bluuebunny/crossref_metadata_2025 huggingface dataset:
Crust‐mantle decoupling by flexure of continental lithosphere
Authors: Brown C. David , Phillips Roger J.
Published: September 2002

Mechanical decoupling between the crust and upper mantle has been proposed as an explanation for anomalously low effective elastic thicknesses (<20 km) locally associated with thermally mature (∼1 Ga) continental lithosphere. The processes and consequences of crust‐mantle decoupling are investigated with a fully dynamic elastic‐viscous‐plastic (EVP) finite element model of orogenic loading and foreland basin subsidence. The basic dependencies of continental flexure on lithospheric thermal state, crustal thickness, and load magnitude are determined and are characterized by the best fit elastic thickness for the simulated deflections. Additional variables such as loading rate, viscoelastic stress relaxation, the boundary condition at the underthrust edge of the plate, and the lithospheric rheology also influence the flexural signature, and these sensitivities are likewise defined by EVP simulations. Decoupling can result in substantial reductions in the effective elastic thickness‐up to a factor of 2 for lithosphere with a thermal age of 1 Ga. Elastic‐viscous‐plastic models show that lower crustal weakening occurs by locally enhanced creep driven by high shear stresses in the middle lithosphere at the load margin. Crust‐mantle decoupling in these models is fundamentally controlled by the temperature and the rheological contrast at the Moho. Previous studies of flexural decoupling have employed simplified multilayered elastic or elastic‐plastic methods, the latter using a yield strength envelope with aprescribed strain rate and an assumption of complete slip at the Moho. However, lithospheric rheology is inherently stress and time dependent, with the deformation rate varying both spatially and temporally. Comparison of elastic‐plastic solutions to the EVP simulations indicates that the former method is satisfactory for oceanic and coupled continental lithosphere but performs poorly with decoupled lithosphere. The EVP model is marginally successful at achieving effective elastic thicknesses <20 km for ∼l‐Ga lithosphere; additional reductions in the thickness may be achieved by a more thorough treatment of the thermal evolution of underthrust continental lithosphere.
Formation‐scale hydraulic and mechanical properties of oceanic crust inferred from pore pressure response to periodic seafloor loading
Authors: Davis E. E. , Wang K. , Becker K. , Thomson R. E.
Published: September 2002

Observations of fluid pressure variations in young igneous oceanic crust have been made in five sealed and instrumented Ocean Drilling Program boreholes on the flanks of the Mid‐Atlantic and Juan de Fuca Ridges. The holes penetrate locally well sedimented, and hence hydrologically well‐sealed crust, and are situated 1 to 85 km from areas where water can flow freely through the seafloor at extensive basement exposures. Amplitudes and phases of formation pressure variations have been determined relative to tidal and noritidal pressure variations measured simultaneously at the seafloor for periods ranging from 4.8 hours to 14 days. Formation pressure variations are reduced to amplitudes between 98% and 28% relative to those at the seafloor and shifted in phase by up to 20°. Simple theory for porous media response to periodic loading predicts that the scale of diffusive signal propagation from locations of basement outcrop through buried parts of the igneous crust should be proportional to basement permeability and the inverse square root of the period of the variation. This behavior is consistent with the observations, and the characteristic half wavelength of the diffusive signal defined by the data from the sites near basement exposures is 14 km at diurnal periods. If signals propagate in a simple one‐dimensional manner, this requires a formation‐scale permeability of 1.7×10−10 m2. No constraints are provided on the thickness of material characterized by this permeability, but the high‐permeability pathway must be laterally continuous. At two sites near basement exposures the bulk modulus of the rock matrix estimated from the elastic component of the pore pressure response is 3 GPa. Where the igneous crust is regionally well sealed by sediment, the elastic response yields a bulk modulus of 16 GPa. The increase in bulk modulus with age and distance from basement outcrop is consistent with an observed increase in crustal alteration, an increase in seismic velocity, and a decrease in permeability. Observed lateral gradients of pressure, coupled with the estimated permeability, suggest that the amplitude of semidiurnal tidal volumetric flux (Darcy velocity) exceeds 10−6 m s−1; semidiurnal fluid particle excursions would reach 0.25 m if the full volume of water contained in 10% porosity of the rock matrix were involved. If flow is channelized along discrete pathways, tidally modulated fluid flow velocities and particle excursions would be locally greater. The amplitude of tidal velocity is similar to that estimated for buoyancy‐driven hydrothermal convection, but the direction is generally different. Thus tidal flow may enhance water‐rock interactions significantly. Energy dissipated in this manner would approach 0.3 μW m−3.
Under‐ice noise resulting from thermally induced fracturing of the arctic ice pack: Theory and a test case application
Authors: Stein Peter J. , Lewis James K. , Parinella James C. , Euerle Steven E.
Published: September 2002

