bluuebunny/crossref_metadata_2025 huggingface dataset:
Summer hemisphere differences in temperature and transport in the lower stratosphere
Authors: Rosenlof Karen H.
Published: February 2004

Lower stratospheric temperatures during southern hemisphere summer are found to be warmer than at the same latitude poleward of the equator during northern hemisphere summer. Coincident with such warmer temperatures is a stronger residual circulation as calculated from radiative heating rates computed using as input observations from the Upper Atmospheric Research Satellite (UARS). Such a summertime asymmetry in extratropical downwelling in the lower stratosphere does not appear in the residual circulation calculated via the downward control technique based on National Meteorological Center (NMC) analyses. Computing the residual circulation with the downward control technique using Eliassen‐Palm flux divergences based on assimilated winds and temperatures does yield the above noted summertime hemispheric asymmetry in downward extratropical mass transport, although not as pronounced as with the radiative calculation. It is proposed that smaller‐scale zonal forces due to gravity waves, absent in the NMC downward control calculation, may be important for driving the southern hemisphere summer circulation in the lower stratosphere.
Seasonal and annual variability of the diurnal cycle of clouds
Authors: Kondragunta Chandrasekhara R. , Gruber Arnold
Published: February 2004

Eight years of cloudiness data from the International Satellite Cloud Climatology Project (ISCCP) were analyzed to study the diurnal variation and its annual variation. An empirical orthogonal function (EOF) analysis technique was used to analyze the ISCCP C‐2 monthly hourly data. Results show that most of the diurnal variation is explained by the first two dominant EOF modes, which explain the diurnal cycle of clouds. On the basis of this analysis, two modes of diurnal cycle of clouds are defined. Mode 1 as defined by the first EOF indicates that cloudiness maximum occurs at 0500 or 1500 local solar time (LST); Mode 2 as defined by the second EOF indicates that cloudiness maximum occurs at 1000 or 2000 LST. In a broad sense, Mode 1 explains the diurnal cycle of low‐level cloudiness, and Mode 2 explains the diurnal variation of high‐level cloudiness. Mode 1 explains 58.5% of the normalized variance, and Mode 2 explains 25% of the normalized variance. Both modes of the diurnal cycle show annual variation. The Mode 1 annual variation is large along the western boundary currents over the oceanic regions and moist convective regions over the continents. Interestingly, this annual variation of the diurnal cycle of cloudiness bears some relationship with the annual variation of the surface temperature. This relationship is such that the cloudiness is maximum in the early morning when the annual surface temperature is at its minimum and in the afternoon when the annual surface temperature is at its maximum.
Numerical simulations of stratocumulus processing of cloud condensation nuclei through collision‐coalescence
Authors: Feingold Graham , Kreidenweis Sonia M. , Stevens Bjorn , Cotton W. R.
Published: February 2004

The role of drop collision‐coalescence as a means of reducing drop number concentrations, and hence cloud condensation nucleus (CCN) concentrations as these are cycled in the stratocumulus‐capped marine boundary layer, is investigated. We focus on the impact of this process on the mass‐mean size of CCN in the absence of wet deposition, and compare this mechanism with size changes resulting from mass addition through aqueous chemistry. The modeling framework is a two‐dimensional eddy‐resolving model that includes explicit treatment of aerosol and drop spectra, as well as the solute transfer between drop size bins. The microphysical processes considered are droplet activation, condensation/evaporation, collision‐coalescence, sedimentation and regeneration of particles following complete evaporation. It is shown that for a case exhibiting negligible wet deposition, collision‐coalescence can significantly reduce drop concentrations (22% h−1) resulting in a measurable increase in particle mass‐mean radius of about 7% h−1. In order to extend the validity of these results, trajectory analyses of parcel in‐cloud residence times have been used together with box model calculations of collision‐coalescence to explore the parameters affecting processing through collision‐coalescence. This trajectory information is also used to deduce the extent of in‐cloud conversion of SO2 to sulfate. Comparisons show that the two mechanisms may produce comparable rates of increase in the mean particle size under certain conditions. Results for remote marine conditions suggest that aqueous phase chemistry may have a greater impact at lower cloud liquid‐water contents, whereas collision‐coalescence may dominate at higher liquid‐water contents, or for broader drop spectra.
Error structure of radar rainfall measurement at C‐band frequencies with dual polarization algorithm for attenuation correction
Authors: Gorgucci E. , Scarchilli G. , Chandrasekar V.
Published: February 2004

A simple procedure is developed to estimate specific attenuation and differential attenuation at C‐band frequencies from the dual polarization radar measurements, reflectivity at horizontal polarization (ZH), and differential reflectivity (ZDR). An algorithm is presented to correct C‐band reflectivity and differential reflectivity measurements for absolute attenuation and differential attenuation, respectively. The effect of the attenuation correction procedure on the error structure of rainfall estimation is studied by utilizing data collected by the dual polarized C‐band radar, POLAR 55C, as well as a network of rain gages over the Arno River basin near Florence, Italy. Radar rainfall estimates are compared over two small basins, one close to the radar, at a distance of 35 km, and the other farther from the radar, at a distance of 65 km, where the effect of attenuation is likely to be more significant. On the basis of a comparative study of radar and rain gage measurements over the two basins it is shown that the attenuation correction improves the radar rainfall estimates based on Z‐R algorithms for both basins. The correction procedure improves the radar rainfall estimates based on dual polarization algorithm for the farther basin; however, the attenuation correction does not improve the radar rainfall estimates based on dual polarization algorithm for the closer basin (when the differential attenuation is not significant).
Empirical orthogonal functions of monthly precipitation and temperature over the United States and homogeneous stochastic models
Authors: Cahalan Robert F. , Wharton Lawrence E. , Wu Man‐Li
Published: February 2004

