bluuebunny/crossref_metadata_2025 huggingface dataset:
Verification of aerosol models for satellite ocean color remote sensing
Authors: Schwindling Myriam , Deschamps Pierre‐Yves , Frouin Robert
Published: February 2004

Direct atmospheric transmittance and sky radiance were measured spectrally at the coastal site of Scripps Institution of Oceanography pier in La Jolla, California, during the winters of 1993 and 1994. Direct atmospheric transmittance was also measured aboard R/V Wecoma and on Catalina Island during the 1994 California Cooperative Oceanic Fisheries Investigation winter cruise. The data were analyzed to (1) verify whether the aerosol models selected by Gordon and Wang [1994] for the sea‐viewing wide field of view sensor are adequate for ocean color remote sensing from space and (2) identify what type of in situ atmospheric optics measurements should be performed to verify atmospheric correction algorithms. Aerosol optical thickness at 870 nm was generally low at La Jolla, with most values below 0.1 after correction for stratospheric aerosols. Values were even lower offshore (R/V Wecoma, Catalina Island), and no systematic correlation was found between aerosol characteristics and meteorological conditions. Therefore a mean background model, specified regionally, may be sufficient for ocean color remote sensing from space. For optical thicknesses above 0.1, two modes of variability characterized by Ångström coefficients of 1.2 and 0.5 and corresponding to tropospheric and maritime models, respectively, were identified in the measurements. The aerosol models selected by Gordon and Wang [1994] allow one to fit, within measurement inaccuracies, the derived values of Ångström coefficient and “pseudo” phase function (the product of single‐scattering albedo and phase function), key atmospheric correction parameters. Importantly, the pseudo phase function can be derived from measurements of the Ångström coefficient. This means that shipborne Sun photometer measurements made at the time of satellite overpass are usually sufficient to verify the atmospheric correction of ocean color.
Simulation of heat storages and associated heat budgets in the Pacific Ocean: 2. Interdecadal timescale
Authors: Auad Guillermo , Miller Arthur J. , White Warren B.
Published: February 2004

We use a primitive equation isopycnal model of the Pacific Ocean to simulate and diagnose the anomalous heat balance on interdecadal timescales associated with heat storage changes observed from 1970–1988 in the expendable bathythermograph (XBT) data set. Given the smallness of the interdecadal signals compared to the El Niño‐Southern Oscillation (ENSO) signal, the agreement between model and observations is remarkably good. The total anomalous heat balance is made up of two parts, the diabatic part (from the model temperature equation) and the adiabatic part (from the model mass conservation equation) due to thermocline heave. We therefore describe our analysis of both the total and diabatic anomalous heat balances in four areas of the tropical and subtropical North Pacific Ocean in the upper 400 m. The interdecadal total (diabatic plus adiabatic) heat balance in the North Pacific Ocean is characterized by a complicated interplay of different physical processes, especially revealed in basin‐scale averages of the heat budget components that have comparable amounts of variance. In smaller subregions, simpler balances hold. For example, in the western equatorial Pacific (area 1) the total heat content tendency term is nearly zero, so that a simple balance exists between surface heat flux, vertical heat transport, and horizontal mixing. In the western subtropical Pacific the total heat content tendency balances the three‐dimensional divergence of the heat flux. We speculate that this complexity is indicative of multiple physical mechanisms involved in the generation of North Pacific interdecadal variability. The diabatic heat balance north of 24°N, a region of special interest to The World Ocean Circulation Experiment (WOCE), can be simplified to a balance between the tendency term, surface heat flux, and meridional advection, the last term dominated by anomalous advection of mean temperature gradients. For the western equatorial region the diabatic heat content tendency is nearly zero and the steady balance involves simply horizontal advection and the surface heat flux, which at these latitudes has a damping role in the model. An important finding of this study is the identification of two interdecadal timescales, roughly 10 and 20 years, both similar to those reported by other investigators in recent years. [Tourre et al., 1998; Latif and Barnett, 1994; Robertson, 1995; White et al, 1997; Gu and Philander, 1997; Jacobs et al., 1994]. The 20‐year timescale is only present in diabatic heat budget components, while the 10‐year timescale is present in both diabatic and adiabatic components. The 10‐year timescale can also be seen in the surface heat flux time series, but it occurs in the ocean adiabatic components which demonstrates the importance of oceanic adjustment through Rossby wave dynamics on decadal timescales.
The influence of Loop Current perturbations on the formation and evolution of Tortugas eddies in the southern Straits of Florida
Authors: Fratantoni Paula S. , Lee Thomas N. , Podesta Guillermo P. , Muller‐Karger Frank
Published: February 2004

