bluuebunny/crossref_metadata_2025 huggingface dataset:
The presence of metastable HNO3/H2O solid phases in the stratosphere inferred from ER 2 data
Authors: Tabazadeh Azadeh , Toon Owen B.
Published: February 2004

We present data to show that at least some of the observed polar stratospheric cloud (PSC) particles sampled by the ER 2 on January 20, 1989, during the Airborne Arctic Stratospheric Expedition (AASE 1) were composed of a water‐rich HNO3/H2O solid phase. The PSC water content derived from the ER 2 data on this day is larger than that of nitric acid trihydrate (NAT), nitric acid dihydrate (NAD) or an aqueous ternary solution of H2SO4/HNO3/H2O. Here, we suggest that these particles were composed of a water‐rich metastable HNO3/H2O solid phase and refer to such clouds as Type 1c PSCs, which are different from Type 1a (crystalline NAT or NAD particles) or Type 1b (aqueous ternary solution droplets) PSCs. Type 1c PSCs could either be crystalline (a higher hydrate of HNO3 and H2O) or amorphous, and the data analysis presented here cannot distinguish between these different solid phases. The observed PSC on this day could have been a mixture of water‐rich HNO3‐containing solid particles with liquid ternary droplets and/or NAT (NAD) aerosols. However, a mixture of cloud particles composed of ternary solution droplets and NAT (NAD) aerosols is inconsistent with the data. A surface adsorption/reaction process occurring on frozen sulfate surfaces is suggested as a formation mechanism for Type 1c PSCs in the stratosphere. A possible mechanism for the formation of large HNO3‐containing aerosols (NAT or NAD) starting with Type 1c PSCs is discussed.
An objective determination of the polar vortex using Ertel's potential vorticity
Authors: Nash Eric R. , Newman Paul A. , Rosenfield Joan E. , Schoeberl Mark R.
Published: February 2004

We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertel's potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s−1. We use 1992–1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.
Modeling of the processing and removal of trace gas and aerosol species by Arctic radiation fogs and comparison with measurements
Authors: Bergin M. H. , Pandis S. N. , Davidson C. I. , Jaffrezo J.‐L. , Dibb J. E. , Russell A. G. , Kuhns H. D.
Published: February 2004

A Lagrangian radiation fog model is applied to a fog event at Summit, Greenland. The model simulates the formation and dissipation of fog. Included in the model are detailed gas and aqueous phase chemistry, and deposition of chemical species with fog droplets. Model predictions of the gas phase concentrations of H2O2, HCOOH, SO2, and HNO3 as well as the fog fluxes of S(VI), N(V), H2O2, and water are compared with measurements. The predicted fluxes of S(VI), N(V), H2O2, and fog water generally agree with measured values. Model results show that heterogeneous SO2 oxidation contributes to approximately 40% of the flux of S(VI) for the modeled fog event, with the other 60% coming from preexisting sulfate aerosol. The deposition of N(V) with fog includes contributions from HNO3 and NO2 initially present in the air mass. HNO3 directly partitions into the aqueous phase to create N(V), and NO2 forms N(V) through reaction with OH and the nighttime chemistry set of reactions which involves N2O5 and water vapor. PAN contributes to N(V) by gas phase decomposition to NO2, and also by direct aqueous phase decomposition. The quantitative contributions from each path are uncertain since direct measurements of PAN and NO2 are not available for the fog event. The relative contributions are discussed based on realistic ranges of atmospheric concentrations. Model results suggest that in addition to the aqueous phase partitioning of the initial HNO3 present in the air mass, the gas phase decomposition of PAN and subsequent reactions of NO2 with OH as well as nighttime nitrate chemistry may play significant roles in depositing N(V) with fog. If a quasi‐liquid layer exists on snow crystals, it is possible that the reactions taking place in fog droplets also occur to some extent in clouds as well as at the snow surface.
Use of on‐line tracers as a diagnostic tool in general circulation model development: 1. Horizontal and vertical transport in the troposphere
Authors: Rind David , Lerner Jean
Published: February 2004

The use of passive on‐line tracers in a general circulation model (GCM) developmental process is discussed. CFC‐11 and 85Kr are used to evaluate interhemispheric transport. It is shown that new boundary layer and convective parameterizations lead to reductions in the interhemispheric exchange times to values in close agreement with observations of little more than one year. Radon 222 is used to evaluate vertical mixing away from the surface. The new convection scheme produces smaller convective fluxes, which substantially reduce the high‐altitude concentrations of 222Rn equatorward of 30° latitude. While this result is in better agreement with other models, scarcity of observations prevent any conclusion as to which formulation is more accurate. The effect of altered numerical schemes for solution of the momentum and energy equations in the GCM is shown to have little influence on the model's interhemispheric transport or vertical mixing. Finer vertical resolution increases convective mixing of 222Rn somewhat and allows for larger pollution concentrations of CFC‐11 in the boundary layer. Employing on‐line passive tracers in the course of model development should allow for improvements in a GCM's horizontal and vertical fluxes and optimization of the model for atmospheric chemistry purposes.
A test of the partitioning between ClO and ClONO2 using simultaneous UARS measurements of ClO, NO2, and ClONO2
Authors: Dessler A. E. , Kawa S. R. , Douglass A. R. , Considine D. B. , Kumer J. B. , Roche A. E. , Mergenthaler J. L. , Waters J. W. , Russell J. M. , Gille J. C.
Published: February 2004

