bluuebunny/crossref_metadata_2025 huggingface dataset:
Microphysical examination of excess cloud absorption in the tropical atmosphere
Authors: Lubin Dan , Chen Jen‐Ping , Pilewskie Peter , Ramanathan V. , Valero Francisco P. J.
Published: February 2004

To investigate the excess shortwave absorption by clouds, a numerical cloud generation model has been coupled to a plane‐parallel discrete ordinates radiative transfer model. The former was used in a time‐dependent fashion to generate a cumulonimbus turret and three types of cirrus anvil (precipitating, extended, detached) representing three stages of cloud evolution outward from the turret. The cloud particle size distributions, as a function of altitude, were used as input to the radiative transfer model using indices of refraction for pure water and pure ice and equivalent sphere Mie theory. The radiative transfer model was used to calculate the ratio of cloud forcing at the surface to cloud forcing at the top of the atmosphere, both for the broadband shortwave and as a function of wavelength. Recent empirical studies have placed this cloud forcing ratio at around 1.5, and our coupled model results approach this value for small solar zenith angles, when the cloud contains large (>100 μm) ice particles that absorb significantly in the near infrared (primarily the 1.6‐μm window). However, the empirical studies are based on diurnal averages, and our plane‐parallel radiative transfer model yields an area and diurnally averaged cloud forcing ratio of only 1.18 for a tropical cumulonimbus and cirrus anvil system, primarily because of the rapid decrease of the ratio with solar zenith angle. The ratio decreases because of the increase in albedo with solar zenith angle, which is a characteristic feature of plane‐parallel clouds. Adding dust or aerosol to the cloud layers, to make them absorb at visible wavelengths, makes the instantaneous cloud forcing ratio larger for an overhead Sun but also makes the solar zenith angle dependence in the cloud forcing ratio more pronounced. These two effects cancel, eliminating interstitial aerosol as a possible explanation for the excess cloud absorption in plane‐parallel radiative transfer modeling. The strong dependence of the surface/top of the atmosphere cloud forcing ratio on solar zenith angle may be a fundamental defect with the plane‐parallel approach to solar radiative transfer in a cloudy atmosphere.
Biases in Earth radiation budget observations: 1. Effects of scanner spatial resolution on the observed anisotropy
Authors: Ye Qian , Coakley James A.
Published: February 2004

The Earth Radiation Budget Experiment (ERBE) used angular direction models (ADMs) to convert satellite scanning radiometer measurements to radiative fluxes at the top of the atmosphere. The ADMs were assumed to depend only on the physical characteristics of the scene being observed, and they were taken to be independent of the instrument's spatial resolution. As is shown here, however, the angular distribution of the radiation fields derived from ERBE observations depends on the field of view size. Because the spatial resolution of the field of view depends on the satellite view zenith angle, the dependence of the angular distribution on field of view size suggests that the radiative fluxes derived from ERBE have biases which depend on satellite view zenith angle. ERBE scanner observations from the Earth Radiation Budget Satellite (ERBS) are averaged for nadir pixels to construct observations that have a constant field of view size for each of the ERBE satellite view zenith angle bins. The angular distribution of the radiation field for the constant size field of view observations exhibits systematic differences of the order of 2–5% from that obtained with the unaltered, full resolution scanner observations. While the differences are small, they are frequently significant at the 90% confidence level, given the variation in the observations and the sample size (3 months). Reflected radiances constructed from the constant size fields of view are generally more anisotropic than those constructed from the full resolution scanner observations. The population of scene type obtained for the constant size fields of view shows that the ERBE scene identification algorithm has to be modified in order to develop angular direction models which are consistent with the observed anisotropy and also independent of field of view size.
Initiation of a continental ice sheet in a global climate model (GENESIS)
Authors: Otto‐Bliesner Bette L.
Published: February 2004

The initiation and maintenance of a continental ice sheet are investigated using the GENESIS global climate model. A necessary condition for ice sheet initiation is to have snow cover survive through the summer. Model simulations examine the sensitivity of the maintenance of summer snow cover to the prescriptions of topography and solar luminosity. The time period chosen for this study is the Late Carboniferous (306 Ma) when an extensive continental ice sheet, as large if not larger than the Pleistocene glaciations, existed on a supercontinent in the southern hemisphere. This ice sheet, persisting for over 60 m.y., was one of the most prolonged periods of continuous glaciation in Earth history. Model simulations indicate that given the geography, solar luminosity (3% less than present), and atmospheric CO2 (same as present) estimated for this time period, summer snow cover remains as far equatorward as 35°S latitude. Global mean temperature is −2.4°C, 17.2°C cooler than for present. Probable regions for the initiation of the Carboniferous ice sheet are apparent in analysis of the spin‐up of the model to equilibrium. Summer snow cover first persists along the polar coastline of the supercontinent during year 4 of the simulation. Summer snow cover shows the greatest sensitivity to solar luminosity. Increasing the solar constant to its present value from the Late Carboniferous value results in a summer warming of southern hemisphere high‐latitude land areas by as much as 37°C and complete melting of any summer snow cover. Topography plays a lesser role. Reducing land elevations to sea level causes persistent summer snow cover to retreat only 8° poleward. Conversely, initializing the model with an elevated land ice sheet has no effect on summer snow cover extent.
Hydrologic budget of a land surface model: A global application
Authors: Levis Samuel , Coe Michael T. , Foley Jonathan A.
Published: February 2004

