. Performance results of the assimilation scheme using both model‐generated data and real observations are presented for tropospheric conditions. In the former case it is demonstrated that time series of only few or even one measured key species convey sufficient information to improve considerably the analysis of unobserved species which are directly coupled with the observed species. In the latter case a Lagrangian approach is adopted where trajectory calculations between two comprehensively furnished measurement sites are carried out. The method allows us to analyze initial data for air pollution modeling even when only sparse observations are available. Besides remarkable improvements of the model performance by properly analyzed initial concentrations, it is shown that the adjoint algorithm offers the feasibility to estimate the sensitivity of ozone concentrations relative to its precursors.
Intercalibration, objective analysis, intercomparison and synthesis of BOREAS surface net radiation measurements
Authors: Hodges Gary B. , Smith Eric A.
Published: February 2004

An analysis of the surface net radiation (Rn) distribution and factors which determine its representativeness and accuracy has been carried out as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). A set of three calibrated net pyrradiometers has been used to intercalibrate 22 Rn measuring systems at 21 sites. GOES satellite‐based fields of Rn are also compared with the associated measured fields to quantify the effect of sparse sampling on the spatial and temporal error structures of Rn. The results show that a standard make of net pyrradiometer, used at 15 of the 21 measuring sites, underestimates net radiation by about 5% in the daytime and about 45% at night. The distribution of measuring sites and intercalibration errors are shown to produce distinctive patterns of bias error in the space‐time fields of Rn over the large‐scale study area. However, we find that sparse sampling of measured Rn has a far more significant effect in generating errors in the large‐scale space‐time framework. At the small scale, considering the distribution of net pyrradiometers within the southern study area, we find that intercalibration is the more serious factor when considering time‐averaged gradient structure. Although the changing ecotone pattern over the BOREAS region introduces gradients in the Rn fields, we find that in characterizing the space‐time sampling error structures, cloud effects dominate ecotone effects. The study concludes with a demonstration of how to take advantage of the GOES satellite's capability of representing between‐site Rn gradients and the net pyrradiometer network's capability of making accurate point‐source measurements, by producing optimally averaged maps of Rn.
Factors controlling threshold friction velocity in semiarid and arid areas of the United States
Authors: Marticorena Beatrice , Bergametti Gilles , Gillette Dale , Belnap Jayne
Published: February 2004

A physical model was developed to explain threshold friction velocities u*t for particles of the size 60–120 μm lying on a rough surface in loose soils for semiarid and arid parts of the United States. The model corrected for the effect of momentum absorption by the nonerodible roughness. For loose or disturbed soils the most important parameter that controls u*t is the aerodynamic roughness height z0. For physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness. The presence of cyanobacteriallichen soil crusts roughens the surface, and the biological fibrous growth aggregates soil particles. Only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms. Therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements.
Introduction to special section: Synoptic and mesoscale weather systems in the polar regions
Authors: Bromwich David H.
Published: February 2004

This special section consists primarily of papers contributed to Symposium M‐9 on Synoptic and Mesoscale Weather Systems in the Polar Regions at the XXI General Assembly of the International Union of Geodesy and Geophysics, which was held in Boulder, Colorado, during July 1995. The symposium was convened by John Turner of the British Antarctic Survey and Erik Rasmussen of the University of Copenhagen.

High‐latitude meteorology is in a period of rapid transition away from being a generally neglected area of atmospheric science. With the general understanding that the atmospheric and oceanic circulation is to first order driven by the thermal contrast between the equatorial heat source and the polar heat sinks, it no longer seems appropriate to deemphasize the role of the polar regions in climate variability and change. However, the natures of the Arctic and Antarctic are profoundly different. The ice‐covered and land‐locked Arctic Ocean contrasts with the elevated and extensive Antarctic ice sheets that are surrounded by the Southern Ocean. The very different geographic characteristics limit the similarities between these two regions.
Vertical profiles of N2O5, HO2NO2, and NO2inside the Arctic vortex, retrieved from nocturnal MIPAS‐B2 infrared limb emission measurements in February 1995
Authors: Wetzel G. , Oelhaf H. , von Clarmann T. , Fischer H. , Friedl‐Vallon F. , Maucher G. , Seefeldner M. , Trieschmann O. , Lefèvre F.
Published: February 2004

Vertical profiles Of N2O5, HO2NO2, and NO2inside the arctic vortex were retrieved from nighttime infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon‐borne version 2 (MIPAS‐B2) instrument from Kiruna (Sweden, 68°N) on February 11, 1995, as part of the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME). Spectra were analyzed by a multiparameter nonlinear least squares fitting procedure in combination with an onionpeeling retrieval algorithm. The N2O5, HO2NO2, and NO2results were derived from spectral features within the bands near 8.0 μm 12.5 μm, and 6.2 μm, respectively. Peak mixing ratios of 1.14 parts per billion by volume (ppbv) N2O5and 80 parts per trillion by volume (pptv) HO2NO2at 17.1 hPa as well as 2.79 ppbv NO2at 12.0 hPa corresponding to 25.8 km and 28.0 km altitude were inferred from the spectra. NO2mixing ratios measured by MIPAS fit well to the data observed by concurrent flights. A comparison with calculations performed with a three‐dimensional chemistry transport model for the time and location of the measurements shows that the best agreement of measured and calculated profiles is reached between 17 and 28 hPa corresponding to 25.8 and 22.7 km altitude, while below and above this altitude region there are some discrepancies between the modeled and observed data.
Global water vapor distributions in the stratosphere and upper troposphere derived from 5.5 years of SAGE II observations (1986–1991)
Authors: Chiou E. W. , McCormick M. P. , Chu W. P.
Published: February 2004