The photochemical theory of the partitioning hetween the chlorine species ClONO2 and ClO in the lower and mid stratosphere is tested for the first time using simultaneous measurements of ClO, NO2, and ClONO2. The theory suggests that over most of the day the production of ClONO2 through the three‐body reaction of ClO and NO2 is balanced by the loss of ClONO2 through photolysis. Our analysis tests this theory by utilizing simultaneous measurements of these species from instruments aboard the Upper Atmosphere Research Satellite (UARS), as well as calculations of the photolysis rate of ClONO2 and laboratory measurements of the rate constant for the reaction between ClO and NO2. Between 32 and 10 hPa (∼24 and 32 km), averaged instantaneous production and loss rates of ClONO2 agree within ∼10%. At 46 hPa (∼21 km), the agreement between averaged instantaneous production and loss of ClONO2 is poorer than at higher altitudes, but still within uncertainties. This analysis provides no evidence for any problems in our understanding of the partitioning between the chlorine species ClONO2 and ClO over the range of pressures (46 to 10 hPa) and latitudes (60°S to 60°N) considered here.
Vertical and horizontal aerosol spectral extinction at a rural location in southern Italy
Authors: Esposito F. , Serio C. , Horvath H. , Romano F.
Published: February 2004

The vertical extinction of the aerosol has been measured with a multiwavelength sunphotometer at a rural location in southern Italy. All measurements were performed during a period with relatively stable weather condition, thus it could be expected to have measured always the same air mass at least within each day. The direct solar radiation was recorded at wavelengths between 400 and 1100 nm with a resolution of 0.5 nm. Calibration of the instrument was performed by means of a Langley plot at an elevation of approximately 3000 m. From these measurements, vertical optical depths have been derived for a series of days in June 1993. The spectral extinction coefficient of the atmospheric aerosol at the same location has been measured by means of a telephotometer. The radiance of a target at a distance of 12 km was measured at nine wavelengths in the visible. Thus the extinction coefficient of the aerosol contained within a conical volume of about 450,000 m3 and a length of 12 km has been measured. The daily variation usually followed the same pattern: a decrease in extinction coefficient in the morning and in the early afternoon and an increase toward the evening. This variation correlated well with the change in humidity, so that it can be assumed that the aerosol particles have decreased their size due to evaporation of water in the morning and uptake of water in the evening. The regular daily pattern of the horizontal aerosol extinction coefficient normally was not followed by the vertical aerosol optical density, but on the other hand, changes in vertical extinction have occurred on some days, which had no effect on the horizontal extinction. Vertical extinction measurements were inverted to obtain aerosol size distributions. Inversions were obtained by the Twomey‐Phillips method and they confirm the presence of a hygroscopic aerosol as suggested directly by the variation of the extinction coefficients against the relative humidity. The aerosol undergoes very clear processes of growth which are well correlated with the behavior of the humidity. Overall, the type of aerosol we have measured could be classified as rural aerosol. This kind of aerosol is typical of the boundary layer of clean continental area and is composed prevalently by water‐soluble substances and in minor abundance by dustlike aerosol.
On the determination of scale ranges for precipitation fields
Authors: Fabry Frédéric
Published: February 2004

To test some recent theories on the nature of precipitation structure, data were collected using radar and a newly developed “sonic” gauge to investigate precipitation variability at small scales. Results show that the structure of precipitation below a few tens of meters has very different statistics than at larger scales. Interpretation of vertically pointing radar data suggests that, at small scales, a mixing‐like process occurs because of the differential fall speed of hydrometeors, which results in the destruction of small‐scale structure. It is also shown that convection in the melting layer seems to recreate some of the structure that had been lost as snow fell. Finally, over scales of meteorological interest, at least four distinct regimes in precipitation variability can be identified.
Peroxide concentrations in the Dome Summit South ice core, Law Dome, Antarctica
Authors: van Ommen Tas D. , Morgan Vin
Published: February 2004