We perform a 20‐year, global‐scale simulation with a land surface model (LSX) and compare its annual hydrologic budget with observations at continental and global scales. The model is forced with observed climate data, including temperature, relative humidity, precipitation, cloudiness, and wind. We find that LSX produces a satisfactory simulation of the annual hydrologic budget on both global and continental scales. The differences between model output and observations suggest the need for improved boundary conditions and land surface parameterizations, as well as the need for more complete data sets of global hydrologic variables. This study presents a methodology for the evaluation of land surface models for their more effective use in climate modeling studies.
Dimethyl sulfide, methane sulfonic acid and physicochemical aerosol properties in Atlantic air from the United Kingdom to Halley Bay
Authors: Davison Brian , Hewitt C. Nicholas , O'Dowd Colin D. , Lowe Jason A. , Smith Michael H. , Schwikowski Margit , Baltensperger Urs , Harrison Roy M.
Published: February 2004

The concentrations of dimethyl sulfide in air were obtained during a cruise between the United Kingdom and the Antarctic in the period October 1992 to January 1993 using a method of sampling and analysis optimized to avoid interferences from oxidants. In equatorial regions (30°N to 30°S) the atmospheric DMS concentration ranged from 3 to 46 ng (S) m−3, with an average of 18 ng (S) m−3. In the polar waters and regions south of the Falkland Islands, concentrations from 3 to 714 ng (S) m−3 were observed, with a mean concentration of 73 ng (S) m−3. The concentrations of a range of DMS oxidation products were also obtained. No clear relationships between reactant and product concentrations were seen. Information on particle number concentration, Fuchs surface area and the thermal volatility characteristics of the ambient aerosol was obtained, but again no clear relationships with sulfur concentrations were observed. Accumulation mode particle concentrations averaged 25 cm−3 in the clean marine and polar air masses south of 58°S while background condensation nuclei (CN) concentrations were of the order of 400–600 cm−3. Simplistic calculations suggest that a particle source strength of about 20–60 particles cm−3 d−1 is required to sustain this background CN concentration. It is not clear whether boundary layer nucleation of new CN or entrainment from the free troposphere provided the source of CN. Periods of elevated CN concentrations (>4000 cm−3) were regularly observed in the boundary layer over the Weddell Sea and were attributed to “bursts” of new particle formation. However, shortly after these nucleation events the CN concentration rapidly decayed to the background level through coagulation losses, suggesting little impact on the background CN or cloud condensation nuclei (CCN) concentration.
Comparison of measured stratospheric OH with prediction
Authors: Pickett H. M. , Peterson D. B.
Published: February 2004

The production and loss of stratospheric OH involve relatively fast reactions that are in near‐photochemical equilibrium. In this study, we compare OH measured by our balloon‐borne far infrared limb observing spectrometer with that predicted from a simple model that uses water and ozone fields obtained from instruments on the upper atmosphere research satellite. This comparison is made for latitudes near 34°N over a full diurnal cycle for five balloon flights that span a period of over 2 years. The ratio of measurement to the photochemical model at 40 km is 0.95, with an uncertainty of 0.08 (at a 90% confidence level) due to the measurement. Comparison at other altitudes and as a function of the diurnal cycle also shows excellent agreement.
Colors of contrails from fuels with different sulfur contents
Authors: Gierens K. , Schumann U.
Published: February 2004

During a recent flight experiment, contrails were produced from fuels with normal and very high sulfur content. A color effect was observed [Schumann et al., 1996]. When observed with the Sun in the back, the low‐sulfur contrail appeared grey with weak contrast against the cloud deck underneath, whereas the high‐sulfur contrail appeared slightly brown with higher contrast. In order to give an explanation, we compute the optical properties of the soot‐containing droplets in the contrails and perform a simple radiative transfer computation. We find that the degree of activation of the soot particles is the basic parameter for the explanation of color and contrast of the contrails. It determines the size and number concentration of the droplets, which both in turn determine the optical properties of the contrail as a whole. For the visual appearance of the relatively optically thin contrails (τ ≲ 1.2) the soot volume fraction in the droplets plays only a minor role.
Lidar observations of the stratospheric aerosol layer over southern Italy in the period 1991–1995
Authors: Di Girolamo P. , Pappalardo G. , Spinelli N. , Berardi V. , Velotta R.
Published: February 2004