The monthly mean precipitation and temperature at p = 62 stations over the United States and Canada for N = 91 years (1900–1990) are analyzed in terms of empirical orthogonal functions (EOFs) and their variances. The eigenvalues and eigenfunctions are compared with a succession of stochastic noise models: (1) uncorrelated noise, having eigenvalues depending on the ratio p/n, with n = N ‐ 1; (2) homogeneous noise having spatial correlations which are fit to the observations; and (3) homogeneous noise having both spatial and temporal correlations fit to the observations. Individual monthly data for January and July were analyzed as well as a combined data set of all months. The eigenvalue spectra of the homogeneous noise models are found to be in close agreement with the observed spectra even when time correlation is excluded from the model. Time correlations only slightly affect the results for temperature and have less impact for precipitation. The EOF patterns of the noise models contain inhomogeneities due only to the distribution of stations, the common correlation length, and the limited sample but are nevertheless in good agreement with the observed patterns, whose inhomogeneities may also be affected by secular trends and physical inhomogeneities such as orography. The observed EOF eigenvectors also show identifiable deviations from the homogeneous EOFs. Further work will be needed to see if these deviations can be convincingly associated with true physical inhomogeneities.
Statistical‐microphysical causes of rainfall variability in the tropics
Authors: Georgakakos Konstantine P. , Krajewski Witold F.
Published: February 2004

To elucidate links between statistical theories of rainfall phenomenology and kinematic/microphysical theories of rainfall production mechanisms, a system of partial differential equations is formulated from the air mass continuity, the water substance continuity, and the conservation of vertical momentum in deep tropical clouds during convection. Using observational evidence regarding the vertical structure of the model state variables and allowing for turbulent mixing of convecting elements with the environment, we integrate the conservation equations along the vertical to arrive at a set of equations for a space‐time description of the rainfall process. In the process of integration, we preserve advective storm velocities, and for the resultant numerical model, we use an explicit, antidiffusive numerical integration scheme to obtain rainfall estimates in space and time from given initial and boundary conditions. The advantage of the proposed model over current detailed dynamical cloud and mesoscale models of rainfall is its suitability for repetitive meso‐γ Monte‐Carlo simulation, necessary for statistical analysis that can support statistical‐dynamical theories. The link to statistical models comes from the synthetic generation of the input and boundary fields. Model input may be normalized to consist only of the spatiotemporal water vapor distribution and its vertical gradient, and the spatiotemporal distribution of the buoyancy acceleration due to thermal and pressure perturbations. Examples presented in this paper show the versatility of the model and its ability to reproduce known statistical properties of rainfields. On the basis of these preliminary studies it is conjectured that the rainfall intermittence in space and time over temporal scales greater than a few minutes and spatial scales greater than a few hundred meters originates in the dynamics and microphysics of the model and, possibly, in the intermittent nature of the water vapor field.
Nitrous oxide and methane fluxes from perturbed and unperturbed boreal forest sites in northern Ontario
Authors: Schiller C. L. , Hastie D. R.
Published: February 2004

Boreal forests cover approximately 11% of the terrestrial surface of the world or 14.7 million km2 in the circumpolar region of the northern hemisphere. Of this total, approximately one third of these forests are found in Canada. This accounts for on the order of 50% of the total land area in Canada. A static chamber technique was used to determine the fluxes of nitrous oxide and methane from four Boreal forest sites near Cochrane, Ontario (49°03′N,80°40′W), during the summer of 1992. The four sites included a lowland forest, a drained lowland forest, a clear‐cut and drained lowland forest, as well as an upland forest. The N2O fluxes ranged from an uptake of 7.7 μg(N2O)/m2/hr from a drainage ditch to an emission of 3.1 μg(N2O)/m2/hr from an unvegetated clear‐cut region. The CH4 fluxes ranged from an uptake of 23 μg(CH4)/m2/hr from an upland forest site, to an emission of 2900 μg(CH4)/m2/hr from a drainage ditch. Drainage ditches which contained a large amount of algae exhibited large CH4 production and large N2O uptake.
The quantity of biomass burned in southern Africa
Authors: Scholes R. J. , Kendall J. , Justice C. O.
Published: February 2004