Large cyclonic eddies on the northern edge of the Florida Current are the dominant mesoscale features within the southern Straits of Florida. The most prominent of these features is a quasi‐stationary eddy that forms near the Dry Tortugas. Our observations, compiled from 3 years of advanced very high resolution radiometer measurements in the Straits of Florida and Gulf of Mexico, demonstrate a strong relationship between the generation of anticyclonic rings from the Gulf of Mexico Loop Current and the evolution of Tortugas eddies within the southern Straits of Florida. In six cases, Tortugas eddies evolve from cyclonic frontal eddies which form along the boundary of the Loop Current. The eddies remain stationary near the Dry Tortugas until they are impacted by an approaching Loop Current frontal eddy. The length of time an eddy spends near the Dry Tortugas is increased when the Loop Current sheds an anticyclonic ring. The involvement of a Loop Current frontal eddy in the ring‐shedding process results in a delay in its, and hence the Tortugas eddy's, downstream propagation. Results suggest that the lifetime of a Tortugas eddy can be as long as 140 days when a ring‐shedding event occurs, or as short as 50 days in the absence of any ring‐shedding events. Upon entering the Straits of Florida, the Tortugas eddies are deformed by the narrowing topography and shrink to approximately 55% of their original size as they propagated downstream. The shrinking of these eddies is accompanied by an accelerated translation from 5 km/d in the western Straits of Florida to 16 km/d in the east.
A three‐dimensional model of internal tides on the Malin‐Hebrides shelf and shelf edge
Authors: Xing Jiuxing , Davies Alan M.
Published: February 2004

In this paper, we describe a three‐dimensional baroclinic sea model and its application to the computation of the internal tide over a shelf and shelf edge region. The model covers the Malin‐Hebrides shelf and shelf edge with water depths ranging from a few meters near shore to over 2000 m in the deep sea. A fine horizontal grid resolution of 1/24° × 1/24° (about 2.4 km east‐west and 4.6 km north‐south grid spacing) and 50 vertical computational levels enables us to examine the internal tide generation at the shelf edge and its propagation. Numerical calculations illustrate the generation of the internal tide over the shelf edge and its propagation toward both the shelf and deep sea in a strongly stratified surface layer (the case of summer stratification). In a weakly stratified surface layer case (winter stratification), the internal tides generated at the shelf edge are much weaker and are dissipated away from the shelf edge region, in particular over the shallow shelf because of strong tidal mixing. A comparison of observed and computed tidal currents is made under a range of stratified conditions, and this shows the difficulty in rigorously validating three‐dimensional internal tidal models. The model also indicates that the Anton Dohrn and Hebrides Terrace seamounts have an important influence upon the internal tide propagation in the region.
Observation of perennial Arctic sea ice melt and freeze‐up using passive microwave data
Authors: Smith Douglas M.
Published: February 2004

The onset of spring melt and autumn freeze‐up of Arctic sea ice are shown, through comparison with coincident ERS‐1 synthetic aperture radar (SAR) and surface air temperature data, to produce significant and detectable changes in the passive microwave signature. Melt onset is marked by a sharp rise in brightness temperatures resulting from the increased emissivity of moisture in the snow cover. Freeze‐up is accompanied by a sharp drop in brightness temperatures because of volume scattering by bubbles in the upper ice layer. An algorithm is developed to detect melt and freeze‐up from passive microwave data and is applied to the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) data from 1979 to 1996. The mean accuracy of melt and freeze‐up dates determined from the SSM/I is estimated to be ±2 and ±3 days, respectively. The reduced temporal sampling of the SMMR degrades the accuracy by 1 day. On average, melt onset varies from late May in the southern Beaufort Sea to mid‐June in the central Arctic north of the Canadian archipelago, while freeze‐up varies between mid‐September and mid‐August. The shortest melt season appears to occur north of the Canadian archipelago rather than at the North Pole, which is consistent with the fact that minimum summer air temperatures are centered over Greenland. The mean standard deviations of the interannual variability of melt and freeze‐up are 6.8 and 11.4 days, respectively.
Observations of El Niño‐Southern Oscillation induced Rossby waves in the northeast Pacific using in situ data
Authors: Jacobson Andrew R. , Spiesberger John L.
Published: February 2004

Bathythermometric observations from standard National Oceanographic Data Center data sets are used to show evidence of Rossby waves in the northeast Pacific Ocean following several El Niño‐Southern Oscillation events between 1968 and 1991. Observations of a westward propagating warm anomaly following the 1982–1983 El Niño are consistent with evidence from ocean models and satellite observations that coastally trapped Kelvin waves of tropical origin excite Rossby waves in the northeast Pacific. Observed Rossby wave phase speeds are about 1.6 cm s−1 higher than those predicted by linear shallow‐water theory. At midlatitudes, observed waves are more than 1.5 times faster than theory. Phase speeds of Rossby waves in a

resolution nonlinear primitive equation model exceed standard linear theory, but only by about 0.1 cm s−1, despite the fact that the model should replicate mechanisms that have recently been proposed to explain the observed anomalously fast phase speeds.
The flow of Antarctic bottom water to the southwest Indian Ocean estimated using CFCs
Authors: Haine Thomas W. N. , Watson Andrew J. , Liddicoat Malcolm I. , Dickson Robert R.
Published: February 2004

