bluuebunny/crossref_metadata_2025 huggingface dataset:
Visual and lidar observations of noctilucent clouds above Logan, Utah, at 41.7°N
Authors: Wickwar Vincent B. , Taylor Michael J. , Herron Joshua P. , Martineau Bethany A.
Published: October 2002

Noctilucent clouds (NLCs) were observed from a midlatitude site (Logan, Utah) on the evenings of 22 and 23 June 1999 mountain daylight time. On both nights the clouds were seen for approximately an hour by experienced observers, and they were photographed. The NLC was also observed on the second evening for approximately an hour in the zenith with the Rayleigh‐scatter lidar at the Atmospheric Lidar Observatory, which is operated by the Center for Atmospheric and Space Sciences on the campus of Utah State University. These observations enabled several of the properties of the cloud to be determined. They were within the range of those observed at higher latitudes, but notably the NLC was very weak and thin. These combined visual and lidar observations unequivocally support the identification of the cloud as a noctilucent cloud. The midlatitude location (41.74°N, 111.81°W) is ∼10° equatorward of previous observations. This equatorward penetration is significant because of potential implications about global change or the global circulation.
Relationships between daytime carbon dioxide uptake and absorbed photosynthetically active radiation for three different mountain/plains ecosystems
Authors: Kelly Robert D. , Hunt E. Raymond , Reiners William A. , Smith William K. , Welker Jeffrey M.
Published: October 2002

Mean midday values of eddy covariance CO2 flux and absorbed photosynthetically active radiation (APAR), derived from solar irradiance and normalized difference vegetation index (NDVI), were measured from May to September 1999 with an aircraft at 60–90 m above ground level over three different ecosystems dominated by native plant species in southeastern Wyoming (mixed conifer forest, mixed short‐grass prairie, and sagebrush shrubland). The midday net CO2 uptake at each site followed seasonal trends, with summer values occurring later over the forest than over the other sites. At the landscape scale, linear relationships were observed between CO2 uptake and APAR for the grassland and shrubland, with increasing APAR leading to increasing CO2 uptake. Over the forest, however, the CO2 uptake was only weakly related to APAR, but an additional linear relationship with infrared surface temperature (TIR) implied that respiration may have been more important than APAR in determining net CO2 uptake by the forest. The regression slope for net CO2 uptake versus APAR for the grassland data agreed with those from other observations.
Longwave radiative forcing of Indian Ocean tropospheric aerosol
Authors: Lubin Dan , Satheesh S. K. , McFarquar Gregory , Heymsfield Andrew J.
Published: October 2002

A spectrally resolved discrete‐ordinates radiative transfer model is used to calculate the change in downwelling surface and top‐of‐the‐atmosphere (TOA) outgoing longwave (3.9–500 μm) radiative fluxes induced by tropospheric aerosols of the type observed over the Indian Ocean during the Indian Ocean Experiment (INDOEX). Both external and internal aerosol mixtures were considered. Throughout the longwave, the aerosol volume extinction depends more strongly on relative humidity than in most of the shortwave (0.28–3.9 μm), implying that particle growth factors and realistic relative humidity profiles must be taken into account when modeling the longwave radiative effects of aerosols. A typical boundary layer aerosol loading, with a 500‐nm optical depth of 0.3, will increase the downwelling longwave flux at the surface by 7.7 W m−2 over the clean air case while decreasing the outgoing longwave radiation by 1.3 W m−2. A more vertically extended aerosol loading, exhibiting a high opacity plume between 2 and 3 km above the surface and having a typical 500‐nm optical depth of 0.7, will increase the downwelling longwave flux at the surface by 11.2 W m−2 over the clean air case while decreasing the outgoing longwave radiation by 2.7 W m−2. For a vertically extended aerosol profile, approximately 30% of the TOA radiative forcing comes from sea salt and approximately 60% of the forcing comes from the combination of sea salt and dust. The remaining forcing is from anthropogenic constituents. These results are for the external mixture. For an internal mixture, TOA longwave forcings can be up to a factor of two larger. Therefore, to complete our understanding of this region's longwave aerosol radiative properties, more detailed information is needed about aerosol mixing states. These longwave radiative effects partially offset the large shortwave aerosol radiative forcing and should be included in regional and global climate modeling simulations.
The South American low‐level jet east of the Andes during the 1999 LBA‐TRMM and LBA‐WET AMC campaign
Authors: Marengo Jose A. , Douglas Michael W. , Silva Dias Pedro L.
Published: October 2002