Global distributions of water vapor in the stratosphere and upper troposphere are presented on the basis of ∼5.5 years (January 1986 to May 1991) of observations from the Stratospheric Aerosol and Gas Experiment II (SAGE II) aboard the Earth Radiation Budget Satellite (ERBS). Tabulations are included for seasonal zonal mean water vapor mixing ratios (in parts per million by volume) with 1‐km vertical resolution and an altitude range from 6 to 40 km. Several climatological features identified in a previous study [McCormick et al., 1993], based on 3 years of observations, have been confirmed by this study: (1) the existence of a region of minimum water vapor (the hygropause) at all latitude bands; (2) the increase in the distance between the tropopause and the hygropause from 1 km at low latitudes to 4 km at high latitudes; and (3) the appearance of a positive poleward gradient throughout all seasons for fixed altitudes between 20 km and 40 km. The latitudinal variation of water vapor mixing ratio at 20 km is characterized by a symmetric pattern with a minimum occurring at the equator. However, the corresponding variations at 25 and 30 km indicate a shift of the minimum toward the summer hemisphere. For the latitude zones 0°–20° and 20°–40° in both hemispheres, the seasonal variations of the hygropause reveal that the altitude as well as the value of the minimum water vapor mixing ratio remain essentially unchanged from December, January, and February to March, April, and May. During September, October, and November the weakening of the hygropause and the spreading of the region of minimum water vapor to a wider altitude range are identified throughout these low‐latitude and midlatitude zones. For the upper troposphere the clear‐sky relative humidities at 300 mbar show a typical range of 5–60%, which is consistent with previous findings based on Meteosat 6.3 μm measurements. In addition, the unique capability of SAGE II observations has provided us with unprecedented vertically resolved moisture information for the upper troposphere. For example, the integrated column water vapor content for the 300‐ to 100‐mbar layer ranges from 0.002 to 0.01 g/cm2 with larger longitudinal variability in the tropics. The integrated column water vapor content from 500 to 100 mbar is found to be significantly larger in the eastern hemisphere than in the western hemisphere. The corresponding integrated water vapor content at high latitudes increases by a factor of 6 from winter to summer (0.02 g/cm2 compared with 0.13 g/cm2).
Ion balance equation in the atmosphere
Authors: Ermakov V. I. , Bazilevskaya G. A. , Pokrevsky P. E. , Stozhkov Y. I.
Published: February 2004

Cosmic rays are the main source of ions in the atmosphere at altitudes of 5–35 km. The flux of cosmic rays in the atmosphere depends on the site of observation and is controlled by the geomagnetic field. Except for the rare cases of solar proton events and Forbush decreases, cosmic ray flux varies slowly with changes of solar activity level. Thus there is quasi‐stationary equilibrium between the ion production and recombination processes. Usually, the equation q=αn2 is used to describe this equilibrium state, where q is the ion production rate, n is the ion concentration, and α is Thompson's coefficient of recombination. However, the joint analysis of data on ion concentration and cosmic ray fluxes observed at various latitudes shows that there is a linear relationship between ion production rate and ion concentration, q=βn, where β is the linear coefficient of ion recombination.
Seasonal snowpack radiobrightness interpretation using a SVAT‐linked emission model
Authors: Galantowicz J. F. , England A. W.
Published: February 2004

This study investigates the link between seasonal snowpack radiobrightness dynamics and land‐atmosphere energy and moisture fluxes using a 7 month observational record and snowpack simulations. Experimental data are presented from the first Radiobrightness Energy Balance Experiment (REBEX 1) which combined continuous micrometeorological observations with ground‐based measurement of terrain radiobrightnesses at Special Sensor Microwave/Imager (SSM/I) frequencies. The REBEX 1 site near Sioux Falls, South Dakota, is characteristic of the northern Great Plains grasslands and climate. The experiment period from October 1992 to April 1993 spanned vegetation senescence, snowpack formation and evolution, and spring thaw. Patterns in the data are analyzed using a snowpack evolution and radiobrightness model driven by REBEX 1 energy and moisture flux measurements. The snowpack model differs from most soil‐vegetation‐atmosphere transfer (SVAT) schemes in the level of detail used in modeling the near‐surface medium. Snowpack temperature and structural details are necessary in order to model both the microwave thermal source and snowpack interactions and the continuing metamorphism of the snow. Modeled radiobrightness variation at 19, 37, and 85 GHz agree with observations to within 29, 16, and 13% (root mean square), respectively, over a 53 day test period. Both observed and modeled radiobrightness dynamics show sensitivity to (1) unfrozen soil moisture content at 19 and 37 GHz, (2) snow grain size and sky brightness variation (primarily at 85 GHz), and (3) snowpack partial melt and refreeze cycles (all frequencies). Furthermore, we find that snowpack stratification does not drive brightness variation in these data even at 19 GHz where snowpack penetration is greatest.