A theory is presented that relates thermally induced fracturing of pack ice to under‐ice noise level variations. It begins with the governing equations for the thermomechanics of pack ice. The thermomechanics relates thermally induced strain rates to the stresses within various vertical layers of the floe. In addition, paradigms are developed which specify the relative quantity of fracturing and stress relief in the floe as the tensile yield strength of the ice is exceeded. The thermomechanics is complemented by an acoustic propagation model that relates the number of fracture events at a given time to the acoustic levels at arbitrary frequency and depth below the ice. The acoustic theory assumes that each fracture acts as a simple monopole source, the fractures are evenly distributed horizontally, and the energy of each fracture propagates through the ice and the water column on the basis of the governing equations for elastic waves in a horizontally stratified medium. The results indicate that noise episodes resulting from fracturing occurring over the top 40 cm of a 160 cm thick floe will propagate over distances of up to 100 km. However, noise episodes associated with fracturing occurring in the lower 100 cm of the floe will only propagate over a range of ∼10 km. The thermomechanics and acoustic propagation theories were used to develop a numerical model for predicting under‐ice noise levels for a given thermal forcing of floes within the arctic ice pack. The model was used to simulate stresses in a multiyear floe and under‐ice noise levels at 500 Hz at 305 m below the floe. Model‐predicted ice stresses and under‐ice noise levels compare quite well to observed stresses and noise variations during the fall of 1988 in the eastern Arctic Ocean. The model predicts that most of the thermally induced, under‐ice noise at 500 Hz was a result of fracturing occurring between 5 and 30 cm below the ice surface for a 1.6 m thick multiyear floe.
A diagnostic study of the Indonesian Throughflow
Authors: Lebedev Konstantin V. , Yaremchuk Maxim I.
Published: September 2002

The objective of this study is to derive estimates of the mean seasonal variations in the Pacific to Indian Ocean throughflow transport using climatological data of the temperature/salinity fields and surface wind stress. A variable grid global general circulation model (GCM) with 1/6° resolution in Indonesian seas configured with ETOP05 bottom topography is utilized to diagnose the three‐dimensional velocity field. An extensive set of sensitivity experiments is conducted to obtain estimates of the error bars. The computations show that Indonesian Throughflow (ITF) is maximum in boreal summer (13.9±1.3 Sv) and minimum in boreal winter (7.5±1.1 Sv). The annual mean ITF transport amounts to 11.5±1.1 Sv (1 Sv = 106 m3 s−1). These values are within the range of various estimations of ITF transport, and they agree in phase. The largest seasonal variations of the transport occur in the upper 175 m of the water column, exhibiting a 8 Sv amplitude of the seasonal cycle. This causes a substantial variability in the heat transport from the Pacific to the Indian Ocean. Its value increases from 0.4±0.1 PW in January to 1.1±0.1 PW in August (1 PW = 1015 watts). In contrast to previous GCM‐based studies, the annual mean transport is found to be more evenly distributed among the three major outlets from the Indonesian seas (Lombok, Ombai, and Timor Straits). The Luzon Strait inflow gives a considerable contribution to ITF. In winter the inflow of 6.3±1.5 is distributed between the two outflows from South China Sea, which follow the routes through Karimata (4.4±0.5) and Mindoro (1.9±1.5 Sv) Straits. In summer the Karimata pathway is blocked, and the net inflow of 4.7±0.6 Sv exits through the Mindoro Strait west of southern Luzon. The total contribution of the Luzon Strait inflow to the net ITF transport varies from 85% in boreal winter to 35% in summer, with an annual average value of ∼50%. Cross‐correlation analysis of the TOPEX/Poseidon monthly mean sea surface height (SSH) anomalies and the diagnosed transport anomalies demonstrates high correlation of the mean SSH difference between the regions in western Banda Sea and north of Luzon with the heat (0.96) and volume (0.91) transports.
Near‐surface phytoplankton distribution in the western Intra‐Americas Sea: The influence of El Niño and weather events
Authors: González Nelson Melo , Müller‐Karger Frank E. , Estrada Sergio Cerdeira , Pérez de los Reyes Roberto , del Río Iván Victoria , Pérez Pedro Cárdenas , Arenal Ida Mitrani
Published: September 2002

The space‐time variation of phytoplankton pigments in the western Intra‐Americas Sea (IAS), in the vicinity of the island of Cuba, is examined using digital images obtained with the Coastal Zone Color Scanner sensor flown aboard the Nimbus 7 satellite from 1978 to 1986. The results are compared to historical in situ hydrographic observations. A marked seasonality in pigment concentration was observed in waters around Cuba, with an average of 0.07 mg m−3 in summer (April‐September) and 0.13 mg m−3 during winter (October–March). The range of variation in pigment concentration was larger in the Gulf of Mexico relative to the western Caribbean Sea. We identified four biogeographical areas on the basis of groups of pixels with similar patterns of time variability. These are area I: southwest of Cuba, Yucatan Channel, and Florida Strait; area II: central Gulf of Mexico; area III: east of Cuba; and area IV: central Caribbean Sea, south of Jamaica and Hispaniola. Two major meteorological events led to anomalies in the seasonal cycle of pigment concentrations. During El Niño‐Southern Oscillation (ENSO) of 1982–1983, positive anomalies were observed in the pigment concentration in the western IAS during winter months. This was associated with intense mixing of the water column by higher‐frequency and stronger winds associated with cold fronts. ENSO 1982–1983 therefore had a fertilizing effect on the IAS region. Another positive anomaly was observed in 1980–1981, a non‐ENSO period that featured higher hurricane and extratropical low‐pressure activity.
Spatial and temporal distribution of the wind‐induced internal wave energy available for deep water mixing in the North Pacific
Authors: Nagasawa Maki , Niwa Yoshihiro , Hibiya Toshiyuki
Published: September 2002