The present study describes some observed surface and upper‐air features of the low‐level jet (LLJ) and southerly jet (SJ). Our results suggest the existence of this low‐level circulation to the east of the Andes that transports moisture from tropical South America toward the south during the warm/wet season of 1999. We explore the synoptic variability, diurnal variation, and alternations between LLJ and SJ episodes by using a combination of surface and high‐resolution upper‐air observations (1 to 8 soundings per day) and global reanalysis. Our results show strong synoptic fluctuations; with the LLJ more frequent than SJs. The LLJ has stronger winds in the afternoon and its core of maximum winds is located between 1600 and 2000 m above the surface. Special observational efforts, such as the pilot balloon sounding network in Bolivia (Pan American Climate Studies Sounding Network [PACS‐SONET] program), the Large‐Scale Biosphere‐Atmosphere (LBA) Experiment‐WET Atmospheric Mesoscale Campaign (AMC), and Tropical Rainfall Measuring Mission (TRMM)‐LBA in Southwest Amazonia, have provided upper‐air information with high temporal and spatial resolution to describe the structure of both the LLJ and the SJ during the January–April 1999 period.
Tides in the mesopause region over Fort Collins, Colorado (41°N, 105°W) based on lidar temperature observations covering full diurnal cycles
Authors: She C. Y. , Chen Songsheng , Williams B. P. , Hu Zhilin , Krueger David A. , Hagan M. E.
Published: October 2002

Eighteen sets of 24‐hour continuous lidar temperature observation, well distributed throughout the year, have been analyzed to reveal the mean amplitude and phase of oscillations with 24‐, 12‐, 8‐, and 6‐hour periods for four seasons. The amplitude and phase of both 24‐ and 12‐hour components are compared to the prediction of the most recent global‐scale wave model (GSWM‐00). The variability of diurnal and semidiurnal oscillations is examined by comparing the amplitude and phases of each observed day in autumn, winter, and spring. Even with the limited data sets reported here, surprisingly good general agreement between the observed and model tidal phases can be established, implicating the prevalence of migrating diurnal and semidiurnal tides. Compared to the diurnal and semidiurnal phases, which typically show clear downward phase progression, the 8‐ and 6‐hour components exhibit small amplitude and show unorganized phase propagation as a function of altitude. The advantage of having coverage over complete diurnal cycle and entire mesopause altitudes as demonstrated here would hopefully stimulate future tidal studies with lidar temperature, and zonal and meridional wind observations.
Aerosol layers over the Pacific Ocean: Vertical distributions and optical properties as observed by multiwavelength airborne lidars
Authors: Menzies Robert T. , Tratt David M. , Spinhirne James D. , Hlavka Dennis L.
Published: October 2002

Scattering properties of tropospheric aerosol layers were observed with airborne lidars during the Global Backscatter Experiment (GLOBE) airborne campaigns covering a wide range of latitude and longitude over the Pacific Ocean. Lidar data at 0.532, 1.064, 1.54, and 9.25 μm wavelengths are used to study the vertical profiles and optical properties of the marine boundary layer (MBL) as well as elevated dust layers due to transport of material from Asia. The lidars were calibrated to provide absolute aerosol backscatter coefficients at their respective wavelengths. The lidar data are compared with predictions based on commonly used dust and sea salt aerosol refractive index and size distribution parameters. Published observations of aerosol characteristics from onboard in situ sensors are used to verify locations where mineral dust aerosol dominated the aerosol optical properties in the free troposphere. Relative humidity has a major impact on the observed MBL aerosol scattering characteristics. The multiwavelength observations of the dust layers are consistent with the assumption that for the particle size range that dominates the scattering properties the particle size distribution varies little over 103 km transport distances and is not strongly dependent on altitude. A nonsphericity effect on the dust scattering, in particular the suppression of backscattering at the shorter lidar wavelengths, is discussed in the context of data comparisons with model predictions. The observations of the sea salt aerosol scattering in the remote MBL indicate that at altitudes above 150 m in optically clear conditions the influence of the supermicron coarse mode is minor compared with that of the submicron accumulation mode.
Forcing of secondary waves by breaking of gravity waves in the mesosphere
Authors: Zhou XueLong , Holton James R. , Mullendore Gretchen L.
Published: October 2002

Previous simulations with a two‐dimensional cloud‐resolving model have shown that gravity waves generated by tropospheric convection can propagate into the mesosphere, where they break and produce local heating and local accelerations. The forcing associated with this wave breaking excites secondary gravity waves that propagate upward and downward away from the wave‐breaking region. Typical horizontal and vertical scales of individual centers of forcing caused by wave breaking are ∼25 km and 10–20 km, respectively. Groups of individual forcing centers tend to be aligned along phase lines of the primary waves. In the region east of the storm center the forcing groups move eastward and downward with the primary wave phase lines, while individual forcing centers move upward. To a first approximation, the secondary gravity waves are a linear response to the localized momentum and thermal forcing associated with wave breaking, though the forcing is itself generated by the nonlinear wave‐breaking process. The secondary waves can be derived from knowledge of the temporal and spatial variability of the forcing.
Monitoring of erythemal irradiance in the Argentine ultraviolet network
Authors: Cede Alexander , Luccini Eduardo , Nuñez Liliana , Piacentini Rubén D. , Blumthaler Mario
Published: October 2002