Measurements of H2O2 concentrations have been made at intervals along the full length of the Dome Summit South (DSS) ice core from Law Dome, Antarctica. These results show mean peroxide concentrations of approximately 37 parts per billion by mass over the past 4 kyr and a tendency for concentrations to decrease with age. The rate of H2O2 decay in Antarctic ice appears slower than in Greenland ice and the pattern of measurements suggests an abrupt increase in concentration during the emergence from the last glacial into the Holocene. There is some evidence for a recent increase in H2O2, but the data so far are not conclusive. Detailed examination of the timing of the seasonality in H2O2 and δ18O shows that peroxide extrema occur very near the solstices. These seasonal curves have been used to derive atmospheric seasonal H2O2 curves based on possible air‐snow partitioning models. These show a broad low in atmospheric concentration with a sharp peak which is again very near the summer solstice.
Determination of photochemically available iron in ambient aerosols
Authors: Siefert Ronald L. , Webb Samuel M. , Hoffmann Michael R.
Published: February 2004

Experiments to determine the concentration of photochemically available Fe in ambient aerosol samples were carried out using a novel photochemical extraction procedure. Ambient aerosol samples, which were collected on Teflon filters, were suspended in an aqueous solution within a photochemical reactor and irradiated. Under these conditions, which were favorable to the photochemical weathering of aerosol particles, the relative amount of Fe(II)aq to Fetotal was shown to increase. The extent and rate of Fe(II)aq photoproduction was used to characterize the Fe in aerosol samples collected from Whiteface Mountain, New York, Pasadena, California, San Nicholas Island, California, and Yosemite National Park, California. Photochemically available Fe concentrations found ranged from <4 ng m−3 (0.07 nmole m−3) to 308 ng m−3 (5.52 nmole m−3), Fetotal concentrations ranged from 10 ng m−3 (0.18 nmole m−3) to 3400 ng m−3 (61 nmole m−3), and the percentage of photochemically available Fe to Fetotal ranged from 2.8% to 100%. Aerosol samples were also collected during biomass burning events in southern California; these samples showed insignificant changes in the photochemically available Fe (compared to nonbiomass burning samples) in conjunction with large increases of Fetotal. Calculations based on these experiments also provide further evidence that redox reactions of Fe in cloudwater could be an important in situ source of oxidants (OH, HO2/O2−). The estimated oxidant production rate in cloudwater based on these experiments is between 0 and 60 nM s−1, with an average value of 16 nM s−1. This estimated in situ oxidant production rate due to Fe chemistry is approximately equal to previous estimates of the oxidant flux to cloudwater from the gas phase.
Evaluating chemical transport models: Comparison of effects of different CFC‐11 emission scenarios
Authors: Hartley Dana E. , Kindler Thomas , Cunnold Derek M. , Prinn Ronald G.
Published: February 2004

In this study we compare the results of model simulations of CFC‐11 in the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM2) with two different published emission distributions. We find that for predicted CFC‐11 concentrations at the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment (ALE/GAGE) observation sites there is very little difference resulting from the two distributions. However, the difference is apparent in major transport events such as pollution events. This has important implications for assessment of regional emissions from a sparse network like ALE/GAGE using inverse methods. It also implies that differences between modeled and observed CFC‐11 concentrations are due predominantly to model transport errors not model emission errors.
Modulation of equatorial subseasonal convective episodes by tropical‐extratropical interaction in the Indian and Pacific Ocean regions
Authors: Meehl Gerald A. , Kiladis George N. , Weickmann Klaus M. , Wheeler Matthew , Gutzler David S. , Compo Gilbert P.
Published: February 2004

Composite relationships among outgoing longwave radiation, sea level pressure, surface winds, and upper tropospheric circulation are examined for northern winter during subseasonal episodes of eastward progression of convection from the Indian Ocean to the western Pacific. This evolution often culminates with westerly wind burst events and strong air‐sea interaction associated with regional‐scale convective blowups in the western equatorial Pacific. We first document some of these interactions in the composites for two timescales, the submonthly (6–30 days) and that of the Madden‐Julian Oscillation (MJO) timescale (30–70 days). We then analyze the December 1992 period during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) to illustrate how these composite relationships are manifested in a case study. Convection in the Indian Ocean for the composites is shown to be associated with a northern hemisphere wave train at 200 mbar, arcing through the midlatitudes, that can contribute to convective blowups farther east on the submonthly (6–30 days) timescale in the Intertropical Convergence Zone (ITCZ) in the eastern Pacific. The eastern Asian trough that is part of this wave train is associated with pressure surges from the northern hemisphere and subsequent convection over Southeast Asia. As the MJO convective envelope moves east to Australasia, midlatitude wave trains in either hemisphere include upper level troughs east of Asia and Australia and pressure surges from either hemisphere that contribute to pressure rises over the Indonesian region and a subsequent shift of the convective envelope to the western Pacific. The vertical wind structure for the December 1992 case study is consistent with the composite surface and upper level winds and also shows strong vertical wind shear in the boundary layer, a sharply defined westerly maximum near 700 mbar and an intensification of the upper level easterlies near 100 mbar. Very deep westerlies (to 200 mbar) are confined to shorter timescales. The case study illustrates the various time and space scale interactions noted in the composites. Reciprocal interactions between the tropics and the midlatitudes on the submonthly and MJO timescales in both the composites and the case study involve pressure surges and wave interaction that influence subsequent convection as the convective envelope migrates eastward from the tropical Indian to Pacific Ocean region.
Algorithm for the estimation of vertical ozone profiles from the backscattered ultraviolet technique
Authors: Bhartia P. K. , McPeters R. D. , Mateer C. L. , Flynn L. E. , Wellemeyer C.
Published: February 2004

