bluuebunny/crossref_metadata_2025 huggingface dataset:
Flux retrieval optimization with a nonscanner along‐track broadband radiometer
Authors: Bodas‐Salcedo A. , Gimeno‐Ferrer J. F. , López‐Baeza E.
Published: May 2003

The theoretical behavior of broadband flux retrieval is analyzed with the aid of a radiance field database. Several angular inversion models are developed for different viewing configurations, with special emphasis in a nonscanner along‐track design concept. Results show that the performance of these angular models depends on the viewing zenith angle (VZA). When cloud cover is predominant within the observed scene, VZAs around 55° (shortwave) and 50° (longwave) minimize the error when deriving fluxes. These results might help in the design of future missions where the main idea is to obtain accurate instantaneous measurements of top of atmosphere reflected and emitted fluxes. Such measurements can be used as a constraint to derive atmospheric profiles from active instruments (lidar and cloud profiling radar), onboard the same platform. For this purpose, along‐track configurations are very good candidates because the flux retrieval can be optimized by using different views of the same pixel almost simultaneously from the same platform.
Observational‐ and modeling‐based budget of lightning‐produced NOx in a continental thunderstorm
Authors: Skamarock W. C. , Dye J. E. , Defer E. , Barth M. C. , Stith J. L. , Ridley B. A. , Baumann K.
Published: May 2003

NOx transport and production by lightning for the 10 July 1996 Stratosphere‐Troposphere Experiment‐Radiation, Aerosols, and Ozone convective storm is examined using radar, in situ observations and cloud model simulations. Observations and model simulations indicate that most of the NOx produced by the storm was transported out into the anvil. The analyzed NOx flux into the anvil, combined with results from a cloud model simulation, indicate that approximately 60 percent of the NOx transported into the anvil during the observational period is produced by lightning. Lightning flash rate and channel length measurements, obtained using the Office National d'Etudes et de Recherche Arospatiales lightning interferometer, are combined with the NOx budget to give estimates of average lightning NOx production per interferometer flash and per unit flash channel length. The analysis yields production rates of approximately 43.2 moles (2.6 × 1025 molecules) NOx per interferometer flash and 1.7 × 10−3 moles (1.0 × 1021 molecules) NOx per meter of flash channel. These production rates fall within the bounds of rates derived in previous studies using completely different approaches.
Photoacoustic and filter‐based ambient aerosol light absorption measurements: Instrument comparisons and the role of relative humidity
Authors: Arnott W. P. , Moosmüller H. , Sheridan P. J. , Ogren J. A. , Raspet R. , Slaton W. V. , Hand J. L. , Kreidenweis S. M. , Collett J. L.
Published: May 2003

Ambient measurements are reported of aerosol light absorption from photoacoustic and filter‐based instruments (aethalometer and a particle soot absorption photometer (PSAP)) to provide insight on the measurement science. Measurements were obtained during the Big Bend Regional Aerosol and Visibility Observational Study at the Big Bend National Park in South Texas. The aethalometer measurements of black carbon concentration at this site correlate reasonably well with photoacoustic measurements of aerosol light absorption, with a slope of 8.1 m2/g and a small offset. Light absorption at this site never exceeded 2.1 Mm−1 during the month of collocated measurements. Measurements were also obtained, as a function of controlled relative humidity between 40% and 90%, during the Photoacoustic IOP in 2000 at the Department of Energy Southern Great Plains Cloud and Radiation Testbed site (SGP). PSAP measurements of aerosol light absorption correlated very well with photoacoustic measurements, but the slope of the correlation indicated the PSAP values were larger by a factor of 1.61. The photoacoustic measurements of light absorption exhibited a systematic decrease when the RH increased beyond 70%. This apparent decrease in light absorption with RH may be due to the contribution of mass transfer to the photoacoustic signal. Model results for the limiting case of full water saturation are used to evaluate this hypothesis. A second PSAP measured the light absorption for the same humidified samples, and indicated very erratic response as the RH changed, suggesting caution when interpreting PSAP data under conditions of rapid relative humidity change.
Last Glacial Maximum over China: Sensitivities of climate to paleovegetation and Tibetan ice sheet
Authors: Jiang Dabang , Wang Huijun , Drange Helge , Lang Xianmei
Published: May 2003

With the boundary conditions appropriate for the Last Glacial Maximum (LGM), including ice sheets, sea surface temperatures, sea‐ice distribution, atmospheric CO2 concentration, the Earth's orbital parameters, topography, and coastline, the atmospheric general circulation model of the Institute of Atmospheric Physics (IAP‐AGCM) computes colder and drier conditions than for present day. Global annual‐average surface temperature decreased by 5.3°C, and terrestrial precipitation was down by 29%. It is shown that IAP‐AGCM LGM simulation compares favorably to results from other AGCMs, and/but generally shows a weak terrestrial cooling when compared to paleoclimatic reconstructions in tropics. The 21 ka (ka: thousands of years ago) vegetation reconstruction is introduced into the model to study the regional climate response to the changes in vegetation and associated soil characteristics over China. The additional cooling due to these two changes reduces, to a certain degree, the model‐data discrepancies. In addition, under the precondition of continental ice existing over part of the Tibetan Plateau at the LGM, the authors examine the regional climate response to the continental ice. It follows that the glacial‐age environment over the Tibetan Plateau is a very important factor for 21 ka climate simulation in East Asia.
Quasibiennial oscillation in tropical ozone as revealed by ozonesonde and satellite data
Authors: Logan J. A. , Jones D. B. A. , Megretskaia I. A. , Oltmans S. J. , Johnson B. J. , Vömel H. , Randel W. J. , Kimani W. , Schmidlin F. J.
Published: May 2003