Abstract. A new method is described for calculating the amount of biomass burned, its type and location, and the time of burning. Active fires in 1989 were detected using daily advanced very high resolution radiometer (AVHRR) satellite imagery. The fire count was calibrated to area burned using a stratified sample of multitemporal multispectral scanner (MSS) imagery. The calibration factor is strongly dependent on mean individual fire area, which is in turn strongly related to cumulative normalized difference vegetation index (NDVI). The best available vegetation maps for southern hemisphere Africa were combined and reclassified into functional vegetation types with a similar fire ecology. The fuel load was calculated in each 0.5° × 0.5° grid square using a production model specific for each vegetation type, driven by monthly rainfall data. Multiyear fuel accumulation, herbivory, and decay were accounted for. Combustion completeness was modeled as a function of fuel mass and fuel type, established from field‐collected data. The method was compared to the conventional procedure for calculating biomass burned, based on classification. The estimated amount of biomass burned in vegetation fires in southern hemisphere Africa annually is 90–264 Tg dry matter (DM) by the new modeling method and 247–2719 Tg DM by the conventional classification method. The modeling method is conservative since it does not include burning due to forest clearing or the burning of agricultural waste or domestic biomass fuels, but it is believed to be more realistic than the classification method and provides space‐and‐time‐resolved output. The bulk of the burning occurs between June and September, with a peak in August. Half of the burning takes place in the broad‐leaved, low‐nutrient‐status savannas which dominate the zone between 5° and 18°S.
NOxemissions from African savanna fires
Authors: Lacaux J. P. , Delmas R. , Jambert C. , Kuhlbusch T. A. J.
Published: February 2004

Fire of Savanna/Dynamique et Chimie Atmosphèrique en Forêt Equatoriale (FOS/DECAFE‐91) and Southern African Fire‐Atmosphere Research Initiative (SAFARI‐92) were two multidisciplinary experiments organized in Africa to determine the gas and aerosol emissions due to prescribed savanna fires. These two experiments took place in two types of savannas which have very different ecological properties: Lamto, Ivory Coast (FOS/DECAFE‐91), in the Guinean area, is wet (80% moisture content) and dense (9 to 10 t ha−1) and has little litter (2 to 5% of total biomass); Kruger Park, in South Africa (SAFARI‐92), on the contrary, is dry (10 to 20% moisture) and has a low biomass (3 to 6 t ha−1) and 40% of its total biomass in the form of litter. The experimental strategy used to determine the emission ratios ΔCO/ΔCO2and ΔNOx/ΔCO2was to measure at ground level, in the same volume of the plume above the fire, the CO, CO2and NOxconcentrations. The mean carbon content of the two savannas was similar (about 43%), and the ΔCO/ΔCO2ratio, the indicator of the burning process, was comparable in back and head fires with mean ratios between 4.5 and 6.1%. These ΔCO/ΔCO2ratios were characteristic of intense flaming combustion with the formation of mainly fully oxidized compounds for both savannas. The ΔCO/ΔCO2ratio for the head fires can be divided into two distinct phases: a flaming period and a smoke period which immediately follows. Nitrogen concentration of the two savannas varied from as low as 0.3% in Lamto to 0.80% of dry mass in the Kruger Park in the Faai plot. In this study we could clearly identify a linear dependency between the nitrogen concentration of the dry and wet savannas of Africa and the NOx/CO2emission ratio, ΔNOx/ΔCO2= 0.66 N% − 0.01, with a correlation coefficient of 0.93, statistically significant at a confidence level better than 99%. This result enables the quantification of the emissions of NOxfrom African savanna burning by measuring the nitrogen concentration of the vegetation and monitoring the type of burn.
Hydrological applications of satellite data: 1. Rainfall estimation
Authors: Tsonis A. A. , Triantafyllou G. N. , Georgakakos K. P.
Published: February 2004

In this study we investigate the ability of satellite visible and infrared data to produce reliable rainfall amount estimates that could be used by hydrological models to predict streamflow for large basins. Rainfall estimates are obtained by (1) classification of clouds to raining and nonraining clouds and (2) applying a multivariate statistical model between rainfall and indices derived from the satellite observations. Satellite data corresponding to 180 randomly selected days in the period May‐September 1982–1988 are used in this study that focuses on the estimation of daily rainfall. The Des Moines River basin in the midwestern United States is the application area. The correlation coefficient between model‐predicted and rain gauge‐observed mean areal precipitation over areas of order 10,000 km2 is found to be about 0.85. In an example application the satellite rainfall estimates are used to force the operational National Weather Service hydrologic forecast model for a subbasin of the Des Moines River basin. The model has been calibrated with rain gauge data. The results show that differences between rain gauge and satellite rainfall input generate differences in flow forecasts and upper soil water model estimates, which are a function of the antecedent soil water conditions. A companion paper [Guetter et al., this issue] quantifies the effects that the differences between rain gauge and satellite rainfall estimates have on flow and upper soil water model predictions for various spatial scales and for hydrologic models calibrated with and without satellite data.
On the production of active nitrogen by thunderstorms over New Mexico
Authors: Ridley B. A. , Dye J. E. , Walega J. G. , Zheng J. , Grahek F. E. , Rison W.
Published: February 2004