The Ultraviolet (UV) Monitoring Network of the Argentine Servicio Meteorológico Nacional (National Weather Service) consists at present of nine stations from 22° to 64° latitude south equipped with biometers, broadband instruments that measure the erythemal irradiance. After a complete calibration of the instruments and reprocessing of the database a preliminary climatology of erythemal irradiance and erythemal exposure was built by analyzing the data of the first 2–4 years for each station. The influence of the measurement interval on the UV Index is quantified. A tropical high‐altitude station in the Andean Altiplano reaches top UV Index values of 20 and daily doses of 10.6 kJ/m2, among the highest UV levels worldwide. Characteristic UV lndex and erythemal exposure in the central, most populated areas of Argentina are similar to those in other Southern Hemisphere stations and are higher than in comparable latitude regions of the Northern Hemisphere. The southern stations in the UV network are often affected by the Antarctic ozone hole. By comparing clear‐sky irradiance measurements with aerosol‐free radiative transfer calculations using the Total Ozone Mapping Spectrometer surface albedo climatology, typical attenuations from 2 to 15% due to aerosols were determined, reaching maxima of 30% at urban locations. On the other hand, increases of ∼3–6% due to higher surface albedo were obtained for snow‐free conditions, and increases up to 15% were obtained for snow‐covered terrain.
Jicamarca radar observations of the diurnal and semidiurnal tide in the troposphere and lower stratosphere
Authors: Riggin Dennis M. , Kudeki Erhan , Feng Zhaomei , Sarango Martin F. , Lieberman Ruth S.
Published: October 2002

The mesosphere‐stratosphere‐troposphere (MST) radar at Jicamarca, Peru (12°S, 77°W), made extended (15 day or longer) observations of the horizontal and vertical winds that were used to infer the diurnal and semidiurnal tides. The measurements were made during several months from mid‐1997 through mid‐1998 and using a higher‐power transmitter and finer range resolution during 10 days of August 1998. The three‐component winds are used to estimate amplitudes, phases, and momentum fluxes associated with the tides. Thermal forcing of the diurnal tide is also examined with diurnal water vapor heating rates calculated using data from the NASA Water Vapor Project (NVAP). For the region near Jicamarca the calculations from NVAP showed the temporal variability of the diurnal heating to be dominated by an annual cycle with maximum around the summer solstice. When projected into tidal modes, about 25% of the total water vapor heating rate amplitude near Jicamarca is found to be nonmigrating. The meridional amplitude of the semidiurnal tide was found to be generally greater than the zonal amplitude, although tidal theory predicts that the zonal amplitude should be much greater at the latitude of Jicamarca (assuming the tide was migrating). The phase of the semidiurnal tide lagged (by about 3 hours) the phase expected from surface pressure climatologies. According to tidal theory the migrating semidiurnal tide should transport little meridional momentum flux. However, substantial southward fluxes (〈v′w′〉 ∼ −1 × 10−3 m2 s−2) were observed at Jicamarca, and the meridional component of momentum flux was typically larger in magnitude than the zonal component was. The diurnal tide was somewhat weaker, was less coherent, and transported less momentum. The semidiurnal tide had a very long vertical wavelength throughout the troposphere and into the lower stratosphere, while the diurnal tide was only observed to propagate at heights above the tropopause with a much shorter (∼10 km) vertical wavelength. Below the tropopause the dominant diurnal motions were not traveling waves, but rather convective motions that exhibited little phase progression with altitude. These motions were broadly peaked in frequency around 24 hours and were presumably standing oscillations with no horizontal propagation and probably with small horizontal scale. Despite the lack of coherence of these quasi‐diurnal motions, the associated vertical wind amplitudes were sizable (∼0.02 m s−1), and thus the fluctuations can presumably transport significant horizontal momentum.
Diagnosis of mixing between middle latitudes and the polar vortex from tracer‐tracer correlations
Authors: Morgenstern Olaf , Pyle John A. , Iwi Alan M. , Norton Warwick A. , Elkins James W. , Hurst Dale F. , Romashkin Pavel A.
Published: October 2002

A method is introduced for diagnosing mixing between the polar vortex and midlatitudes from tracer data. Tracers with different photochemical activities and lifetimes usually exhibit curved tracer‐tracer correlation functions on an isentropic surface. The effect of mixing events is to populate the inner side of such a curve. Using simultaneous measurements of trace gases or model results, we exploit this process to calculate the distribution of recent origins in tracer space prior to such a mixing event. The method relies on both hemispheric and local data and is applicable to situations where mixing is nonlocal in tracer space. It is applied to measurements taken during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO 2000) winter campaign and to a chemical transport model simulation covering the same winter. In one of the cases studied, a vortex breakup and subsequent remerger of the vortex fragments in March 2000 results in significant diagnosed mixing. In a further example, an elongated filament shed off the polar vortex is characterized by anomalous composition. For the two high‐altitude aircraft flights of the SOLVE campaign that probe the vortex boundary, a correspondence is found for mixing diagnosed in the measurements and in the model. Mixing timescales considered here are given by the life span of planetary waves, up to a few weeks.
Condensation/evaporation of insoluble organic vapor as functions of source rate and saturation vapor pressure
Authors: Pirjola Liisa , Korhonen Hannele , Kulmala Markku
Published: October 2002

