bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer simulation of a downward negative stepped leader and its interaction with a ground structure
Authors: Mazur V. , Ruhnke L. H. , Bondiou‐Clergerie A. , Lalande P.
Published: February 2004

A scheme for computer simulation of a negative stepped leader is developed, based on a physical description of the processes occurring during step development. The scheme is applied to a three‐dimensional model of the interaction between a downward negative leader and a grounded mast under two assumptions, one with and one without an upward positive leader initiated from the tip of the mast. The modeling is performed for 2‐m and 10‐m‐diameter leaders under a −10‐MV and −40‐MV electrical potential, respectively. The downward leader reacts to the mast only at the final jump‐over, irrespective of the emission of the positive leader from the mast, thus suggesting the critical role of the final jump. The dominant factor in the interaction of the negative leader with a grounded mast of fixed height is shown to be the leader's potential, which determines the length of the leader step and, thus, the length of the final jump to the ground structure.
Spatial and temporal variability of satellite‐derived cloud and surface characteristics during FIRE‐ACE
Authors: Maslanik J. A. , Key J. , Fowler C. W. , Nguyen T. , Wang X.
Published: February 2004

Advanced very high resolution radiometer (AVHRR) products calculated for the western Arctic for April‐July 1998 are used to investigate spatial, temporal, and regional patterns and variability in energy budget parameters associated with ocean‐ice‐atmosphere interactions over the Arctic Ocean during the Surface Heat Budget of the Arctic Ocean (SHEBA) project and the First ISCCP (Internatonal Satellite Cloud Climatology Project) Regional Experiment ‐ Arctic Cloud Experiment (FIRE‐ACE). The AVHRR‐derived parameters include cloud fraction, clear‐sky and all‐sky skin temperature and broadband albedo, upwelling and downwelling shortwave and longwave radiation, cloud top pressure and temperature, and cloud optical depth. The remotely sensed products generally agree well with field observations at the SHEBA site, which in turn is shown to be representative of a surrounding region comparable in size to a climate‐model grid cell. Time series of products for other locations in the western Arctic illustrate the magnitude of spatial variability during the study period and provide spatial and temporal detail useful for studying regional processes. The data illustrate the progression of reduction in cloud cover, albedo decrease, and the considerable heating of the open ocean associated with the anomalous decrease in sea ice cover in the eastern Beaufort Sea that began in late spring. Above‐freezing temperatures are also recorded within the ice pack, suggesting warming of the open water areas within the ice cover.
Ice particles in stratiform clouds in the Arctic and possible mechanisms for the production of high ice concentrations
Authors: Rangno Arthur L. , Hobbs Peter V.
Published: February 2004

The presence of ice particles in clouds affects precipitation processes, the radiative properties of the clouds, and the derivation of cloud properties from remote sensing measurements. High ice particle concentrations occur often in slightly to moderately supercooled clouds in the Arctic. This paper combines data collected in a common type of ice‐producing arctic cloud (stratocumulus) with calculations based on laboratory experiments to elucidate mechanisms that might be responsible for the ice. Ice splinters produced during riming could account for the relatively high concentrations of ice particles in clouds that encompass temperatures between −2.5°C and −8°C. However, it has generally been assumed that ice splinters grow into pristine ice crystal habits, whereas detailed measurements in an arctic stratocumulus cloud showed that only 32% of the ice particles were pristine crystals (needles, sheaths, short columns, and plates) and 10% were broken pieces of needles or sheaths. Thirty‐seven percent of the ice particles were not identifiable crystal types, 20% were frozen drops, and 1% were aggregates and graupel. The large numbers of unidentifiable ice particles could have originated from the fragmentation of delicate ice crystals and the shattering of some drops during freezing in free fall. These two mechanisms may also play a role in the production of relatively high ice particle concentrations in moderately supercooled arctic clouds that lie outside of the temperature zone where ice splinter production by riming occurs.
Mixing of anthropogenic pollution with stratospheric ozone: A case study from the North Atlantic wintertime troposphere
Authors: Parrish D. D. , Holloway J. S. , Jakoubek R. , Trainer M. , Ryerson T. B. , Hübler G. , Fehsenfeld F. C. , Moody J. L. , Cooper O. R.
Published: February 2004

As part of the North Atlantic Regional Experiment (NARE), instrumentation for the measurement of O3 and CO was included on research flights conducted by the National Oceanic and Atmospheric Administration WP‐3D Orion aircraft from St. John's, Newfoundland, Canada, and Keflavik, Iceland, from February 2 to 25, 1999. These flights sampled the lower troposphere over the western North Atlantic Ocean. One significant feature observed during these flights was the close proximity of air masses with contrasting source signatures: high levels of anthropogenic pollution immediately adjacent to elevated O3 of stratospheric origin. Here we present a case study showing the most pronounced example of this proximity, which was associated with a frontal passage across North America and out into the North Atlantic region. Trajectory analyses and satellite imagery are used to investigate the transport mechanisms that create the interleaving of air masses from the different sources. One important chemical feature was noted: in air masses with differing amounts of anthropogenic pollution admixed, O3 was negatively correlated with CO, which indicates that emissions from surface anthropogenic sources had reduced O3 in this wintertime period, even in air masses transported into the free troposphere.
Investigation of the collision‐induced absorption by O2 near 6.4 μm in pure O2 and O2/N2 mixtures
Authors: Maté B. , Lugez C. L. , Solodov A. M. , Fraser G. T. , Lafferty W. J.
Published: February 2004

