bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct climate forcing by biomass‐burning aerosols: Impact of correlations between controlling variables
Authors: Iacobellis Sam F. , Frouin Robert , Somerville Richard C. J.
Published: September 2002

Estimates of the direct climate forcing by condensed organic species resulting from biomass burning have been made using bulk radiative transfer models of various complexity and the SUNRAY radiation code of the European Centre for Medium‐Range Weather Forecasts general circulation model. Aerosols arising from the burning of tropical forests and savannas as well as those from biomass fires outside the tropics are considered. The bulk models give values ranging from −1.0 to −0.6 W m−2, which compare with −0.7 W m−2using the SUNRAY code. There appears to be significant uncertainty in these values due to uncertainties in the model input parameters. The difference is only 13% between the forcing obtained by taking into account the spatial and temporal distribution of the controlling variables and the forcing obtained using global averages for all the variables. This indicates that the effects of variations in the controlling variables tend to compensate. Yet the forcing varies by up to 34% depending on which variables are set to global averages. The SUNRAY results show that the efficiency at which the biomass‐burning aerosols backscatter sunlight in cloudy conditions is 0.53, a value significantly higher than that reported for sulfate aerosols. Most of the difference is due to the relatively low latitude (hence low sun zenith angle) of the biomass‐burning aerosol sources relative to the sulfate aerosol sources. The implication is that clouds should not be assumed to have a reflectivity of unity in bulk models. Comparison of SUNRAY and bulk model results points to other potential problems with bulk models. First, the use in bulk models of mean aerosol optical properties across the entire solar spectrum has significant impact on the calculated forcing and may account for 23% of the difference between SUNRAY and bulk model estimates in clear‐sky conditions. Second, neglecting multiple scattering in bulk models introduces significant differences in the clear‐sky forcing at high sun zenith angles.
Use of on‐line tracers as a diagnostic tool in general circulation model development: 2. Transport between the troposphere and stratosphere
Authors: Rind D. , Lerner J. , Shah K. , Suozzo R.
Published: September 2002

The effect of altering the vertical resolution of a general circulation model and processes associated with it are used to investigate mixing between the troposphere and stratosphere. Four on‐line tracers are employed: chlorofluorocarbon‐11 and SF6 for mixing from the troposphere into the stratosphere, Rn222 for vertical mixing within the troposphere, and 14C for mixing from the stratosphere into the troposphere. Four standard models are tested, with varying vertical resolution, gravity wave drag, and location of the model top, and additional subsidiary models are employed to examine specific features. The results show that proper vertical transport between the troposphere and stratosphere in the Goddard Institute for Space Studies models requires lifting the top of the model considerably out of the stratosphere and including gravity wave drag in the lower stratosphere. Increased vertical resolution without these aspects does not improve tropospheric‐stratospheric exchange. The transport appears to be driven largely by the residual circulation within the stratosphere; associated Eliassen‐Palm flux convergences require both realistic upward propagating energy from the troposphere and realistic pass‐through possibilities. A 23‐layer version with a top at the mesopause and incorporating gravity wave drag appears to have reasonable stratospheric‐tropospheric exchange in terms of both the resulting tracer distributions and atmospheric mass fluxes.
A multipole expansion method for analyzing lightning field changes
Authors: Koshak William J. , Krider E. Philip , Murphy Martin J.
Published: September 2002

Changes in the surface electric field are frequently used to infer the locations and magnitudes of lightning‐caused changes in thundercloud charge distributions. The traditional procedure is to assume that the charges that are effectively deposited by the flash can be modeled either as a single point charge (the Q model) or a point dipole (the P model). The Q model has four unknown parameters and provides a good description of many cloud‐to‐ground (CG) flashes. The P model has six unknown parameters and describes many intracloud (IC) discharges. In this paper we introduce a new analysis method that assumes that the change in the cloud charge can be described by a truncated multipole expansion, i.e., there are both monopole and dipole terms in the unknown source distribution, and both terms are applied simultaneously. This method can be used to analyze CG flashes that are accompanied by large changes in the cloud dipole moment and complex IC discharges. If there is enough information content in the measurements, the model can also be generalized to include quadrupole and higher order terms. The parameters of the charge moments are determined using a three‐dimensional grid search in combination with a linear inversion, and because of this, local minima in the error function and the associated solution ambiguities are avoided. The multipole method has been tested on computer‐simulated sources and on natural lightning at the NASA Kennedy Space Center and U.S. Air Force Eastern Range.
The mineral dust aerosol cycle during the Last Glacial Maximum
Authors: Reader M. C. , Fung I. , McFarlane N.
Published: September 2002