Condensation growth of atmospheric particles by insoluble organic vapors was studied by a monodisperse aerosol dynamics model MONO32 including atmospheric chemistry mechanism. The source rate of the vapor and its saturation vapor density at a particle surface were varied. The initial particle number size distribution was assumed to be bimodal, and aerosol was initially an internal mixture of particles consisting of soluble sulphuric acid and insoluble organic material in a ratio of 1:1. Model simulations predicted that if the ratio of the vapor source rate and the condensation sink of particles, Q/CS, exceeded 108 cm−3 and the vapor was nonvolatile or low volatile, the 10 nm nuclei grew with the growth rates of 2.5–3 nm h−1 and could act as cloud condensation nuclei in supersaturations of 1.6% or lower after 24 h simulation. Then the vapors must have saturation vapor pressures less than 7 × 106 cm−3. However, the higher the ratio of Q/CS, the higher the saturation vapor pressure could be. When the vapor was highly volatile, i.e., the saturation vapor density was high enough, in our simulations greater than 6 × 107 cm−3 the Ostwald ripening was observed. Then the nucleation mode particles were not able to grow but coagulated with larger particles without causing any significant impact on climate. An analytical expression for the saturation vapor density for the Ostwald ripening (cOst) was derived as a function of the ratio of Q/CS. Sensitivity tests for the accommodation coefficient and thermodynamic parameters as well as their effects on cOst were investigated.
Broadband shortwave calibration results from the Atmospheric Radiation Measurement Enhanced Shortwave Experiment II
Authors: Michalsky J. , Kiedron P. , Berndt J. , Stoffel T. , Myers D. , Reda I. , Treadwell J. , Andreas A. , Asano S. , Uchiyama A. , Yamazaki A. , Haeffelin M. , Tooman T. , McCoy R. , Bucholtz A. , Bush B. C. , Pope S. K. , Leitner A. S. , Valero F. P. J.
Published: October 2002

The second ARM (Atmospheric Radiation Measurement) Enhanced Shortwave Experiment (ARESE II) used a single aircraft flying above the north central Oklahoma Southern Great Plains ARM central facility to measure the atmospheric absorption in the column of air between the surface and the Twin Otter altitude ceiling around 7 km on both clear and overcast days. For this experiment, three types of broadband radiometers were used to measure upwelling and downwelling shortwave flux on the aircraft at 7 km. This provided redundancy that was lacking in the first ARESE. Further, all instruments used on the ground and on the aircraft were calibrated on the ground against the same radiation standard. Preflight and postflight comparisons of the broadband instruments used on the aircraft during the flight series with the standard, and comparisons of the ground instruments with the standard during the flights, suggest agreement much better than the target uncertainty of 20 W/m2 at the 95% confidence level. Comparisons of the standard and the aircraft instruments for low‐altitude passes on clear days indicate more uncertainty as expected for a nonstationary platform. The estimated uncertainty in the measured column absorption based on the difference in measured net irradiances at the surface and near 7 km at the 95% confidence level is 20 W/m2.
Extended resistance analogy for construction of the vertical diffusion scheme for dispersion models
Authors: Sofiev M.
Published: October 2002

A new physical analogy for the discrete approximation of the vertical diffusion equation is presented. It is based on the resistor‐capacitor chain and incorporates the classical resistance approach to dry deposition simulation in a natural way. It is shown that in the simplest case the finite chain of resistors and capacitors is equivalent to a second‐order discrete representation of the diffusion equation resulting from the K theory closure. However, the scheme provides several features, which either cannot be obtained from straightforward discretization at all, or would require a sophisticated construction for their justification and realization. The examples discussed include a thick model layer with a highly variable vertical diffusion coefficient and an internal emission source, interaction with a chemical transformation unit, and spectral features of the model's vertical structure.
Response of the mesosphere to human‐induced perturbations and solar variability calculated by a 2‐D model
Authors: Khosravi Rashid , Brasseur Guy , Smith Anne , Rusch David , Walters Stacy , Chabrillat Simon , Kockarts Gaston
Published: October 2002

We have used the improved NCAR interactive 2‐D model (SOCRATES) to investigate the chemical and thermal response of the mesosphere to composition changes from the preindustrial era (∼1850) to the present, to doubling the CO2 concentration, and to the 11‐year solar flux variability. The calculations show that all regions in the model mesosphere have cooled relative to the preindustrial times. The mesopause region has cooled by ∼5 K and the winter pole by up to 9 K near 60 km. Ozone mixing ratio has decreased by about 5% in the lower mesosphere and by about 30% near the summer mesopause region (caused by a dramatic increase in [OH]). Doubling the CO2 abundance cools the whole mesosphere by about 4–16 K and has a complicated effect on O3, which exhibits an alternating increase/decrease behavior from the lower mesosphere up to the mesopause region. Similar results are obtained, in both magnitude and structure, for the O3 response to a decrease in solar UV flux. Similarities are also found in the response of T, OH, and H to these two perturbations. These results lead to the conclusion that the long‐term increase in the well‐mixed greenhouse gases, in particular CO2, alters the thermal structure and chemical composition of the mesosphere significantly and that these anthropogenic effects are of the same magnitude as the effects associated with the 11‐year solar cycle. Thus, the difference in the timescales involved suggests that the anthropogenic signal over periods of typically 10 years is smaller than the signal generated by the 11‐year solar variability. Finally, analysis of the results from a simulation of the combined perturbations (2 × CO2 + 11‐year solar variability) shows that, for the most part, the solar variability does not interact with increasing CO2 and vice versa; that is, the two effects are additive.
Solar influence on a major mode of atmospheric variability
Authors: Ruzmaikin Alexander , Feynman Joan
Published: October 2002