The collision‐induced fundamental vibrational band of molecular oxygen has been measured between 1300 and 2000 cm−1 using a Fourier‐transform infrared spectrometer and an optical path length of 84 m. Spectra were recorded for pure O2 and O2/N2 mixtures at densities up to 10 times the density of an ideal gas at standard temperature (273.15 K) and pressure (101.325 kPa), and for temperatures between 228 and 296 K. The band is dominated by the ΔJ = 0, Q branch and the ΔJ = 2, S and ΔJ = −2, O branch shoulders, with the S branch exhibiting ripples previously attributed to bound dimer transitions, pure quadrupole transitions of O2 perturbed by line mixing, and intercollisional interferences. The ripples are seen at the same wavenumbers in O2‐Ar mixtures, with intensities dependent on both the O2 and Ar densities, suggesting that the ripples are not due to bound dimer transitions. The integrated band intensity S is related to the collision‐induced absorption coefficients by S = SO2‐O2 ρO22 + SO2‐N2 ρO2 ρN2, where SO2‐O2 and SO2‐N2 are the integrated binary collision‐induced absorption coefficients for O2‐O2 and O2‐N2 collisions, respectively, and ρO2 and ρN2 are the O2 and N2 gas densities. We find values for SO2‐O2 = 6.972(66) × 10−4 cm−2 and SO2‐N2 = 7.12(22) × 10−4 cm−2, respectively, at 296 K, when the gas density is equal to that found at STP (i.e., SO2‐O2 = 6.972(66) × 10−4 cm−2 amagat−2 and SO2‐N2 = 7.12(22) × 10−4 cm−2 amagat−2).
The episodic nature of air pollution transport from Asia to North America
Authors: Yienger James J. , Galanter Meredith , Holloway Tracey A. , Phadnis Mahesh J. , Guttikunda Sarath K. , Carmichael Gregory R. , Moxim Waller J. , Levy Hiram
Published: February 2004

We employ the Geophysical Fluid Dynamics Laboratory (GFDL) global chemistry transport model (GCTM) to address the episodic nature of trans‐Pacific pollution. The strongest Asian CO episodes over North America (NA), occurring most frequently between February and May, are often associated with disturbances that entrain pollution over eastern Asia and amplify over the western Pacific Ocean. Using 55 ppb of Asian CO as a criterion for major events, we find that during a typical year three to five Asian pollution events analogous to those observed by Jaffe et al. [1999] are expected in the boundary layer all along the U.S. West Coast between February and May. In contrast to CO, Asia currently has a small impact on the magnitude and variability of background ozone arriving over NA from the west. Direct and indirect Asian contributions to episodic O3 events over the western United States are generally in the 3–10 ppbv range. The two largest total O3 events (>60 ppbv), while having trajectories which pass over Asia, show negligible impact from Asian emissions. However, this may change. A future emission scenario in which Asian NOx emissions increase by a factor of 4 from those in 1990 produces late spring ozone episodes at the surface of California with Asian contributions reaching 40 ppb. Such episodic contributions are certain to exacerbate local NA pollution events, especially in elevated areas more frequently exposed to free tropospheric and more heavily Asian‐influenced air.
Intercomparison of Arctic regional climate simulations: Case studies of January and June 1990
Authors: Rinke Annette , Lynch Amanda H. , Dethloff Klaus
Published: February 2004

Advances in regional climate modeling must be strongly based on analysis of physical processes in comparison with data. In a data‐poor region such as the Arctic; this procedure may be enhanced by a community‐based approach, i.e., through collaborative analysis by several research groups. To illustrate this approach, simulations were performed with two regional climate models, HIRHAM and ARCSyM, over the Arctic basin to 65°N, laterally driven at the boundaries by observational analyses. It was found that both models are able to reproduce reasonably the main features of the large‐scale flow and the surface parameters in the Arctic. Distinct differences in the simulations can be attributed to specific characteristics of the boundary layer and surface parameterizations, which result in surface flux differences, and to the lateral moisture forcing, both of which affect moisture availability in the atmosphere. Further disparities are associated with the additional degrees of freedom allowed in the coupled model ARCSyM. Issues of model configuration and experimental design are discussed, including domain size, grid spacing, boundary formulations, model initialization and spin‐up, and ensemble approaches. In order to reach definitive conclusions in a regional climate model intercomparison, ensemble simulations with adequate spin‐up and equivalent initialization of surface fields will be required.
Tropical stratospheric gravity wave activity and relationships to clouds
Authors: Alexander M. Joan , Beres Jadwiga H. , Pfister Leonhard
Published: February 2004