The Canadian Centre for Climate Modelling and Analysis general circulation model (GCM) is used to investigate the factors influencing the mineral dust distribution at the time of the last glacial maximum (LGM). Simulated deposition rates are used in conjunction with existing paleoclimate data to infer likely characteristics of the mineral dust conditions at that time and identify contributions to the pattern of enhanced “dustiness” of the LGM relative to the present, indicated by ice core and loess data. Maximal consistency between the GCM, the Climate: Long Range Investigation, Mapping and Prediction (CLIMAP) data set, and the ice core data requires approximately a 15‐ to 30‐fold increase in dust production globally during the last glacial maximum relative to the preindustrial Holocene. In particular, regional 10‐ to 50‐fold and 20‐fold LGM increases are indicated in eastern Asia and South America, respectively. These implied source enhancements are quite insensitive to the details of the dust model. Analysis of the contributing factors suggests an increase in fine‐particle availability in the source regions.
Intercomparison of cloud liquid water path derived from the GOES 9 imager and ground based microwave radiometers for continental stratocumulus
Authors: Greenwald Thomas J. , Christopher Sundar A. , Chou Joyce , Liljegren James C.
Published: September 2002

Solar reflectance measurements (0.6 and 3.9 μm) from 15‐min Geostationary Operational Environmental Satellite (GOES) 9 imager data were used to estimate cloud liquid water path (LWP) for an extensive stratocumulus system over Oklahoma on May 2, 1996. The objective was to determine the consistency between these satellite estimates and retrievals from high temporal resolution (20 s) surface microwave radiometer (SMWR) measurements. The SMWRs were located at the Atmospheric Radiation Measurement (ARM) program cloud and radiation test bed (CART) sites at Morris and Purcell in Oklahoma. Results show that while the comparisons are in favorable agreement at both sites in the morning and early afternoon (root‐mean‐square difference of 17 g m−2 and correlation of 0.94), large cloud LWP maxima in the midafternoon as measured by the SMWR at the Morris site are not captured by the satellite retrievals. On the basis of indirect evidence (in situ microphysical measurements were unavailable), it is hypothesized that the discrepancies may be the result of the formation of light drizzle in the middle to lower portions of the cloud, unseen at visible and near‐infrared wavelengths from space but easily sensed by microwave radiometry. These results demonstrate that extra care must be taken in future efforts to validate satellite derived cloud properties on a routine basis using SMWR data. Additional information about the cloud microphysical properties may also be required to help properly interpret the comparisons, particularly in the later stages of development of stratocumulus.
Lower stratospheric radiative heating rates and sensitivities calculated from Antarctic balloon observations
Authors: Hicke Jeffrey , Tuck Adrian , Vömel Holger
Published: September 2002

Temperature, water vapor, and ozone profiles from balloon soundings at McMurdo Station, Antarctica, during winter 1994 were used to calculate the diabatic heating rate in the lower stratosphere. These three variables represent the necessary inputs to a clear‐sky radiative heating rate calculation. Accurate profiles of these variables without recourse to ancillary climatological databases provide a means of testing the importance of realistic water vapor and ozone distributions to the lower stratospheric heating rate. Water vapor at wavelengths greater than 17 µm contributes as much cooling as CO2 in the lower stratosphere during winter. Dehydration begins in midwinter and causes a decrease in the cooling rate (increase in the heating rate) by 30%. Ozone depletion affects both longwave and shortwave heating rates in the altitude region where the ozone loss occurs. The longwave heating decreases by 0.3 K d−1 (potential temperature) and is comparable in magnitude to that of the decrease in shortwave heating for a balloon sounding in early October. Significant longwave heating increases of 0.6 K d−1 also occur above the region of ozone depletion as a result of higher penetration of upwelling radiation emitted by the surface. Sensitivity studies of the impact of tropospheric clouds at McMurdo show that the lower stratospheric heating rates can decrease by 0.08 to 0.26 K d−1 (vertically averaged) (18 to 60%), depending on the cloud parameterization used. Time series of the calculated heating rate at McMurdo show the effects of dehydration, ozone loss, and the increase of the heating rate (decrease of the cooling rate) as the temperatures decrease and approach radiative equilibrium through the winter.
Growing season energy and CO2 exchange at a subarctic boreal woodland
Authors: Lafleur Peter M.
Published: September 2002

Measurements of net ecosystem exchange (NEE) of CO2 were made via the eddy covariance technique over a boreal treeline woodland near Churchill, Manitoba, Canada during the growing season of 1997. Precipitation and temperature deviated only slightly from long‐term normals and energy balance measurements were similar to those in other years. The mean daily Bowen ratio was 1.16+0.12 (S.E.) and mean daily evaporation was 1.8±0.19 (S.E.) mm. Mean daily energy balance closure was 0.92±0.036 (S.E.). A curvilinear relationship was used to model the relationship between half‐hourly NEE and photosynthetically active radiation, yielding r2 of 0.51. Poor relationships were found between nighttime ecosystem respiration and air and soil temperatures. The best fit was found for in‐canopy air temperature using average nighttime data (r2 = 0.20). Daily NEE showed a seasonal trend with the peak uptake (6.2 g CO2 m−2 d−1) occurring in late July. The mean daily NEE was an uptake of 1.7 g CO2 m−2 d−1. Accumulated NEE estimated for the 90‐day measurement period was between 86 and 151 g CO2 m−2. CO2 fluxes were found to be considerably smaller than for central and southern boreal forest sites and only slightly larger than tundra sites.
Optical classification, existence temperatures, and coexistence of different polar stratospheric cloud types
Authors: Stein B. , Wedekind C. , Wille H. , Immler F. , Müller M. , Wöste L. , del Guasta M. , Morandi M. , Stefanutti L. , Antonelli A. , Agostini P. , Rizi V. , Readelli G. , Mitev V. , Matthey R. , Kivi R. , Kyrö E.
Published: September 2002