Lidar measurements of the stratospheric aerosol layer have been carried out in Napoli (40°50′ N‐14°10′ E) and Potenza (40°36′ N‐15°44′ E) during the period 1991–1995, covering the history of the aerosol cloud produced by the eruption of Mount Pinatubo (June 1991, Philippines). Measurements are expressed in terms of aerosol backscattering coefficient βA(z), aerosol integrated backscattering IB and aerosol optical thickness τAat λ = 351 nm and 355 nm; βA(z) and τAare determined from a single‐wavelength elastic return. IB, τA, and βmax, the peak aerosol backscattering, reached their maximum value in December 1991, displaying a subsequent decay withe‐folding times of 237 ± 25, 250 ± 111, and 257 ± 33 days, respectively,Rmax, the peak scattering ratio, is characterized by a decay time of 235 ± 13 days. Measurements of the extinction‐to‐backscattering ratio, αA(z)/βA(z), and of the column parameter, τA/IB, allowed us to retrieve aerosol dimensional characteristics. The time evolution of the height,zc, of the aerosol cloud center of mass was also determined. Downward gravitational settling of stratospheric aerosols with time suggests aerosol particles fall within the size range 0.1–0.3 μm. An abrupt change in IB and βmaxis observed approximately 1000 days after the eruption as a result of the winter‐summer transition and the tropospheric removal of the lower portion of the stratospheric aerosol layer. Changes in the values of IB, τA/IB, andzcsuggest that this transition is characterized by a change in the aerosol mean radius from 0.3 to 0.1 μm.
Thermal mesoscale circulations on the Baltic coast: 2. Perturbation of surface parameters
Authors: Grisogono Branko , Tjernström Michael
Published: February 2004

A three‐dimensional numerical study of an observed seabreeze (SB) is presented in a companion paper, part 1. This paper, part 2, addresses the influence of various forcing parameters emanating from the physical surface and perturbing the control run. The control run in part 1 reveals that the SB stage is preceded by a coastal jet (CJ) stage and that even moderate terrain seems to have a strong influence on the coastal mesoscale circulation. The parameter sensitivity analysis attempts to identify the hierarchy of surface effects on the previously observed and modeled dynamics, while the background airflow parameters remain unchanged. Seven perturbation runs are presented: two topographic, three land surface, and two sea surface temperature perturbations. It is shown that the terrain elevations (h0 ≤ 206m) have profound impact on the flow and its time history. The main offshore island has a notable impact but does not change the main flow structure. Surface temperature perturbations modify the CJ pattern dictated by the mesoscale thermal wind; they also influence the SB shape more but the SB front propagation less. The SB at one coast is “locked up in phase” with another SB at an almost perpendicular coast (i.e., the first SB waits until the other SB removes the opposing ambient flow). This is governed by the specific distribution of land and water, and by the direction of the ambient flow against the coastline. Introduced SST gradients have minor to moderate, localized influence on the dynamics for this case.
On the role of wind‐induced surface heat exchange in a two‐dimensional model of super cloud clusters
Authors: Chao Winston C. , Deng Litao
Published: February 2004

Experiments with a two‐dimensional model of Chao and Lin [1994] are conducted to investigate the role of WISHE (wind‐induced surface heat exchange) in the super cloud clusters. It is found that while WISHE as an instability mechanism is not responsible for the existence of the super cloud clusters, the process of surface fluxes being enhanced by surface wind has the important roles of prolonging the life span of the cloud clusters and making the super cloud clusters more robust. WISHE is also found not responsible for upstream propagation of the super cloud cluster. The latter is explained by the cloud cluster teleinduction mechanism proposed by Chao and Lin [1994].
Are VLF rapid onset, rapid decay perturbations produced by scattering off sprite plasma?
Authors: Dowden R. L. , Brundell J. B. , Lyons W. A.
Published: February 2004

Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler‐induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.
Development of a computationally efficient, reactive subgrid‐scale plume model and the impact in the northeastern United States using increasing levels of chemical detail
Authors: Kumar Naresh , Russell Armistead G.
Published: February 2004

A subgrid‐scale plume model has been developed for a better treatment of dynamics of the emissions from concentrated emission sources in air quality models. The model is based on the Gaussian model description of the dispersion of NOx emissions from the power plants. Detailed inorganic and, if desired, organic atmospheric chemistry is included in the model. The plume model has been interfaced with the Urban and Regional Multiscale (URM) model and has been used to quantify the impact of subgrid‐scale plume modeling on evolution of ozone and other species in the Northeast. Various simulations were performed in the northeastern United States where there are a large number of power plants over the domain. First, a case was chosen where only one large point source was followed using the subgrid plume model to better identify the local impacts of subgrid treatment. Two simulations were performed for this case; the first included a detailed inorganic chemistry in the plume, whereas the second included only simple NO‐NO2‐O3 chemistry. A second case was chosen where all point sources emitting more than 25 tons NOx per day were followed independently using the plume model to study the regionwide impact on predicted ozone. In addition to the two plume treatments used in the test above, a third was added that followed the plume chemistry using a full organic plus inorganic mechanism. In this case, the limited‐NOx, detailed inorganic and detailed inorganic plus organic chemistry, are compared with the base case where the power plant emissions were injected directly into the airshed grid. Using the subgrid‐scale plume model for large point sources had significant local impacts on predicted ozone concentrations, but regionwide impact was very small (less than 2%). Results from the simulation with the detailed organic chemistry found little difference than the detailed inorganic chemistry, though significant local differences were found between those and the simplified NOx chemistry or the calculation without the subgrid‐scale treatment.
Statistical characterization of spatial patterns of rainfall cells in extratropical cyclones
Authors: Bacchi Baldassare , Ranzi Roberto , Borga Marco
Published: February 2004