Wind measurements from NASA's ER‐2 aircraft in the stratosphere are used to obtain information on the momentum flux carried by gravity waves with horizontal wavelengths between 5 and 150 km. Tropical data are compared with the cloud brightness temperature below the aircraft as an indicator of deep convective activity. A striking correlation between cold, high clouds and large gravity wave momentum flux is seen in data from the Stratosphere‐Troposphere Exchange Project (STEP) tropical campaign during the monsoon season over northern Australia and Indonesia. There is an enhancement in the flux carried by waves propagating against the background wind in these observations. The same analysis was performed with data from more recent ER‐2 flights over the tropical Pacific Ocean during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric Tracers of Atmospheric Transport (STRAT), and Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns which took place in 1994, 1995–1996, and 1997, respectively. These data also show a correlation between gravity wave momentum flux and deep convective clouds, but the relationship is much weaker, and the magnitudes of the momentum flux over the deepest clouds are about 7 times smaller than those seen in the STEP data. The reasons for these differences remain uncertain, but possibilities include both real geophysical differences and differences associated with the flight paths during the 1987 versus later campaigns.
Modeling mineral aerosol production by wind erosion: Emission intensities and aerosol size distributions in source areas
Authors: Alfaro Stéphane C. , Gomes Laurent
Published: February 2004

A dust production model (DPM) is obtained by combining preexisting models of saltation and sandblasting, the two processes that lead to mineral aerosol release in arid areas. From a description of the soil characteristics and wind conditions, the DPM allows computation of the amounts of aerosol released and of their size distributions. Semiquantitative comparisons of the model outputs with the few field data available in the literature validate its main implications. The first one is that the aptitude of a soil to release particles smaller than 20 μm depends on (1) the dry size distribution of aggregates constituting its loose fraction, (2) its roughness length, and (3) the wind velocity. The second implication is that the size distribution of aerosols released in source areas also strongly depends on these parameters.
Surface heat budget of Antarctic snow and blue ice: Interpretation of spatial and temporal variability
Authors: Bintanja Richard
Published: February 2004

A surface energy balance model is used to calculate the surface heat budget terms from data gathered in austral summer 1997–1998 at seven locations near Svea (74°35′S, 11°13′W), located in the Heimefrontfjella, Dronning Maud Land, Antarctica. The modeled results are validated using the turbulent fluxes calculated with the profile method, those directly measured with sonic anemometer, the measured surface and subsurface temperatures, and the stake ablation readings to constrain the simulated budgets. The simulated quantities generally agree reasonably well with those observed. The seven measuring sites can be subdivided into three climatic regions: I, blue ice areas; II, the low‐altitude coastal snow‐covered plateau; and III, the high‐altitude interior plateau. Spatial and temporal differences in the average surface heat budget and in the variability on daily and longer‐term timescales are presented and discussed. The three regions exhibit quite different surface heat budget regimes induced by their climatic setting and the prevailing boundary layer flow, a well‐established katabatic flow regime, and the associated typical surface heat fluxes being found over regions II and III. The differences between blue ice and snow surface are quite distinct, blue ice absorbing twice as much solar energy due to its lower albedo. This and the lower relative humidity values cause the latent heat flux to be much higher over blue ice than over snow, enabling strong sublimation and a negative mass balance. Differences in the turbulent heat fluxes between the three climatic regions can be well understood in terms of the temperature‐dependent Bowen ratio. The surface heat budgets of the various sites are compared with estimates of other regions in the Antarctic.
Interactions among longwave radiation of clouds, turbulence, and snow surface temperature in the Arctic: A model sensitivity study
Authors: Wang Shouping , Wang Qing , Jordan Rachel E. , Persson P. O. G.
Published: February 2004

Recent observations have shown that low‐level clouds have significant impact on snow surface temperature in the Arctic by enhancing downwelling longwave radiative flux at the surface. This study focuses on the detailed interactions among the longwave radiation, clouds, turbulent structure, and snow surface temperature. The approach is to perform sensitivity simulations with a coupled one‐dimensional planetary boundary layer (PBL) turbulence closure‐snow model. The numerical experiments show that the responses of the snow surface temperature to the cloud longwave radiation may depend on how the clouds are related to the boundary layer turbulent structure. For a cloud‐topped boundary layer without upper level clouds, longwave radiative cooling at the cloud top is a main source for the turbulence. In this situation the energetic turbulent eddies effectively transport the radiatively cooled air near the cloud top down to the ground, resulting in a strong sensible heat flux at the surface. When multilayered clouds exist, the radiative cooling at the lower‐level cloud top is significantly reduced due to the enhanced downward longwave radiative flux above the cloud, leading to a higher temperature of surface air and weaker positive sensible heat flux at the surface. Consequently, the snow surface temperature is higher in the presence of the multilayered clouds than in cases of only boundary layer clouds. Therefore the boundary layer clouds, dependent on their vertical distribution, may not only increase the snow surface temperature by increasing the downwelling longwave radiative flux but also create negative feedback mechanisms to reduce this temperature increase. A key process in this feedback mechanism is turbulent transport as it directly links the longwave radiative cooling at the cloud top to the surface air temperature.
A deflation module for use in modeling long‐range transport of yellow sand over East Asia
Authors: Wang Zifa , Ueda Hiromasa , Huang Meiyuan
Published: February 2004