Multispectral lidar measurements of polar stratospheric clouds (PSCs) from two winter campaigns in 1994/1995 and 1996'1997 at Sodankylä, Finland, have been evaluated together with temperature data from local radiosondes to find optical parameters for a PSC classification of different particle types and their existence temperatures. Precise depolarization measurements show that both solid and liquid particles exist below the NAT (nitric acid trihydrate) temperature. A comparison of temperatures at the PSC base and at the cloud top shows a good agreement with the NAT‐existence temperature for solid type Ia clouds and a 3–4 K lower temperature for liquid type Ib clouds. The two particle families are therefore consistent with solid NAT particle formation and condensational growth of HNO3, H2O and H2SO4 liquid ternary solutions. The coexistence of solid and liquid particles has been observed by means of the temporal development of parallel and perpendicular polarized lidar signals. These time series of subsequent lidar measurements show stronger and faster fluctuations in the liquid particle mode compared to the solid particles and thus indicate a higher sensitivity toward temperature fluctuations for the liquid PSCs. While the optical properties of most observations are consistent with the definition of PSC type Ia (solid) and type Ib (liquid) clouds, a third type has been observed which does not fit into the current type Ia/Ib optical classification. This cloud type consists of solid particles but has a higher backscatter than type Ia PSC.
Modeling the methanesulfonate to non‐sea‐salt sulfate molar ratio and dimethylsulfide oxidation in the atmosphere
Authors: Koga Seizi , Tanaka Hiroshi
Published: September 2002

A photochemical box model is used to examine the latitudinal and seasonal variations of the methanesulfonate (MSA, CH3SO3−) to non‐sea‐salt sulfate (nss‐SO42−) molar ratio in NOx‐poor environments of the remote marine atmosphere. Reasonable agreement between observed and modeled results was obtained. The unimolecular decomposition rates of both CH3SO2 and CH3SO3 are assumed to strongly depend on air temperature. These radicals are thought to be produced through the hydrogen abstraction reaction and the addition reaction of dimethylsulfide (DMS, CH3SCH3) with both OH and NO3. In this model, the production of MSA is assumed to occur through the OH addition reaction of DMS along with the hydrogen abstraction reaction. If the MSA production through the OH addition reaction is neglected, the predicted MSA to nss‐SO42− molar ratios are not in agreement with the latitudinal and seasonal variations observed in the atmosphere. Assuming a MSA production yield of 10% through the OH addition reaction in addition to the further oxidation of CH3SO3 without breaking the C‐S bond, the model calculations can reproduce MSA/nss‐SO42− molar ratios similar to the observed latitudinal and seasonal variations. Since the reaction of DMS with OH is the most important sink of DMS in the summer the addition and abstraction reactions appear to control the MSA production during this season. At middle and high latitudes during the winter, DMS is mainly oxidized by reaction with NO3. Therefore MSA in the winter may primarily be produced from the further oxidation of CH3SO3. It appears that competition between the decomposition and the further oxidation of CH3SO3 is a determining factor of the winter MSA/nss‐SO42− molar ratio.
Establishing the stability of multifilter UV rotating shadow‐band radiometers
Authors: Bigelow David S. , Slusser James R.
Published: September 2002

A modified Langley technique is used in a preliminary evaluation of the stability of ion‐assisted deposition interference filters. The technique uses lamp‐calibrated direct‐normal irradiance measurements to return estimates of the extraterrestrial solar irradiance over the passband of the filter. Results from prototype and production UV‐multifilter rotating shadow‐band radiometers deployed in the U.S. Department of Agriculture UV Radiation Monitoring program indicate a 1%/yr drift in filter measurements. Measurements are reported to be within 8% of the SUSIM Atlas‐3 extraterrestrial solar irradiance.
Comparison between observed and computed polar mesosphere summer echoes
Authors: Klostermeyer J.
Published: September 2002