Using a three‐dimensional multilevel numerical model, we examine the distribution of the wind‐induced near‐inertial internal wave energy in the North Pacific. Energetic low vertical mode near‐inertial internal waves are excited at 30°–45°N in the western and central North Pacific by traveling midlatitude storms during winter and at 10°–30°N in the western North Pacific by tropical cyclones during fall. Thus excited internal waves propagate equatorward down to 5°–15°N, where their frequencies are twice the local inertial frequencies. Parametric subharmonic instability can then transfer their energy across the local internal wave vertical wavenumber spectrum to small dissipation scales. The calculated results show that low vertical mode double‐inertial frequency internal waves are very weak at the times and locations of previous microstructure measurements, which suggests that the observed value of diapycnal diffusivity of ∼10−5 m2 s−1, an order of magnitude lower than required to satisfy the large‐scale advective‐diffusive balance of the thermohaline circulation, may not be representative.
Sensitivity of the Arabian Sea mixed layer to 1994–1995 operational wind products
Authors: Rochford Peter A. , Kindle John C. , Gallacher Patrick C. , Weller Robert A.
Published: September 2002

The evolution of the upper ocean in the strong seasonally forced Arabian Sea, as observed by a mooring deployed in 1994–1995, is investigated using the Naval Research Laboratory Layered Ocean Model (NLOM). Model simulations were sensitive to the choice of surface wind products used for forcing, and results are reported for simulations forced by monthly mean climatologies and 12 hourly 1994–1995 wind products from two operational atmospheric forecast models, the European Centre for Medium‐Range Weather Forecast model and the Navy Operational Global Atmospheric Prediction System model of Fleet Numerical Meteorology and Oceanography Center (FNMOC). The NLOM yields the best prediction of sea surface temperature (SST) and mixed layer depth when using FNMOC forcing. Surface cooling is found to be responsible for the seasonal SST minimum during the NE monsoon. Heat advection is found to be important for supporting the surface cooling during the second half of the NE monsoon. Strong entrainment and appreciable advective cooling are responsible for the SST minimum of the SW monsoon. The NLOM wind experiments strongly suggest that thermal convection may be important in the central Arabian Sea during the winter months.
Bathymetry, stratification, and internal seiche structure
Authors: Fricker P. D. , Nepf H. M.
Published: September 2002

Internal seiches play a significant role in a broad range of physical, chemical, and biological processes in lakes. A detailed assessment of the impact of seiching requires an understanding of seiche structure, which is determined by bathymetry and stratification. In this study, internal seiche solutions are evaluated for arbitrary bathymetry and continuous stratification using a two‐dimensional numerical model. Formulated in terms of a stream function, the model produces a finite set of linear internal wave eigenmodes and allows the computation of the complete velocity field (over a grid) associated with each seiche mode. Several idealized configurations of continuous stratification and variable bathymetry are used to explore the effect of nonuniform systems on internal wave structure. In particular, we focus on bed velocity distribution and the resulting potential impact on scalar fluxes, sediment transport, and internal wave damping. Model results are also compared to thermistor chain data collected in the Upper Mystic Lake (UML, Winchester, Massachusetts). Using an idealized description of the UML bathymetry and density profiles which emulate the seasonal variation of stratification in the lake, the evolution of bed velocities during the autumnal breakdown in stratification is assessed, providing insight into the fate of the contaminants entering the lake.
Oxidized nitrogen and ozone production efficiencies in the springtime free troposphere over the Alps
Authors: Carpenter L. J. , Green T. J. , Mills G. P. , Bauguitte S. , Penkett S. A. , Zanis P. , Schuepbach E. , Schmidbauer N. , Monks P. S. , Zellweger C.
Published: September 2002

The Free Tropospheric Experiment (FREETEX’98) was conducted at the Jungfraujoch Observatory in the Swiss Alps (3580 in above sea level) during the well‐documented spring maximum in ozone. In spring the Jungfraujoch frequently lies in the free troposphere but can also be influenced by air from the planetary boundary layer. Measurements of NOx, NOy, peroxy‐acetylnitrate (PAN), HCHO, O3, CO, nonmethane hydrocarbons, peroxy radicals, j(O1D), j(NO2), and a variety of other tropospheric constituents crucial to ozone photochemical cycles were made over a 1‐month period. Two independent measurements of NOx, NOy, and PAN showed good agreement. Average free tropospheric daytime NO levels were about 50 pptv, sufficient to sustain photochemical ozone formation. Although high mixing ratios were encountered, PAN decomposition did not contribute to NOx production during FREETEX’98. Ozone production efficiencies (E N) derived from observed ΔO3/(NOz) ratios in free tropospheric air were 20–30 molecules of O3 produced per NOx molecule oxidized and agreed well with a photochemical model. A much lower ozone production efficiency of 4 was determined in a photochemically aged air mass arriving from southern Europe, in line with other measurements and calculations in regimes containing high levels of oxidized nitrogen. Model simulations indicated that by sequestering NOx and HO2, low‐temperature formation of peroxynitric acid (PNA) decreased ozone production by 20% and instantaneous ozone production efficiencies by 40%, whereas PAN formation had little effect. The model reproduced well the observed sharp transformation from ozone production to ozone destruction (defined as ΔO3/Delta;(NOz) = 0) at 20–25 pptv NO. The observed and calculated strong dependence of EN on NOx concentration in the low‐NOx regime highlights the difficulty in assigning a single O3 production efficiency value to remote regions, where most of the CO and CH4 in the atmosphere are oxidized.
Product studies of the OH‐ and ozone‐initiated oxidation of some monoterpenes
Authors: Orlando John J. , Nozière Barbara , Tyndall Geoffrey S. , Orzechowska Grazyna E. , Paulson Suzanne E. , Rudich Yinon
Published: September 2002