An implementation of the optimal estimation scheme to obtain vertical ozone profiles from satellite measurements of backscattered solar ultraviolet (buv) radiation is described. This algorithm (Version 6.0) has been used to produce a 15‐year data set of global ozone profiles from Nimbus 7 SBUV, NOAA 11 SBUV/2, and Space Shuttle SSBUV instruments. A detailed discussion of the information content of the measurement is presented. Using high vertical resolution ozone profiles from the SAGE II experiment as “truth” profiles, it is shown that the buv technique can capture short‐term variabilities of ozone in 5‐km vertical layers, between 0.3 mbar and 100 mbar, with a precision of 5–15%. However, outside the 1–20 mbar range, buv‐derived results are heavily influenced by a priori assumptions. To minimize this influence, it is recommended that the studies of long‐term trends using buv data be restricted to 1–20 mbar range. Outside this range, only the column amounts of ozone between 20 mbar and surface, and above 1 mbar, can be considered relatively free of a priori assumptions.
Thermal mesoscale circulations on the Baltic coast: 1. Numerical case study
Authors: Tjernström Michael , Grisogono Branko
Published: February 2004

A three‐dimensional mesoscale numerical model is utilized to investigate a well‐documented seabreeze event taken from experimental data. The simulations are not set up as a model validation. Instead, the model is driven by generalized and simplified initial and boundary conditions extracted from the field experiment data. The aim is to simulate the main characteristics, if not all the details, of the observed case. This generates a comprehensive set of fully three‐dimensional and time dependent data that can be used to obtain a more complete description of the case characteristics. Analyzing these data facilitates understanding of the flow mechanisms in more detail. Once such a control simulation is done, the case can be resimulated for slightly different forcing and the cause of observed features can be illuminated. Significant features in the simulation are the significant temperature difference between land and sea, moderate coastal terrain, and an almost 90° change in coastline orientation. Among the conclusions from the control simulation presented here is that the seabreeze stage is preceded by a coastal wind jet stage with a significant influence on the flow by the quite moderate topography. This is due to the ratio of the depth of the marine boundary layer to the terrain height; the terrain is acting on the flow in three ways. First, the stability impedes vertical motions and the terrain thus acts as a barrier inhibiting geostrophic balance. The flow is blocked and deflected down the synoptic‐scale pressure gradient. Second, it generates a perturbation in the temperature field causing local baroclinicity and a thermal wind. Finally, a slope flow is also generated that acts to enhance the temperature perturbation by advection of cold air to the coastal zone. A seabreeze evolves on both coastlines. The first forms on the offshore flow coast but remains stationary at the shoreline until the second, later, seabreeze on the almost perpendicular coastline, where the coastal jet dominates the flow well into the morning, moves inland and removes the offshore flow balancing the first seabreeze at the shoreline. After this obstruction is removed, the initial seabreeze rapidly propagates inland.
Wind and pressure fields near tropical cyclone Oliver derived from scatterometer observations
Authors: Hsu Carol S. , Liu W. Timothy
Published: February 2004

The results of this study demonstrate that the surface wind velocity and pressure fields derived from spaceborne scatterometers are useful in monitoring the location and intensity of tropical cyclones. Satellite‐borne microwave scatterometers can penetrate the cloudy core regions of tropical cyclones to resolve the circulation in detail over data sparse regions. The location of the cyclone observed by the ERS‐1 scatterometer is very close to that revealed in Geostationary Meteorological Satellite images. The surface winds provided by the ERS‐1 scatterometer are used here with a modified two‐layer planetary boundary layer model which includes effects of curvature, stability, and secondary flow to derive surface pressures near tropical cyclone Oliver. The curvature effect is found to be more significant than stability and secondary flow, which are crucial in deriving accurate surface pressure fields in midlatitudes.
On the relationship between seasonal occurrence of northern hemispheric polar mesosphere summer echoes and mean mesopause temperatures
Authors: Balsley Ben B. , Huaman Mercedes
Published: February 2004