A recently developed model for simulating VHF radar signals from the polar summer mesopause region predicts a predominant effect of the temperature on the generation of these so‐called polar mesosphere summer echoes (PMSE). This effect is due to the strongly nonlinear influence of the temperature on the saturation water vapor pressure over ice determining the concentration of ice particles which in turn affects the concentration of free electrons by electron scavenging. In particular, the height and the temperature of the mesopause as well as height regions with enhanced temperature gradients should significantly control the PMSE layer structure whereas other relevant parameters such as the meteoric dust size distribution, the water vapor mixing ratio, and the energy dissipation rate essentially affect the overall echo intensity. In the present study, temperature profiles and radar signal‐to‐noise ratios observed simultaneously in summer 1987 over Andøya (northern Norway) are used for comparing simulated and measured PMSE under the condition of low cosmic noise absorption. In most cases the model reproduces the observed number of PMSE layers as well as the layer heights and widths with accuracies of 1 km, and the deviations between observed and computed maximum signal‐to‐noise ratios do not exceed 10 dB. Since the temperature profiles and the signal‐to‐noise ratios have been measured at distances of about 40 km, at least part of the discrepancies can be attributed to the horizontal nonuniformity of the mesopause region.
Seasonality of carbon emissions from biomass burning in a Zambian savanna
Authors: Hoffa E. A. , Ward D. E. , Hao W. M. , Susott R. A. , Wakimoto R. H.
Published: September 2002

Seasonal trends in carbon emissions from savanna fires in Western Province, Zambia, were investigated in the early dry season (early June to early August) 1996. The objective was to determine the effect of fuel moisture content on combustion factors and modified combustion efficiency (the ratio of the molar concentration of CO2released to the molar concentration of CO and CO2). Early dry season biomass burning may emit fewer emission products of complete combustion (CO2) and more products of incomplete combustion (e.g., CO). Thirteen experimental sites were burned between June and August 1996, six in a miombo woodland and seven in a dambo grassland. Fires were lit in each ecosystem as the fuels dried so as to monitor changes in fire behavior as the dry season progressed. Total fuel loading ranged from 1884 kg ha−1to 3314 kg ha−1in the dambo and 8953 kg ha−1to 13233 kg ha−1in the miombo. Moisture content of green grass decreased from 127% to 69% in the dambo and from 119% to 33% in the miombo through the length of the study. Combustion factors (CF, the percentage of fuel consumed) increased from 44% to 98% in the dambo. CF values for the miombo increased from 1% to 47%. Fire line intensity increased from 288 kW m−1to 5271 kW m−1in the dambo and from 25 kW m−1to 5274 kW m−1in the miombo. Results indicate that combustion factors and combustion efficiency values follow seasonal trends correlating to metrics of vegetation moisture content, which may alter the type and quantity of carbon emissions. Incorporation of seasonal dynamics of the fire regime should be included in global estimates of carbon flux in the subtropics and in the amount of products of incomplete combustion per unit area burned.
Assessment of flash flood warning procedures
Authors: Johnson Lynn E.
Published: September 2002

Assessment of four alternate flash flood warning procedures was conducted to ascertain their suitability for forecast operations using radar‐rainfall imagery. The procedures include (1) areal mean basin effective rainfall, (2) unit hydrograph, (3) time‐area, and (4) 2‐D numerical modeling. The Buffalo Creek flash flood of July 12, 1996, was used as a case study for application of each of the procedures. A significant feature of the Buffalo Creek event was a forest fire that occurred a few months before the flood and significantly affected watershed runoff characteristics. Objectives were to assess the applicability of the procedures for watersheds having spatial and temporal scale similarities to Buffalo Creek, to compare their technical characteristics, and to consider forecaster usability. Geographic information system techniques for hydrologic database development and flash flood potential computations are illustrated. Generalizations of the case study results are offered relative to their suitability for flash flood forecasting operations. Although all four methods have relative advantages, their application to the Buffalo Creek event resulted in mixed performance. Failure of any method was due primarily to uncertainties of the land surface response (i.e., burn area imperviousness). Results underscore the need for model calibration; a difficult requirement for real‐time forecasting.
Trace gas mixing ratio variability versus lifetime in the troposphere and stratosphere: Observations
Authors: Jobson B. T. , McKeen S. A. , Parrish D. D. , Fehsenfeld F. C. , Blake D. R. , Goldstein A. H. , Schauffler S. M. , Elkins J. W.
Published: September 2002

Several archived data sets have been reviewed to examine the relationship between mixing ratio variability and lifetime for hydrocarbon and halocarbon species in the troposphere and stratosphere. The dependence on lifetime was described by the power law relationship slnX = Aτ−b where slnX is the standard deviation of the ln of the mixing ratios, A is a proportionality coefficient, and b is an exponent that relates to the dominance of sink terms in the regional variability budget. At the Harvard forest ground site, winter and summer data displayed the same lifetime dependence, τ−0.18, which was significantly weaker than the τ−0.5 dependence of remote tropospheric data, indicating that source terms dominated regional variability at Harvard. In addition, the ratio of summer to winter slnX values was found to be similar for all species except ethane, averaging 1.54±0.04. This ratio is consistent with a factor of 11 seasonal change in the species lifetimes, given a τ−0.18 lifetime dependence. Stratospheric data displayed a stronger lifetime dependence than tropospheric trends, indicating a more dominant role for sink terms in describing spatial variability in this region of the atmosphere. We show that a unique power law relationship between slnX ratios for two species Xi and Xj and the kinetic slope of ln(Xi) versus ln lpar;Xj) correlation plots is found to hold in both observations and theory. Thus knowledge of the coefficient b allows for a clearer understanding of the relationship between observed slopes of ln (Xi) versus ln (Xj) correlation plots and the ratio of the species lifetimes.
Numerical modeling of atmospheric response to polynyas in the Southern Ocean sea ice zone
Authors: Dare R. A. , Atkinson B. W.
Published: September 2002