Observations of chlorofluorocarbons (CFCs), carbon tetrachloride, temperature, and salinity from five sections following the outflow path of Antarctic Bottom Water (AABW) into the southwest Indian Ocean are reported. The transient tracer data clearly show the plume of recently ventilated water whose hydrographic properties are progressively altered by mixing with the overlying waters. We use the CFC measurements to estimate the mean speeds (or transit times) and mixing rates (or dilutions) of the abyssal flow at each section using simple kinematic circulation models. Given our assumptions, the CFC ventilation age equals the transit time. The results suggest a transit time of 23±5 years (outflow speed of 1.2±0.3 cm s−1) to the Crozet‐Kerguelen Gap with a dilution of 8–15 from the surface waters of the Weddell Sea. The estimated horizontal diffusivity is 30–70 m2 s−1, and the vertical diffusivity is 3–7×10−4 m2 s−1 Combined with the estimate of R. R. Dickson (unpublished data, 1998) for the AABW transport at this point, we conclude that a volume flux of 0.8–1.6 Sv (106 m3 s−1) is leaving the continental shelves of the Weddell Sea to eventually enter the abyssal Indian Ocean past Crozet Island.
Forced Rossby waves in the southern tropical Indian Ocean
Authors: Masumoto Yukio , Meyers Gary
Published: February 2004

Seasonal and interannual variation of the upper southern tropical Indian Ocean (STIO) is described by harmonic and empirical orthogonal function (EOF) analysis of the depth of the 20°C isotherm (D20) derived from expendable bathythermograph (XBT) data and from an ocean general circulation model (OGCM). The harmonic analysis shows a band of large annual amplitude between 8° and 20°S extending across the STIO with a steady westward propagation in both the XBT and model data. The generation of the annual wave is discussed in terms of Ekman pumping and the westward propagation of long, nondispersive, baroclinic Rossby waves. The Ekman pumping on a large scale over the open ocean strongly modifies waves radiating from the eastern boundary and generates much of the structure in the amplitude and the phase of the annual signal in the STIO. The EOF analysis shows strong interannual variation of the anomaly of D20 on two XBT lines covering the western and eastern sides of the basin. Large interannual anomalies are observed in a band between 6° and 14°S on the western side, with deep D20 in 1988, 1991, and 1994 and shallow D20 in 1990 and 1992. On the eastern side, large interannual anomalies are observed close to the coast of Java, and sign is opposite to that on the western side, suggesting an oscillation in the zonal tilt of the thermocline across the STIO. Those interannual variations of D20 are also well simulated in the model. The generation of the interannual anomalies in the model are also due mainly to the wind stress curl integrated along the Rossby wave trajectories in the STIO.
Remote sensing of the vertical distribution of atmospheric water vapor from the TOVS observations: Method and validation
Authors: Chaboureau Jean‐Pierre , Chédin Alain , Scott Noëlle A.
Published: February 2004

This paper presents a method to remotely sense the vertical distribution of atmospheric water vapor using spaceborne measurements from the TOVS instrument aboard the NOAA polar satellite series. It describes a new approach to the water vapor retrieval scheme in the improved initialization inversion (3I) method. The technique is based on a neural network scheme, which is analyzed theoretically. Cross‐comparisons of its results with a wide variety of independent observations (in situ measurements or other global data sets, e.g., the special sensor microwave/imager (SSM/I) retrievals, analyses) are then carried out to fully evaluate the method. It is shown that the mean of the differences between total water vapor contents obtained from each data set represents less than 20% of the global mean value of the water vapor content. Different behaviors between TOVS and SSM/I in tropical situations are also highlighted. Concerning the vertical profile, the standard deviation between water vapor content retrieved by 3I and measured by radiosondes varies from 20% in the 1000–850 hPa layer to less than 40% in the 500–300 hPa layer. The vertical increase of the error is linked to the difficulty of measuring weak values by radiosonde instruments, radiometers, or analyses.
Stable isotopic signatures (δ13C, δD) of methane from European landfill sites
Authors: Bergamaschi P. , Lubina C. , Königstedt R. , Fischer H. , Veltkamp A. C. , Zwaagstra O.
Published: February 2004

The stable isotopic signatures (δ13C, δD) of CH4 from four German and Dutch landfill sites have been characterized using different techniques for isotope analysis (tunable diode laser absorption spectroscopy and isotope ratio mass spectrometry). Samples taken directly from the gas collection systems show fairly uniform, biogenic δ13C‐δD isotopic signatures [δ13C = (−59.0±2.2)‰ VPDB (n = 104); δD = (−304±10)‰ VSMOW (n = 46)]. In contrast, emission samples taken with static chambers on soil‐covered landfill areas exhibit a considerable δ13C‐δD variability, mainly due to the influence of aerobic bacterial CH4 oxidation, which occurs when the biogas CH4 encounters atmospheric oxygen available in the uppermost region of the cover soil. Soil gas samples from the landfill covers clearly show the progressive isotopic enrichment within the aerobic regions of the soil. Isotope fractionation factors due to CH4 oxidation were determined to be α(δ13C) = 1.008±0.004 and α(δD) = 1.039±0.026. On average, about 80% (70–97%) of CH4 is oxidized during the transport through cover soils, while no significant CH4 oxidation was found in uncovered areas consisting of freshly dumped waste. Area‐integrated δ13C values of total emissions were derived from upwind‐downwind measurements around the landfill and show very little temporal and site‐to‐site variation (δ13C = (−55.4±1.4)‰ VPDB (n = 13; four different landfills)). CH4 budgets were established for two landfill sites, indicating that projected CH4 surface emissions from uncovered and covered areas are significantly lower compared to total CH4 production (for a landfill without gas collection) or compared to the difference between CH4 production and recovery (for a landfill with a gas collection system). For these two landfill sites the overall fraction of CH4 oxidation is estimated to be 46 and 39% (53%) of total CH4 production (minus recovery). Furthermore, the δ13C balance (comparing the δ13C values of the different emission pathways with the area‐integrated δ13C results) implies that direct CH4 emissions via cracks or leakages constituted the major transport pathway (∼70%) into the atmosphere in both landfills.
Global simulation of tropospheric O3‐NOx‐hydrocarbon chemistry: 3. Origin of tropospheric ozone and effects of nonmethane hydrocarbons
Authors: Wang Yuhang , Jacob Daniel J. , Logan Jennifer A.
Published: February 2004