The assumption of a particular type of distribution of rainfall cells in space is needed for the formulation of several space‐time rainfall models. In this study, weather radar‐derived rain rate maps are employed to evaluate different types of spatial organization of rainfall cells in storms through the use of distance functions and second‐moment measures. In particular the spatial point patterns of the local maxima of rainfall intensity are compared to a completely spatially random (CSR) point process by applying an objective distance measure. For all the analyzed radar maps the CSR assumption is rejected, indicating that at the resolution of the observation considered, rainfall cells are clustered. Therefore a theoretical framework for evaluating and fitting alternative models to the CSR is needed. This paper shows how the “reduced second‐moment measure” of the point pattern can be employed to estimate the parameters of a Neyman‐Scott model and to evaluate the degree of adequacy to the experimental data. Some limitations of this theoretical framework, and also its effectiveness, in comparison to the use of scaling functions, are discussed.
Predicting precipitation level
Authors: Fokianos Konstantinos , Kedem Benjamin , Short David A.
Published: February 2004

We present a generalized logistic regression model for the statistical analysis of multicategorical time series. The model is suitably parameterized and partial likelihood inference is proposed for estimation of the unknown parameters. A goodness of fit statistic is derived to judge the quality of fit. The analysis is applied to data from the Tropical Ocean and Global Atmosphere/Coupled Ocean‐Atmosphere Response Experiment.
Passive microwave remote sensing of rainfall considering the effects of wind and nonprecipitating clouds
Authors: Li Qihang , Bras Rafael L. , Veneziano Daniele
Published: February 2004

It has long been shown both in theory and in observation that emission from rain drops in a raining cloud results in upwelling brightness temperature above that caused by the sea surface alone. High brightness temperatures at microwave frequencies (e.g., 37 and 19 GHz) have usually been quantitatively associated with rainfall using physical or statistical models. By comparing concurrent special sensor microwave/imager and radar data, however, we noticed many cases where there is no appreciable rainfall in a field of view (FOV) which exhibits high brightness temperature (TB) at 37 and 19 GHz. On the basis of calculations and past literature it is shown that such high brightness temperatures can be caused by nonprecipitating clouds and by wind. The effect of the wind is to create wave and high‐emissivity foam on the sea surface. A model is developed to relate TB to the fractional coverage of rain, f, within a FOV. The parameters of the model are calibrated by fitting the model to the observed brightness temperature and fractional rain coverage data. The critical parameter of the model, TB, min, which is the threshold brightness temperature for the presence of rain, depends on the strength of the storm. The strength of the storm is characterized by the fraction of the FOVs within a large area that have TB higher than 240 K, which is readily obtainable from satellite data alone. The instantaneous FOV rain rate R can then be obtained through the f ∼ R relationship which is empirically derived using radar data. An algorithm has been proposed based on the TB ∼ f and f ∼ R relationship. Application of the algorithm to TOGA‐COARE and Darwin storms results in reasonable instantaneous FOV rain estimate. When averaged over the entire radar scan, a more accurate and unbiased areal rain estimate can be achieved.
Ozone peaks associated with a subtropical tropopause fold and with the trade wind inversion: A case study from the airborne campaign TROPOZ II over the Caribbean in winter
Authors: Gouget Hervé , Cammas Jean‐Pierre , Marenco Alain , Rosset Robert , Jonquières Isabelle
Published: February 2004

Aircraft measurements of ozone, methane, carbon monoxide, relative humidity, and equivalent potential temperature were performed during the TROPOZ II campaign. During the aircraft descent down to Pointe‐à‐Pitre (16.3°N, 61.5°W), at 2100 UTC on January 12, 1991, two ozone peaks (75 ppb) are observed, one at an altitude of 7.5 km and the other at 3.0 km. A physicochemical interpretation for each ozone peak is proposed in connection with the meteorological context, using radiosounding data, total ozone content from TOMS/NIMBUS 7 and diagnoses issued from analyses by the European Centre for Medium‐Range Weather Forecasts, Reading, England. The stratospheric origin of the 7.5‐km ozone peak is inferred from negative correlations between ozone and its precursors and from diagnoses based on potential vorticity and ageostrophic circulations depicting the structure of the tropopause fold embedded in the subtropical jet front system. Using an appropriate method to isolate cross‐ and along‐front ageostrophic circulations, we show that much of the observed structure of the tropopause fold can be ascribed to transverse and vertical circulations associated with the irrotational part of the flow. Though the downward extent of the subtropical tropopause fold (400 hPa) is restricted in comparison with typical extratropical tropopause ones (700 hPa), the present results suggest that subtropical tropopause folds may significantly contribute to the global stratosphere‐troposphere ozone exchange. The origin of the 3.0‐km ozone peak trapped just below the trade wind inversion cannot be ascribed precisely. Analogies with other measurements of dust and aerosols transported over the Atlantic or Pacific in the summer season are discussed. Various possibilities are examined: (1) an earlier stratospheric intrusion event, (2) long‐range transport by the trade winds of biomass burning species emitted over West Africa, and (3) fast photochemical ozone formation occurring just below the trade wind inversion within already polluted air parcels originating from remote regions (United States and Gulf of Mexico) after eastward and southward transport around the western Atlantic anticyclone.
Impact of biomass burning in equatorial Africa on the downward surface shortwave irradiance: Observations versus calculations
Authors: Konzelmann Thomas , Cahoon Donald R. , Whitlock Charles H.
Published: February 2004