The polar regions play an important role in the climate system but energy exchanges between the atmosphere and surface in polar sea ice regions are not well understood, particularly in the observation‐sparse Southern Ocean. A high resolution mesoscale numerical model of the planetary boundary layer was used to simulate mean winter atmospheric conditions over open water (widths of 10, 20, 30, 40, and 50 km) surrounded by sea ice in the Southern Ocean, rather than the colder Arctic environment which is the focus of most past work. Previous work has concentrated on surface turbulent fluxes. Here we present data on momentum, heat, and moisture fluxes up to 800 m. Shear and increase in buoyancy above the polynya generated a turbulent plume that caused downward mixing of high momentum air. The consequent acceleration of the surface wind and horizontal divergence contributed to descent, despite thermal buoyancy, while downwind of the polynya, there was a region of convergence and ascent. Surface drag on the atmosphere over the water increased due to the accelerated wind and increased drag coefficient. The surface heat flux increased for distances up to 20 km over open water owing to the increase in the heat transfer coefficient and wind speed. Beyond the point where the wind speed reached a maximum, the decrease in surface‐air temperature difference, which decreased with distance from the upwind ice edge, became the dominant influence on heat flux. Transfer of heat from air to the ice downwind of a polynya increased with polynya width despite an increase in thermal stability of the lower atmosphere.
Joint retrieval of atmospheric constituent profiles from ground‐based millimeterwave measurements: ClO, HNO3, N2O, and O3
Authors: Kuntz M. , Kopp G. , Berg H. , Hochschild G. , Krupa R.
Published: September 2002

The constrained linear inversion technique and the optimal estimation method are applied to joint retrievals of atmospheric constituent profiles from vertical sounding spectra in the millimeterwave range. The algorithms are illustrated on dual profile retrievals of ozone (O3) and chlorine monoxide (ClO) from synthesized spectra and on dual profile retrievals of ozone and chlorine monoxide, of ozone and nitrous oxide (N2O), and of ozone and nitric acid (HNO3), respectively, from measurements performed with the ground‐based millimeterwave radiometer MIRA II during February and March 1996 at Kiruna. Reconstructed profiles are compared to observations performed by other groups. The joint retrieval concept proves to be a useful tool that provides valuable information about the altitude distribution of atmospheric constituents.
Microwave radiometric signatures of different surface types in deserts
Authors: Prigent Catherine , Rossow William B. , Matthews Elaine , Marticorena Béatrice
Published: September 2002

In arid environments, specific microwave signatures have been observed with the Special Sensor Microwave /Imager (SSM/I). For a given diurnal change in surface skin temperature, the corresponding change in the microwave brightness temperature is smaller than expected. With the help of a one‐dimensional, time‐dependent heat conduction model, this behavior is explained by microwave radiation coming from different depths in the soil, depending on the soil type and on the microwave radiation frequency. Using the 8‐times daily estimates of the surface skin temperature by the International Satellite Cloud Climatology Project (ISCCP) and a simple Fresnel model, collocated month‐long time series of the SSM/I brightness temperatures and the surface skin temperatures give a consistent estimate of the effective microwave emissivity and penetration depth parameters. Results are presented and analyzed for the Sahara and the Arabian Peninsula, for July and November 1992. The case of the Australian desert is also briefly mentioned. Assuming a reasonable thermal diffusivity for the soil in desert areas, the microwave radiation is estimated to come from soil layers down to depths of at least five wavelengths in some locations. Regions where the microwave radiation comes from deeper soil layers also have large microwave emissivity polarization differences and large visible reflectances, suggesting that these areas correspond to sand dune fields. Two soil classification data sets show good correspondence of sand dunes and the microwave signature of significant penetration. This suggests that this analysis of microwave observations, along with other remote sensing technics, can be used to map the sand dunes in large, poorly surveyed deserts; a map of the sand dune fields in the Sahara and Saudi Arabia is derived from SSM/I observations.
Streamers observed by the CRISTA experiment and simulated in the KASIMA model
Authors: Kouker W. , Offermann D. , Küll V. , Reddmann T. , Ruhnke R. , Franzen A.
Published: September 2002