A global three‐dimensional model of tropospheric O3‐NOx‐hydrocarbon chemistry is used to investigate the factors controlling ozone concentrations in the troposphere. Model results indicate a close balance between chemical production and chemical loss of ozone in the tropospheric column at all latitudes (except high latitudes in winter). Using separate tracers for ozone produced in the stratosphere and in different regions of the troposphere, we find that the contribution of transport from the stratosphere to ozone concentrations in the troposphere is about 30% at midlatitudes in winter, 10% in summer, and 5% in the tropics. Production of ozone in the upper, middle, and continental lower troposphere all make significant contributions (10–50%) to ozone concentrations throughout the troposphere. The middle troposphere is a major global source region for ozone even though it is not a region of net production. The springtime maximum of ozone observed at remote sites in the northern extratropics is explained by a phase overlap between ozone transported from the stratosphere which peaks in late winter and ozone produced in the troposphere which peaks in late spring. Our model results do not support previous explanations of the springtime maximum based on wintertime accumulation of ozone or its precursors in the Arctic. The particularly strong springtime maximum at Mauna Loa Observatory (Hawaii) is attributed to long‐range transport of Asian pollution over the North Pacific in spring. A sensitivity simulation without nonmethane hydrocarbons (NMHCs) indicates small decreases of ozone concentrations (<15%) in the remote troposphere and a 20% increase in the global mean OH concentration. Without NMHCs as a source of peroxyacetylnitrate, concentrations of NOx decrease by 30% in the remote lower troposphere but increase by 70% in the continental lower troposphere and by 40% in the upper troposphere. Biogenic isoprene accounts for about half of the NMHC effects in the model.
Biomass burning smoke measured using backscattered ultraviolet radiation: SCAR‐B and Brazilian smoke interannual variability
Authors: Gleason James F. , Hsu N. Christina , Torres Omar
Published: February 2004

A recently developed method for monitoring biomass burning smoke from space has been applied to measurements over Brazil. Long‐term measurements of aerosol index measured from space by a series of satellites show the interannual variability of smoke‐covered areas over Brazil. Using a newly developed algorithm, the aerosol index has been inverted into aerosol optical depth values. Coincident ground‐based Sun photometer aerosol optical thickness measurements are used to validate this inversion. The interannual variability of the area covered by smoke of varying intensity is described over the period 1978 to 1995. The results show that 1995, the year of the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) mission, was similar to recent years for overall smoke area, but the area covered by heavier smoke was larger than other years in the data record.
The Turnbull correlation and the freezing of stratospheric aerosol droplets
Authors: MacKenzie A. Robert , Laaksonen Ari , Batris Evangelos , Kulmala Markku
Published: February 2004

An empirical correlation that is important in the calculation of homogeneous freezing probabilities, the “Turnbull correlation” for interfacial tensions, has been reevaluated and applied to systems of interest as possible components of polar stratospheric clouds (PSCs). The systems studied were: sulphuric acid solutions freezing to water ice and sulphuric acid tetrahydrate (SAT); and nitric acid solutions freezing to nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD). The calculations have been compared to experimental data: agreement is generally good, although aerosol freezing experiments, which would rigorously test the theory, have not been made for NAT. Of the three measurements of aerosol freezing to NAD, the calculations are closer to those measurements showing a lower freezing temperature, of about 175–177 K. The comparison substantially improves our confidence in our understanding of the mechanisms of PSC formation. Freezing of stratospheric aerosol to water ice remains the most plausible first step in solid‐particle PSC formation if homogeneous freezing is the mechanism by which solid‐particle PSC formation occurs.
Observations of precipitation associated with a cold front using a VHF wind profiler and a ground‐based optical rain gauge
Authors: Chu Yen‐Hsyang , Song June‐Shyen
Published: February 2004