Long‐term ground‐based measurements of downward surface shortwave irradiance are compared with the satellite‐derived downward surface shortwave irradiance for equatorial Africa. Good agreement between satellite‐derived and measured values are shown in the spring and fall (seasons relative to the northern hemisphere). Large differences between satellite‐derived and measured values are found in regions where extensive savanna fires take place during the dry season. Discrepancies are found in regions north of the equator during the winter months, and the differences shift to the southern hemisphere during summer. Significant differences of 40–80 W m−2are found for extended areas, corresponding to 25–40% of the measured monthly mean downward surface shortwave irradiance. Savanna fires release large amounts of particles into the atmosphere, increasing the aerosol optical depth. The increased optical depth leads to an overestimation of the downward surface shortwave irradiance by the satellite reduction algorithms which do not account for the fire‐produced aerosol.
Venting of gases by convective clouds
Authors: Flossmann Andrea I. , Wobrock Wolfram
Published: February 2004

A two‐dimensional dynamic model with spectral microphysics and a spectral treatment of aerosol particle and gas scavenging (DESCAM) was used to estimate the transport of gases from the marine boundary layer to the free troposphere by a medium‐sized warm precipitating convective cloud. In the simulation, three gases were considered, covering a range of Henry's law constants: an inert tracer, SO2, and H2O2. SO2was also used as the inert tracer by artificially suppressing any interaction with the cloud drops. The horizontal and vertical fluxes, their vertical means and the transport across the cloud boundaries were studied. It was calculated that for SO2as an inert tracer 37 kg, for SO2as a scavenged gas 34 kg, and for H2O212 kg were transported from the marine boundary layer across cloud base to the free troposphere for an estimated three‐dimensional cloud. This represents a depletion of the marine boundary layer in the vicinity of the cloud by about 60%. After about half an hour of cloud life time, however, only 75% of the SO2and only 30% of the H2O2transported aloft still existed in the cloudy air. These residual gases could eventually participate in a long range transport if the cloud would dissipate. The rest had been scavenged by the cloud.
Effects of the radar observation process on inferred rainfall statistics
Authors: Krajewski Witold F. , Anagnostou Emmanouil N. , Ciach Grzegorz J.
Published: February 2004

Contrary to the popular notion that radar can measure rainfall, we demonstrate that both the physics of the radar measurement process as well as the processing of radar data can have significant effects on inferred statistics of rainfall. Only the following effects were considered: (1) the radar beam averaging scheme, (2) the nonlinear transformations inherent in estimation of rainfall based on radar observations, and (3) the coordinate transformation from a polar to a Cartesian grid. The statistics included the following: mean, variance, spatial covariance, and a certain scaling parameter. We limited our considerations to spatial aspects of the problem. The methodology used in this paper is based on analysis of two radar rainfall estimation algorithms and a numerical Monte Carlo simulation. A series of simple experiments allows us to consider the above effects individually and collectively. In the experiments, the true rainfall process is simulated using a variety of statistical models. Its statistics are compared to those obtained after the radarlike processing has been applied. The models used to simulate the rainfall field include point process models, which conceptualize mesoscale rainfall organization according to marked Poisson processes, a random cascade model, and an iterated random pulse model, which is a combination of the above two models.
On the retrieval of lightning radio sources from time‐of‐arrival data
Authors: Koshak William J. , Solakiewicz Richard J.
Published: February 2004

