bluuebunny/crossref_metadata_2025 huggingface dataset:
Consistent sampling methods for comparing models to CO2 flask data
Authors: Haas‐Laursen Danielle E. , Hartley Dana E. , Conway Thomas J.
Published: February 2004

We address the issue of how to compare atmospheric carbon dioxide (CO2) observations from the National Oceanic and Atmospheric Administration / Climate Monitoring and Diagnostics Laboratory (NOAA / CMDL) air sampling network to model output in a consistent method for model evaluation and inverse problems. Two steps are needed to “sample” the model using the same methodology that is applied to the flask samples: (1) ensure that the wind direction is one that is deemed acceptable in the field and (2) reject outliers. Since a “clean air sector” has only been determined for four sites, we compute a statistical analysis of the wind direction when samples are collected at 38 NOAA/CMDL CO2 collection sites operational from 1993–1995. We compare this to the wind directions from the European Centre for Medium‐Range Weather Forecasts (ECMWF) assimilated wind fields for the same 3 year time period. From this analysis, we deduce which of the sites are selecting for wind direction. We then analyze model output to compare the effects on monthly mean concentration from this two‐step methodology. We find that at the majority of sites the removal of outliers is particularly important. Finally, we explore the impact of sampling frequency in the model. At most sites, the sampling frequency does impact the model results. We conclude that it is best to use high‐frequency model output rather than sampling with the same low‐frequency as the flask network. The bias resulting from low‐frequency sampling is potentially large, and if done in both the model and the observations, the comparison error is exacerbated. We estimate this bias error for use in error estimation and inverse problems.
Retrieval of microwave surface emittance over land using coincident microwave and infrared satellite measurements
Authors: Jones Andrew S. , Vonder Haar Thomas H.
Published: February 2004

A method for routinely generating microwave surface emittance over land from coincident microwave and infrared satellite data has been developed. The method includes a surface skin temperature retrieval, and a dynamic cloud discrimination method, in addition to an explicit atmospheric correction based on in situ atmospheric sounding information. An example of results from the method are presented for a 70‐day period over the central United States. Atmospheric‐corrected microwave surface emittance results are shown to enhance the use of the microwave data sets for determining land surface characteristics, especially in regards to analysis of the data's frequency dependencies. Several problems that affect the use of the microwave brightness temperature data were examined, including natural characteristics of the spatial and temporal variability of the microwave background signature. The microwave surface emittance was found to be sensitive to numerous rain events captured in the data set. Application of this method to generate longer‐term climatologies of microwave surface emittance should improve the understanding of the microwave surface emittance behavior. In particular, realistic spatial distributions of microwave surface emittance should enhance current atmospheric microwave remote sensing efforts over land. The microwave surface emittance data set also shows potential in nonforested regions for flood‐monitoring purposes using change detection methods.
Intercomparison of stratospheric ozone profiles obtained by Stratospheric Aerosol and Gas Experiment II, Halogen Occultation Experiment, and ozonesondes in 1994–1995
Authors: Lu Jianjun , Mohnen Volker A. , Yue Glenn K. , Atkinson Roger J. , Matthews W. A.
Published: February 2004

Stratospheric ozone mixing ratio profiles measured by satellite‐borne Stratospheric Aerosol and Gas Experiment (SAGE II data version 5.93) and Halogen Occultation Experiment (HALOE data versions 17 and 18) are compared with each other and with balloon‐borne ozonesonde profiles at Payerne, Lauder, and Macquarie Island stations during 1994–1995. The SAGE II‐HALOE ozone mixing ratio profiles are compared at 13–55 km altitudes, and the satellite‐ozonesonde profiles are compared at 13–33 km altitudes. It is found that HALOE data version 18 ozone values are systematically larger than HALOE data version 17, particularly in the lower stratosphere. HALOE data version 18 ozone profiles are about 2%, 5–8%, and 10–20% larger than the corresponding HALOE data version 17 at 40–55, 25–40, and 15–25 km altitudes, respectively. The new version of HALOE ozone agrees better with SAGE II and Payerne ozonesonde profiles than HALOE data version 17 does. In general, the agreements between satellite‐borne sensors and ozonesondes are all within their stated level of uncertainties. Intercomparison results between SAGE II‐Payerne ozonesonde and SAGE II‐Lauder ozonesonde are compared with earlier comparative studies between SAGE I‐Payerne ozonesonde and SAGE II‐Lauder ozonesonde by Veiga et al. [1995]. Better agreement is achieved at Payerne station, but no improvement is discernible at Lauder ozonesonde station. At altitudes above 20 km the intercomparisons between SAGE II and HALOE data version 18 and between SAGE II and ozonesondes seem to indicate that aerosol has no statistically significant impact on SAGE II ozone retrieval during the period of 1994–1995.
Robust and efficient inversion of vertical sounding atmospheric high‐resolution spectra by means of regularization
Authors: Schimpf B. , Schreier F.
Published: February 2004