In July and August of 1989 the National Center for Atmospheric Research (NCAR) Sabreliner jet aircraft was used to probe electrically active and inactive convective storms over west central New Mexico to examine the production of odd nitrogen in the middle and upper troposphere by thunderstorms. In the anvil outflow or cloud top region of active and nonactive storms, the majority of flights showed that O3 was reduced relative to the extracloud air owing to transport of ozone‐poor air from lower altitudes. A similar result was found for active nitrogen (NOx) and total odd nitrogen (NOy) in nonelectrically active storms, but the reduction in NOy was also enhanced by removal of soluble constituents during convective transport. Examples of efficient removal from the gas phase are described. There was no evidence of O3 production by lightning discharges. Indeed, O3 was a good tracer over the lifetime (∼1 hour) of the storms. During the active‐to‐mature stage of air mass thunderstorms, large enhancements in active nitrogen were observed in the anvil altitude region (9–11.8 km) and, in one case, in the midlevel outflow (near 7 km) of a dissipating thunderstorm. Two thunderstorms allow good estimates of the NOx production by lightning within or transport to the upper altitude region (8–11.8 km). Thunderstorms of August 12 and August 19 yield amounts in the range of 253–296 kg(N) and 263–305 kg(N), respectively. If, as an exercise, these amounts are extrapolated to the global scale on the basis of the number of cloud‐to‐ground and intracloud lightning flashes counted or estimated for each storm and a global flash frequency of 100 s−1 the result is 2.4–2.7 and 2.0–2.2 Tg(N)/yr. Alternatively, an estimate for the two storms made on the basis of the average number of thunderstorms that occur per day globally (44,000) yields amounts in the range of 4.1–4.7 and 4.2–4.9 Tg(N)/yr, respectively. These estimates only apply to the production or transport of lightning‐generated NOx in or to the altitude region between 8 km and the top of the thunderstorm anvil (∼11.8 km in these studies). Since in some large‐scale models, lightning‐generated NOx is equally distributed by mass into each tropospheric layer, our estimates are roughly equivalent to those model runs that use a global source strength of about twice our estimate for the upper altitude region. In several flights where the region below the base of thunderstorms was examined, no large enhancements in odd nitrogen which could be clearly attributed to lightning were observed. Apparently, the aircraft was not in the right place at the right time. Thus no estimate of the NOx production by lightning that remains below ∼8 km could be made.
Analysis of stratospheric chlorine monoxide measurements recorded by a ground‐based radiometer located at the Plateau de Bure, France
Authors: Ricaud P. , de La Noë J. , Lauqué R. , Parrish A.
Published: February 2004

Chlorine monoxide (ClO) measurements have been recorded since December 1993 using a ground‐based radiometer built in the United States and installed at a European site belonging to the Network for the Detection of Stratospheric Change, namely the Plateau de Bure (45°N, 10°E, 2550 m altitude). This paper describes the installed instrument, details the calibration and data processing routines that have been developed for use with the instrument, and characterizes the optimum estimation method used to retrieve vertical profiles and the associated error budget. Great care is taken to lessen biases induced by instrumental baseline ripples. We show the best data obtained so far, recorded from January 3 to 5, 1995, during the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME) Campaign. The diurnal variation of ClO measured from 25 to 50 km is presented and compared with results from a zero‐dimensional and a three‐dimensional model. The remainder of the data is still being processed, although a great deal of this data has been degraded by systematic instrumental artifacts in the spectral baseline.
Atmospheric measurements of water vapor in the 442‐nm region
Authors: Harder J. W. , Brault J. W.
Published: February 2004

In recent years much interest has been generated in the atmospheric community concerning low resolution water vapor cross sections in the blue spectral region. Proper removal of water absorption from long path tropospheric and zenith sky stratospheric measurements has posed a significant problem for recovery of absorption spectra of low concentration molecular species which overlap the water vapor spectrum. The 7‐quanta 4v1+3v3 and 5v1+2v3 water vapor overtone bands absorb light near 442.6 nm, and were found to have an integrated band intensity of 4.05×10−24 cm−1 cm2 molecule–1 with a measured accuracy of 4%. This cross section was synthesized from high resolution (0.001 nm or 0.055 cm−1) solar spectra taken on the Fourier transform spectrometer at the National Solar Observatory (NSO). This cross section was convolved with the instrument function of a low resolution spectrometer and used to simultaneously measure tropospheric water vapor and nitrogen dioxide over a folded 20.6 km optical path. A comparison of this spectroscopic measurement and a frost point hygrometer located at one end of the path agree to within 4%±17% with the spectroscopic measurement generally lower. Some of the observed discrepancy can be explained by the proximity of the hygrometric measurement to local surface sources of water, and by the tendency of the elevated long path measurement to sample drier air coming from aloft.
A 6‐year climatology of cloud occurrence frequency from Stratospheric Aerosol and Gas Experiment II observations (1985–1990)
Authors: Wang Pi‐Huan , Minnis Patrick , McCormick M. Patrick , Kent Geoffrey S. , Skeens Kristi M.
Published: February 2004