We examine the problem of retrieving three‐dimensional lightning locations from radio frequency time‐of‐arrival (TOA) measurements. Arbitrary antenna locations are considered. By judiciously differencing measurements that are related to the location of the antennas and their excitation times, the problem is converted from the initial spherical nonlinear form to a system of linear equations. In the linear formalism, the source location and time‐of‐occurrence is viewed geometrically as an intersection of hyperplanes in the four‐dimensional Minkowski space (x, y, z, t). The linear equations are solved to obtain explicit analytic expressions for the location and time variables. Retrieval errors are not interpreted with conventional Geometrical Dilution of Precision (GDOP) arguments as discussed by Holmes and Reedy [1951], but with more recent inversion analyses considered by Twomey [1977]. Measurement errors are propagated analytically so that the specific effect of these errors on the solution is clarified. The sensitivity of the solution on the number of antennas used, antenna network geometry, source position, and measurement differencing schemes are discussed in terms of the eigenvalues of the linear system.
Hydrologic applications of satellite data: 2. Flow simulation and soil water estimates
Authors: Guetter Alexandre K. , Georgakakos Konstantine P. , Tsonis Anastasios A.
Published: February 2004

The uncertainty in streamflow simulations and soil water estimates associated with satellite rainfall forcing is investigated for the Upper Des Moines River basin in the midwestern United States. Synthetic series of satellite rainfall estimates were produced with a rain gauge‐satellite stochastic model and 10 years of daily rain gauge data (1979–1988) for three basins with drainage areas ranging from 2,000 km2 to 14,000 km2. The synthetic satellite rainfall series was based on observed satellite visible and infrared data which provided estimates of satellite rainfall for 180 randomly selected days in the period 1980‐1987. Streamflow and soil water estimates were obtained with a rainfall‐runoff‐routing model (3R), based on soil water balance and accounting for snowmelt and frozen ground effects. Sensitivity of flow prediction with respect to rainfall was examined for three different conditions: (1) 3R calibrated and forced with rain gauge data, (2) 3R calibrated with rain gauge data and forced with satellite rainfall, and (3) 3R calibrated and forced with satellite rainfall. The most important results regarding the effect of satellite rainfall on flow simulation and soil water estimation for climate studies are as follows: (1) Flow simulation accuracy is sensitive to the basin scale, yielding higher correlation of simulated with observed streamflow for larger scales, (2) the hydrologic model forced with satellite data possesses skill during the period May‐July for the midwestern United States, (3) derived upper soil water estimates are similar to the ones obtained using rain gauge forcing, and derived lower soil water estimates are lower than those obtained from rain gauge forcing.
OH and HO2 measurements using laser‐induced fluorescence
Authors: Mather James H. , Stevens Philip S. , Brune William H.
Published: February 2004

During August and September 1993 a laser‐induced fluorescence (LIF) instrument was used to observe OH and HO2 as part of the Tropospheric OH Photochemistry Experiment (TOHPE) in the mountains west of Boulder, Colorado. A prototype version of this instrument has been described previously. Modifications were made to the instrument for TOHPE, including the integration of a new dye laser. The instrument was calibrated by producing known amounts of OH and HO2 above the instrument inlet. HO2 was measured on 19 days during TOHPE, while OH was measured on 5 days. Although poor laser performance limited the detection limit to 1 to 2×l06 cm−3 for an integration time of 300 s and a signal‐to‐noise ratio of 1, OH concentrations were observed to reach 3 to 7×l06 cm−3 near midday. HO2 typically peaked near 1×108 cm−3. HO2 consistently reached its maximum near local noon. Observations made with the LIF instrument are compared with observations made with an ion‐assisted technique.
Ozone and potential vorticity at the subtropical tropopause break
Authors: Folkins Ian , Appenzeller Christof
Published: February 2004

Ozone measurements near 200 mbar from two flights between California and Tahiti are interpreted using maps of potential vorticity (PV) on isentropic surfaces. We focus on extremely abrupt changes in ozone mixing ratio observed at latitudes of 13°N and 23.5°N. Their proximity to strong PV gradients on the 350 K isentropic surface shows that they are associated with crossings of the subtropical tropopause. Small‐scale anticorrelations between ozone and carbon dioxide near one of the two ozone transitions indicate that some stratosphere‐troposphere exchange does occur in this region. Ozone mixing ratios on the stratospheric side of the subtropical tropopause varied from 50 to 100 parts per billion by volume, a range that is more commonly associated with the midlatitude troposphere and is much less than seen on the stratospheric side of the midlatitude tropopause.
Measurements of water vapor in the middle atmosphere and implications for mesospheric transport
Authors: Nedoluha Gerald E. , Bevilacqua Richard M. , Gomez R. Michael , Waltman William B. , Hicks Brian C. , Thacker D. L. , Matthews W. Andrew
Published: February 2004

We present data obtained during more than 3 years of nearly continuous measurements of middle atmospheric water vapor. The data are obtained from ground‐based measurements at 22 GHz taken at two sites, one in each hemisphere, using the Naval Research Laboratory water vapor millimeter‐wave spectrometer (WVMS). With the construction of a second instrument, it has been possible to maintain continuous monitoring from both sites since January 1994. The measurements from both instruments show significant seasonal variability. There is a clear annual cycle, with the water vapor above ∼60 km increasing in summer and decreasing in winter. The observed amplitude of the annual oscillation is larger at 45.0°S than at 34.4°N, a result which is qualitatively consistent with the higher latitude of the southern hemisphere site. There is also an indication of a semiannual cycle, particularly at altitudes near 80 km. The annual cycle is consistent with transport due primarily to advection, while the weaker semiannual cycle may be indicative of the effect of gravity waves on diffusive transport.
Three‐dimensional observations of ice crystal characteristics in polar ice sheets
Authors: Nishida Kenro , Narita Hideki
Published: February 2004