Retrieval of atmospheric temperature or constituent profiles from vertical sounding spectroscopic measurements is an ill‐posed problem, and additional information has to be introduced in the inversion process in order to compute meaningful solutions. Modern mathematical techniques are shown to be suitable for an analysis of these problems and the actual inversion to retrieve atmospheric profiles. A stable and efficient numerical implementation of Phillips‐Tikhonov regularization techniques is discussed; generalized singular value decomposition is the appropriate tool to compute the formal solution of the modified minimization problem, and the L‐curve permits determination of the optimum balance between information from measurement and side constraints. It is also shown that these techniques can provide further insight in the basic ill‐posed nature of the inverse problem, give tools for the diagnostics of the retrieved profiles, and allow a discussion of the relation to other standard retrieval techniques, especially optimal estimation. The methods are demonstrated on examples for retrieval of ozone and hydroxyl profiles from simulated far infrared high‐resolution spectra. A comparison with optimal estimation retrieval is performed by a preliminary analysis of a Spitsbergen millimeter‐wave spectrum of ozone.
A new mechanism for regional atmospheric chemistry modeling
Authors: Stockwell William R. , Kirchner Frank , Kuhn Michael , Seefeld Stephan
Published: February 2004

A new gas‐phase chemical mechanism for the modeling of regional atmospheric chemistry, the “Regional Atmospheric Chemistry Mechanism” (RACM) is presented. The mechanism is intended to be valid for remote to polluted conditions and from the Earth's surface through the upper troposphere. The RACM mechanism is based upon the earlier Regional Acid Deposition Model, version 2 (RADM2) mechanism [Stockwell et al., 1990] and the more detailed Euro‐RADM mechanism [Stockwell and Kley, 1994]. The RACM mechanism includes rate constants and product yields from the most recent laboratory measurements, and it has been tested against environmental chamber data. A new condensed reaction mechanism is included for biogenic compounds: isoprene, α‐pinene, and d‐limonene. The branching ratios for alkane decay were reevaluated, and in the revised mechanism the aldehyde to ketone ratios were significantly reduced. The relatively large amounts of nitrates resulting from the reactions of unbranched alkenes with NO3 are now included, and the production of HO from the ozonolysis of alkenes has a much greater yield. The aromatic chemistry has been revised through the use of new laboratory data. The yield of cresol production from aromatics was reduced, while the reactions of HO, NO3, and O3 with unsaturated dicarbonyl species and unsaturated peroxynitrate are now included in the RACM mechanism. The peroxyacetyl nitrate chemistry and the organic peroxy radical‐peroxy radical reactions were revised, and organic peroxy radical +NO3 reactions were added.
Anthropogenic sources of halocarbons, sulfur hexafluoride, carbon monoxide, and methane in the southeastern United States
Authors: Bakwin P. S. , Hurst D. F. , Tans P. P. , Elkins J. W.
Published: February 2004

Mixing ratios of several radiatively active and ozone depleting trace gases were measured at 1‐hour intervals from December 1994 to August 1996 at 496 m above the ground on a communications tower in eastern North Carolina. The time series show variability on a range of timescales, most apparently diurnal cycles, pollution episodes with synoptic time scale, seasonal cycles, and long‐term trends. We focus on the pollution episodes and analyze the time series to reveal relationships between species in polluted air. These relationships are used to determine relative sources of the constituents on a regional scale. Using tetrachloroethene (C2Cl4) as a reference compound and assuming that the tower site is representative of the whole continent with respect to emissions ratios, we estimate source strengths of the other species for North America. For methyl chloroform (CH3CCl3) our emissions estimate is in excellent accord with an estimate from industrial production figures, giving confidence in our methods and assumptions. For carbon monoxide (CO) our estimate also agrees very well with an independent estimate of North American sources based on national inventories. Sources of chlorofluorocarbons 11, 12, and 113 are much lower than expected from global emissions inventories, a finding that may indicate that the ratio of North American to global emissions of these compounds has dropped dramatically since the mid‐1980s, as was indeed the case for CH3CCl3, or that emissions of these compounds have decreased more rapidly in recent years than industry estimates, or most likely both. We also estimate the North American source of sulfur hexafluoride (SF6).
Modeling of microscale turbulence and chemistry interaction in near‐field aircraft plumes
Authors: Wang Z. , Chen J.‐Y.
Published: February 2004

The potential impact of aircraft exhaust gas on entrained ambient air in the stratosphere has been studied by using several numerical models to manifest the importance of mixing processes occurring at different scales. These models include perfectly stirred reactor (PSR), transient well‐mixed reactor (WMR), partially stirred reactor (PaSR), and joint scalar probability density function (PDF) simulation for the near‐field jet exhaust. Sensitivities of major species to chemical reactions are presented to illustrate the chemical transformation in the near‐field jet regime. An accurate reduced chemical mechanism that captures major reaction pathways has been systematically developed and tested. The predicted results with the reduced mechanism are found in good agreement with those from the detailed mechanism, but the computing time is decreased by a factor of 3. The effect of microscale turbulent mixing and chemistry interaction on species evolution is studied, and its impact is found most noticeable in the near field where the timescale of chemistry for highly reactive radicals, such as OH and HO2, is comparable or faster compared with that of microscale mixing. When microscale mixing is included, the predicted H2SO4 level increases at locations where sulfate aerosol formation may be important in the near field.
Mesoscale cyclone activity over Antarctica during 1991: 1. Marie Byrd Land
Authors: Carrasco Jorge F. , Bromwich David H. , Liu Zhong
Published: February 2004