The occurrence of polar mesospheric summer echoes (PMSE) has been closely associated with the seasonal cooling of the high‐latitude summer mesopause region wherein the echoes are embedded. Indeed, a “threshold” temperature has been suggested, with PMSE appearing as mesosphere temperatures fall below some critical value. A careful comparison between seasonally varying mean mesopause temperatures and PMSE occurrence statistics, however, suggests that the probability curve of PMSE occurrence is displaced from the seasonal curve of low summer mesopause temperatures by 1 to 2 weeks, with the temperature decrease leading the PMSE occurrence curve in springtime and the temperature increase leading the PMSE dropout in the fall. A similar displacement can also be seen in the occurrence statistics for polar mesospheric clouds (PMC), a closely related phenomenon observed by satellites. This lack of a direct correlation between mesopause temperature and PMSE occurrence suggests that PMSE occurrence is governed by more than just low mesospheric temperatures. The most likely possibility is that summertime mesospheric water vapor maximizes somewhat later than the minimum in mesospheric temperature, providing a more hospitable environment for PMSE generation. This possibility is supported by a model of the supersaturation region [Garcia, 1989], which shows a comparable lag between the seasonal mesospheric temperature minimum and the maximum in water vapor mixing ratio.
Comparative effects of migrating solar sources on tidal signatures in the middle and upper atmosphere
Authors: Hagan M. E.
Published: February 2004

A steady state two‐dimensional linearized model that extends from the ground into the thermosphere and captures the salient features of migrating diurnal and semidiurnal tidal components is used to investigate the comparative importance of the principal sources of these waves. The results, which have previously gone unreported in the literature, demonstrate the nonnegligible effects of atmospheric absorption of solar radiation at infrared (in the troposphere) and ultraviolet (in the stratosphere) wavelengths on mesospheric and lower thermospheric semidiurnal and diurnal tidal fields, respectively. In addition, latent heat release associated with cloudiness or rainfall in the troposphere is shown to be another plausible source of semidiurnal variability in the upper atmosphere. The important effects of these sources on the dynamics of the mesosphere and lower thermosphere emphasize the need to include realistic parameterizations of migrating tides at the lower boundaries of middle and upper atmospheric general circulation models. The results of this investigation also suggest that updated parameterizations of tropospheric tidal forcing are needed to further current understanding of tidal variability in the upper atmosphere.
Influence of environmental winds on propagation and motion of thunderstorms in northern Greece
Authors: Sioutas Michalis V. , Flocas Helena A.
Published: February 2004

The prediction of propagation and motion of thunderstorm activity, especially when heavy rainfall accumulation amounts could occur, is a matter of great importance. The objective of this study is to examine the possibility of using the environmental wind data for the problem of assessing predictability of the propagation and movement of thunderstorms in northern Greece. The propagation of radar thunderstorm echoes (radar echo reflectivity maxima) and cell movement (fine radar echo patterns) were determined for the spring and summer thunderstorms in 1992 and 1993 (April to September). Radar data were used by examining and analyzing digitally recorded plan position indicator for the entire time that the radar was operated. Multicell and single‐cell thunderstorms were identified and thunderstorm propagation and cell motion were calculated for each. Environmental winds at standard levels of 850, 700, and 500 hPa were considered and, additionally, a mean 0‐ to 6‐km layer density‐weighted wind was also examined. In northern Greece, winds and radar thunderstorm echoes rarely moved from the clockwise sector 070° to 220° (ENE‐SW). The movement of thunderstorm cells was not only steered by the 700‐hPa level wind but was also well represented by the wind at the 500‐hPa level and by the average wind in the layer 0–6 km. Using standard levels, cell motion can be determined by adding 5° to 500‐hPa wind direction and reducing the 500‐hPa wind speed by 30%. With the use of the mean 0‐ to 6‐km density‐weighted wind, cell motion can be represented by adding 5° to the 0‐ to 6‐km wind direction and increasing 35% the 0‐ to 6‐km wind speed. The propagation of multicell and single‐cell thunderstorm echoes was very similar, in spite of initial expectations, and was approximately equal to the cell motion, suggesting that the new cells grew on all sides of existing multicell thunderstorms. The majority of northern Greece thunderstorms do not propagate significantly and their motion is substantially translational and similar to cell motion. However, in cases of severe thunderstorms, propagation was indicated. Characteristic cases of storm propagation are presented and vertical wind shear is investigated as a particularly important factor in influencing storm structure and evolution and the resulting storm propagation.
Fertilization practices and soil variations control nitrogen oxide emissions from tropical sugar cane
Authors: Matson P. A. , Billow C. , Hall S. , Zachariassen J.
Published: February 2004