A deflation module of soil and sand dust loading, especially for Asian dust (called yellow sand or Kosa), has been designed to provide explicit information on emission intensity for use in modeling long‐range transport of yellow sand over East Asia. In contrast to previous modules for Sahara and Australian deserts, it includes three major predictors, the friction velocity, the surface humidity, and the dominant weather system. Comparison of the deflation module results, using these three parameters together or separately, with observed data on dust deflation in April and July 1988, shows that the best estimate can be obtained by considering the three predictors together when we take the minimum of the total error ratio as the selection criteria. It indicates that last two predictors provide a limitation for deflation and can decrease the number of false declarations from 23 to 7%, especially in the wet season. A regional long‐range transport model for Kosa is introduced, which considers various parameters such as particle size, transport, diffusion, and removal in detail. The model results show a reasonable agreement with the observations during Kosa episodes in April 1988. A size‐resolved analysis explains the peculiar multi layered vertical distribution of dust at large distances from the source areas; that is, for fine particles one peak appears close to the ground, while the other is in the middle troposphere.
A method for the airborne measurement of PAN, PPN, and MPAN
Authors: Williams J. , Roberts J. M. , Bertman S. B. , Stroud C. A. , Fehsenfeld F. C. , Baumann K. , Buhr M. P. , Knapp K. , Murphy P. C. , Nowick M. , Williams E. J.
Published: February 2004

An aircraft‐based instrument for the measurement of peroxyacetic nitric anhydride (PAN) and related compounds peroxypropionic nitric anhydride (PPN), and peroxymethacrylic nitric anhydride (MPAN) is described. The instrument functions by direct injection of a small sample (2 ambient cm2) into a wide bore fused silica capillary gas chromatographic separation column with subsequent detection by electron capture. The efficiency of separation and early elution of water made possible a sampling frequency of once every 5 min. The selective destruction of PAN type compounds at 125°C in actual ambient samples showed that no interfering species were present. Calibration of the instrument was accomplished by the total odd‐nitrogen (NOy) measurement of air mixtures of high‐purity PANs. Inflight calibrations of PAN in either thermally decomposed ambient air or dry zero air showed that the sensitivity of the instrument to PAN was not affected by other matrix species such as carbon dioxide or water. The instrument response to PPN and MPAN relative to that of PAN was consistent (within 5%) over the span of several months. The instrument response to PAN was linear up to 3.5 ppbv and the slope of the response versus [PAN] indicated a relative standard deviation of 2% over the course of a 30‐day field study. The detection limits of the method for PAN and PPN were estimated to be 5 pptv (3s) based on the uncertainty of peak detection. The overall accuracy of the PAN measurement was estimated to be ±(5 pptv+15%) in one campaign and ±(5 pptv+20%) in others.
Chemical ionization mass spectrometric measurements of SO2 emissions from jet engines in flight and test chamber operations
Authors: Hunton D. E. , Ballenthin J. O. , Borghetti J. F. , Federico G. S. , Miller T. M. , Thorn W. F. , Viggiano A. A. , Anderson B. E. , Cofer W. R. , McDougal D. S. , Wey C. C.
Published: February 2004

We report the results of two measurements of the concentrations and emission indices of gas‐phase sulfur dioxide (EI(SO2)) in the exhaust of an F100–200E turbofan engine. The broad goals of both experiments were to obtain exhaust sulfur speciation and aerosol properties as a function of fuel sulfur content. In the first campaign, an instrumented NASA T‐39 Sabreliner aircraft flew in close formation behind several F‐16 fighter aircraft to obtain near‐field plume composition and aerosol properties. In the second, an F‐100 engine of the same type was installed in an altitude test chamber at NASA Glenn Research Center where gas composition and nonvolatile aerosol concentrations and size distributions were obtained at the exit plane of the engine. In both experiments, SO2 concentrations were measured with the Air Force Research Laboratory chemical ionization mass spectrometer as a function of altitude, engine power, and fuel sulfur content. A significant aspect of the program was the use of the same fuels, the same engine type, and many of the same diagnostics in both campaigns. Several different fuels were purchased specifically for these experiments, including high‐sulfur Jet A (∼1150 ppmm S), low‐sulfur Jet A (∼10 ppmm S), medium‐sulfur mixtures of these two fuels, and military JP‐8+100 (∼170 and ∼300 ppmm S). The agreement between the flight and test cell measurements of SO2 concentrations was excellent, showing an overall precision of better than ±10% and an estimated absolute accuracy of ±20%. The EI(SO2) varied from 2.49 g SO2/kg fuel for the high‐sulfur fuel in the test chamber to less than 0.01 g/kg for the lowest‐sulfur fuel. No dependence of emission index on engine power, altitude or simulated altitude, separation distance or plume age, or the presence of contrails was observed. In all experiments the measured EI(SO2) was consistent with essentially all of the fuel sulfur appearing as gas‐phase SO2 in the exhaust. However, accurate determination of S(IV) to S(VI) conversion was hampered by inconsistencies in the assays of total fuel sulfur content.
Latitudinal distribution of atmospheric CO2 sources and sinks inferred by δ13C measurements from 1985 to 1991
Authors: Morimoto Shinji , Nakazawa Takakiyo , Higuchi Kaz , Aoki Shuji
Published: February 2004