In ice core studies, mean crystal size is generally measured on a section image of an ice specimen. However, an ice crystal with a complex shape may appear as several cells on a section image; consequently, the mean crystal size may be underestimated by overcounting the crystals on the image. We examined this phenomenon, which we called duplication effect, through three‐dimensional observation of crystal shapes. In terms of results, we found that the frequency in which duplicate cells appear on a section image is linear with respect to the frequency of which randomly drawn lines on the image pass a cell more than once. On the basis of this empirical relationship, the error on the mean crystal size caused by the duplication effect can be revised from the section image itself. As to its application, we examined crystal sizes in some Antarctic ice specimens and found that the extent of change of crystal size steadily increased with depth up to 100 m and then fluctuated between 6% and 10% in the deeper parts.
The annual cycle of the surface energy balance of Antarctic blue ice
Authors: Bintanja Richard , Jonsson Stig , Knap Wouter H.
Published: February 2004

A 15‐month meteorological data set was obtained from an automatic weather station over a blue ice area in Dronning Maud Land, Antarctica. The meteorological measurements are used as input for a surface energy balance model in order to compute the hourly varying surface fluxes and the (sub)surface temperatures to a depth of 10 m. The model reproduces reasonably well the directly measured temperatures in the upper meter of ice. Model results show that the net shortwave radiation is the largest positive term in the annual mean energy budget (42.2 W m−2). Other positive fluxes are the downward sensible heat flux (12.1 W m−2) and the upward subsurface energy flux (0.2 W m−2). Energy is lost by net longwave radiation (−49.1 W m−2) and the upward directed latent heat flux (−5.0 W m−2). We analyze the annual cycle of the surface heat fluxes on the basis of daily, monthly, and seasonally mean values. In addition, we calculate the surface energy budget for two distinctly different weather regimes, which are typical for this region. Finally, we demonstrate that the annual cycle of the turbulent fluxes can be explained in terms of the limiting values of the Bowen ratio.
On abandoning the thunderstorm tripole‐charge paradigm
Authors: Rust W. David , Marshall Thomas C.
Published: February 2004

We reexamine the original, half‐century‐old, in situ measurements of Simpson and colleagues used to justify the tripole model of charge structure of a thunderstorm that became the paradigm for the charge structure of mature thunderstorms. We find that the original measurements violate the criteria of the tripole model in 49% of the cases. We also reexamine our soundings of electric field in thunderstorms and mesoscale convective systems in the same way as the original soundings were. Our soundings do not fit the tripole model in at least 86% of the storms. Even when we consider only the major features in the electric field profiles, our data show noncompliance with the tripole model in 45% of the cases. We conclude that we must abandon the paradigm of the tripole‐charge structure of thunderstorms.
Wet and dry deposition of nitrogen compounds in the southeast Pacific coast: Montemar, central Chile
Authors: Prado‐Fiedler Roberto , Fuenzalida Humberto A.
Published: February 2004

Atmospheric concentration in the marine boundary layer and direct deposition measurements of total nitrate and ammonium for an eastern South Pacific coastal site near Valparaíso, Chile, were carried out during the period January 1992 through March 1993. Concentrations in air for both nitrogen forms exhibit a seasonal pattern with low values in summer and a pronounced increase in winter, having a mean annual concentration of 32.6 nmol m−3 for total nitrate and of 44.8 nmol m−3 for total ammonium. Wet deposition is characterized by annual precipitation weighted concentrations of 6.0 and 8.1 μmol dm−3 for nitrate and ammonium, respectively. The main factor regulating wet deposition for both nitrate and ammonium is rainfall amount, so that changes in precipitation amount alone originated by climatological factors can produce changes in their relative deposition levels. Existing climatological pattern determines an overall dominance of dry over wet deposition, with totals of 12.9 and 7.34 mmol m−2 per year, the reduced forms of nitrogen being generally more abundant.
Estimation of the source strength of soil NOx in the Nashville, Tennessee, urban area
Authors: Thornton F. C. , Shurpali N. J.
Published: February 2004