A 1‐year statistical study of mesoscale cyclonic activity over southern Marie Byrd Land, West Antarctica, was conducted using high‐resolution satellite imagery collected insitu during 1991 at the U.S. McMurdo and Palmer stations. Results indicate that, on average, one or two mesoscale cyclones can be observed each week over the southernmost part of Marie Byrd Land (Siple Coast and adjacent areas), where cold katabatic winds from the high plateau of East Antarctica and relatively warm katabatic airflows from West Antarctica interact, forming a boundary layer baroclinic zone in the form of a stationary front. The 1991 annual mesoscale cyclone behavior exhibits maximum activity in March and minimum in September. Analysis of the large‐scale pattern for these months suggests that the higher activity in March is associated with synoptic‐scale conditions that support the cold katabatic drainage from East Antarctica and favor warm air advection into West Antarctica, a situation that can form and/or enhance the boundary layer baroclinic zone. More than half (70%) of the mesoscale vortices observed near Siple Coast were of comma cloud type with an average diameter of about 250 km. Their satellite signature characteristics show that these cyclonic perturbations were stratiform low cloud features, indicating that most of them developed within the lower troposphere, more than likely below 700 hPa, according to a cloud top temperature evaluation.
A split‐window algorithm for land surface temperature from advanced very high resolution radiometer data: Validation and algorithm comparison
Authors: Coll César , Caselles Vicente
Published: February 2004

A split‐window algorithm for deriving land surface temperatures (LSTs) from advanced very high resolution radiometer (AVHRR) channels 4 and 5 is proposed and validated with in situ measured temperatures. On the basis of the radiative transfer theory the algorithm defines a set of surface‐independent coefficients which are equivalent to the classical split‐window coefficients for sea surface temperature (SST). These coefficients are calculated using SST matchups (coincident AVHRR and buoy measurements) provided by the National Oceanic and Atmospheric Administration (NOAA)‐NASA Pathfinder Database of worldwide measurements. Thus calibration of the split‐window coefficients is done using real data. The variability of atmospheric attenuation is represented in the proposed algorithm by a quadratic dependence on the brightness temperature difference. For LST determination the emissivity effect is modeled through an additive coefficient which depends on surface emissivity in the AVHRR channels 4 and 5. The algorithm is validated for both SST and LST by using independent ground‐based and AVHRR data. The database used in the validation of LST was obtained for a wide range of surface types in a semiarid environment. The same databases are used to compare the accuracies of other published split‐window algorithms. The proposed algorithm yields standard errors of temperature estimate between ±1.0 and ±1.5 K, and no significant biases are observed. Although results are encouraging, more validation is required principally for moist atmospheric conditions.
Evolution and stoichiometry of heterogeneous processing in the Antarctic stratosphere
Authors: Jaeglé L. , Webster C. R. , May R. D. , Scott D. C. , Stimpfle R. M. , Kohn D. W. , Wennberg P. O. , Hanisco T. F. , Cohen R. C. , Proffitt M. H. , Kelly K. K. , Elkins J. , Baumgardner D. , Dye J. E. , Wilson J. C. , Pueschel R. F. , Chan K. R. , Salawitch R. J. , Tuck A. F. , Hovde S. J. , Yung Y. L.
Published: February 2004

Simultaneous in situ measurements of HCl and ClO have been made for the first time in the southern hemisphere, allowing a systematic study of the processes governing chlorine activation between 15 and 20 km in the 1994 Antarctic winter. Data for several other gases (O3, NO, NOy, OH, HO2, N2O, CH4, CO, H2O, CFCs), particulates, and meteorological parameters were collected from the ER‐2 aircraft out of New Zealand as part of the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements of Atmospheric Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign. Observations from the ER‐2 in the fall (April–May), prior to polar night, show that chlorine activation begins with 60–75% of inorganic chlorine as HCl. By midwinter (July–August), near‐total removal of HCl is observed. The wintertime loss of HCl in air recently exposed to extreme temperatures is found to be correlated with high levels of reactive chlorine (ClO and its dimer, Cl2O2) in the linear fashion expected from the stoichiometry of the heterogeneous reaction of hydrochloric acid with chlorine nitrate on polar stratospheric clouds (PSCs): HCl + ClONO2 → Cl2 + HNO3. To constrain the role of different heterogeneous reactions and PSC types, we have used a photochemical trajectory model which includes heterogeneous sulfate and PSC chemistry. Model calculations of the evolution of reactive gases are compared with the in situ observations. In addition, simultaneous measurements of OH and HO2 are used as a diagnostic for the occurrence of the heterogeneous reaction HOCl + HCl → Cl2 + H2O, which contributes to suppressed levels of HOx inside the vortex. It is shown that the amount of chlorine activation is not strongly dependent on the composition of PSCs. However, HOx levels exhibit different signatures depending on the type of heterogeneous surfaces that affected chlorine activation. Furthermore, this analysis implies that in the edge region of the Antarctic vortex, the observed near‐total removal of HCl can result from latitudinal excursions of air parcels in and out of sunlight during the winter, which photochemically resupply HOCl and ClONO2 as oxidation partners for HCl.
Large‐scale variability of wind erosion mass flux rates at Owens Lake: 2. Role of roughness change, particle limitation, change of threshold friction velocity, and the Owen effect
Authors: Gillette Dale A. , Hardebeck Ellen , Parker Jim
Published: February 2004