Data from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) showed three narrow streamers of air with tropical mixing ratios of HNO3 and N2O pointing from the tropics toward middle latitudes in the middle stratosphere on November 6, 1994. By means of the mechanistic prognostic model, the diagnostic chemical transport model (CTM) and the combined nudged model, which are all versions of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the hypothesis is checked of whether these streamers are due to adiabatic transport processes on a timescale of days. Whereas the prognostic model reproduces the northern hemisphere streamers only qualitatively in their position, the CTM and the nudged model show a good agreement between their simulated tracer structures and the observed streamers. Because of the clear streamer signal in the nudged model compared to the CTM, its data are used for the investigation of isentropic tracer deformations. They show that the northern hemisphere streamers are mainly built by adiabatic transport on a timescale of days. Rossby wave breaking plays a role in the dissolution of the streamers. In the southern hemisphere, the production of Ertel's potential vorticity (EPV) and the net heating rate is large, and the observed streamers are therefore not reproduced in the EPV. Moreover, the isentropic deformations of the EPV due to the horizontal flow are that strong during a minor warming in the end of October that the reproduction of the southern hemisphere streamer by means of artificial tracers fails.
Comparison of satellite and in situ ozone measurements in the lower stratosphere
Authors: Lingenfelser Gretchen S. , Grose William L. , Remsberg Ellis E. , Fairlie T. Duncan , Pierce R. Bradley
Published: September 2002

An air parcel trajectory model with chemistry initialized using data obtained by the Halogen Occultation Experiment (HALOE) has been demonstrated to be an effective tool for studying chemical and transport processes in the lower stratosphere. One of the sources of the differences between the trajectory calculations and the in situ observations from instruments aboard the NASA ER‐2 aircraft is the uncertainty in the initial HALOE data. We compared HALOE ozone (O3) with near‐coincident O3 obtained by the NOAA dual‐beam UV‐absorption photometer during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign in a previous study. We have extended this study to obtain a more reliable evaluation of the differences in the remote and in situ O3 data by increasing the statistical base and using a revised version of HALOE data. In the present study we have extended the comparisons of O3 data to encompass four separate ER‐2 airborne campaigns that have occurred since HALOE became operational in 1991. By extending the previous study to encompass the time span from 1991 through 1996, a broader range of latitude in both hemispheres, and a variety of seasons, we believe the conclusions resulting from the comparisons to be more robust and reliable. Statistical analyses show that the best agreement between HALOE and ER‐2 O3 occurs in the 50–70 mbar region, where the difference of the means and RMS differences between the data sets are less than 7% and 28%, respectively, and where the HALOE data have an estimated error greater than 18% but less than 30%. Differences are larger in the 70–100 mbar region, where the maximum mean and RMS differences are 20% and 50%, respectively. These differences are somewhat less than those determined in the previous study.
Uncertainty of measurements of spectral solar UV irradiance
Authors: Bernhard G. , Seckmeyer G.
Published: September 2002

Most investigations on the nature and effects of solar ultraviolet (UV) radiation at the Earth's surface require measurements of high accuracy combined with well‐defined procedures to assess their quality. Here we present a general evaluation of all relevant errors and uncertainties associated with measurements of spectral global irradiance in the UV. The uncertainties are quantified in terms of dependence of the characteristics of the spectroradiometer, the uncertainty of calibration standards, the solar zenith angle, and atmospheric conditions. The methodologies and equations presented can be applied to most spectroradiometers currently employed for UV research. The sources of error addressed include radiometric calibration, cosine error, spectral resolution, wavelength misalignment, stability, noise, stray light, and timing errors. The practical application of the method is demonstrated by setting up a complete uncertainty table for the mobile spectroradiometer of the Fraunhofer Institute for Atmospheric Environmental Research (IFU). This instrument has successfully participated in several international intercomparisons of UV spectroradiometers. The expanded uncertainty (coverage factor k = 2) for measurements of global spectral irradiance conducted with this instrument varies between 6.3% in the UVA and 12.7% at 300 nm and 60° solar zenith angle. The expanded uncertainties in erythemally and DNA weighted irradiances are 6.1% and 6.6%, respectively. These expanded uncertainties are comparable to uncertainties at the 2σ level in conventional statistics. A substantial reduction of these uncertainties would require smaller uncertainties in the irradiance standards used to calibrate the instrument. Though uncertainties caused by wavelength misalignment and noise become prominent in the shortwave UVB, which is the most important spectral range for UV trend detection, the results indicate that the accuracy of the IFU radiometer is sufficient to detect long‐term trends in UV arising from a 3% change in atmospheric ozone. The detection of trends caused by a 1% change in ozone may be beyond the capabilities of current instrumentation.
Sensitivity of simulated atmospheric mercury concentrations and deposition to model input parameters
Authors: Pai Prasad , Karamchandani Prakash , Seigneur Christian , Allan Mary Ann
Published: September 2002