A 6‐year climatology of subvisual and opaque cloud occurrence frequencies is established using observations from the Stratospheric Aerosol and Gas Experiment (SAGE) II between 1985 and 1990. The subvisual clouds are observed mostly at high altitudes near the tropopause. The opaque clouds terminate the profiling, reducing the measurement frequency of the SAGE II instrument in the troposphere. With its 1‐km vertical resolution, the climatology shows many interesting features, including (1) the seasonal expansion and migration behavior of the subvisual and opaque cloud systems; (2) the association of the zonal mean cloud frequency distributions with the tropospheric mean circulation (Hadley and Ferrel cells); (3) the tropical cloud occurrence that follows the equatorial circulation, including the Walker circulation over the Pacific Ocean; and (4) the overall higher cloud occurrence in the northern hemisphere than in the southern hemisphere. The radiative impact of subvisual clouds is estimated to be a 1‐W m−2reduction in outgoing longwave radiation. The maximum overall effect is a net positive cloud forcing of 0.5–1 W m−2in the tropics. During the 1987 El Niño‐Southern Oscillation (ENSO), cloud frequency was generally enhanced in the tropics and midlatitudes and reduced in the subtropics and high latitudes. The present study shows a distinct negative correlation between the high‐altitude cloud occurrence and the lower stratospheric water vapor mixing ratio in the tropics, providing intrinsic evidence on the delicate connection between the stratospheric‐tropospheric exchange and dehydration processes and the high‐altitude cloud activities.
Seasonality of vegetation fires in Africa from remote sensing data and application to a global chemistry model
Authors: Cooke W. F. , Koffi B. , Grégoire J.‐M.
Published: February 2004

This paper sets out to show the potential use of remote sensing of active vegetation fires for continental‐ to global‐scale modeling of biomass burning studies. It focuses on the analysis of the seasonality of vegetation fires for the African continent, as derived from NOAA‐AVHRR‐GAC‐5km satellite data. These data are ideally suited for savanna fires, which constitute between 60 and 80% of the biomass burnt in Africa. Monthly counts of fire pixels, within 1° latitude × 1° longitude grid cells, over continental Africa have been calculated from November 1984 through October 1989. These 1° grid cells are summated to a 5° × 5° grid to enable comparison with previous studies and are analyzed at this resolution to show various features of the fire season. The analysis shows that previous attempts to characterize the seasonality of biomass burning have tended to underestimate the intensity of the peak months of burning or have predicted too long a fire season in certain areas. It also shows that there can be, for a given area, a temporal shift in the timing of the fire season from year to year. Such an interannual variability of fire seasonality makes satellite data more appropriate than statistical data for the modeling of atmospheric transport of vegetation fire products and the comparison with experimental measurements. Modeled values of black carbon mass concentration from a global transport model (MOGUNTIA), using the seasonality of biomass burning as an independent variable, are compared with measurements taken at Amsterdam Island (38°30′S, 77°30′E) and Lamto, Ivory Coast (6°N, 5°W). Although 5‐year averaged satellite data were used, the seasonality as derived from satellite data determined in this paper gives modeled values of black carbon mass concentration that are in good agreement with the measurements.
Causal space‐time multifractal processes: Predictability and forecasting of rain fields
Authors: Marsan David , Schertzer Daniel , Lovejoy Shaun
Published: February 2004

Building on earlier cascade models of rainfall, we propose a model of space‐time rain fields based on scaling dynamics. These dynamics are indeed related to the space‐time symmetries of the turbulent medium within which rainfall occurs: the underlying phenomenology corresponds to a cascade of structures with lifetimes depending only on the scale of the structures. In this paper we clarify two major issues: the scaling anisotropy between space and time, and the need to respect causality, i.e., a fundamental asymmetry between past and future. We detail how this ”arrow of time” breaks the mirror symmetry with respect to the spatial hyperplane, and how it can be introduced in continuous multiplicative cascade models so as to remove the artificial temporal mirror symmetry of earlier models. We show that such a causal multifractal field can be understood as the result of an anomalous diffusion acting on the singularities of the field. Finally we will exploit and test these models through (1) a succinct analysis of rainfall data, (2) numerical simulations of the temporal decorrelation of two initially similar fields (accounting for the loss of predictability of the process), and (3) a forecasting method for multifractal rain fields.
Extreme rainfall prediction using the European Centre for Medium‐Range Weather Forecasts ensemble prediction system
Authors: Petroliagis T. , Buizza R. , Lanzinger A. , Palmer T. N.
Published: February 2004

The combined use of the European Centre for Medium‐Range Weather Forecasts high‐resolution, at T213 spectral triangular truncation and with 31 vertical levels (T213L31, operational model and ensemble prediction system (EPS), during cases of intense Mediterranean storms, is studied. In particular, it is discussed how EPS products can be used to provide a measure of confidence in the high‐resolution precipitation forecast. Three case studies (two extreme events plus one false alarm case) are analyzed. For the first two cases, the EPS probability values for precipitation occurrence supported the medium‐range T213L31 prediction, which proved to be successful. By contrast, for the third case, the high‐resolution forecast suggested heavy rainfall over northern Italy but was not supported by the EPS. The T213L31 prediction for this case was poor. EPS forecasts of extreme weather events are necessarily compromised by the moderate resolution of the T63L19 model (a version of the operational high‐resolution T213L31 model, at T63 spectral triangular truncation and with 19 levels) used to generate the ensembles. In future studies, ensembles will be made using at least T106L31 resolution combined with an increase in ensemble size.
Black carbon and absorption of solar radiation by clouds
Authors: Chýlek Petr , Lesins Glen B. , Videen Gorden , Wong J. G. D. , Pinnick R. G. , Ngo Dat , Klett James D.
Published: February 2004