Variability in airborne particles larger than sand flux for distance scales larger than 200 m was observed during wind erosion episodes on the northeast side of a dry lake bed (Owens Lake). Measurements were made during erosion episodes on a ∼3‐km line of wind measuring and sand flux collecting instruments. Data were selected for winds that (1) aligned with the instrument line and (2) had mean speeds cubed at 4‐m heights at the beginning, middle, and end of the line that differed by less than 5% (i.e., mean wind speeds differed by less than 1.7%). Four mechanisms were determined to cause the large‐scale differences in the mass flux profiles. In order of their importance, the mechanisms are as follows: (1) change of the drag coefficient (or the ratio u*/U, where u* is wind friction velocity and U is mean wind speed from place to place), this is a measure of variability in roughness height; (2) particle limitation (depletion of the loose “available” erodible material on the surface); (3) variation of the threshold friction velocity; and (4) the Owen effect (the increase of u*/U with U).
Onset of instability in precipitating water drops submitted to horizontal electric fields
Authors: Georgis Jean‐François , Coquillat Sylvain , Chauzy Serge
Published: February 2004

Most intracloud discharges are supposed to be initiated by hydrometeors in a vertical ambient electric field. However, horizontal propagating flashes are commonly observed. The aim of this work is to measure the horizontal onset field of instability for various drop sizes and to compare the results with those previously determined in experiments where the drops did not reach their terminal velocity [Nolan, 1926; Macky, 1931; Ausman and Brook, 1967] and with those recently published by Kamra et al. [1993] for drops suspended in a wind tunnel. In the present experiment, uncharged drops fall about 14 m and are at terminal velocity before entering an interelectrodes region where a horizontal electric field is generated. It appears that the lowest onset field intensities are obtained when the aerodynamical effects are maximum. However, there is a large discrepancy between our results and those by Kamra et al. It is believed that the onset fields of instability are underestimated by the latter authors because of probably unperfect experimental conditions. The present results, which are certainly more representative of the fall of uncharged drops in still air than those previously obtained, seem to indicate that horizontal electric fields in thunderclouds are not strong enough to cause lightning initiation by corona emission in these conditions and to affect the drop‐size distributions. This conclusion conflicts with that of Kamra et al. [1993]. However, the case of charged drops falling at higher altitude would certainly lead to lower horizontal field onsets.
Modeling the spatial distribution of total ozone in northern hemisphere winter: 1979–1991
Authors: McCormack John P. , Hood Lon L.
Published: February 2004

A form of the linearized steady state ozone continuity equation, expressed in terms of perturbations in lower stratospheric temperature and geopotential height, is used to model the spatial distribution (i.e., deviations from the zonal mean) of total ozone in northern hemisphere (NH) winter for the period 1979–1991. Lower stratospheric temperatures from channel 4 of the microwave sounding unit (MSU), geopotential height and temperature analyses provided by the Stratospheric Research Group at the Free University of Berlin (FUB), and climatological values of the zonal mean ozone mixing ratio are used to calculate the ozone mixing ratio perturbation for a given location at three levels in the lower stratosphere: 100, 50, and 30 mbar. These values are then summed together to provide an estimate of the total ozone perturbation at northern extratropical latitudes for the months of January and February. By applying this modeling approach to individual months on a year‐to‐year basis, much of the observed year‐to‐year variability in the spatial distribution of total ozone is approximately simulated. A significant decrease of the wave‐induced variance in the wintertime total ozone distribution over the 1979–1991 period is indicated, consistent with the observed longitude dependences of total ozone trends in NH winter.
A comparative study of mesospheric water vapor measurements from the ground‐based water vapor millimeter‐wave spectrometer and space‐based instruments
Authors: Nedoluha Gerald E. , Bevilacqua Richard M. , Gomez R. Michael , Waltman William B. , Hicks Brian C. , Thacker D. L. , Russell James M. , Abrams Mark , Pumphrey Hugh C. , Connor Brian J.
Published: February 2004

We compare water vapor measurements from the Naval Research Laboratory ground‐based Water Vapor Millimeter‐wave Spectrometer (WVMS) instruments with measurements taken by five space‐based instruments. For coincident measurements the retrievals from all of the instruments show qualitatively similar altitude profiles. The retrieved mixing ratios from most instruments generally differ from an average calculated using retrievals from all of the instruments by <1 ppmv at most altitudes from 40 km to 80 km. Comparisons with the Microwave Limb Sounder (MLS) and the Halogen Occultation Experiment (HALOE) allow for the validation of observed temporal variations. The observed variations show similar annual and semiannual cycles. A comparison of several years of data from HALOE and WVMS also shows that the instruments are detecting similar interannual variations. A regression analysis of the WVMS and HALOE data sets shows that the observed variability is consistent within the estimated errors in the mesosphere and that in the upper stratosphere, where the natural variability is small, there is a positive correlation between the WVMS and the HALOE data.
Atmospheric layers measured from the NASA DC‐8 during PEM‐West B and comparison with PEM‐West A
Authors: Wu Zhongxiang , Newell Reginald E. , Zhu Yong , Anderson Bruce E. , Browell Edward V. , Gregory Gerald L. , Sachse Glen W. , Collins James E.
Published: February 2004