The OH‐ and O3‐initiated oxidation of five monoterpenes (myrcene, terpinolene, Δ3‐carene, α‐pinene, and β‐pinene) has been studied in environmental chambers equipped with either a Fourier transform infrared spectrometer or a gas chromatography/flame ionization detector system. The OH‐oxidation of myrcene and terpinolene is shown to lead to substantial yields of acetone (36 and 39%, respectively), while the acetone yield from the pinene compounds is quite small (4% and ∼2%, for α‐ and β‐pinene, respectively). Formaldehyde has been identified as a major product (yields of 20–40%) in the OH‐initiated oxidation of all five species. Formic acid was also observed in the OH‐initiated oxidation of all five monoterpenes, with yields of 2% from β‐pinene and 5–9% from the other species studied. The production of acetone from the reaction of monoterpenes with ozone in the presence of an OH scavenger was measured. The yields of acetone for the O3 reactions were α‐pinene, 0.03±0.01; β‐pinene, 0.009±0.009; Δ3‐carene, 0.10±0.015; myrcene, 0.25±0.06; and terpinolene, 0.50±0.06. The mechanism leading to the production of these compounds is discussed, as is the atmospheric relevance of the results. In particular, an estimate of the contribution of monoterpene oxidation to observed atmospheric levels of acetone and formic acid is made.
Continuous high‐frequency observations of hydrogen at the Mace Head baseline atmospheric monitoring station over the 1994–1998 period
Authors: Simmonds P. G. , Derwent R. G. , O'Doherty S. , Ryall D. B. , Steele L. P. , Langenfelds R. L. , Salameh P. , Wang H. J. , Dimmer C. H. , Hudson L. E.
Published: September 2002

Continuous high‐frequency (every 40‐min) automatic measurements of hydrogen have been made at the Mace Head atmospheric research station on the Atlantic Ocean coast of Ireland throughout 1994–1998. These observations represent one the most comprehensive in situ records of a trace gas that has received comparatively little attention. Individual measurements have been sorted by four independent methods to separate clean, maritime air masses from regionally polluted European air masses. Hydrogen concentrations in midlatitude Northern Hemisphere baseline air show a distinct seasonal cycle with highest concentrations during spring and lowest concentrations during late autumn, with a peak‐to‐trough amplitude of 38±6 ppb, averaged over the observed seasonal cycles from 1994 to 1998. The mean hydrogen concentration in midlatitude Northern Hemisphere baseline air on January 1, 1995, was estimated as 496.5 ppb with an upward trend of 1.2±0.8 ppb yr−1. Evidence has also been obtained for European pollution sources with source strength of about 0.8 Tg yr−1 and for deposition of hydrogen to soils. The observation of slightly elevated hydrogen concentrations relative to baseline levels in tropical maritime air masses points to a latitudinal gradient in hydrogen with higher concentrations in lower latitudes of the Northern Hemisphere and in the Southern Hemisphere. This is confirmed by comparable hydrogen observations at Cape Grim, Tasmania, which are consistently higher than measurements recorded at Mace Head. Mean hemispheric concentrations of 504 and 520 ppb have been estimated for the Northern and Southern Hemispheres, respectively, for January 1, 1996, corresponding to a total atmospheric hydrogen burden of 182 Tg.
Case studies of the impact of landscape changes on weather modification in western Australia in summer
Authors: Li Dayin , Komiyama Hiroshi , Kurihara Kazuo , Sato Yasuo
Published: September 2002

The impact of landscape changes on weather modification in western Australia in summer was studied. A three‐dimensional spectral model was used to simulate limited‐area weather modification by changing surface conditions, such as surface wetness, roughness, and albedo. Using global analysis data sets as the initial and boundary conditions, comparative simulations were made for original and modified surface conditions in a nearly 500×500 km region on January 14 and 22, 1990. For those two days, precipitation naturally occurred under convective atmospheric conditions. The simulation results show that the net precipitation (i.e., increased precipitation minus increased evaporation) increased significantly in the modified region when we changed surface conditions from bare soil to forests. Also, we did sensitivity studies of the effect of the evapotranspiration efficiency β, roughness z0, and albedo on precipitation, sensible and latent heat fluxes, and precipitation patterns during the day. We did sensitivity studies for 0.1 < β < 0.8, 0.01 < z0 < 2.00 m, and albedo from 0.05 to 0.25. Our simulations indicate that for z0 = 1.0 m and albedo of 0.15, increasing β from 0.1 to 0.8 increased precipitation from 9.1 to 14.6 mm on January 14 and 4.4 to 8.5 mm on January 22. For z0 = 1.0 m and β = 0.60, decreasing albedo from 0.25 to 0.05 increased precipitation from 12.8 to 16.1 mm on January 14 and 7.6 to 8.1 mm on January 22. Precipitation varied when z0 was increased from 0.01 to 0.5 m but changed insignificantly for further increases in z0 above 0.5 m.
Regional model simulations of African wave disturbances
Authors: Druyan Leonard M. , Fulakeza Matthew B. , Thiaw Wassila
Published: September 2002