The exact solution of the scattered electromagnetic field from a water droplet containing an arbitrarily located spherical black carbon particle is used to investigate the effect of black carbon on the absorption of solar radiation by clouds. When droplet absorption is averaged over all possible locations of black carbon within a droplet, the averaged absorption is close to the value calculated using the effective medium approximation. The preferential black carbon location on the top or close to the bottom of the droplet leads to an increased absorption. The estimated upper bound on the increased absorption of solar radiation (global and annual average) is 1–3 W/m2 over the absorption of pure water clouds.
An analysis of POAM II solar occultation observations of polar mesospheric clouds in the southern hemisphere
Authors: Debrestian D. J. , Lumpe J. D. , Shettle E. P. , Bevilacqua R. M. , Olivero J. J. , Hornstein J. S. , Glaccum W. , Rusch D. W. , Randall C. E. , Fromm M. D.
Published: February 2004

The second Polar Ozone and Aerosol Measurement (POAM II) instrument is a space‐borne visible/near IR photometer which uses the solar occultation technique to measure vertical profiles of ozone, nitrogen dioxide, and water vapor as well as aerosol extinction and atmospheric temperature in the stratosphere and upper troposphere. Here we report on the detection of polar mesospheric clouds (PMCs) in the high‐latitude southern hemisphere by POAM II during the 1993 and 1994 summer seasons. These measurements are noteworthy because they are the first measurements of PMCs in atmospheric extinction. The POAM II PMC data set has been analyzed using a simple geometric cloud model. We find that mean cloud altitudes deduced from these data are 82–83 km, consistent with previous ground‐based and satellite measurements. In addition, the 0.7 km vertical resolution of POAM II allows for accurate determination of cloud thickness. For the PMCs detected by POAM II we find a mean thickness of 2.4 km. The mean peak slant optical depth was determined to be 1.2×10−3 for the 1993 season and 1.8×10−3 for the 1994 season, corresponding to a cloud extinction coefficient of 3.9×10−6 and 6.1×10−6 km−1, respectively. The multichannel capability of POAM II also makes it possible to study the wavelength dependence of the measured slant optical depth for the clouds with largest extinction. The results of this analysis suggest an upper limit to the modal particle radii for these clouds of approximately 70 nm.
Energy dissipation in lightning flashes
Authors: Cooray Vernon
Published: February 2004

In this paper the amounts of energy dissipation in different stages of ground flashes are estimated by electrostatic energy considerations. In the analysis the cloud is modeled using the typical charge configuration given by Malan [1963]. The leader stage is modeled by a column of charge that extends from cloud to ground. The linear charge density along the leader channel is assumed to remain uniform or decrease exponentially with height. On the basis of the results, the energy budget of return strokes and ground flashes can be described as follows: (1) A typical stepped leader‐return stroke process that neutralizes 5 C of charge dissipates about 5.5×108 J. Of this energy about 3.5×108 J dissipates in the return stroke stage, and 2×108 J in the leader stage. An unit length of the first return stroke channel dissipates about 7×104 J/m. (2) A typical dart leader‐return stroke process that neutralizes 1 C of charge dissipates about 12×107 J. Of this energy 4×107 J dissipates in the return stroke stage and 8×107 J in the dart leader stage. An unit length of the subsequent return stroke channel dissipates about 8×103 J/m. (3) A typical ground flash with four strokes dissipates about 9.5×108 J. Of this energy 4.5×108 J dissipates in the leader stages, and 5×108 J dissipates in the return stroke stages. In our analysis we also discovered the following: (1) The charge that maximizes the energy dissipation during the leader stage depends on the charge density of the cloud. For the values of cloud charge densities measured in experimental investigations this optimum charge is about 5 C. (2) For a given amount of charge neutralization, a cloud flash dissipates more energy than a ground flash.
Surface level measurements of ozone and precursors at coastal and offshore locations in the Gulf of Maine
Authors: Ray John D. , Heavner Ron L. , Flores Miguel , Michaelsen Clifton W.
Published: February 2004

The northeastern United States has episodic high ozone several times each year at locations remote from urban and industrial centers. Extended measurements of ozone and the ozone precursors, volatile organic compounds (VOC) and nitrogen oxides, were made at Acadia National Park, Cape Elizabeth, and other coastal Maine locations during the North Atlantic Regional Experiment (NARE) intensive. In addition, ozone was measured from a commercial ferry, the Scotia Prince, in the Gulf of Maine where ozone concentrations up to 129 ppb were observed. Two high‐ozone episodes were observed during late August 1993 when ozone was greater than 90 ppb along much of the Maine coast. NOy concentrations at Acadia averaged 2.0 ppb (±1.97), maximum 12.0 ppb. During the high‐ozone episodes, NOy had linear relationships to ozone with slopes of 4–16. The timing of maximum values and extent of the high‐ozone air mass suggests that urban plumes transported over the Gulf of Maine are brought inland by sea breezes to the coastal regions but not to the interior areas of Maine.
On the possibility of complicated dynamic behavior of atmospheric photochemical systems: Instability of the Antarctic photochemistry during the ozone hole formation
Authors: Feigin A. M. , Konovalov I. B.
Published: February 2004