Net CO2 fluxes between the atmosphere and the ocean, and between the atmosphere and the terrestrial biosphere, were estimated by constraining a two‐dimensional atmospheric transport model with the CO2 concentration and its δ13C data obtained from our shipboard measurements in the western Pacific region during the period April 1984 to December 1991. The results obtained for the non‐El Niño and Southern Oscillation (ENSO) years during this time period showed that the ocean acted as a CO2 sink in middle latitudes of both hemispheres and a CO2 source around the equator. The results also suggested that during the 1984–1991 period there were biospheric CO2 sources in southern low and northern middle latitudes, and significant biospheric sinks in northern high latitudes. By comparing the latitudinal distributions of CO2 source/sink for ENSO years with those for non‐ENSO years, it was found that a considerable amount of CO2 was released from the terrestrial biosphere in low latitudes in association with the 1986–1987 ENSO event. While the oceanic region from the equator to 30°N acted as an excess CO2 sink of about 1.0 Gt C y−1r during 1987 and 1988, the oceanic contributions to the CO2 anomaly in the atmosphere were less pronounced in the northern high latitudes and in the southern hemisphere. Also, compared with the estimated interannual fluctuations in the atmosphere‐biosphere CO2 flux, the net CO2 flux between the atmosphere and the ocean showed relatively less interannual variability.
Modification of the aerosol size distribution within exhaust plumes produced by diesel‐powered ships
Authors: Osborne Simon R. , Johnson Douglas W. , Bower Keith N. , Wood Robert
Published: February 2004

Aircraft measurements are presented of the interaction of aerosols and stratocumulus cloud within the effluent of ships powered by partial combustion of low‐grade diesel fuel. Two case studies are shown where aerosol is modified by cloud processing over times of 1–2 hours, or <1 hour accumulatively spent within cloud. One case shows a good example of the Twomey effect, whereby the cloud droplet effective radius reduced and the extinction coefficient increased quite dramatically for a given liquid water content. The other cloudy case shows relatively little cloud perturbation due to the ship aerosol. In both cases a Hoppel dip appears in the dehydrated aerosol spectrum between 0.1 and 0.2 μm diameter and a new mode grows out to about 0.45 μm. Scattering coefficients of particles between 0.1–0.7 μm show that the modified spectra have greater efficiency at scattering solar radiation. A modeling study of one of these cases indicates that aqueous‐phase sulfur chemistry within cloud can explain the aerosol features. By comparison with the concomitant processing of the background aerosol, it is shown that the Hoppel dip and hence critical size for droplet activation lies at larger particle sizes in polluted clouds. A third case study is presented where the plume evolves in a cloud‐free boundary layer. Here no modal growth of the aerosol was seen and the aerosol was diluted primarily by mixing with background air. The aerosol was highly deliquesced, which suggests that haze particle chemistry was probably insignificant in modifying the aerosol in any of the cases.
Modeled and empirical approaches for retrieving columnar water vapor from solar transmittance measurements in the 0.72, 0.82, and 0.94 μm absorption bands
Authors: Ingold T. , Schmid B. , Mätzler C. , Demoulin P. , Kämpfer N.
Published: February 2004

A Sun photometer (18 channels between 300 and 1024 nm) has been used for measuring the columnar content of atmospheric water vapor (CWV) by solar transmittance measurements in absorption bands with channels centered at 719, 817, and 946 nm. The observable is the band‐weighted transmittance function defined by the spectral absorption of water vapor and the spectral features of solar irradiance and system response. The transmittance function is approximated by a three‐parameter model. Its parameters are determined from MODTRAN and LBLRTM simulations or empirical approaches using CWV data of a dual‐channel microwave radiometer (MWR) or a Fourier transform spectrometer (FTS). Data acquired over a 2‐year period during 1996–1998 at two different sites in Switzerland, Bern (560 m above sea level (asl)) and Jungfraujoch (3580 m asl) were compared to MWR, radiosonde (RS), and FTS retrievals. At the low‐altitude station with an average CWV amount of 15 mm the LBLRTM approach (based on recently corrected line intensities) leads to negligible biases at 719 and 946 nm if compared to an average of MWR, RS, and GPS retrievals. However, at 817 nm an overestimate of 2.7 to 4.3 mm (18–29%) remains. At the high‐altitude station with an average CWV amount of 1.4 mm the LBLRTM approaches overestimate the CWV by 1.0, 1.4, and 0.1 mm (58, 76, and 3%) at 719, 817, and 946 nm, compared to the FTS instrument. At the low‐altitude station, CWV estimates, based on empirical approaches, agree with the MWR within 0.4 mm (2.5% of the mean); at the high‐altitude site with a factor of 10 less water vapor the agreement of the SPM with the FTS is 0.0 to 0.2 mm (1 to 9% of the mean CWV there). Sensitivity analyses show that for the conditions met at the two stations with CWV ranging from 0.2 to 30 mm, the retrieval errors are smallest if the 946 nm channel is used.
The histogram of the brightness distribution of clouds in high‐resolution remotely sensed images
Authors: Koren Ilan , Joseph Joachim H.
Published: February 2004