A regional climate simulation model is run at 50 km horizontal resolution over West Africa to study synoptic systems, such as African wave disturbances (AWDs). The study examines the results of 18 simulations, the longest of them covering the period August 8–15, 1988. Simulations were initialized with European Center for Medium‐Range Weather Forecasts (ECMWF) gridded analysis data which also supply the lateral boundary conditions. Daily means of 700 mbar circulation and other model diagnostics are compared with ECMWF and National Centers for Environmental Prediction analyses to assess the skill of the model in simulating the evolution of synoptic systems. Trajectories of AWDs are tracked on time‐longitude cross sections of 700 mbar vorticity, lower tropospheric divergence and precipitation rates. Cloud clusters on satellite imagery relate rather well to synoptic features on observational analyses as well as to spatial patterns of rain gage measurements but not consistently to simulated precipitation. Movement of major AWDs and the generation of new waves are handled realistically by the model. Model dynamics and physics appear to be more important than lateral boundary conditions in determining the course of simulations.
Remote sensing of optical and microphysical properties of cirrus clouds using Moderate‐Resolution Imaging Spectroradiometer channels: Methodology and sensitivity to physical assumptions
Authors: Rolland P. , Liou K. N. , King M. D. , Tsay S. C. , McFarquhar G. M.
Published: September 2002

A methodology for the retrieval of cirrus cloud microphysical and optical properties based on observations of reflected sunlight is introduced. The retrieval method is based on correlation of the bidirectional reflectance of three channels, 0.65, 1.6, and 2.2 μm, that are available onboard Earth Observing System (EOS) Moderate‐Resolution Imaging Spectroradiometer (MODIS). Validation studies using microphysical measurements and MODIS airborne simulator (MAS) observations illustrate the nature of the potential errors associated with the retrieved optical depth and mean effective ice crystal size. The effects of the physical assumptions involving ice crystal size distribution and shape employed in the algorithm are subsequently assessed. In terms of the microphysical models used for radiation calculations the ice crystal shape assumption is found to have the most significant impact on the retrieved parameters. The effect of the background surface reflectance on the retrieval results is further examined, and we show that in order to reliably infer nonblack cirrus parameters from solar reflectance measurements it is essential to properly account for the background radiation over both land and ocean surfaces. Finally, we present the measured ice microphysical data for tropical cirrus as a function of cloud development and ambient temperature to illustrate the importance of vertical inhomogeneity for validation studies.
Stratospheric ozone climatology from lidar measurements at Table Mountain (34.4°N, 117.7°W) and Mauna Loa (19.5°N, 155.6°W)
Authors: Leblanc Thierry , McDermid I. Stuart
Published: September 2002

Using more than 1600 nighttime profiles obtained by the Jet Propulsion Laboratory differential absorption lidars located at Table Mountain Facility (TMF, 34.4°N) and Mauna Loa Observatory (MLO, 19.5°N) stratospheric ozone climatology is presented in this paper. These two systems have been providing high‐resolution vertical profiles of ozone number density between 15 and 50 km for several nights a week since 1988 (TMF) and 1993 (MLO). The climatology presented here is typical of early night ozone values at both sites and typical of a low solar activity period for MLO. The observed seasonal and vertical structure of the ozone concentration at TMF is consistent with that typical of middle to subtropical latitudes. A clear annual cycle in opposite phases below and above the ozone concentration peak is observed. The observed winter maximum below the ozone peak is associated with a maximum day‐to‐day variability, typical of a dynamically driven lower stratosphere. The maximum concentration observed in summer above the ozone peak emphasizes the more dominant role of photochemistry. Unlike TMF, the ozone concentration observed at MLO tends to be higher during the summer months and lower during the winter months throughout the entire stratospheric ozone layer. Only a weak signature of the extratropical latitudes is observed near 19–20 km, with a secondary maximum in late winter. The only large variability observed at MLO is in the lowermost stratosphere, which could be associated with the natural variability of the tropical tropopause.
Boundary layer evolution and regional‐scale diurnal circulations over the and Mexican plateau
Authors: Whiteman C. D. , Zhong S. , Bian X. , Fast J. D. , Doran J. C.
Published: September 2002