In this paper the VHF radar returns from hydrometeors and refractivity fluctuations associated with a cold front are analyzed. A composite analysis of the precipitation echo intensity and the vertical air velocity indicates that the vertical air velocity plays a vital role in the formation of the bright band. The observations show that the VHF radar reflectivity from precipitation at the height around the melting layer may be enhanced in the condition of weak vertical air velocity, while the bright band may be disrupted if the upward vertical air speed is as large as 1.2 m/s. The intense updraft may also diminish the echo intensity from refractivity fluctuations through the mechanism of turbulent mixing, as first suggested by Chu and Lin [1994]. The corresponding observational evidence will be presented and discussed in this paper. Comparing the time series of the VHF precipitation echo intensity aloft with that of the surface rainfall rate indicates there is a systematic time delay in the two, which increases approximately linearly with height. The drop‐off rate of the time difference is approximately equal to the average fall velocity of the raindrop. A regression analysis of the precipitation data between range‐corrected VHF radar reflectivity Pr and ground‐based rainfall rate R using the equation Pr = αRβ is also made. It shows that β has a tendency to decrease with height, while α is nearly height‐independent.
Sulfur‐containing species (sulfate and methanesulfonate) in coastal Antarctic aerosol and precipitation
Authors: Minikin Andreas , Legrand Michel , Hall Julie , Wagenbach Dietmar , Kleefeld Christoph , Wolff Eric , Pasteur Elizabeth Claire , Ducroz François
Published: February 2004

The concentrations of sulfur‐containing species (sulfate and methanesulfonate (MSA)) of aerosols collected at three coastal Antarctic sites (Neumayer, Dumont d'Urville, and Halley) have been studied in order to investigate the natural sulfur cycle at high southern latitudes. The multiple‐year data sets indicate annual mean concentrations of MSA and non‐sea‐salt (nss) sulfate of 38 and 151 ng m−3 at Neumayer (1983–1995) and 20 and 147 ng m−3 at Dumont d'Urville (1991–1995). On the basis of the study of a more limited time period (1991 and 1992), the Halley data set indicates significantly lower MSA and nss sulfate concentrations, 15 and 50 ng m−3, respectiveley. The concentrations of both species exhibit a consistent and strong seasonal cycle with maxima from December to March and minima from May to September (from 2 to 4 and from 17 to 50 ng m−3, for MSA and nss sulfate, respectively). These data, together with radionuclide studies (210Pb and 10Be), indicate that the marine biogenic source dominates the sulfur budget of the boundary layer of these regions throughout the year. The contribution of other sources, such as the long‐range transported sulfur from continents and to a lesser extent the stratospheric sulfate reservoir, remains weak when averaged over the year. Differences in the seasonal pattern of the two sulfur‐containing species, as well as intersite differences in the summer concentrations, are compared to several factors, mainly the spatial and temporal variations of the chlorophyll content of the surface ocean water and the seasonality of the sea ice cover. This permits investigation of the respective influence of marine regions emitting dimethylsulfide located either near the Antarctic continent (south of 60°S) or at more temperate latitudes. A strong correlation is found between the chlorophyll content of the Antarctic ocean and the level of total nss sulfur. Furthermore, nss sulfate and MSA deposition fluxes determined from firn and ice cores extracted at sites located at various distances from the ocean and various altitudes allow assessment of the spatial variation of the marine biogenic sulfur input in Antarctica.
Deuterium excess in Greenland snow: Analysis with simple and complex models
Authors: Armengaud Alexandre , Koster Randal D. , Jouzel Jean , Ciais Philippe
Published: February 2004

A simple Rayleigh‐type isotope model, typical of those used to develop algorithms for extracting climatic information from stable water isotope paleodata, is evaluated against the more complex and presumably more reliable calculations of a general circulation model (GCM) fitted with isotope tracer diagnostics. The evaluation centers on an analysis of how the temperature Te of an oceanic moisture source affects the deuterium excess d of Greenland precipitation. The annual Te‐d relationship derived from the GCM diagnostics is largely reproduced by the simple isotope model when the latter is properly initialized. This, coupled with the fact that the GCM itself reproduces observed isotope behavior, suggests that the simpler model's atmospheric calculations are indeed adequate for isotope studies. Furthermore, the GCM results support the idea, originally developed with the simpler models, that polar deuterium excess values contain information on meteorological conditions at distant evaporative sources.
Dissolution of sulfuric acid tetrahydrate at low temperatures and subsequent growth of nitric acid trihydrate
Authors: Iraci Laura T. , Fortin Tara J. , Tolbert Margaret A.
Published: February 2004

Crystalline sulfuric acid tetrahydrate (SAT) has been observed to change phase at temperatures below its melting point, in agreement with recent theoretical predictions of deliquescence. Dissolution of SAT was observed in 63% of experiments expected to show a phase change, leading to formation of a ternary HNO3/H2SO4/H2O solution. This solution, which still contained a portion of the original solid SAT, crystallized to form nitric acid trihydrate (NAT). NAT then continued to grow by condensation of additional nitric acid and water at temperatures several degrees above the ice frost point. This process of SAT dissolution followed by NAT crystallization and growth may offer a mechanism for the formation of type Ia polar stratospheric clouds on frozen sulfate aerosols when SNAT>15.
Studying the effects of aerosols on vertical photolysis rate coefficient and temperature profiles over an urban airshed
Authors: Jacobson Mark Z.
Published: February 2004