We find that the North Annular Mode (NAM) of the wintertime geopotential height anomalies between 10 and 1000 hPa is influenced by solar changes and that the effect is statistically significant. This evidence suggests that a mechanism of solar influence on climate operates through the excitation of this mode. The influence depends on the phase of the quasi‐biennial oscillation (QBO). In early winter for the west QBO and late winter for the east QBO the solar changes affect the NAM in both the stratosphere and the troposphere almost equally. The results are compared with earlier studies of the role of the QBO and solar UV changes on near‐polar temperatures and geopotential heights. Thus the late winter effect for the west QBO found by Labitzke and van Loon [1988] is clearly evident in the NAM in the stratosphere but does not appear in the troposphere.
Lidar observations of atmospheric boundary layer structure and sea spray aerosol plumes generation and transport at Mace Head, Ireland (PARFORCE experiment)
Authors: Kunz Gerard J. , de Leeuw Gerrit , Becker Edo , O'Dowd Colin D.
Published: October 2002

A scanning backscatter lidar was used to measure the depth and structure of the coastal atmospheric boundary layer and the evolution of primary aerosol (sea spray) plumes produced by breaking waves during the New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign at the Mace Head Atmospheric Research Station (Ireland) in September 1998 and in June 1999. The PBL structure was observed to vary from a single‐layer well‐mixed structure to multilayered structures. Comparison with in situ aircraft measurements of temperature and humidity exhibited good agreement. Using the lidar in the scanning mode allowed two‐dimensional profiling over a spatial scale of 10 km, revealing significant primary aerosol plumes produced by breaking waves, particularly in the surf zone and at high wind speeds on the open sea. The initial plume heights were some tens of meters and evolved to hundreds of meters while transported over only a few kilometers from the source. The plumes were traceable to distances of more than 10 km down wind from the source.
Advantages of diffuse radiation for terrestrial ecosystem productivity
Authors: Gu Lianhong , Baldocchi Dennis , Verma Shashi B. , Black T. A. , Vesala Timo , Falge Eva M. , Dowty Pete R.
Published: October 2002

Clouds and aerosols alter the proportion of diffuse radiation in global solar radiation reaching the Earth's surface. It is known that diffuse and direct beam radiation differ in the way they transfer through plant canopies and affect the summation of nonlinear processes like photosynthesis differently than what would occur at the leaf scale. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetically active radiation (PAR) for a Scots pine forest, an aspen forest, a mixed deciduous forest, a tallgrass prairie and a winter wheat crop. The comparison was based on the seasonal patterns of the parameters that define the canopy photosynthetic responses to diffuse PAR and those that define the responses to direct PAR. These parameters were inferred from half‐hourly tower CO2 flux measurements. We found that: (1) diffuse radiation results in higher light use efficiencies by plant canopies; (2) diffuse radiation has much less tendency to cause canopy photosynthetic saturation; (3) the advantages of diffuse radiation over direct radiation increase with radiation level; (4) temperature as well as vapor pressure deficit can cause different responses in diffuse and direct canopy photosynthesis, indicating that their impacts on terrestrial ecosystem carbon assimilation may depend on radiation regimes and thus sky conditions. These findings call for different treatments of diffuse and direct radiation in models of global primary production, and studies of the roles of clouds and aerosols in global carbon cycle.
Comparison of mean climate trends in the Northern Hemisphere between National Centers for Environmental Prediction and two atmosphere‐ocean model forced runs
Authors: Lucarini Valerio , Russell Gary L.
Published: October 2002

Results are presented for two greenhouse gas experiments of the Goddard Institute for Space Studies atmosphere‐ocean model (AOM). The computed trends of surface pressure; surface temperature; 850, 500, and 200 mbar geopotential heights; and related temperatures of the model for the time frame 1960–2000 are compared with those obtained from the National Centers for Enviromental Prediction (NCEP) observations. The domain of interest is the Northern Hemisphere because of the higher reliability of both the model results and the observations. A spatial correlation analysis and a mean value comparison are performed, showing good agreement in terms of statistical significance for most of the variables considered in the winter and annual means. However, the 850 mbar temperature trends do not show significant positive correlation, and the surface pressure and 850 mbar geopotential height mean trends confidence intervals do not overlap. A brief general discussion about the statistics of trend detection is presented. The accuracy that this AOM has in describing the regional and NH mean climate trends inferred from NCEP through the atmosphere suggests that it may be reliable in forecasting future climate changes.
Representation of the quasi‐biennial oscillation in the tropical stratospheric wind by nonlinear principal component analysis
Authors: Hamilton Kevin , Hsieh William W.
Published: October 2002