Data collected in a measurement campaign in February and March 1997 showed that the Mexico Basin (also called the Valley of Mexico), located atop the Mexican plateau, fails to develop the strong nocturnal inversions usually associated with basins and does not exhibit diurnally reversing valley wind systems. Data analyses, two‐and three‐dimensional numerical simulations with the Regional Atmospheric Modeling System (RAMS), and a Lagrangian particle dispersion model are used to interpret these observations and to examine the effects of topography and regional diurnal circulations on boundary layer evolution over the Mexico Basin and its surroundings during fair weather periods in the winter dry season. We show that the boundary layer evolution in and above the basin is driven primarily by regional diurnal circulations that develop between the air above the Mexican Plateau and the generally cooler surrounding coastal areas. A convective boundary layer (CBL) grows explosively over the plateau in the late morning to reach elevations of 2250 m agl (4500 m msl) by noon, and a strong baroclinic zone forms on the edges of the plateau separating the warm CBL air from its cooler surroundings. In early afternoon the rates of heating and CBL growth are slowed as cool air leaks onto the plateau and into the basin through passes and over low‐lying plateau edges. The flow onto the plateau is retarded, however, by the strongly rising branch of a plain‐plateau circulation at the plateau edges, especially where mountains or steep slopes are present. An unusually rapid and deep cooling of the air above the plateau begins in late afternoon and early evening when the surface energy budget reverses, the CBL decays, and air accelerates onto the plateau through the baroclinic zone. Flow convergence near the basin floor and the associated rising motions over the basin and plateau produce cooling in 3 hours that is equivalent to half the daytime heating. While the air that converges onto the plateau comes from elevations at and above the plateau, it is air that was modified earlier in the day by a cool, moist coastal inflow carried up the plateau slopes by the plain‐plateau circulation.
Particle size distributions of Mount Etna's aerosol plume constrained by Sun photometry
Authors: Watson I. M. , Oppenheimer C.
Published: September 2002

Near simultaneous instrument calibration and volcanic aerosol measurement were performed near the summit of Mount Etna, Sicily, in October 1997 using an eight channel Sun‐tracking photometer. Three objectives were achieved using this methodology: calibration of the instrument, measurement of the background atmosphere, and measurement of the volcanic aerosol plume. Out‐of‐plume calibration measurements were used to extrapolate extraterrestrial solar voltages for the five nonpolarized visible and near‐infrared channels using a Langley calibration routine. The Langley coefficients were then used to calculate the optical depth of the background atmosphere and subsequently the volcanic plume using the Beer‐Bouger‐Lambert law. Coefficients were calculated from the Ångström equation with means of α = 1.67 (0.13 < α < 2.42) and β = 0.06 (0.001 < β < 0.14). A constrained linear inversion of the spectral optical depth of the plume was performed using Mie code to produce particle number, surface area and volume spectra, and the effective scattering radius. The volume spectra indicate a trimodal distribution consisting of particles of radius < 0.1 μm (nucleation mode), a possible second fine mode (< 1.0 μm), and large particles of radius >1.0 μm (acid droplets) with minima at 0.5 and 1.5 μm. The mean effective radius was determined to be at 0.83 μm within the range 0.35 < r < 1.6 μm, and the total aerosol mass flux was estimated as 4.5–8.0 kg s−1 with the smaller radius mode contributing 6–18% of the total mass flux.
Radiative heating rates and direct radiative forcing by mineral dust in cloudy atmospheric conditions
Authors: Quijano Ana Lía , Sokolik Irina N. , Toon O. Brian
Published: September 2002

We explore several issues relevant to assessments of solar and infrared radiative effects due to mineral aerosols. One issue is the importance of the vertical distribution of dust for calculations of dust radiative heating rates. Another issue is the role that clouds may play in augmenting the radiative forcing by dust. We also explore the importance of the composition of mineral aerosols by employing spectral optical properties for dust that comes from two different regions of the globe, the Saharan and Afghan deserts. A combined longwave and shortwave radiative transfer model was used to determine the instantaneous radiative forcing in the atmosphere, radiative fluxes at the surface, and radiative heating rates by airborne mineral aerosols for clear‐sky and cloudy atmospheric conditions. Extensive calculations with our model show that increasing dust loading results in increasing both solar heating rates and infrared cooling rates. However, the net instantaneous rates during the day are always positive, yielding net radiative heating of the dust layer. With similar atmospheric conditions and dust loading, Saharan dust causes larger heating rates than Afghan dust. The magnitudes of the Saharan dust heating rates can easily be 25% larger than Afghan dust heating rates at high Sun angles and over bright surfaces. Also, Saharan dust yields more positive values of TOA (top of the atmosphere) radiative forcing than Afghan dust; and for a diurnal average, this can lead to a change of sign of the TOA radiative forcing from negative to positive just due to mineralogical composition. Clouds significantly influence the direct radiative impact of dust depending on cloud altitude and optical depth. Moreover, this influence is strongly dependent on Sun position and surface albedo.
Method for measuring changes in the atmospheric O2/N2 ratio by a gas chromatograph equipped with a thermal conductivity detector
Authors: Tohjima Yasunori
Published: September 2002

We present a method for measuring changes in the atmospheric O2/N2 ratio based on data from a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). In this method, O2 and N2 in an air sample are separated on a column filled with molecular sieve 5A with H2 carrier gas. Since the separated O2 includes Ar, which has a retention time similar to that of O2, the (O2+Ar)/N2 ratio is actually measured. The change in the measured (O2+Ar)/N2 ratio can be easily converted to that in the O2/N2 ratio with a very small error based on the fact that the atmospheric Ar/N2 ratio is almost constant. The improvements to achieve the high‐precision measurement include stabilization of the pressure at the GC column head and at the outlets of the TCD and the sample loop. Additionally, the precision is improved statistically by repeating alternate analyses of sample and a reference gas. The standard deviation of the replicate cycles of reference and sample analyses is about 18 per meg (corresponding to 3.8 parts per million (ppm) O2 in air). This means that the standard error is about 7 per meg (1.5 ppm O2 in air) for seven cycles of alternate analyses, which takes about 70 min. The response of this method is likely to have a 2% nonlinearity. Ambient air samples are collected under pressure in glass flasks equipped with two stopcocks sealed by Viton O‐rings at both ends. Pressure depletion in the flask during the O2/N2 measurement does not cause any detectable change in the O2/N2 ratio, but the O2/N2 ratio in the flask was found to gradually decrease during the storage period. We also present preliminary results from air samples collected at Hateruma Island (latitude 24°03′N, longitude 123°49′) from July 1997 through March 1999. The observed O2/N2 ratios clearly show a seasonal variation, increasing in spring and summer and decreasing in autumn and winter.
Estimating subcanopy soil moisture with radar
Authors: Moghaddam Mahta , Saatchi Sasan , Cuenca Richard H.
Published: September 2002