We suggest a new approach to studying atmospheric photochemical processes by analyzing an atmospheric photochemical system as a dynamic system possessing many degrees of freedom and demonstrate a good agreement between the Antarctic photochemical system (APCS) behavior described by a three‐order set of ordinary differential equations and ozone variations taking place during ozone hole phenomenon. We investigate the dynamic properties of this set under the parameter values corresponding to the Antarctic stratosphere conditions during the last decade and show that APCS may become unstable as a result of the loss of equilibrium state stability and/or as a result of the self‐oscillations appearance. We present some arguments in favor of close connection between the APCS instability and anomalous deep depletion of Antarctic ozone concentration in spring observed since the mid‐1980s.
Atmospheric 222Rn measurements during the 1993 NARE Intensive
Authors: Zaucker Fritz , Daum Peter H. , Wetterauer Uli , Berkowitz Carl , Kromer Bernd , Broecker Wallace S.
Published: February 2004

In this paper we describe a recently developed technique to collect samples for atmospheric 222Rn measurements suited for deployment on (research) aircraft. Following a technical description of the sampler we present measurements from nine flights during the 1993 North Atlantic Regional Experiment (NARE) intensive over the North Atlantic Ocean and over the continent in the vicinity of Nova Scotia and New Brunswick, Canada. Samples were taken both in the boundary layer and in the free troposphere up to about 5500 m (msl). Concentrations ranged from 0.01 to 1.66 Bq/m3 in the free troposphere, with means of 1.66 and 0.52 Bq/m3, respectively. Boundary layer 222Rn concentrations were strongly correlated with back trajectory origins confirming the use of 222Rn as tracer for continental air masses. Samples collected in the marine boundary layer (MBL) were found to be significantly correlated with aerosol particle, NOy, and O3 concentrations reaffirming the utility of radon as a tracer of continental boundary layer air. Radon concentrations for the MBL samples were not found to be significantly correlated with transit times from the continent as estimated from back trajectory analysis implying that variations in the radon concentrations are dominated by factors other than radioactive decay. It is suggested that this factor is the height of the continental boundary layer into which the radon is emitted. On the basis of the radioactive decay we estimated apparent 222Rn ages for our samples and discuss the inherent difficulties in reliable age estimates. A comparison with hydrocarbon ratio age estimates for a highly polluted plume of air encountered during the campaign shows a similar age range for these samples and suggests that simultaneous measurements of species with different chemical and physical properties might allow narrowing the uncertainties in such ages estimates.
Sources and sinks of carbonyl compounds in the Arctic Ocean boundary layer: Polar ice floe experiment
Authors: Shepson P. B. , Sirju A.‐P. , Hopper J. R. , Barrie L. A. , Young V. , Niki H. , Dryfhout H.
Published: February 2004

Measurements of HCHO, CH3CHO, and CH3C(O)CH3 were made at the Narwhal ice floe camp in the Lincoln Sea at 84°N latitude from April 10 to 24, 1994. During the period April 13 to 18, O3 was below the detection limit of the measurement (i.e., <1 ppb), and the average HCHO, CH3CHO, and CH3C(O)CH3 concentrations were 193, 93, and 1730 ppt, respectively. A box model of the chemistry involved in the surface O3 depletion shows that the majority of BrOx termination reactions occur via Br atom reaction with the aldehydes. The reaction of Br atoms with CH3CHO is shown to be very effective in removing NOx from the Arctic marine boundary layer (MBL), via formation of peroxyacetyl nitrate (PAN). This denitrification of the surface layer has a significant impact on the radical chemistry. In particular, the model indicates that the observed levels of HCHO and CH3CHO cannot be reproduced if, as discussed in recent reports of Arctic ozone chemistry at sunrise, both Br atom and Cl atom chemistry occur simultaneously (at estimated concentrations of 1 × 104 and 1 × 107 atoms/cm3, respectively). However, if only chlorine atoms are present (at 1 × 104 atoms/cm3), reasonable steady state CH3CHO levels (∼80 ppt), but rather low HCHO levels (∼50 ppt) are produced. The model HCHO levels for chlorine‐atom‐only chemistry are as much as a factor of 10 lower than those observed (by these authors and others) in the Arctic MBL at sunrise. Model simulations show that the ratio CH3C(O)CH3/C2H5CHO could be a useful indicator of the relative importance of Br atom and Cl atom chemistry.
Development, validation, and potential enhancements to the second‐generation operational aerosol product at the National Environmental Satellite, Data, and Information Service of the National Oceanic and Atmospheric Administration
Authors: Stowe Larry L. , Ignatov Alexander M. , Singh Ramdas R.
Published: February 2004

A revised (phase 2) single‐channel algorithm for aerosol optical thickness, τSATA, retrieval over oceans from radiances in channel 1 (0.63 μm) of the advanced very high resolution radiometer (AVHRR) has been implemented at the National Oceanic and Atmospheric Administration's National Environmental Satellite Data and Information Service for the NOAA 14 satellite launched December 30, 1994. It is based on careful validation of its operational predecessor (phase 1 algorithm), implemented for NOAA 11 in 1989. Both algorithms scale the upward satellite radiances in cloud‐free conditions to aerosol optical thickness using an updated radiative transfer model of the ocean and atmosphere. Application of the phase 2 algorithm to three matchup Sun‐photometer and satellite data sets, one with NOAA 9 in 1988 and two with NOAA 11 in 1989 and 1991, respectively, show systematic error is less than 10%, with a random error of στ≈0.04. First results of τSATA retrievals from NOAA 14 using the phase 2 algorithm, and from checking its internal consistency, are presented. The potential two‐channel (phase 3) algorithm for the retrieval of an aerosol size parameter, such as the Junge size distribution exponent, by adding either channel 2 (0.83 μm) from the current AVHRR instrument, or a 1.6‐μm channel to be available on the Tropical Rainfall Measurement Mission and the NOAA‐KLM satellites by 1997 is under investigation. The possibility of using this additional information in the retrieval of a more accurate estimate of aerosol optical thickness is being explored.
Absorption of solar radiation by clouds: Interpretations of satellite, surface, and aircraft measurements
Authors: Cess R. D. , Zhang M. H. , Zhou Y. , Jing X. , Dvortsov V.
Published: February 2004