The zonal winds at several levels between 70 and 10 hPa (roughly 20–30 km) measured at near‐equatorial stations during 1956–2000 were analyzed to produce a one‐dimensional approximation. The neural network‐based technique applied was the circular nonlinear principal component analysis (NLPCA.cir) designed to characterize quasiperiodic phenomena. The reconstructed height‐time series of wind based on the one‐dimensional NLPCA.cir captures many of the characteristic features of the observed quasi‐biennial oscillation (QBO). The nonlinear results were evaluated relative to comparable linear principal component analysis and found to produce a superior one‐dimensional representation of the data. The NLPCA.cir analysis produces a single time series of QBO phase based on data at all levels. This phase was then applied to demonstrate a strong correlation of the state of the tropical QBO and boreal winter high‐latitude stratospheric temperatures.
On the seasonal dynamics of mean soil moisture
Authors: Laio F. , Porporato A. , Ridolfi L. , Rodriguez‐Iturbe I.
Published: October 2002

Starting from a stochastic model of the water balance at a point proposed by Rodriguez‐Iturbe et al. [1999], the unsteady dynamics of mean soil moisture under varying climatic conditions is analyzed. The exact differential equations that describe the evolution of the mean soil moisture are obtained, and the simplifications that are needed to solve these equations are detailed. Within this framework we are able to study the importance of the seasonal variations in precipitation and evapotranspiration and the role of the root depth and soil characteristics, as well as the possible occurrence and relevance of steady state conditions for the soil moisture process during the growing season.
Nonthermal steady state translational energy distributions of O(1D) atoms in the stratosphere
Authors: Takahashi Kenshi , Taniguchi Nori , Sato Yoshihiro , Matsumi Yutaka
Published: October 2002

The steady state translational energy distributions of O(1D) in the stratosphere between 20 and 50 km have been studied by laboratory experiments and Monte Carlo simulations. The results indicate that at all altitudes studied the translational energies of stratospheric O(1D) are distributed at high energies more than the Maxwell–Boltzmann distributions of the local temperatures. The predominant source of stratospheric O(1D) atoms is the solar UV photolysis of ozone, and the nascent O(1D) atoms produced have large translational energies due to the excess energy. The average translational energy of O(1D), for instance, at the altitude of 50 km, is found to be about twice as large as the thermal energy at the temperature of the ambient air. The nonthermal steady state translational energy distributions of O(1D) atoms result from the relatively slow translational energy relaxation process compared with the electronic quenching process (1D → 3P) by collisions with ambient air molecules (N2 and O2). It is suggested that the atmospheric reactions involving O(1D) atoms in the stratosphere proceed under nonlocal thermodynamic equilibrium conditions.
A comparative study of two land surface schemes in regional climate integrations over South America
Authors: Misra Vasubandhu , Dirmeyer Paul A. , Kirtman Ben P.
Published: October 2002

A case study comparison using two different land surface models in the Regional Spectral Model (RSM) is presented here. This comparison is motivated from recent studies that show a significant impact of land surface processes on the predictability of precipitation at seasonal to interannual scales. We find in this comparative study that coupled land‐atmosphere interactions using the simplified simple biosphere (SSiB) scheme and a two‐layer soil (control) model with uniform vegetation fraction yield mixed results. It does not clearly indicate the superiority of one scheme over the other for this particular case. The warmer mean surface temperatures simulated by RSM‐SSiB improve the seasonal (January‐February‐March [JFM]) simulation over Amazon River Basin, along Brazilian Highlands, over Bolivian Plateau, and over central America relative to the control model. However, the RSM‐SSiB runs exhibit a strong warm bias in the surface temperature over the subtropics of South America. The mean JFM onshore easterly flow from the tropical Atlantic Ocean and the low‐level jet are relatively stronger in the SSiB model. The mean JFM precipitation from the RSM‐SSiB model shows an improvement over Guianan Highlands, Venezeulan Llanos, and Amazon River Basin. However, the mean JFM precipitation of the control model over the Caribbean Sea, Central America, and equatorial eastern Pacific Ocean is better than the SSiB model.
Global soil‐biogenic NOx emissions and the role of canopy processes
Authors: Ganzeveld L. N. , Lelieveld J. , Dentener F. J. , Krol M. C. , Bouwman A. J. , Roelofs G.‐J.
Published: October 2002

Soils are an important though uncertain source of oxidized nitrogen (NOx) to the atmosphere. One of the main uncertainties in the source estimates is the role of the canopy interactions between NOx emissions, dry deposition, turbulence, and chemistry. Previous studies, in which only dry deposition has been considered, indicate a reduction of about 50% of the globally emitted NOx by soils. We have implemented a multilayer trace gas exchange model in a chemistry general circulation model to explicitly calculate the role of canopy interactions in regulating the effective NOx emissions to the atmosphere. Our new NOx emission algorithm interactively calculates a global soil emission flux of about 12 Tg N yr−1. For a sensitivity analysis we have also included a fixed global soil NOx emissions inventory of about 21 Tg N yr−1. It appears that the enhancement of NOx and O3 concentrations in response to the soil emission flux is suppressed by the compensating effect of dry deposition. For sites that are exposed to relatively large emission fluxes, our multilayer and the previously used big leaf approach, which does not consider canopy interactions, calculate similar surface NOx fluxes. This confirms the validity of the big leaf approach for most polluted regions at midlatitudes. However, for relatively pristine sites in the subtropics and tropics, where NOx is a limiting factor in ozone and hydroxyl chemistry, there are distinct differences between the multilayer and big leaf NOx surface fluxes. This justifies the use of more comprehensive atmosphere‐biosphere exchange descriptions in global models.
Modification of spectral UV irradiance by clouds
Authors: Schwander Harry , Koepke Peter , Kaifel Anton , Seckmeyer Gunther
Published: October 2002