The subcanopy soil moisture of a boreal old jack pine forest stand is estimated using polarimetric L and P band airborne synthetic aperture radar (AIRSAR) data. Model simulations have shown that for this stand the principal scattering mechanism responsible for radar backscatter is the double‐bounce mechanism between the tree trunks and the ground. The data to be used here were acquired during five flights from June to September 1994 as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS) project. The dielectric constants, or equivalently moisture contents, of the trunks and soil can change significantly during this period. To estimate these dynamic unknowns, parametric models of radar backscatter for the double‐bounce mechanism are developed using a series of simulations of a numerical forest scattering model. A nonlinear optimization procedure is used to estimate the dielectric constants. Ground measurements of soil and trunk moisture content are used to validate the results. The trunk moisture content measurements are used to gain confidence that the respective estimation results are accurate enough not to corrupt the soil moisture estimation, which is the main focus of this paper. After conversion of the trunk moisture measurements to dielectric constants it is found that the estimated values are within 14% of the measurements. Owing to possible calibration uncertainties in the soil moisture measurements on the ground as well as in AIRSAR data, the variations rather than the absolute levels of the estimated soil moisture are considered. The results indicate that the estimated variations closely track the measurements. The worst case average estimated change differs by <1% volumetric soil moisture from that measured on the ground.
No evidence for solar absorption anomaly by marine water clouds through collocated aircraft radiation measurements
Authors: Asano Shoji , Uchiyama Akihiro , Mano Yuzo , Murakami Masataka , Takayama Yozo
Published: September 2002

No observational evidence was found for the so‐called anomalous solar absorption by maritime water clouds through collocated aircraft measurements taken during the Japanese Cloud‐Climate Study (JACCS) program. The aircraft experiment has been carried out by using two aircraft equipped with various instruments for wintertime stratocumulus clouds over an area centered at (29°N, 129°E) in the East China Sea. Here we have carefully analyzed solar absorption by the water stratocumulus clouds observed on February 2, 1998. The visible‐band net fluxes measured above and below the cloud layer were almost the same within measurement accuracy; this means no substantial absorption in the visible spectral region. On the other hand, there were significant differences as much as 50–80 W m−2 between the near‐infrared‐band net fluxes measured above and below the cloud layer; this difference corresponds to absorptance of 6–10% of the total‐band solar irradiance above the cloud layer. Without cloud particles, water vapor absorption was estimated to be about 4% of the total‐band irradiance for the layer. Distributions along the flight legs of the measured visible‐band and near‐infrared‐band absorptance were in phase in their positions with zero mean visible‐band absorptance. The measured radiation budget averaged over long distances along the flight legs for the inhomogeneous cloud layers agreed well with theoretical counterparts calculated for plane‐parallel, homogeneous cloud models based on the observed microphysical parameters.
Reactive uptake of ozone by proxies for organic aerosols: Surface versus bulk processes
Authors: Moise Tamar , Rudich Yinon
Published: September 2002

Uptake measurements of ozone were conducted with two types of proxies for atmospheric organic aerosols: organic liquids and self‐assembled organic monolayers. Alkanes and terminal alkenes were used. The monolayer surface was characterized, prior to and after reaction, using IR spectroscopy. Uptake experiments were conducted using a flow tube reactor coupled to a chemical ionization mass spectrometer. The reactive uptake coefficient, γ, is shown to be due to reaction with the double bond. For the monolayers, γ is composed solely of a surface reactive component and is smaller by at least an order of magnitude than values obtained for a liquid of the same chain length. Uptake by the liquids is higher due to solubility and reaction in the bulk. The phase of the atmospheric organic aerosol will determine the appropriate use of a bulk or surface uptake probability in atmospheric models. Since the aerosol surface is processed and sites are consumed, γ is time variant. We define a parameter γ as the surface uptake probability per reactive site and determine its value as 9×10−19 cm2 molecule−1. This enables the modeling of surface reactions as surface site concentrations diminish following interaction with the gaseous species.
FORTE radio‐frequency observations of lightning strokes detected by the National Lightning Detection Network
Authors: Jacobson Abram R. , Cummins Kenneth L. , Carter Michael , Klingner Phillip , Roussel‐Dupré Diane , Knox Stephen O.
Published: September 2002