The Pacific Exploratory Mission‐West B (PEM‐West B) explored atmospheric layer structure using measurements of O3, H2O, CO, and CH4 from the NASA DC‐8 fast‐response instruments. The mission took place in February‐March 1994 over the western Pacific, mainly in the northern hemisphere. Results were compared with similar measurements made during the Pacific Exploratory Mission‐West A (PEM‐West A) in September‐October 1991. PEM‐West B sampled 94 vertical profiles, with an average atmospheric depth per profile of 6.4 km, and this sampling yielded 254 discrete layers. PEM‐West A sampled 105 profiles, also with a 6.4 km average depth, yielding 538 layers. Both missions revealed that layers containing high ozone and low water vapor were the most abundant, and low ozone and high water vapor layers were the next most abundant. Lidar images and potential vorticity cross sections showed the former associated with subsidence from the stratosphere in middle latitudes, spreading into the tropics. The latter was associated with convection from the boundary layer. The partition among different types of layers only changed slightly in the two missions, although PEM‐West B had half as many layers. Compared to PEM‐West A, PEM‐West B showed only one‐third the number of layers associated with polluted continental air. However, PEM‐West B revealed significantly more layers showing characteristics of descended clean marine air. In some cases these layers originated from the southern hemisphere. For ozone‐ and water‐vapor‐rich layers, the ozone deviation amplitude was higher in low latitudes and lower in high latitudes in PEM‐West B. The mean thickness of layers increased from about 450 m in PEM‐West A to 680 m in PEM‐West B. Layers also existed in the Intertropical Convergence Zone. CO2 measurements had sufficient vertical resolution to show layer structure as well.
An improved open chamber system for measuring soil CO2 effluxes in the field
Authors: Rayment M. B. , Jarvis P. G.
Published: February 2004

An “open” chamber for the measurement of soil CO2 efflux has been designed which utilizes an air seal to maintain chamber pressure within 0.004 Pa of atmospheric pressure, eliminating any mass flow and ensuring that atmospheric pressure fluctuations are transferred through to the soil surface. The system was used at the Boreal Ecosystem‐Atmosphere Study (BOREAS) southern study area old black spruce site through the 1994 and 1996 field campaigns and was found to be robust and simple to use. Consistency was found between soil CO2 effluxes measured with this system and a closed dynamic chamber system.
Leaf area index of boreal forests: Theory, techniques, and measurements
Authors: Chen Jing M. , Rich Paul M. , Gower Stith T. , Norman John M. , Plummer Steven
Published: February 2004

Leaf area index (LAI) is a key structural characteristic of forest ecosystems because of the role of green leaves in controlling many biological and physical processes in plant canopies. Accurate LAI estimates are required in studies of ecophysiology, atmosphere‐ecosystem interactions, and global change. The objective of this paper is to evaluate LAI values obtained by several research teams using different methods for a broad spectrum of boreal forest types in support of the international Boreal Ecosystem‐Atmosphere Study (BOREAS). These methods include destructive sampling and optical instruments: the tracing radiation and architecture of canopies (TRAC), the LAI‐2000 plant canopy analyzer, hemispherical photography, and the Sunfleck Ceptometer. The latter three calculate LAI from measured radiation transmittance (gap fraction) using inversion models that assume a random spatial distribution of leaves. It is shown that these instruments underestimate LAI of boreal forest stands where the foliage is clumped. The TRAC quantifies the clumping effect by measuring the canopy gap size distribution. For deciduous stands the clumping index measured from TRAC includes the clumping effect at all scales, but for conifer stands it only resolves the clumping effect at scales larger than the shoot (the basic collection of needles). To determine foliage clumping within conifer shoots, a video camera and rotational light table system was used. The major difficulties in determining the surface area of small conifer needles have been largely overcome by the use of an accurate volume displacement method. Hemispherical photography has the advantage that it also provides a permanent image record of the canopies. Typically, LAI falls in the range from 1 to 4 for jack pine and aspen forests and from 1 to 6 for black spruce. Our comparative studies provide the most comprehensive set of LAI estimates available for boreal forests and demonstrate that optical techniques, combined with limited direct foliage sampling, can be used to obtain quick and accurate LAI measurements.
Winter CO2 fluxes in a boreal forest
Authors: Winston G. C. , Sundquist E. T. , Stephens B. B. , Trumbore S. E.
Published: February 2004

We measured soil respiration during two winters in three different ecotypes of the BOREAS northern study area. The production of CO2 was continuous throughout the winter and, when totaled for the winter of 1994–1995, was equivalent to the release of ∼40–55 g C/m2 from the soil surface. As soils cooled in the early winter, the CO2 production rate decreased in a manner that appeared to be exponentially related to shallow soil temperatures. This exponential relationship was not observed when soils began to warm, possibly indicating that there may be additional or different processes responsible for increased CO2 production during winter warming events. We also measured CO2 concentrations in soil gas and the Δ14C of the soil CO2. These measurements show that the CO2 produced in winter is not simply the return to the atmosphere of the carbon fixed during the previous growing season. We suggest that the wintertime production of CO2 originates, at least in part, from the decomposition of old organic carbon stored at depth in the soil.
Gravity wave characteristics in the middle atmosphere derived from the Empirical Mode Decomposition method
Authors: Zhu Xun , Shen Zheng , Eckermann Stephen D. , Bittner Michael , Hirota Isamu , Yee Jeng‐Hwa
Published: February 2004