Previously, we have simulated the atmospheric transport and fate of mercury emissions in North America and derived estimates of ambient concentrations and dry and wet deposition of mercury. In this study we quantify sensitivity of the derived estimates to model input parameters that we believe have the largest potential to influence model estimates. We vary five input parameters: emission speciation, Hg(II) dry deposition velocity, precipitation amount, concentration of redox species, and Hg(II) boundary conditions, within their plausible range of values. Our results show that emission speciation has the largest influence and Hg(II) boundary conditions have the smallest influence on the derived estimates. The sensitivity of simulated wet deposition to emission speciation and redox species concentration is non‐linear and varies by region. In regions with low wet deposition (5–15 μg m−2 yr−1), emission speciation and chemistry show comparable influence, whereas in regions with high wet deposition (15–30 μg m−2 yr−1), emission speciation shows greater influence than chemistry. The interregional differences in sensitivity suggest that different pathways control total wet deposition for different regions. While in our previous study we evaluated the modeling system against observations, the sensitivity studies described in this paper enabled us to obtain new insights on atmospheric mercury by focusing on the dynamics of the system, i.e., response of the system to variation in its inputs. This analysis is essential before model‐simulated results are used to investigate source‐receptor relationships. Our findings also indicate that there is a critical need to get additional data on mercury speciation of major emission sources.
Formation of acetone from the OH radical‐ and O3‐initiated reactions of a series of monoterpenes
Authors: Reissell Anni , Harry Cheryl , Aschmann Sara M. , Atkinson Roger , Arey Janet
Published: September 2002

The formation yields of acetone from the gas‐phase reactions of the OH radical (in the presence of NO) and O3 with a series of monoterpenes have been measured at room temperature and atmospheric pressure of air. The acetone formation yields ranged from <2–3% for the OH radical reaction with limonene and the O3 reactions with limonene and α‐phellandrene to 50% for the O3 reaction with terpinolene. Combining these acetone formation yields with literature estimates of emission rates of monoterpenes from vegetation leads to an estimate of acetone formation from the atmospheric photooxidation of monoterpenes of ∼10–11 Tg yr−1 globally, a significant fraction of the global acetone source strength of 40–60 Tg yr−1. Reaction mechanisms leading to acetone formation from these monoterpene reactions are discussed.
Sensitivity of a meridional temperature gradient index to latitudinal domain
Authors: Gitelman Alix I. , Risbey James S. , Kass Robert E. , Rosen Richard D.
Published: September 2002

We compare two alternative definitions of the meridional surface temperature gradient (MTG) in the northern hemisphere. One focuses on equator‐pole differences ((0°–25°N)–(65°–90°N)) and the other focuses on differences over midlatitudes ((30°–35°N)–(50°–55°N)). The MTG is sensitive to the choice of latitudes used to define it, and these two MTGs show substantial differences in behavior on timescales from interannual to multidecadal. The equator‐pole MTG is plagued by missing data in the high‐latitude belt, which affects its characteristics over at least the early part of the data record. Because the missing data are concentrated most heavily in the northernmost zones of the high‐latitude belt, the variance of the equator‐pole MTG is unduly high. This suggests a need to weight the equator‐pole MTG by data availability in each latitude zone. The midlatitude‐oriented MTG is more robust with respect to data availability and provides a measure of midlatitude circulation features.
Measurements of J(NO2) at Ny‐Ålesund, Svalbard
Authors: Beine Harald J. , Dahlback Arne , Ørbæk Jon B.
Published: September 2002

The photolytic rate constant of NO2, J(NO2), has been determined using two independent methods at the Ny‐Ålesund International Arctic Research and Monitoring Facility. At the Zeppelin mountain station the Norwegian Institute for Air Research (NILU) has operated a commercially available J(NO2) radiometer since 1994. Additionally, a five‐channel UV irradiance radiometer (GUV‐541) was operated at Ny‐Ålesund during 1995/1996. The results of the latter, together with surface albedo measured by the Norwegian Polar Institute (NP), are used as input in a radiative transfer model using the discrete ordinate method to calculate J(NO2) data. In this paper we present a detailed error discussion for the radiometer data and the quality controlled measurements of J(NO2). J(NO2) reached an annual maximum between 1994 and 1996 of up to 0.012 s−1 during springtime. Surface albedo was found to have a large influence on the radiative balance in the Arctic. Measured noontime data are compared with model results. Unweighted linear regression showed for 1996 a J(NO2)model/J(NO2)measure ratio of 0.90. This good agreement suggests that multichannel irradiance radiometers have the potential to be used to derive J values for a variety of trace gases.
Impact of biomass burning on rainwater acidity and composition in Singapore
Authors: Balasubramanian R. , Victor T. , Begum R.
Published: September 2002

The Indonesian forest fires that took place from August through October 1997 released large amounts of gaseous and particulate pollutants into the atmosphere. The particulate emissions produced a plume that was easily visible by satellite and significantly affected regional air quality in Southeast Asia. This prolonged haze episode provided an unprecedented opportunity to examine the effects of biomass burning on regional atmospheric chemistry. We undertook a comprehensive field study to assess the influence of biomass burning impacted air masses on precipitation chemistry in Singapore. Major inorganic and organic ions were determined in 104 rain samples collected using an automated wet‐only sampler from July through December 1997. Mean pH values ranged from 3.79 to 6.20 with a volume‐weighted mean of 4.35. There was a substantially large number of rain events with elevated concentrations of these ions during the biomass burning period. The relatively high concentrations of SO2−4, NO−3, and NH+4 observed during the burning period are attributed to a long residence time of air masses, leading to progressive gas to particle conversion of biomass burning emission components. The decrease in pH of precipitation in response to the increased concentrations of acids is only marginal, which is ascribed to neutralization of acidity by NH3 and CaCO3.
Temperature effects correction in a Brewer MKIV spectrophotometer for solar UV measurements
Authors: Cappellani F. , Kochler C.
Published: September 2002