This work compares simultaneous observations of lightning from two complementary systems. FORTE is a low‐Earth‐orbit satellite carrying radiowave and optical instruments for the study of lightning. The radio receivers aboard FORTE observe very high frequency (VHF) emissions from the air‐breakdown process preceding (and sometimes accompanying) a lightning current. The National Lightning Detection Network (NLDN) is a ground‐based array of sensors in the contiguous United States observing the low‐frequency (LF) and very low frequency (VLF) radiation from vertical currents. Prior to the launch of FORTE in 1997, essentially no work had been done on the statistical correlations between (1) ground‐based LF/VLF and (2) spaced‐based VHF remote sensing of lightning. During a 6‐month campaign in April‐September 1998, FORTE took most of its triggered VHF data over and near the contiguous United States, and NLDN data were specially postprocessed in a loosened‐criterion mode providing enhanced detection range beyond the coastline and borders of the array itself. The time history of reported events from the two systems was compared, and event pairs (each pair containing one event from FORTE, the other from NLDN) which were candidate correlations (closer than 200 ms from each other) were scrutinized to determine whether the members of a pair actually came from the same discharge process. We have found that there is a statistically significant correlation, for a subset of FORTE events. This correlation is most likely to occur for intracloud and less likely to occur for cloud‐to‐ground discharges. The correlated VHF and NLDN events tend to occur within ±30 μs of each other, after correction for the propagation of the VHF signal to FORTE from the NLDN‐geolocated stroke location. Most correlations outside of ±30 μs turn out to be merely a statistical accident. The NLDN‐furnished geolocation allows the correlated FORTE‐detected VHF pulses to be better interpreted. In particular, we can deduce, from the lag of the VHF ground‐reflection echo, the height of the VHF emission region in the storm.
Wavelet analysis of covariance with application to atmospheric time series
Authors: Whitcher Brandon , Guttorp Peter , Percival Donald B.
Published: September 2002

Multiscale analysis of univariate time series has appeared in the literature at an ever increasing rate. Here we introduce the multiscale analysis of covariance between two time series using the discrete wavelet transform. The wavelet covariance and wavelet correlation are defined and applied to this problem as an alternative to traditional cross‐spectrum analysis. The wavelet covariance is shown to decompose the covariance between two stationary processes on a scale by scale basis. Asymptotic normality is established for estimators of the wavelet covariance and correlation. Both quantities are generalized into the wavelet cross covariance and cross correlation in order to investigate possible lead/lag relationships. A thorough analysis of interannual variability for the Madden‐Julian oscillation is performed using a 35+ year record of daily station pressure series. The time localization of the discrete wavelet transform allows the subseries, which are associated with specific physical time scales, to be partitioned into both seasonal periods (such as summer and winter) and also according to El Niño‐Southern Oscillation (ENSO) activity. Differences in variance and correlation between these periods may then be firmly established through statistical hypothesis testing. The daily station pressure series used here show clear evidence of increased variance and correlation in winter across Fourier periods of 16–128 days. During warm episodes of ENSO activity, a reduced variance is observed across Fourier periods of 8–512 days for the station pressure series from Truk Island and little or no correlation between station pressure series for the same periods.
Intensity of the second and third OH overtones of H2O2, HNO3, and HO2NO2
Authors: Zhang Hui , Roehl Coleen M. , Sander Stanley P. , Wennberg Paul O.
Published: September 2002

The 3νOH and 4νOH of H2O2, HNO3, and HO2NO2 have been observed. The band strengths of 3νOH are (7.0±1.8) × 10−20, (2.9±0.7) × 10−20, and (3.8±1.1) × 10−20 cm2 molecules−1 cm−1 for H2O2, HNO3, and HO2NO2, respectively. Those of 4νOH are (4.5± .6) × 10−21, (2.8± .0) × 10−21, and (3.0±1.8) × 10−21 cm2 molecules−1 cm−1 for H2O2, HNO3, and HO2NO2, respectively. The uncertainty is for one standard deviation. Assuming excitation of these modes by solar absorption is dissociative for HO2NO2, these measurements confirm that this process will play a small role in the atmospheric photochemistry of the lower stratosphere.
On the “tuning” of autoconversion parameterizations in climate models
Authors: Rotstayn Leon D.
Published: September 2002

Autoconversion is a highly nonlinear process, which is usually evaluated in global climate models (GCMs) from the mean in‐cloud value of the liquid‐water mixing ratio q′l. This biases the calculated autoconversion rate, and may explain why it usually seems to be necessary to reduce the autoconversion threshold to an unrealistically low value to obtain a realistic simulation in a GCM. Two versions of a threshold‐dependent autoconversion parameterization are compared in the CSIRO GCM. In the standard (“OLD”) treatment, autoconversion occurs in a grid box whenever the mean in‐cloud q′l exceeds the threshold qcrit, which is derived from a prescribed threshold cloud‐droplet radius rcrit. In the modified (“NEW”) version, the assumed subgrid moisture distribution from the model's condensation scheme is applied in each grid box to determine the fraction of the cloudy area in which q′l > qcrit, and autoconversion occurs in this fraction only. Simulations are performed using both treatments, for present‐day and preindustrial distributions of cloud‐droplet concentration, and using different values for rcrit. Changing from the OLD to the NEW treatment means that rcrit can be increased from 7.5 μm to a more realistic 9.3 μm, while maintaining the global‐mean liquid‐water path at about the same value. Simulations for preindustrial and present‐day conditions are compared, to see whether the change of scheme alters the modeled cloud‐lifetime effect. It is found that the NEW scheme with rcrit = 9.3 μm gives a 0.5 W m−2 (62%) stronger cloud‐lifetime effect than the OLD scheme with rcrit = 7.5 μm.