Approximately 10,000 UV irradiance spectra resulting from 2 years of continuous measurements in Germany were used as a database to analyze the effect of cloudiness on spectral UV irradiance. Values of spectral cloud modification factors (CMF) were derived by modeling a corresponding clear‐sky irradiance spectrum for every UV measurement under cloudy conditions. The total set of CMF values was used to train neural networks using different sets of input data (parameter records) to describe the clouds, resulting in different, optimized, algorithms (CMF parameterizations). These different CMF parameterizations were evaluated by asking how the quality of the derived CMFs depended on the information content of different parameter records. It was shown that a visual description of cloudiness is not adequate to determine CMFs for an actual case (deviations of 50% and more), even if it was known whether or not the solar disk was obscured by clouds. Improvements for the determination of actual CMFs are possible, with deviations mostly below 15% if the parameter record comprises an actual broadband irradiance measurement. It was shown that such a CMF parameterization is able to provide a good estimation of actual CMFs, also for places with a different cloud climatology. The sensitivity of CMFs to wavelength and solar zenith angle was investigated on the basis of the derived CMF parameterizations. The relations found depend on the kind of CMF parameterization, i.e., the parameter record. In particular the separation of those cases when the solar disk is visible from those cases when the solar disk is obscured may lead to different dependencies of CMFs on solar zenith angle and wavelength.
On the possible origin of humic matter in fine continental aerosol
Authors: Gelencsér A. , Hoffer A. , Krivácsy Z. , Kiss G. , Molnár A. , Mészáros E.
Published: October 2002

In this paper we present a novel hypothesis for a possible formation mechanism of humic‐like substances, which were shown to be ubiquitous in fine continental aerosol. It tentatively relates the formation of atmospheric humic matter to the vast pool of soil organic matter via the evaporation, condensation, and aerosol‐phase polymerization of low molecular weight polar degradation products of organic debris in the soil. The theory is based on a comparative study on the chemical structure of bulk organic matter in fine aerosol and natural humic and fulvic acids and involves simplified mass flux calculations with estimated air‐water equilibrium data. Although the total soil flux of all degradation products is estimated to be of the order of ng m−2s−1, which is very low and difficult to measure, it is sufficient to sustain a mass concentration comparable to that of fine organic aerosol. The results imply that the proposed formation mechanism may be a significant natural source for fine organic aerosol at continental sites. We also suppose that precursors from other anthropogenic and natural sources contribute to producing secondary humic‐like polymeric matter in continental fine aerosol.
Analysis of the performance characteristics of the five‐channel Microtops II Sun photometer for measuring aerosol optical thickness and precipitable water vapor
Authors: Ichoku Charles , Levy Robert , Kaufman Yoram J. , Remer Lorraine A. , Li Rong‐Rong , Martins Vanderlei J. , Holben Brent N. , Abuhassan Nader , Slutsker Ilya , Eck Thomas F. , Pietras Christophe
Published: October 2002

Five Microtops II Sun photometers were studied in detail at the NASA Goddard Space Flight Center (GSFC) to determine their performance in measuring aerosol optical thickness (AOT or τaλ) and precipitable column water vapor (W). Each derives τaλ from measured signals at four wavelengths λ (340, 440, 675, and 870 nm), and W from the 936 nm signal measurements. Accuracy of τaλ and W determination depends on the reliability of the relevant channel calibration coefficient (V0). Relative calibration by transfer of parameters from a more accurate Sun photometer (such as the Mauna‐Loa‐calibrated AERONET master Sun photometer at GSFC) is more reliable than Langley calibration performed at GSFC. It was found that the factory‐determined value of the instrument constant for the 936 nm filter (k = 0.7847) used in the Microtops' internal algorithm is unrealistic, causing large errors in V0(936), τa936, and W. Thus, when applied for transfer calibration at GSFC, whereas the random variation of V0 at 340 to 870 nm is quite small, with coefficients of variation (CV) in the range of 0 to 2.4%, at 936 nm the CV goes up to 19%. Also, the systematic temporal variation of V0 at 340 to 870 nm is very slow, while at 936 nm it is large and exhibits a very high dependence on W. The algorithm also computes τa936 as 0.91 τa870, which is highly simplistic. Therefore, it is recommended to determine τa936 by logarithmic extrapolation from τa675 and τa870. From the operational standpoint of the Microtops, apart from errors that may result from unperceived cloud contamination, the main sources of error include inaccurate pointing to the Sun, neglecting to clean the front quartz window, and neglecting to calibrate correctly. If these three issues are adequately taken care of, the Microtops can be quite accurate and stable, with root‐mean‐square (rms) differences between corresponding retrievals from clean calibrated Microtops and the AERONET Sun photometer being about ±0.02 at 340 nm, decreasing down to about ±0.01 at 870 nm.
Validation of NO2 and HNO3 measurements from the Improved Limb Atmospheric Spectrometer (ILAS) with the version 5.20 retrieval algorithm
Authors: Irie H. , Kondo Y. , Koike M. , Danilin M. Y. , Camy‐Peyret C. , Payan S. , Pommereau J. P. , Goutail F. , Oelhaf H. , Wetzel G. , Toon G. C. , Sen B. , Bevilacqua R. M. , Russell J. M. , Renard J. B. , Kanzawa H. , Nakajima H. , Yokota T. , Sugita T. , Sasano Y.
Published: October 2002