This paper discusses the effects of size‐ and composition‐resolved aerosols on photolysis and temperatures within and above an urban airshed. With respect to photolysis, three‐dimensional simulations indicated that (1) in regions of the boundary layer where absorption of ultraviolet (UV) radiation was strong, aerosols reduced photolysis coefficients of UV‐absorbing gases; (2) in regions of the boundary layer where UV scattering dominated UV absorption by aerosols, aerosols enhanced photolysis coefficients of UV‐absorbing gases; (3) aerosols increased photolysis coefficients for visible‐absorbing gases since visible scattering always exceeded visible absorption by aerosols; (4) scattering and weakly absorbing aerosols above the boundary layer increased photolysis coefficients above the boundary layer for all absorbing gases; and (5) increases in aerosol absorption extinction within the boundary layer reduced photolysis coefficients above the boundary layer for all absorbing gases. Photolysis coefficients changes due to aerosols decreased near‐surface ozone mixing ratios in Los Angeles by 5–8%. With respect to temperatures, simulations indicated that aerosols increased radiative heating rates at all altitudes but decreased surface solar irradiances during the day. Surface irradiance reductions cooled the ground, reducing mechanical and thermal turbulent heat fluxes back to the boundary layer, cooling near‐surface air, and stabilizing the boundary layer. During the night, aerosols decreased boundary‐layer heating rates but increased downward infrared irradiances to the ground. Warmer ground temperatures increased mechanical turbulent heat fluxes to the boundary layer, increasing nighttime near‐surface temperatures. Thus, aerosols affected temperatures primarily through ground‐atmosphere turbulent heat transfer.
Laboratory simulations of radiatively induced entrainment in stratiform clouds
Authors: Sayler Bentley J. , Breidenthal Robert E.
Published: February 2004

Laboratory experiments have been conducted to simulate entrainment in stratiform clouds. In particular, the case of entrainment across a capping temperature inversion and induced by cloud top cooling has been simulated. This geometry is termed interfacial convection, and its physics differ from the more thoroughly studied case of penetrative convection. The dimensionless entrainment rate associated with interfacial convection has been found to vary inversely with a bulk Richardson number over a broad range of Richardson numbers. A dependence of the entrainment rate on the diffusivity of the stratifying agent has also been found. This dependence is explained in terms of Taylor layers. A physical model for the dynamics of interfacial convection is proposed. In the laboratory case, a stably stratified interface separates two fluid layers. Convection is driven in the upper layer by the deposition of radiation near the interface. After sufficient energy has been deposited, buoyant fluid rises and induces formation of entraining cusps at the interface. The spacing between cusps is determined by equating a buoyancy instability timescale with a heating timescale. When the depth of the convecting layer is large compared to the distance separating the cusps, a larger‐scale circulation also develops. In such cases, eddies of size comparable to the depth of the convecting layer advect the cusps horizontally. Despite Reynolds numbers that differ by 4 orders of magnitude or more between the laboratory simulation and the real atmosphere, it is argued that the entrainment dynamics are analogous. Dimensionless entrainment rates measured in the laboratory are within 1 order of magnitude of those measured in the atmosphere for a given Richardson number. Thinner Taylor layers and the lack of evaporative effects in the laboratory may account for the difference.
A linked global model of terrestrial hydrologic processes: Simulation of modern rivers, lakes, and wetlands
Authors: Coe Michael T.
Published: February 2004

A terrain‐based hydrologic model is developed to simulate rivers, lakes, and wetlands on the continental scale as a linked dynamical system. This surface water area model (SWAM) is an extension of earlier work [Coe, 1997] and is based on a linear reservoir model. The model requires, as input, estimates of runoff, precipitation, and evaporation from either observations or climate simulations. The model develops its own river transport directions based on elevation. The river discharge is calculated at each grid cell as the accumulated flow of water across the land surface. Lake and wetland area and volume are calculated as a function of the local precipitation minus evaporation, the streamflow into and out of the grid cells, and the potential for storage of water on the surface as derived from 5′ × 5′ resolution digital terrain models. SWAM is applied on a 5′ × 5′ spatial resolution to simulate present‐day surface waters and river transport for all continents (except Antarctica and Greenland). The model simulates the discharge of the 27 major rivers of the world in fair agreement with the observations; 12 of the rivers have simulated discharge within ±20% of the observational estimates. The discharge from rivers in arid and semi‐arid climates is generally overestimated probably due to underestimated evaporation from wetlands, irrigated croplands, and reservoirs within the river basins. The modern simulated lake area (about 3% of the land area) is larger than the observed area (about 2% of the land area). Wetlands are poorly simulated by the model due to the coarse vertical and horizontal resolution of the digital terrain models. The model simulates the potential for increased surface water (up to 7.5% of the global land area) and river basin area in semi‐arid and arid regions where closed basins exist in the present day climate which illustrates the utility of SWAM for climate change experiments. While higher resolution and more accurate digital terrain models are needed to improve these surface water simulations (particularly for wetlands), these preliminary results indicate that current digital terrain models are adequate for including lakes and rivers as interactive components in the surface hydrology parameterizations of climate models and for studying feedbacks between lakes and climate.
LITE and SAGE II measurements of aerosols in the southern hemisphere upper troposphere
Authors: Kent G. S. , Trepte C. R. , Skeens K. M. , Winker D. M.
Published: February 2004