The recently developed Empirical Mode Decomposition (EMD) method is applied to analyzing gravity wave characteristics in the middle atmosphere. By establishing a close connection between the fundamental Intrinsic Mode Functions (IMFs) derived from the EMD method and WKB solutions of a dispersive‐dissipative wave equation, we show that the EMD method can provide useful insights into physical processes in the middle atmosphere where dispersive‐dissipative wave phenomena are dominant. A local power spectrum function P is introduced which provides a quantitative description of the spectrum at any particular location within a data series. The sharp localization of P in space and wavenumber leads to an identification of unphysical small scale oscillations by falling spheres embedded in the wind profiles above 60 km. Further analyses of the horizontal wind profiles derived from the Dynamics Adapted Network for the Atmosphere (DYANA) campaign suggest that for horizontal wind fluctuations with vertical wavenumber m≤3 km−1 (or vertical wavelength Lz≥2 km) the previously observed m−3 Fourier spectra could be produced by a linear wave packet whose characteristic vertical wavenumber decreases with altitude. For small vertical scale disturbances with m>3 km−1 (Lz<2 km) a near −3 slope in the marginal distribution exists locally in the middle atmosphere with a great degree of universality, suggesting that nonlinear energy cascade processes may dominate the spectral formation in this wavenumber range.
Sources and sinks of H2SO4 in the remote Antarctic marine boundary layer
Authors: Jefferson A. , Tanner D. J. , Eisele F. L. , Berresheim H.
Published: February 2004

A steady state analysis of H2SO4 sources and sinks in the Antarctic marine boundary layer was performed using measurements from project SCATE, Sulfur Chemistry in the Antarctic Troposphere Experiment. The calculations show that the SO2 levels needed to account for the observed gas phase H2SO4 ranged from about 17 to 300 parts per trillion by volume (pptv) with an average SO2 concentration of 100 pptv, far more than previous measurements of SO2 in this region which range between 7 and 17 pptv [Berresheim, 1987; Pszenny et al., 1989; P. Quinn, personal communication, 1996]. Boundary layer oxidation of DMS via an SO2 intermediate was found to be an insufficient source of H2SO4 in this region. Likely alternative sources of H2SO4 include oxidation of boundary layer DMDS and vertical entrainment of air from higher altitudes.
Annual carbon cost of autotrophic respiration in boreal forest ecosystems in relation to species and climate
Authors: Ryan Michael G. , Lavigne Michael B. , Gower Stith T.
Published: February 2004

Autotrophic respiration (Ra) in forest ecosystems can be >50% of the carbon fixed in photosynthesis and may regulate productivity and carbon storage in forest ecosystems, because Ra increases with temperature. We estimated annual Ra from chamber measurements in aspen, black spruce, and jack pine forests in Canada for 1994. Mean foliage respiration at 10°C for expanded leaves was 0.21–0.95 μmol m−2 (leaf surface) s−1 for all species and differed little from May to September. Wood respiration at 15°C (0.2–1 μmol m−2 (stem surface) s−1 for all species) was strongly seasonal, with high rates in midsummer that coincided with wood growth. Fine root respiration at 10°C was 2.5–7.7 μmol kg−1 s−1 for all species and declined throughout the growing season for the conifers. Annual costs of Ra for foliage, wood, and roots (overstory and understory) were 490, 610, and 450 g C m−2 (ground) yr−1 for aspen, black spruce, and jack pine (old) in northern Manitoba and 600, 480, and 310 g C m−2 yr−1 for aspen, black spruce, and jack pine (old) in central Saskatchewan. Carbon use efficiency (CUE), the ratio of net production to production plus Ra, averaged 0.44, 0.34, and 0.39 for aspen, black spruce, and jack pine (old) for all tissues and 0.61, 0.36, and 0.44 for aboveground tissues. Differences in CUE between the northern and the southern sites were small for all species, and CUE did not vary with stand biomass. Species differences in CUE suggest that models assuming a constant CUE across species may poorly estimate production and carbon balance for any given site.
Growth behavior of the marine submicron boundary layer aerosol
Authors: Kerminen Veli‐Matti , Wexler Anthony S.
Published: February 2004