A modeling algorithm for the brightness histogram of stratocumulus, cumulus, and fair‐weather‐cumulus clouds (in short, “convective clouds”) is proposed. The algorithm is based on the similarity between the beta probability density function, and the convective cloud brightness histogram. On assuming that the image brightness histogram's right wing (the high values in the solar spectral range) is free of background contribution, the cloud histogram may be extracted. The algorithm gives new information on the number of cloudy pixels in an image, using the same data as used for conventional cloud analysis methods. This information improves the estimation of cloud fraction of the image and aids in the selection of thresholds for cloud‐background separation. It provides a new perspective on the data that will be helpful when using conventional methods for cloud detection and classification.
Variability of spectral solar ultraviolet irradiance in an Alpine environment
Authors: Gröbner J. , Albold A. , Blumthaler M. , Cabot T. , De la Casiniere A. , Lenoble J. , Martin T. , Masserot D. , Müller M. , Philipona R. , Pichler T. , Pougatch E. , Rengarajan G. , Schmucki D. , Seckmeyer G. , Sergent C. , Touré M. L. , Weihs P.
Published: February 2004

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch‐Partenkirchen, Germany, during spring 1999. The measured clear‐sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow‐covered stations and the snow‐free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single‐scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.
In situ measurement of the aerosol extinction‐to‐backscatter ratio at a polluted continental site
Authors: Anderson Theodore L. , Masonis Sarah J. , Covert David S. , Charlson Robert J. , Rood Mark J.
Published: February 2004

The extinction‐to‐backscatter ratio S is a crucial parameter for quantitative interpretation of lidar data, yet empirical knowledge of S for tropospheric aerosols is extremely limited. Here we review that knowledge and extend it using a recently developed in situ technique that employs a 180° backscatter nephelometer. This technique allows robust quantification of measurement uncertainties and permits correlations with other aerosol and meteorological properties to be explored. During 4 weeks of nearly continuous measurements in central Illinois, S was found to vary over a wide range, confirming previous indications that geographical location by itself is not necessarily a good predictor. The data suggest a modest dependence of S on relative humidity, but this explains only a small portion of the variation. Most variation was associated with changes between two dominant air mass types: rapid transport from the northwest and regional stagnation. The latter category displayed much higher aerosol concentrations and a systematically higher and more tightly constrained range of S. Averages and standard deviations were 64±4 sr for the stagnant category and 40±9 sr for the rapid transport category. Considering the 95% confidence precision uncertainty of the measurements, the difference between these averages is at least 13 sr and could be as large as 35 sr. The wavelength dependence of light scattering, as measured by a conventional nephelometer, is shown to have some discriminatory power with respect to S.
Simulation of regional‐scale water and energy budgets: Representation of subgrid cloud and precipitation processes within RegCM
Authors: Pal Jeremy S. , Small Eric E. , Eltahir Elfatih A. B.
Published: February 2004

A new large‐scale cloud and precipitation scheme, which accounts for the sub‐grid‐scale variability of clouds, is coupled to NCAR's Regional Climate Model (RegCM). This scheme partitions each grid cell into a cloudy and noncloudy fraction related to the average grid cell relative humidity. Precipitation occurs, according to a specified autoconversion rate, when a cloud water threshold is exceeded. The specification of this threshold is based on empirical in‐cloud observations of cloud liquid water amounts. Included in the scheme are simple formulations for raindrop accretion and evaporation. The results from RegCM using the new scheme, tested over North America, show significant improvements when compared to the old version. The outgoing longwave radiation, albedo, cloud water path, incident surface shortwave radiation, net surface radiation, and surface temperature fields display reasonable agreement with the observations from satellite and surface station data. Furthermore, the new model is able to better represent extreme precipitation events such as the Midwest flooding observed in the summer of 1993. Overall, RegCM with the new scheme provides for a more accurate representation of atmospheric and surface energy and water balances, including both the mean conditions and the variability at daily to interannual scales. The latter suggests that the new scheme improves the model's sensitivity, which is critical for both climate change and process studies.
Hygroscopic properties of aerosol particles over the central Arctic Ocean during summer
Authors: Zhou Jingchuan , Swietlicki Erik , Berg Olle H. , Aalto Pasi P. , Hämeri Kaarle , Nilsson E. Douglas , Leck Caroline
Published: February 2004

The hygroscopic properties of submicrometer aerosol particles in the Arctic summer marine boundary layer (MBL) were observed on board the icebreaker Oden during the Arctic Ocean Expedition 1996 (AOE‐96). The measurements were performed between July 15 and August 25 1996 and covered the region between longitudes 16°‐147°E and latitudes 70°‐87.5°N, mostly over melting pack ice. The hygroscopic tandem differential mobility analyzer (H‐TDMA) was used to determine the hygroscopic diameter growth of aerosol particles at four dry diameters (15, 35, 50, and 165 nm) and three relative humidities (50%, 70%, and 90% RH). The hygroscopic behavior of the aerosol particles over the pack ice showed large temporal variations, in contrast to previous observations in marine boundary layers over warmer oceans. These variations were mostly due to the high degree of vertical atmospheric stratification often observed over the pack ice. However, when comparing the average diameter growth factors of the more hygroscopic particle group, representing an aged aerosol with growth factors between 1.4–1.9 at 90% RH and present in 81–86% of all cases, the agreement between the measurements over the Arctic and the warmer oceans was very good and depended on the average wind speed. The average diameter growth factors of the more hygroscopic particles as a function of relative humidity were modeled empirically by power law expressions. The concentration of cloud condensation nuclei (CCN) estimated from aerosol number size distribution and hygroscopic growth data correlated well with direct measurements but overpredicted the CCN concentrations by about 30%. In 43 cases when the sampled air mass had undergone processing in Arctic Ocean MBL clouds, the minimum CCN diameter was estimated to be 76±15 nm, corresponding to effective water vapor supersaturations of 0.28±0.08%.
Chemical and mineralogical analysis of individual mineral dust particles
Authors: Falkovich Alla H. , Ganor Eliezer , Levin Zev , Formenti Paola , Rudich Yinon
Published: February 2004