Nitrogen (N) fertilization of agricultural systems is thought to be a major source of the increase in atmospheric N2O; NO emissions from soils have also been shown to increase due to N fertilization. While N fertilizer use is increasing rapidly in the developing world and in the tropics, nearly all of our information on gas emissions is derived from studies of temperate zone agriculture. Using chambers, we measured fluxes of N2O and NO following urea fertilization in tropical sugar cane systems growing on several soil types in the Hawaiian Islands, United States. On the island of Maui, where urea is applied in irrigation lines and soils are mollisols and inceptisols, N2O fluxes were elevated for a week or less after fertilization; maximum average fluxes were typically less than 30 ng cm−2 h−1. NO fluxes were often an order of magnitude less than N2O. Together, N2O and NO represented from 0.03 to 0.5% of the applied N. In fields on the island of Hawaii, where urea is broadcast on the surface and soils are andisols, N2O fluxes were similar in magnitude to Maui but remained elevated for much longer periods after fertilization. NO emissions were 2–5 times higher than N2O through most of the sampling periods. Together the gas losses represented approximately 1.1–2.5% of the applied N. Laboratory studies indicate that denitrification is a critical source of N2O in Maui, but that nitrification is more important in Hawaii. Experimental studies suggest that differences in the pattern of N2O/NO and the processes producing them are a result of both carbon availability and placement of fertilizer and that the more information‐intensive fertilizer management practice results in lower emissions.
Lightning charge analyses in small Convection and Precipitation Electrification (CaPE) experiment storms
Authors: Murphy Martin J. , Krider E. Philip , Maier Michael W.
Published: February 2004

A least squares method for analyzing multiple, ground‐based measurements of electric field changes produced by lightning has been applied to seven small thunderstorms that occurred on July 19 and August 9, 1991, during the Convection and Precipitation Electrification (CaPE) experiment. Two of the storms produced little or no cloud‐to‐ground (CG) lightning, and a third produced CG lightning only during its early stages. A total of 79 flashes were analyzed on July 19 and 315 on August 9. About 58% of these discharges could be fitted to either a point charge (Q) or a point dipole (P) model, and in this sample, the spatial pattern of the Q and P solutions was consistent with a tripole or double dipole charge pattern in the cloud. In cases where there was little or no CG lightning, the inferred region of upper positive charge was quite close to the inferred negative region. Comparisons of the locations of upper P solutions with measurements of radar reflectivity at S band show that the P solutions tended to cluster at altitudes where the reflectivity was between 25 and 35 dBZ. Comparisons of Q (and P) model solutions with the locations of CG flash strike points, determined using a network of wideband direction finders, showed an average horizontal displacement of 3.9 km with a standard deviation of 3.3 km. This displacement is consistent with the expected random development of a stepped‐leader channel from the altitude of the negative Q region to the ground.
Seasonal variation of the global electrical circuit
Authors: Adlerman Edwin J. , Williams Earle R.
Published: February 2004

The effects of boundary layer aerosol particles on the electric field measurement of the DC global circuit are considered. Aitken (condensation) nuclei concentrations are found to have systematic local seasonal variations which obscure the global behavior of the DC circuit. These local variations appear to be the result of several seasonal factors, including variations in atmospheric mixed layer heights, variations in the productions rates of anthropogenic aerosols, and variations in surface wind speed. Air‐Earth conduction current measurements made by W. Cobb at Mauna Loa (1977–1983), a site remote from sources of pollution and mostly above the boundary layer, appear to be relatively free of aerosol particle effects. The Mauna Loa data are examined and the air‐Earth current is found to peak in the northern hemisphere summer, consistent with the peak of the global thunderstorm activity in the same season. A reanalysis of the entire Carnegie and Maud ocean data set as well as ongoing Schumann resonance results support this finding. However, the general absence of a distinct semiannual signal remains unresolved.
Simulation with the regional climate model RegCM2 of extremely anomalous precipitation during the 1991 east Asian flood: An evaluation study
Authors: Liu Yongqiang , Avissar Roni , Giorgi Filippo
Published: February 2004

One of the potential applications of the regional climate model RegCM2 developed at the National Center for Atmospheric Research (NCAR) is to analyze and possibly forecast high‐resolution spatial features of anomalous, short‐term climate variations. To evaluate the ability of RegCM2 to predict anomalous precipitation, a simulation of the east Asian flood that occured during May through July 1991 was performed. This simulation was driven by large‐scale atmospheric background derived from the European Center for Medium‐Range Weather Forecast (ECMWF) analysis. We found that RegCM2 is capable of reproducing an anomalous rainbelt in the flood region from the Yangtze‐Huai River Basin (eastern China) to southern Japan, and to produce local, strong rainfall centers within the belt. It reasonably well simulates the spatial patterns of the circulations, the anomalous early leap of the summer monsoon to the flood region, and the stable maintenance of the summer monsoon, which directly results in the formation of the flood. One of the major deficiencies in the model is its overestimation of precipitation in the rainfall centers, by about 30%, as compared to the analysis. Another deficiency of the model is the shift northward of the simulated rainbelt near Japan, resulting in a significant bias of rainfall over Japan. In addition, the ending of the flood is not well predicted by the model. Our analysis indicates that a bias pattern in the simulated circulations, which is dependent upon the Earth's surface type (i.e., land or ocean), is possibly the major reason for these deficiencies.
Operational relationships between NOAA‐advanced very high resolution radiometer vegetation indices and daily fraction of absorbed photosynthetically active radiation, established for Sahelian vegetation canopies
Authors: Bégué A. , Myneni R.
Published: February 2004