A box model for investigating the chemistry and growth of submicron particles in the marine boundary layer was developed. Processes simulated by the model were gas phase chemistry, in‐cloud sulfate production, gas‐to‐particle transfer of condensable vapors, coagulation, dry deposition of particles and gases, and entrainment between the boundary layer and the free troposphere. According to model simulations, the most influential factor for the growth of nuclei and Aitken mode particles is the production rate of methane sulfonic acid (MSA) and other low‐volatility compounds in the gas phase. Processes controlling SO2 concentrations dictate the amount of non‐sea‐salt sulfate produced in the boundary layer but are less important for particle growth. The ratio of MSA to non‐sea‐salt sulfate in the particulate phase may vary largely, even when a constant MSA yield from dimethylsulfide (DMS) oxidation is assumed. Clouds decrease nuclei lifetime but do not affect their growth significantly, unless the time between two cloud passages is very short. Sources other than DMS may produce condensable vapors that assist particle growth to some extent. With our current knowledge of the concentrations of condensible matter in the marine boundary layer, however, it seems unlikely that small nuclei are able to grow into cloud condensation nuclei size over their lifetime. More information is needed on heterogeneous surface reactions that may occur between submicron particles and vapors such as SO2, as well as on potential transport limitations between condensable vapors and particles caused by thermodynamics or organic surfactants.
Turbulence eddy dissipation rates from radar observations at 5–20 km at White Sands Missile Range, New Mexico
Authors: Nastrom G. D. , Eaton F. D.
Published: February 2004

A climatology of the eddy dissipation rate is presented based on the widths of the Doppler radar spectra observed by the 50‐MHz, clear‐air profiler located at White Sands Missile Range, New Mexico. The observations are from radar beams 15° off zenith and extend from about 5 to 20 km altitude. The data are nearly continuous over the 5‐year period 1991–1995. Mean corrections to the observed spectral widths are much larger for beam‐broadening effects than for vertical shear of the wind or gravity‐wave effects. The mean eddy dissipation rate falls between about 10−3.5 and 10−2.5 m2 s−3 at all altitudes and all seasons. The hourly means at a given height have an approximately lognormal frequency distribution. The hourly values have larger standard deviations in the troposphere than in the stratosphere. The maximum eddy dissipation rate is found at the lowest altitude observed, and the minimum value is found about 2–3 km below the tropopause, depending upon season. At about 6–10 km altitude the summer values of eddy dissipation rate are several decibels larger than those during other seasons. When the observations are sorted according to distance from the tropopause, there is very little difference among the seasonal values. The summer tropospheric values show very large interannual changes; the large values of eddy dissipation rate correlate with high surface temperatures and with large mean hourly standard deviations of vertical velocity (an indicator of convective activity in the troposphere). The diurnal changes of eddy dissipation rate and of hourly vertical velocity standard deviations also have similar patterns; that is, largest values occur in the afternoon, and smallest values occur during the night. The largest diurnal range of eddy dissipation rates is found during the summer in the troposphere. The correlation of hourly values of eddy dissipation rate with hourly values of wind speed and vertical wind shear is relatively low, which suggests that the eddy dissipation rate is strongly controlled by the small‐scale flow and is not well predicted from features of the large‐scale flow.
Biospheric environmental monitoring at BOREAS with AVHRR observations
Authors: Czajkowski Kevin P. , Mulhern Theresa , Goward Samuel N. , Cihlar Josef , Dubayah Ralph O. , Prince Stephen D.
Published: February 2004

Global models of biospheric processes such as production efficiency models need environmental input and validation data sets at high temporal and spatial resolution. Methods developed to assess biospheric environmental conditions from advanced very high resolution radiometer (AVHRR) observations, specifically air temperature and surface moisture, are explored through exploitation of field measurements collected at the Boreal Ecosystem‐Atmosphere Study site. The surface temperature/spectral vegetation index (TVX) concept provides the potential for estimating air temperature and surface moisture from AVHRR‐type satellite observations. Initial results show that the slope of TVX over boreal landscapes is related to near‐surface soil moisture in addition to vegetation type and solar irradiance. Also, the TVX air temperature estimation correlates well with shelter height observations. However, our analysis shows that challenges remain in using the TVX approach, largely because it is difficult to isolate the effects of variations in atmospheric conditions from physical environmental conditions at the Earth's surface. Clouds significantly limit the application of the TVX technique. It is also limited when the variability of the normalized difference vegetation index is low or a sharp vegetation boundary is contained within the TVX contextual array. The TVX air temperature can be used to approximate shelter height temperature to within ±5 K under most conditions; however, there are outliers with differences as large as 15 K which, at this time, cannot be explained. The air temperature estimation also exhibits a 3.2 K warm bias. Part of this bias is a result of errors in the split window surface temperature estimate used in the TVX regression.
Modeling soil temperature and snow dynamics in northern forests
Authors: Levine E. R. , Knox R. G.
Published: February 2004

A simulation model of soil moisture and heat flux, FroST (frozen soil temperatures) was developed and applied to field data for the BOREAS northern and southern study areas (NSA, SSA). FroST is a modification of another soil physics model (“Residue”) [Bidlake et al., 1982] to which simple representations of snow dynamics and transpiration, canopy, and litter properties were added. Simulations were run for the old jack pine and black spruce sites at the SSA and NSA. Predicted results showed good fit to measured data for snow depth and soil temperature at various depths at the old jack pine sites for which measured data were available for 1994. The presence of ice through the soil profile was also predicted, indicating a longer and deeper frozen period in the NSA and the development of permafrost at the NSA black spruce site. The model was sensitive to initial conditions of soil moisture and temperature and soil profile characteristics. With the availability of these data, estimates of temperature and moisture flux in soils over time can be predicted. Because the model relies on soil characterization data and yet represents fundamental physical processes, it can be readily extended to new conditions or a wider range of sites.
Enhanced discrimination of boreal forest covers with directional reflectances from the airborne polarization and directionality of Earth reflectances (POLDER) instrument
Authors: Bicheron P. , Leroy M. , Hautecoeur O. , Bréon F. M.
Published: February 2004