To investigate the absorption of shortwave radiation by clouds, we have collocated satellite and surface measurements of shortwave radiation at several locations. Considerable effort has been directed toward understanding and minimizing sampling errors caused by the satellite measurements being instantaneous and over a grid that is much larger than the field of view of an upward facing surface pyranometer. The collocated data indicate that clouds absorb considerably more shortwave radiation than is predicted by theoretical models. This is consistent with the finding from both satellite and aircraft measurements that observed clouds are darker than model clouds. In the limit of thick clouds, observed top‐of‐the‐atmosphere albedos do not exceed a value of 0.7, whereas in models the maximum albedo can be 0.8.
Mean radiation fluxes in the near‐IR spectral range: Algorithms for calculation
Authors: Titov G. A. , Zhuravleva T. B. , Zuev V. E.
Published: February 2004

Algorithms are presented to compute the mean fluxes of shortwave radiation, modulated by statistically homogeneous broken clouds, in the near‐IR spectral range (0.7–3.2 μm). One method of the mean spectral flux computation consists of dividing the spectral interval in question into Nint subintervals according to the spectral resolution specified. The mean spectral fluxes are then calculated with the Monte Carlo method by assuming constant cloud optical characteristics within each subinterval. This last method (algorithm 1) accurately accounts for the spectral behavior of optical characteristics of clouds and atmospheric gases and can therefore be regarded as a reference. The mean spectral flux computations of high spectral resolution (say, Δν = 10–20 cm−1 ) may require several hundreds of spectral intervals. Therefore the method of dependent tests (algorithm 2) is proposed, which effectively uses notable features of the spectral dependence of cloud optical parameters, thus allowing significant simplifications and extra savings of computer time. Comparison of results from algorithms 1 and 2 shows that the mean radiant fluxes agree to within the relative computation error (3%). This indicates that algorithm 2 is reasonably accurate; in addition, its efficiency is several orders of magnitude better than that of the reference algorithm 1.
Aerosol measurements in the Arctic relevant to direct and indirect radiative forcing
Authors: Hegg Dean A. , Hobbs Peter V. , Gassó Santiago , Nance Jon D. , Rangno Arthur L.
Published: February 2004

Airborne measurements in the Arctic in June permit calculation of some of the parameters needed to assess both the direct and indirect radiative forcing by aerosols in the region. Values for the single‐scattering albedo of the aerosols suggest that in June the direct effect will produce a net cooling, in contrast to winter arctic hazes. Internal closure calculations comparing aerosol size distribution measurements with directly measured light scattering by aerosols over the first 4 km of the atmosphere show good agreement. Measurements of cloud condensation nucleus (CCN) activation spectra show steeper slopes than previous measurements in the Arctic in winter and early spring. Based on the CCN and collated cloud microphysical measurements, the susceptibility of the clouds encountered in this project to aerosol‐induced albedo modification appears quite high.
Monitoring the isotopic composition of atmospheric CO2: Measurements from the NOAA Global Air Sampling Network
Authors: Trolier M. , White J. W. C. , Tans P. P. , Masarie K. A. , Gemery P. A.
Published: February 2004

The stable isotopic composition of atmospheric CO2 is being monitored via measurements made at the University of Colorado‐Institute of Arctic and Alpine Research, using air samples collected weekly by the Global Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmospheric CO2 mixing ratios, offer the potential to characterize quantitatively the mechanisms operating in the global carbon cycle, by recording the isotopic signatures imparted to CO2 as it moves among the atmosphere, biosphere, and oceans. This data set increases the number of measurements of atmospheric CO2 isotopes by nearly an order of magnitude over those previously available. We describe the analytical techniques used to obtain and calibrate these data and report measurements from 25 land‐based sites, and two ships in the Pacific Ocean, from samples collected during 1990–1993. The typical precision of our mass spectrometric technique is 0.03‰ for δ13C and 0.05‰ for δ18O. Collecting the flask samples without drying leads to loss of δ18O information at many sites. The seasonal cycle in δ13C at sites in the northern hemisphere is highly correlated with that of the CO2 mixing ratio, with amplitudes approaching 1‰ at high latitudes. The seasonal cycle in δ18O is of similar amplitude, though variable from year to year and lags the other species by 2–4 months. Interhemispheric differences of the 1992 and 1993 means of the isotopic tracers are in strong contrast: the north pole‐south pole difference for δ13C is −0.20‰, which though highly quantitatively significant is dwarfed by the −2‰ difference for δ18O. In contrast to the record of atmospheric δ13C during the 1980s we observe no significant temporal trend in annual mean δ13C during 1990–1993.