To improve the estimation of primary production at a regional scale, an assessment of the utility of fraction of absorbed photosynthetically active radiation (fAPAR) estimated from spectral vegetation indices (VI) for the case of the Sahelian vegetation was studied. Simulations using a three‐dimensional radiative transfer model were conducted for two types of structurally distinct vegetation canopies: millet crop (regularly distributed clumps of vegetation over bare soil) and savanna (mixture of a grass layer and a sparse woody layer). A realistic range of values is extracted for each vegetation input variable (leaf area index, ground cover, height and spatial distribution) from published literature. Bidirectional reflectance factors were calculated in the NOAA‐advanced very high resolution radiometer (AVHRR) spectral bands for a geometric configuration representative of the NOAA satellite series. Two vegetation indices were tested: normalized difference vegetation index (NDVI) and modified soil adjusted vegetation index (MSAVI). The simulations indicate that the fAPAR‐VI relationship is sensitive to the geometry of measurement and soil optical properties, especially in the case of a millet crop. Simple linear models that include this variability and are directly applicable to atmospherically corrected AVHRR data are proposed, and the error of estimation of fAPAR is evaluated. MSAVI gives better results, especially when the brightness of soil is known. For natural vegetation the relationship between fAPAR and MSAVI is then reduced to a coefficient that is a constant (∼2.2). For millet canopies this coefficient is a function of Sun and view angles. Finally, the utility of our approach is illustrated with two examples of fAPAR‐VI relationships applied to a NOAA time series.
Intense extratropical northern hemisphere winter cyclone events: 1899–1991
Authors: Lambert Steven J.
Published: February 2004

The climatology of intense northern hemisphere winter cyclone events is extracted from a long series of daily mean sea level pressure analyses. An intense event is the occurrence of a low with a central pressure less than or equal to 970 mbar. The majority of intense events are found near the “semipermanent” Aleutian and Icelandic Lows. Compared with the Pacific, intense events are more numerous in the Atlantic and occur over a larger geographical area. There is little or no trend in the number of intense events before about 1970. After 1970, there is a noticeable increase in the number of Pacific and Atlantic events. There is a weak out‐of‐phase relationship between the Pacific and the Atlantic events, with an indication of a 25‐year periodicity. Temporal correlations between the number of intense Pacific events and sea surface temperatures (SST) suggest a relationship between SST gradients averaged between the equator and midlatitudes and cyclone behavior. Weak correlations suggest that a similar relationship does not exist for the intense events in the Atlantic.
Initial results from simultaneous observation of X‐rays and electric fields in a thunderstorm
Authors: Eack Kenneth B. , Beasley William H. , Rust W. David , Marshall Thomas C. , Stolzenburg Maribeth
Published: February 2004

With an X ray detector designed for flight on a free balloon, we obtained a sounding of X ray intensity and electric‐field strength in a mesoscale convective system (MCS) near Norman, Oklahoma, in the spring of 1995. The balloon passed through a region of high electric field strength, at which time an increase in X ray intensity of about 2 orders of magnitude occurred, lasting for approximately 1 min. The X ray intensity returned to background levels at the time of a lightning flash that reduced the electric field strength measured at the balloon. This observation suggests that the production mechanism for the X rays we observed is related to the storm electric field and not necessarily to lightning discharge processes.
The surface energy and mass balance at Halley, Antarctica during winter
Authors: King J. C. , Anderson P. S. , Smith M. C. , Mobbs S. D.
Published: February 2004

We present measurements of the components of the energy and mass balance of the snow surface at Halley Research Station, Antarctica. During the winter months, when insolation is small or zero, the surface energy balance is dominated by radiative cooling. This is mostly balanced by a downward transport of atmospheric sensible heat, with an upward conductive flux of heat through the snowpack making a secondary contribution. The average flux of atmospheric latent heat is downward but of negligible importance in the surface energy balance. During the winter, a significant imbalance is seen in the measured energy budget, with insufficient sensible and conductive heat fluxes to balance the radiative cooling. The wintertime surface mass balance is dominated by precipitation. Sublimation of blowing snow makes a small negative contribution to the budget and is observed to be highly dependent on wind speed. It is suggested that this may be an important mechanism for removing surface mass in some parts of Antarctica.
Changes in atmospheric lead and other pollution elements at Bermuda
Authors: Huang Suilou , Arimoto Richard , Rahn Kenneth A.
Published: February 2004