The increasing demand for accuracy in the measurement of the solar UV‐spectral irradiance, has motivated an investigation of the dependence of the spectral responsivity of our Brewer MKIV spectrophotometer on the ambient temperature. The study has been performed using a set of 50 W tungsten‐halogen lamps whose specific intensities have been calibrated by a National Institute of Standards and Technology referenced lamp. The main observed feature is a decrease of the responsivity of the instrument with increasing temperature: this effect is greater at low wavelengths and can produce mean inaccuracies over the 290–325 nm spectral interval ranging from 2% in the cold season up to 8% in summer. The proposed correction methodology makes use of a family of responsivity files (uvres files) made at different temperatures and normalized to a calibration performed with a referenced lamp at a given temperature. Each UV spectrum taken along the day will be calculated using the uvres files corresponding to the same temperature at which the spectrum has been measured. The appropriate uvres will be obtained by linear interpolation of the already determined responsivity files.
Measurements of the diffuse UV sky radiance during broken cloud conditions
Authors: Weihs Philipp , Webb Ann R. , Hutchinson Samantha J. , Middleton Glenn W.
Published: September 2002

Sky radiance distributions pertaining to surface UV have been investigated under clear‐sky and broken cloud conditions. Comparison between the measured clear‐sky distribution and either a radiative transfer model or an empirical function showed reasonable agreement in each case. The radiance distribution measurements performed during broken cloud conditions were then investigated and compared to the clear‐sky radiance. Radiances from cloudy sky locations were either higher (by up to 2.5 times) or lower (with minimum values that were 8% of the clear‐sky radiances) than the clear‐sky case. It has been shown that clouds have an enhancing effect on the ground UV radiance at locations with scattering angles between 30° and 60° and a sight zenith angle less than 30°, or with scattering angles greater than 60° irrespective of the sight zenith angle. These results represent an advance toward the accurate description and modeling of ground UV under broken cloud conditions.
Regional surface momentum flux derived from atmospheric boundary layer bulk similarity approach
Authors: Sugita Michiaki , Endo Nobuhiko , Hiyama Tetsuya
Published: September 2002

Wind speed data in the atmospheric boundary layer (ABL) observed during two recent experiments over a wide stability range and over different experimental areas allowed an extensive test of the ABL bulk similarity equation with the similarity functions Bw previously proposed and calibrated with the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) data set in order to obtain regional surface momentum fluxes. It was found that FIFE‐calibrated Bw functions produced similarly good results with the two new data sets; furthermore, one similarity function Bw based on a simple dual‐layer ABL model which worked best with the FIFE data set was also found to perform best with the new data sets. Recalibration of the constant of this Bw function with all three combined data sets resulted in the same value as that calibrated with the FIFE data set.
Subsoil CO2 and CH4 and their advective transfer from faulted grassland to the atmosphere
Authors: Etiope Giuseppe
Published: September 2002

Measurements of CO2 and CH4 in 3 m deep groundwater, soil‐gas, and soil‐atmosphere fluxes were completed in two grasslands in central Italy having the same soil conditions but different subsoil fault‐linked secondary permeability. Unfaulted grassland displays gas‐phase equilibrium between soil‐air and groundwater, typical soil‐gas diffusion profiles, and diffusive soil to atmosphere gas transfer; on the basis of oxygen depletion assessment, assuming a ratio of 1:1 between biogenic O2 consumption and CO2 production, the measured soil CO2 concentrations are consistent with a normal production in the soil by biologic activity. The faulted grassland, instead, has higher CO2 and CH4 concentrations (up to 6% and 10 ppmv in soil‐air) and flux (1.2 mL m−2 s−1 and 1.3 μL m−2 s−1) resulting from a combination of soil biologic and endogenous components, with evidence of gas transfer from the saturated to the unsaturated zone, and advective gas transfer from soil to the atmosphere. The extra‐soil source in the faulted zone, which is about 0.3–4 times the background soil biologic production, is likely related to migrating crustal gas. Whatever the C gas origin and depth may be (biogenic or abiogenic, shallow or deep), the present results demonstrate that the subsoil‐derived component occurring in the soil in areas with active tectonics cannot be ignored a priori in the assessment of the C terrestrial sources. In particular, the assumption that dryland soils are sinks for methane, owing to methanotrophic consumption, may be not true in areas affected by active and gas‐bearing faults. Accordingly, it would be very important to assess at global scale the actual role in the carbon dioxide and methane cycle of soils within the active tectonic bounds.