During the Boreal Ecosystem‐Atmosphere Study (BOREAS), directional and spectral reflectance measurements were acquired from May to July 1994 with the polarization and directionality of Earth reflectances (POLDER) instrument on board a NASA C‐130 aircraft. The instrument has a wide field‐of‐view optics, a two‐dimensional CCD array, and a rotating wheel carrying filters in the visible and near infrared. Measurements were obtained (1) over coniferous forests at the young and old jack pine and old black spruce sites, (2) over a deciduous forest at the old aspen site, and (3) over a fen at the fen site. A prominent hot spot feature was apparent at each site, with an additional strong peak in the specular direction for the fen site. Strong variations of the bidirectional reflectance distribution function (BRDF) with sun zenith angle were observed. For a constant sun zenith angle, the variation of the BRDF of conifer stands between May and July was relatively weak. A key objective of this paper is to quantify the improvement of discrimination of various forest covers when remotely sensed directional signatures are added to the more conventional spectral signatures. The experimental protocol consisted of the following steps. First, 150 pixels pertaining to five different classes of forest covers were selected on land cover maps available in the BOREAS Information System (BORIS) data base. Second, the BRDF measurements acquired by POLDER at each pixel were adjusted against a three‐parameter semiempirical BRDF model and processed to retrieve the reflectance seen in three different viewing directions. Third, the results of supervised classifications were compared on all selected pixels, using as input, either the reflectances in only one direction (this simulates the case of conventional spectral signatures), or reflectances acquired in three directions (this simulates the case of spectral + directional signatures). The results showed that when only one spectral band was used, the proportion of correctly classified pixels increased from 36–59% with one viewing direction to 64–84% with three viewing directions. When three spectral bands were considered, this proportion improved from 72–87% to 83–97%. These results demonstrate that the account of directional information enhances the ability to discriminate forest covers by remotesensing.
Variation of snow cover ablation in the boreal forest: A sensitivity study on the effects of conifer canopy
Authors: Davis R. E. , Hardy J. P. , Ni W. , Woodcock C. , McKenzie J. C. , Jordan R. , Li X.
Published: February 2004

The duration and meteorological history of winter and thaw periods in the boreal forest affect carbon exchange during the growing season. Characteristics of conifer canopies exert important control on the energy exchange at the forest floor, which in turn controls snow cover processes such as melting. This analysis investigated the role of the conifer tree characteristics, including height and canopy density. Canopy and snow models estimated radiation incoming to the snow surface, the net energy budget of the snow, and melting rates of snow cover under conifer forests with different canopy density and tree height. This analysis assumed that canopy effects dominated snow surface energy exchange under conifers in the boreal forest. We used data layers of forest characteristics from the Boreal Ecosystem‐Atmosphere Study (BOREAS) modeling subareas in Saskatchewan and Manitoba to guide the choice of modeled tree height and canopy density. Modeled stand characteristics assumed random location of trees and used a uniform tree height within a stand and regular crown geometry scaled to tree height. Measurements during winter and thaw in 1994 of incoming solar and longwave radiation, humidity, and wind speed above the forest canopy provided input to the models, along with air temperature measured in the canopy. Results showed the importance of canopy density and tree height as the first‐order controls on cumulative incoming solar radiation at the forest floor for the range of these variables in the BOREAS test area. The combined canopy and snow models showed a large range of snow ablation within conifers, which showed the trade‐offs between canopy density and tree height. Solar fluxes dominated the net transfer of energy to the snow in the north, while sensible heat exchange, net solar, and net longwave radiation played important roles in the south.
Microwave land surface emissivities estimated from SSM/I observations
Authors: Prigent Catherine , Rossow William B. , Matthews Elaine
Published: February 2004

Microwave emissivities of land surfaces are estimated from special sensor microwave/imager (SSM/I) observations by removing the contributions from the atmosphere, clouds, and rain using ancillary satellite data (International Satellite Cloud Climatology Project (ISCCP) and TIROS Operational Vertical Sounder (TOVS) products). In the first step, cloud‐free SSM/I observations are isolated with the help of collocated visible/infrared satellite observations (ISCCP data). The cloud‐free atmospheric contribution is then calculated, from an estimate of the local atmospheric temperature‐humidity profile (TOVS retrieval). Finally, with the surface skin temperature derived from IR observations (ISCCP estimate), the surface emissivity is calculated for all the SSM/I channels. As an exploration the method is applied to the SSM/I data for four months in 1991 within the Meteosat observation area. The magnitude and fluctuations of the atmospheric contributions are estimated along with the effect of surface temperature variations. Correspondences between geographical and seasonal patterns of the emissivities and topography, vegetation, flooding, and snow cover are analyzed. The potential for using microwave emissivities to monitor vegetation phenology and surface properties at regional and continental scale is investigated, and the possibility of retrieving atmospheric parameters (water vapor content, cloud liquid water path, and precipitation) over land is discussed.