The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) measured nitrogen dioxide (NO2) and nitric acid (HNO3) profiles from November 1996 to June 1997 at high latitudes in both hemispheres. The ILAS NO2 profiles (version 5.20) are compared with those obtained by balloon‐borne and satellite measurements to validate ILAS NO2 data. Comparisons with balloon‐borne measurements indicate that ILAS NO2 at 25–30 km has a positive bias of 0.3–0.4 ppbv (6–11%). The random difference in NO2 at 25–30 km is 0.2–0.3 ppbv (3–9%). The random error in the ILAS NO2 measurements is larger than 100% below 20 km and above 45 km, where the NO2 mixing ratios were less than 1.0 ppbv. It is possible that ILAS NO2 values were lowered by optically thick aerosols with aerosol extinction coefficients at 780 nm of greater than 0.001 km−1. The lack of diurnal correction along the line of sight contributes to the positive bias in the ILAS NO2 values below 25 km. Agreement of the ILAS NO2 values with those by the Polar Ozone and Aerosol Measurement (POAM) II instrument is within 10–30% at 25–35 km. The agreement with the Halogen Occultation Experiment (HALOE) is as good as ±10% at 25–40 km. ILAS HNO3 (version 5.20) agrees with balloon‐borne HNO3 to within 0.1 ppbv (0–1%), and the random difference is within 10% at 25–30 km.
Spatial expression of ENSO, drought, and summer monsoon in seasonal δ13C of ponderosa pine tree rings in southern Arizona and New Mexico
Authors: Leavitt Steven W. , Wright William E. , Long Austin
Published: October 2002

Major climatic impacts from ENSO and the North American Monsoon in the U.S. Southwest provide fortuitous circumstances for exploring the suitability of stable‐carbon isotope (δ13C) composition of tree rings to represent and reconstruct interannual climate variability. We sampled ponderosa pine tree rings at eight sites in seven mountain ranges over a 500‐km transect in southern Arizona and New Mexico. We developed δ13C chronologies for 1985–1995 by subdividing rings into pre‐ and post‐monsoon‐onset segments based on the presence of a false‐latewood band in each ring induced by the arid period immediately preceding the summer monsoon, pooling these segments from multiple trees at each site, and analyzing the cellulose component. Chronologies from two subsets of trees from the same site and separate chronologies from two different sites in the same mountain range indicate strong coherence and signal reproducibility. Additionally, the seasonal δ13C patterns showed remarkable coherence across the transect, with the spatially disjunct easternmost chronology tending to be most frequently dissimilar. The δ13C values of ring subdivisions before the false‐latewood band are generally positively correlated with winter Southern Oscillation Indices (SOI), resulting from the ENSO teleconnection with southwestern winter/early spring precipitation. The δ13C from these sites correlates with monthly Palmer Drought Severity Indices (PDSI), with the strongest correlations occurring in progressively later months from first‐formed to last‐formed subdivisions. The post‐false latewood subdivision δ13C was particularly strongly correlated with summer precipitation amount. These results confirm the strong influence of moisture on tree ring δ13C and support their use in both temporal and spatial modes to infer past climate variability.
Stratospheric transport in a three‐dimensional isentropic coordinate model
Authors: Mahowald Natalie M. , Plumb R. Alan , Rasch Phillip J. , del Corral John , Sassi Fabrizio , Heres Will
Published: October 2002

A new version of the Model of Atmospheric Transport and Chemistry (MATCH) is developed, which uses a coordinate that is terrain‐following near the surface and becomes isentropic in the upper troposphere. Starting from a simulation of age of air and water vapor in a standard version of National Center for Atmospheric Research's Middle Atmospheric Community Climate Model (MACCM), we use the meteorological fields to drive two versions of the off‐line MATCH model: the standard hybrid‐pressure coordinate (MATCH) and the new hybrid‐isentropic coordinate (IMATCH). An analysis of the age of air estimates from MACCM, MATCH, and IMATCH and observations shows that the hybrid‐isentropic model, IMATCH, is better able to capture the observed ages than either the original online MACCM or the hybrid‐pressure MATCH model. Comparisons of water vapor distributions in the tropical lower stratosphere in the three different model versions and observations suggest that IMATCH produces a better propagation of the seasonal cycle than the other models. Analysis of the model results and vertical velocities suggests that the improvements in the age of air and the water vapor simulations are largely due to the reduced numerical vertical diffusion in IMATCH in the lower tropical stratosphere region. In this region, diabatic vertical motions are much smaller than reversible adiabatic motions, making simulations less susceptible to numerical errors if the transport is calculated on isentropic surfaces. Because the lower tropical stratosphere acts as the source region for transport into the stratosphere, correct simulation of the transport in the lower tropical stratosphere is crucial for improving model simulations of the stratosphere.