Individual mineral dust particles collected in a dust storm over Israel were analyzed by a scanning electron microscopy and energy‐dispersed system (SEM‐EDS). The analysis shows that the particles were mostly aggregates of varying mineralogical composition rather than pure minerals. It is also shown that sulfur (not associated with gypsum) and, to a lesser extent, iron tended to reside on the particles' surface, while Ca, Mg, K, Al, and Si were all an integral part of the particles. The lack of NaCl and sulfuric acid aerosols in the sample indicates that the air mass did not interact with marine air or with clouds. This conclusion is further supported by back trajectory calculations. These findings suggest that the sulfur in the aerosols did not result from atmospheric processes but rather originated from processes in the source region. Black residue, surrounding some of the particles, suggests the possible existence of organic matter in the sample, probably originating from biological activity in the soil at the source of the particles. The method of individual particle analysis provides important information about the composition and morphology of the particles, information that otherwise cannot be obtained by bulk analysis methods.
Environmental variables controlling the uptake of carbonyl sulfide by lichens
Authors: Kuhn U. , Kesselmeier J.
Published: February 2004

The uptake of atmospheric carbonyl sulfide (COS) by the lichen species Ramalina menziesii, representative for the open oak woodland in central California, was studied under laboratory conditions. By use of a dynamic cuvette system, the controlling parameters for the COS uptake were investigated under climate chamber conditions. The thallus water content, essential for the overall physiology of lichens, was found to be of basic importance for the trace gas exchange. A water content of 30% was the approximate minimum for COS uptake, with increasing activity up to a water content of 200%. Additionally, actual atmospheric mixing ratios have a significant influence on the exchange. The COS uptake was found to be a linear function of the ambient COS mixing ratio resulting in a compensation point as low as 37 ppt. A temperature optimum of 25°C was indicative of a physiological basis of the COS uptake. The inhibition of the COS consumption in the presence of a specific inhibitor for the enzyme carbonic anhydrase proved this enzyme to be of key relevance for the uptake. All these variables controlling the COS deposition were integrated into an uptake algorithm to model the exchange behavior of this lichen. The applicability of the model to field data is demonstrated. Uptake rates on a dry weight basis normalized to optimized conditions (25°C; 450 ppt COS) reached 0.17±0.09 pmol g−1 s−1 (i.e. 4.2±2.2 pmol m−2 s−1 thallus surface area, respectively). The contribution of lichens to the global COS sink strength is assigned to be about 0.3 Tg a−1, representing not a major but a significant sink.
Joint statistics of photon path length and cloud optical depth: Case studies
Authors: Min Qilong , Harrison Lee C. , Clothiaux Eugene E.
Published: February 2004

We show the joint statistics of photon path length and cloud optical depth for cloudy sky cases observed at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site between September and December 1997. The photon path lengths are retrieved from moderate resolution oxygen A‐band observations taken by a rotating shadow band spectroradiometer (RSS). For high optical depth cloud cases, two different populations in the scattergram of the path length versus cloud optical depth are apparent. One population is a result of single‐layer cloud cases that exhibit a small variation of path length enhancement over a large optical depth range, together with a strong correlation between the radiation field and the cloud liquid water path, while the second population is attributed to multiple‐layer cloud cases with large variability of enhanced photon path lengths. When the optical depth is less than 5, the population of cases appears to bifurcate as the solar air mass increases, with the lower branch exhibiting pressure‐weighted path lengths shorter than the direct beam path lengths at these larger solar zenith angles. Using information from a millimeter‐wave cloud radar, together with lidar and balloon‐borne sonde data to further analyze these cases demonstrates that this bifurcation is caused by the altitude of the scattering; thin clouds aloft produce the lower branch and low‐level aerosols produce the upper branch.
Radiation transfer model intercomparison (RAMI) exercise
Authors: Pinty Bernard , Gobron Nadine , Widlowski Jean‐Luc , Gerstl Sigfried A. W. , Verstraete Michel M. , Antunes Mauro , Bacour Cédric , Gascon Ferran , Gastellu Jean‐Philippe , Goel Narendra , Jacquemoud Stéphane , North Peter , Qin Wenhan , Thompson Richard
Published: February 2004