Measurements of atmospheric lead at Bermuda during the Atmosphere‐Ocean Chemistry Experiments (AEROCE) in 1993–1994 showed that annual‐mean concentrations had decreased by an order of magnitude from the 1970s and by a factor of 4 since the 1980s. Seasonal patterns had changed as well, with lead no longer being highest during winter. Both changes are consistent with decreased use of leaded gasoline in North America. Pollution‐derived zinc and antimony also decreased, probably because of reduced smelting in the United States or changed atmospheric transport to Bermuda. Lead/aluminum mass ratios depended on direction: 0.04 with western air‐mass trajectories (pollution from North America) versus 0.0001 and 0.0003 with eastern trajectories. The eastern Pb/Al ratios were indistinguishable from typical crustal values of 0.0002. The lower eastern ratio probably represents pure Saharan dust, while the higher ratio may indicate minor amounts of superimposed pollution aerosol, possibly from Europe or the Mediterranean area. Crustal lead was not evident in the 1970s because more lead was emitted from gasoline and dust transport from the Sahara was weaker.
Annual and seasonal air temperature trend patterns of climate change and urbanization effects in relation to air pollutants in Turkey
Authors: Tayanç Mete , Karaca Mehmet , Yenigün Orhan
Published: February 2004

With a view to estimating climate change and ifs urban‐induced bias in selected Turkish cities, we have used data from the period 1951 to 1990 recorded by 54 climate stations, four of which are corrected for their inhomogeneities. Two sets are produced; S1, including the large urban stations, and S2, consisting of rural, small urban and medium urban stations. Normalized Kendall trend test coefficients with a spatial prediction scheme, kriging, are used to construct spatial patterns of both sets together and separately. Results reveal a statistically significant cooling in mean temperatures mostly in northern regions and warming in minimum temperatures specific to large urban areas. Seasonal analysis shows that most of this cooling has been occurring in the summer and urban warming in the spring. The causes of cooling is investigated in relation to some air pollutants, SO2 and particulate matter (PM). Linear regressions performed on the time series resulted in a significant urban bias of 0.24°C per 40 years on mean temperatures and 0.56°C/40 years on minimum temperatures. In association with the above results, a decrease in the temperature range of 0.48°C over the period owing to urban bias was found. A 0.24°C urban bias magnitude of mean temperature trends is much greater than the results found on other three regions of the Earth [Jones et al., 1990]. An overall average cooling in mean temperatures, −0.07°C per decade, detected here is the same as Nasrallah and Balling's [1993] average result for the two grid points located over Turkey.
Temporal and spatial subsampling errors for global empirical orthogonal functions: Application to surface temperature field
Authors: Kim Kwang‐Y.
Published: February 2004

In this paper, spatial and temporal subsampling errors in estimating empirical orthogonal functions (EOFs) and the corresponding eigenvalues are studied. The subsampling errors are measured in terms of the minimum difference between an estimated eigenvalue and an exact one, and the minimum Euclidean norm of distance between an estimated eigenfunction and a linear space spanned by a certain number of exact eigenfunctions. This linear space represents a set of exact EOFs which admix into an estimated EOF. The extent of modal mixing, or variance split, is determined by the proximity of adjacent eigenvalues and the degree of subsampling error, and is estimated using one of the theorems of a posteriori error estimates for the eigenvalues and the eigenfunctions. Such a representation of the EOF errors allows a systematic approach to the sampling problems of EOFs. This methodology is applied to the surface air temperature field, in which errors in computing the EOFs and the corresponding eigenvalues are measured in terms of the sampling insufficiency spatially and temporally. To this end, different configurations of spatial sampling points, and different lengths of the temperature record have been used. This study addresses the spatial and temporal “sampling theorem” for the EOFs and the eigenvalues of the global surface temperature field.
Origin of Bermuda's clay‐rich Quaternary paleosols and their paleoclimatic significance
Authors: Herwitz Stanley R. , Muhs Daniel R. , Prospero Joseph M. , Mahan Shannon , Vaughn Bruce
Published: February 2004

Red clayey paleosols that are chiefly the product of aerosolic dust deposition are interbedded in the Quaternary carbonate formations of the Bermuda oceanic island system. These paleosols provide a basis for reconstructing Quaternary atmospheric circulation patterns in the northwestern Atlantic. Geochemical analyses were performed on representative paleosol samples to identify their parent dust source. Fine‐grained fractions were analyzed by energy‐dispersive X ray fluorescence to determine trace element (Zr, Y, La, Ti, and Nb) concentrations and to derive geochemical signatures based on immobile element ratios. These ratios were compared with geochemical signatures determined for three possible sources of airborne dust: (1) Great Plains loess, (2) Mississippi River Valley loess, and (3) Saharan dust. The Zr/Y and Zr/La ratios provided the clearest distinction between the hypothesized dust sources. The low ratios in the paleosol B horizons most closely resemble Saharan dust in the <2‐μm size class fraction. Contributions from the two North American loessial source areas could not be clearly detected. Thus Bermuda paleosols have a predominantly Saharan aerosolic dust signature. Saharan dust deposition on Bermuda during successive Quaternary glacial periods is consistent with patterns of general circulation models, which indicate that during glacial maxima the northeast summer trade winds were stronger than at present and reached latitudes higher than 30°N despite lower‐than‐present sea surface temperatures in the North Atlantic.