Two global satellite data sets have been used to characterize the behavior of aerosols in the upper troposphere of the southern hemisphere during the spring season. The first data set was obtained by the Lidar‐In‐Space Technology Experiment (LITE) during 10 days in September 1994 and provides high‐resolution information about aerosol layering and optical characteristics. The second data set was obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) II over the time period 1984–1996 and provides information on the aerosol distribution and long‐term climatology. During September, elevated aerosol layers are found to occur within a latitude band between 20°S and 40°S that extends to almost all longitudes. The latitude and altitude distribution and the optical characteristics of the aerosol suggest that a major source is smoke from biomass burning within the southern hemisphere. This conclusion is supported by the results of back‐trajectory analyses that show airmasses originating in the region of southern Africa and traveling longitudinally across the Indian Ocean and Australia into the western Pacific Ocean. The dominant source of the smoke is uncertain, but quite possibly some of it may have originated from Brazil, with additions from southern Africa. The aerosol distribution shows strong similarities to published distributions for ozone and carbon monoxide, also believed to have originated from biomass burning.
Carbon dioxide exchange in a subarctic peatland ecosystem in northern Europe measured by the eddy covariance technique
Authors: Aurela Mika , Tuovinen Juha‐Pekka , Laurila Tuomas
Published: February 2004

Micrometeorological measurements of carbon dioxide and energy fluxes were made on a mesotrophic flark fen in northern Finland (69°08′N, 27°17′E). The measurements were conducted from August 15 to September 13, 1995, the period extending from the intense phase of the growing season through its decline. The objective was to assess the net CO2 balance on a daily basis during the crucial time of the fen turning from a net sink to a net source of CO2. A clear decline in the CO2 fluxes was observed due to the senescence of the vascular plants at the end of August. The hourly median of daytime fluxes decreased from −0.12 to −0.04 mg m−2 s−1 from August to September. The nighttime fluxes decreased simultaneously from 0.07 to 0.04 mg m−2 s−1. A simple photosynthetically active radiation (PAR) driven net ecosystem exchange model was used to patch up the time series for calculating the diurnal balances. The value of the model parameter of maximal gross photosynthetic uptake changed from −0.320 to −0.134 mg m−2 s−1 from August to September. The value of α (initial slope of net ecosystem exchange versus PAR) changed, correspondingly, from −0.888 to −0.480 μg μmol−1. The highest negative daily balances at the beginning of the period were about −2 g m−2 d−1. During the first days of September they rose to 3 g m−2 d−1, but decreased to 1 g m−2 d−1 at the end of the measurements.
Carbon monoxide measurements from 76° N to 59° S and over the South Tasman Sea
Authors: Kok Gregory L. , Prévôt Andrê S. H. , Schillawski Richard D. , Johnson James E.
Published: February 2004

In November and December of 1995, carbon monoxide (CO) measurements were made in a Pacific transect and over the South Tasman Sea as part of the First Aerosol Characterization Experiment (ACE 1) program. Airborne CO measurements were made from 76° N to 59° S. A clear latitudinal gradient in CO concentrations was measured, with the southern hemisphere average about 80 parts per billion by volume (ppbv), and increasing to 120–130 ppbv at the most northern latitudes. Plumes of CO with a 30–40 ppbv concentration increase over the general background concentrations could be seen at several latitudes. The National Oceanic and Atmospheric Administration R/V Discoverer made CO measurements over the South Tasman Sea from November 15 to December 9, 1995. A systematic decrease of 0.31 ppbv/d CO was observed. Vertical profile measurements of CO from near the ocean surface to 2500 m altitude during the Lagrangian B intensive of ACE 1 suggested the mixing of stratospheric air with reduced CO concentrations.
Intraseasonal variability of high‐resolution Doppler imager winds in the equatorial mesosphere and lower thermosphere
Authors: Lieberman Ruth S.
Published: February 2004

Analyses of high‐resolution Doppler imager (HRDI) daytime equatorial zonal winds indicate significant variability on time scales of 20–121 days. These appear as transient oscillations in the equatorial zonal mean zonal wind between 70 and 100 km. The oscillations are characterized by two maxima near 75 and 95 km that are out of phase, separated by a minimum near 80 km. These features move downward in time and exhibit equatorial symmetry with maximum values at the equator. During 1994, maximum amplitudes of the intraseasonal oscillation are 20 m s−1 in January and 30 m s−1 in June. The intraseasonal variability present in HRDI winds is qualitatively consistent with variations on similar timescales reported at Christmas Island. It has been hypothesized that the wind patterns at Christmas Island result from modulated gravity wave driving [Eckermann et al, 1997]. The results of the present study strongly support this view.
Baseline radon detectors for shipboard use: Development and deployment in the First Aerosol Characterization Experiment (ACE 1)
Authors: Whittlestone S. , Zahorowski W.
Published: February 2004