Soil biogenic NOx emissions are an important part of the total global NOx budget. The objective of this study was to investigate the role of the soil NOx source in an urban‐metropolitan area. We measured the emissions from soils within the urban‐metropolitan land use area of Nashville, Tennessee, and surrounding Davidson County, to estimate the importance of this source in the overall NOx budget of Davidson County. Nashville is currently in nonattainment for ozone (O3), and a State Implementation Plan (SIP) has been promulgated to help achieve compliance of the O3 standard. Using geographic information systems imagery and aerial photographs, we determined the extent of urban‐metropolitan soils within the Nashville area. Results indicated that 409 km2, or approximately 34% of the county, was classified as residential or urban, with the predominant vegetation being grass (68%). The residential sector was the largest with 323 km2, the urban area comprising 87 km2, and a total area of 8 km2 of golf courses. Soil NOx emission measurements were made in approximate proportion to the major land use types using static chamber techniques. Results indicated that urban‐metropolitan soils were not an important source of soil NOx. Approximately 10 Mg NOx, or 0.003% of the total annual NOx budget of Davidson County, is estimated to be emitted by urban soils annually. Additionally, the remaining soil sources in the county, forests and agricultural land, contributed 179 Mg, or 0.66% of the total annual NOx budget. While soil NOx emissions were concluded to be a negligible component of the Davidson County budget, their contribution to the NOx budget of the surrounding four counties in the SIP area was significant. Extrapolation of soil NOx emissions for three of the four counties within the SIP area indicated that soil biogenic NOx emissions contributed from 7.0 to 9.8% of the daily average NOx budget during the months of June through August. Because of the soil temperature dependence of soil NOx emissions, it was estimated that during the hottest July days the soil biogenic component could contribute over 17% of the total NOx in some SIP counties.
Deposition of 210Pb to the Agassiz Ice Cap, Canada
Authors: Peters A. J. , Gregor D. J. , Wilkinson P. , Spencer C.
Published: February 2004

A record of the annual 210Pb deposition to an Arctic glacier for the past 30 years has been obtained. Samples representing discrete annual layers of accumulated snowfall from the period 1963 to 1993 were collected from a snow pit on the Agassiz Ice Cap, Ellesmere Island, Canada (80°49′N, 72°56′W). The annual layers were independently and accurately dated using stratigraphie comparison and electrical conductivity measurements. This allowed the testing for accuracy of the constant initial concentration (CIC) model used for determining 210Pb chronologies. The measured mean annual concentration and flux (±1 s.d.) of 210Pb to the ice cap for the 30‐year period were determined to be 44 (±19) mBq L−1 and 5.1 (±2.4) Bq m−2 y−1, respectively, and the measured mean accumulation rate was 113 (±31) mm y−1. Despite the high variability of the 210Pb data, the CIC model proved to be accurate, yielding corresponding mean values of 40, 4.4, and 110, respectively. The CIC model also predicted values of the age of firn layers to within approximately ±1 year of the observed ages determined for the 30‐year period. No evidence of a dependence of deposition of 210Pb on the mass accumulation of snow was found. A better understanding of 210Pb and snow accumulation would be achieved by further studies using a more intensive sampling regime.
Cloud‐to‐ground lightning flash characteristics in southeastern Brazil for the 1992–1993 summer season
Authors: Pinto O. , Gin R. B. B. , Pinto I. R. C. A. , Mendes O. , Diniz José H. , Carvalho André M.
Published: February 2004

A lightning positioning and tracking system (LPATS) has been used for the first time to obtain the cloud‐to‐ground flash characteristics in southeastern Brazil during the 1992–1993 summer season. About 300,000 flashes to ground were recorded. The lightning data are presented in terms of polarity, multiplicity, first‐stroke peak current, stroke of maximum peak current, and local time occurrence. The preliminary results are compared with lightning data obtained by several authors in other parts of the world.
Urban/industrial aerosol: Ground‐based Sun/sky radiometer and airborne in situ measurements
Authors: Remer Lorraine A. , Gassó Santiago , Hegg Dean A. , Kaufman Yoram J. , Holben Brent N.
Published: February 2004

Both airborne in situ and ground‐based remote sensing methods are used to measure the properties of urban/industrial aerosols during the Sulfate Clouds and Radiation—Atlantic (SCAR‐A) experiment in 1993. Airborne in situ methods directly measure aerosol characteristics such as size distribution and scattering coefficient at a particular altitude and infer the total column optical properties, such as optical thickness. Ground‐based remote sensing is sensitive to the aerosol optical properties of the entire column and infers the physical properties from inversion of sky radiance. Comparison of optical thickness measurements are encouraging but inconclusive because of measured profiles which extend no higher than 2 km. By comparing aerosol volume size distributions we find that the two systems are in agreement in the radius size range 0.05–2 μm, after the stratospheric aerosol mode is removed from the remote sensing data. At larger aerosol sizes both systems suffer from greater uncertainty, and the larger aerosols themselves are less spatially uniform because of their short lifetimes. The combination of factors makes the comparison at larger radii impossible. The disadvantages of the in situ systems are that there is a measuring efficiency for each device which is dependent on aerosol size and that airborne in situ measurements are rare events in time and space. Also, in situ instruments dry the aerosol before measurement. Automatic remote sensing procedures measure the total column ambient aerosol unaffected by drying or sampling issues, and these instruments can be installed globally to make observations many times per day. However, the disadvantages to remote sensing are that the inferred physical properties are dependent on the assumptions and numerical limitations of the inversion procedures. The favorable comparison between the two types of measurement systems suggests that these drawbacks are manageable in both cases.