The community involved in modeling radiation transfer over terrestrial surfaces designed and implemented the first phase of a radiation transfer model intercomparison (RAMI) exercise. This paper discusses the rationale and motivation for this endeavor, presents the intercomparison protocol as well as the evaluation procedures, and describes the principal results. Participants were asked to simulate the transfer of radiation for a variety of precisely defined terrestrial environments and illumination conditions. These were abstractions of typical terrestrial systems and included both homogeneous and heterogeneous scenes. The differences between the results generated by eight different models, including both one‐dimensional and three‐dimensional approaches, were then documented and analyzed. RAMI proposed a protocol to quantitatively assess the consequences of the model discrepancies with respect to application, such as those motivating the development of physically based inversion procedures. This first phase of model intercomparison has already proved useful in assessing the ability of the modeling community to generate similar radiation fields despite the large panoply of models that were tested. A detailed analysis of the results also permitted to identify apparent “outliers” and their main deficiencies. Future undertakings in this intercomparison framework must be oriented toward an expansion of RAMI into other and more complex geophysical systems as well as the focusing on actual inverse problems.
Understanding the Indian Ocean Experiment (INDOEX) aerosol distributions with an aerosol assimilation
Authors: Rasch Philip J. , Collins William D. , Eaton Brian E.
Published: February 2004

We use the aerosol assimilation procedure described by Collins et al. [2000] to help explain INDOEX aerosol distributions. The procedure combines modeled aerosol with AVHRR satellite estimates. The result is consistent with satellite measurements, regular in space and time, and provides information where retrievals are difficult (over land, coincident with clouds, and at night). Extra information on aerosol composition, vertical distribution, and region of origin is also produced. Carbonaceous, sulfate, and sea salt aerosols agree with the in situ measurements to 10–20%. Carbonaceous aerosols were estimated to be the dominant contributor (36%) to the aerosol optical depth (AOD); dust (31%) and sulfate (26%) were also important. The residence time for sulfate and carbon is ∼7 and ∼8 days respectively, longer than globally averaged residence times of many modeling studies. Thus aerosols produced here during the winter monsoon may have a larger climate impact than the same emissions occurring where the residence time is shorter. Three points of entry are found for anthropogenic aerosol to the INDOEX region: a strong near surface southward flow near Bombay; a deeper plume flowing south and east off Calcutta and a westward flow originating from southeast Asia and entering the Bay of Bengal. All three plumes are strongly modulated by a low‐frequency change of meteorological regime associated with the Madden Julian Oscillation. The analysis suggests that India is the dominant source of aerosol in the Arabian Sea and Bay of Bengal near the surface but that Asia, Africa and the rest of world also contribute at higher altitudes. India and Asia contribute ∼40% each to the total column mass of air reaching the Maldives, the balance of air comes from other source regions. The assimilation procedure produces an analysis that is a synergy in information about aerosols, that is not easily accessible by independent estimates from remote sensing, in situ measurements, or global transport models by themselves.
Simulation of large‐scale dynamics in the mesosphere and lower thermosphere with the Doppler‐spread parameterization of gravity waves: 1. Implementation and zonal mean climatologies
Authors: Akmaev R. A.
Published: February 2004

Small‐scale gravity waves (GWs) originating in the lower atmosphere maintain the large‐scale structure and circulation in the mesosphere and lower thermosphere (MLT). At solstice the drag imposed by these waves supports a strong meridional circulation from summer pole to winter pole that acts as a giant “refrigerator” transporting heat from a colder summer mesosphere to a warmer winter mesosphere. The Doppler‐spread parameterization (DSP) of gravity waves developed by C. O. Hines has been implemented into the Spectral Mesosphere/Lower Thermosphere Model. Simulations for solstice and equinox conditions compare favorably with empirical models and observations. A particularly good agreement for both seasons is found with recent in situ observations of polar mesospheric temperatures. The thermal structure at high latitudes, especially at solstice, is almost entirely driven by the GW momentum deposition in the MLT. The DSP appears to provide a very realistic distribution of the GW drag imposed on the large‐scale circulation. A modification is suggested for the calculation of GW energy deposition rate that appears to be more consistent with the physics of Doppler spreading. The simulations exhibit strong seasonal variations in the energy deposition rates with a summer maximum of ∼200 mW kg−1 at middle and high latitudes in the upper mesosphere in very good agreement with recent in situ estimates.
Further evidence of total ozone variation during the solar eclipse of 1995
Authors: Chakrabarty D. K. , Peshin S. K. , Srivastav S. K. , Shah N. C. , Pandya K. V.
Published: February 2004

A solar eclipse occurred across India on October 24, 1995. Vertical ozone column was measured at New Delhi (28.4°N, 77.1°E) by a Brewer spectrometer during this event. We found that at the time of maximum obscuration of the Sun the ozone column decreased by about 25 Dobson units (DU) from its preeclipse value. These results are compared with the measurements obtained elsewhere in India by other techniques during the same event. The NO2 slant column density measured at Ahmedabad (23.0°N, 72.4°E) by visible absorption spectroscopy showed a wavelike variation during the eclipse. These variations did not follow the variation of the obscuration of the Sun during the eclipse. Correlation between ozone column, UV flux, and NO2 column could not be established because observations were not taken at sufficiently close intervals.