A new design of two‐filter radon detector has been developed for measurement of extremely low levels of radon in the harsh environments on board ships and remote islands. These were needed for the First Aerosol Characterization (ACE 1) multiplatform experiment in the Southern Ocean. By employing an internal recirculation system and a wire mesh screen as the second filter it has been possible to reduce the power consumption by as much as a factor of 10 and the weight and cost by a factor of 2 compared to current designs of comparable sensitivity. A very high efficiency of 0.38 count s−1 Bq−1 radon in the instrument has been achieved by counting while sampling. This is a key parameter because the larger this number, the smaller the volume and power consumption of the detector. Two air flow paths are used to separate the high flow rate needed to prevent loss of radon daughters to the walls of the detector and the lower flow rate needed to change the air sample in the instrument. As a result, the inlet air lines and delay chamber needed to remove thoron are compact. With a volume of 750 L the detectors used on board ships for ACE 1 had a sensitivity of about 0.2 counts s−1 Bq−1 m−3 and a lower limit of detection of 40 mBq m−3 for a 1 hour count. An instrument with a volume of 10 m3 and incorporating improvements made since ACE 1 could be expected to have sensitivity of 3.7 c s−1 Bq−1 m−3 and a lower limit of detection of 2 mBq m−3. At 45 min the time resolution is twice as good as that of instruments using a low internal flow rate, but not as good as instruments with a moving filter, where the sampling period is precisely defined. Dual‐flow loop radon detectors with screens have the virtues of simplicity and freedom from routine maintenance. This new technology extends the range of sites at which baseline radon measurements can be made to remote areas with little regular technical support and a harsh environment.
Changes in aerosol properties with relative humidity in the remote southern hemisphere marine boundary layer
Authors: Baumgardner Darrel , Clarke Antony
Published: February 2004

In situ measurements of atmospheric aerosols have been made from an airborne platform over the remote southern hemisphere ocean in November and December 1995 as part of the First Aerosol Characterization Experiment (ACE 1). A subset of these measurements have been evaluated during three of the flights to characterize the aerosol microphysical and optical properties in the cloud‐free, marine boundary. The relationship between the microphysical and optical characteristics in the size range from 0.3 to 20 μm and relative humidity was evaluated. A new technique is introduced by which the scattering coefficient is derived directly from the optical particle counter measurements. The results of this study indicate that changes in particle volume, effective radius, and optical scattering are strongly related to changes in relative humidity (RH). The observations are in very good agreement with laboratory studies of particle volume changes as a function of relative humidity.
Filter and impactor measurements of anions and cations during the First Aerosol Characterization Experiment (ACE 1)
Authors: Huebert B. J. , Howell S. G. , Zhuang L. , Heath J. A. , Litchy M. R. , Wylie D. J. , Kreidler‐Moss J. L. , Cöppicus S. , Pfeiffer J. E.
Published: February 2004

During the First Aerosol Characterization Experiment (ACE 1), we measured the concentrations of aerosol non‐sea‐salt sulfate (NSS), methane sulfonate (MS), ammonium, sodium, nitrate, and a variety of other anions and cations from the National Center for Atmospheric Research C‐130Q aircraft and the Cape Grim Baseline Air Monitoring Station. We used the data to test a variety of hypotheses concerning the sources and properties of aerosol in remote environments. We noted a large gradient in free tropospheric (FT) MS, with much more in the spring (southern) hemisphere, but FT NSS showed no latitudinal gradient. Most species had strong vertical concentration gradients, generally with more material near the biogenic sources of the marine boundary layer (MBL) than in the FT. During the Lagrangian experiments and in time series measurements at Cape Grim, we saw daytime increases of photochemically derived MS, with more constant or decreasing concentrations at night. NSS also increased in the daytime during the Lagrangian experiments. At Cape Grim, we were unable to account for the gravimetric mass of particles on impactor substrates using the measured ions, which implies that a substantial fraction of the aerosol at that location must be either organic or mineral matter. Our FT ratios of MS/NSS exhibited a strong latitudinal gradient, with the southernmost values similar to those found in Antarctic plateau snowfall.
Lidar in Space Technology Experiment correlative measurements by lidar in Potenza, southern Italy
Authors: Cuomo V. , Di Girolamo P. , Pappalardo G. , Spinelli N. , Berardi V. , Armenante M. , McCormick M. P.
Published: February 2004

An intensive lidar measurement campaign was carried out in Potenza (40°36′N‐15°44′E, 820 m above sea level (asl)) in conjunction with the Lidar in Space Technology Experiment (LITE) mission and primarily aimed at the validation of LITE stratospheric aerosol measurements. Potenza lidar measurements in coincidence with all five nighttime overpasses near southern Italy (September 11, 12, 17, and 18, 1994) are compared with simultaneous LITE data. Potenza lidar data appear to be highly correlated with LITE data both at 355 and 532 nm. Potenza lidar versus LITE measurements of the aerosol‐scattering ratio show a correlation coefficient of 0.72–0.81 at 355 nm and 0.88–0.93 at 532 nm, with an average calibration coefficient of 0.92±0.19 at 355 nm and 1.02±0.07 at 532 nm. Comparisons are also made in terms of the average Ångstrom coefficient, whose values are consistent with submicrometer aerosol particles. Finally, Potenza lidar measurements of the aerosol layer base and top heights, the peak aerosol‐scattering ratio and peak height, as well as of the aerosol scattering ratio at the cloud base appear to be consistent with measurements performed by other ground lidar stations in Europe during the LITE campaign as well as with the LITE data.