We present an analysis of the quasi‐biennial oscillation (QBO) in tropical 0ozone using recent in situ measurements made by ozonesondes, supplemented by satellite profile and column data. The first in situ equatorial ozone profiles reveal the dramatic change in shape of the profile that accompanies the descent of the westerly shear zone. The partial pressure maximum in ozone increases by ∼25% in 5–6 months as it descends from 17.5 to 24 hPa. The amplitude of the QBO anomaly that extends from 15 to 80 hPa is found to exceed ±20%, larger than indicated by earlier analyses of satellite data. The influence of the QBO on equatorial ozone is dominant between 10 and 45 hPa, but the seasonal cycle is more important below 50 hPa. The equatorial ozone anomalies are influenced by El Niño‐Southern Oscillation (ENSO) in the lowest part of the stratosphere. The ozone anomaly in the lower stratosphere at 20°S lags that at the equator by only a few months during the easterlies from 1994 to 1998, contrary to the previous picture of the subtropical and equatorial anomalies being out of phase. There is often a three‐cell structure in ozone anomalies at 20°N and 20°S, with the upper two related to the QBO and that below 50 hPa sometimes related to ENSO. We present an analysis of the contribution of the ozone anomalies as a function of altitude to the subtropical QBO in column ozone. There is a strong subtropical column anomaly (>5 Dobson units (DU)) when the anomalies above 20 hPa and from 50 to 20 hPa reinforce. There were four such cases at 20°N and at 20°S in 1985–1991, but five at 20°N and only one at 20°S in 1993–1999. About 70% of these cases are associated with strong shear at 25–35 hPa in late fall/early winter. There is a weak subtropical anomaly in column ozone when the ozone anomalies above and below 20 hPa are of opposite sign, or one of them is very weak. Over half of these cases are associated with strong wind shear in late fall/early winter in the middle stratosphere at 12.5 hPa. In the southern subtropics, there is strong shear at 12.5 hPa and a weak column ozone anomaly for 5 of 6 years from 1994 to 1999. Near 20°N the seasonal cycle contributes more to the ozone variance above 20 hPa than does the QBO, but the reverse appears true near 20°S. The seasonal cycle dominates the variance in ozone below 40 hPa at 20°S. The effects of ENSO are more important for ozone than those of the QBO below 60 hPa at 20°N.
Airborne measurements of carbonaceous aerosols in southern Africa during the dry biomass burning season
Authors: Kirchstetter Thomas W. , Novakov T. , Hobbs Peter V. , Magi Brian
Published: May 2003

Particulate matter collected aboard the University of Washington's (UW) Convair‐580 research aircraft over southern Africa during the dry biomass burning season was analyzed for total carbon (TC), organic carbon (OC), and black carbon (BC) contents using thermal and optical methods. Samples were collected in smoke plumes of burning savanna and in regional haze. A known artifact, produced by the adsorption of organic gases on the quartz filter substrates used to collect the particulate matter samples, comprised a significant portion of the TC collected. Consequently, conclusions derived from the data are greatly dependent on whether or not OC concentrations are corrected for this artifact. For example, the estimated aerosol coalbedo (1 − single scattering albedo (SSA)), which is a measure of aerosol absorption, of the biomass smoke samples is 60% larger using corrected OC concentrations. Thus, the corrected data imply that the biomass smoke is 60% more absorbing than do the uncorrected data. The BC to (corrected) OC mass ratio (BC/OC) of smoke plume samples (0.18 ± 0.06) is lower than that of samples collected in the regional haze (0.25 ± 0.08). The difference may be due to mixing of biomass smoke with background air characterized by a higher BC/OC ratio. A simple source apportionment indicates that biomass smoke contributes about three quarters of the aerosol burden in the regional haze, while other sources (e.g., fossil fuel burning) contribute the remainder.
Oxygen‐18 concentrations in recent precipitation and ice cores on the Tibetan Plateau
Authors: Tian L. , Yao T. , Schuster P. F. , White J. W. C. , Ichiyanagi K. , Pendall E. , Pu J. , Yu W.
Published: May 2003

A detailed study of the climatic significance of δ18O in precipitation was completed on a 1500 km southwest‐northeast transect of the Tibetan Plateau in central Asia. Precipitation samples were collected at four meteorological stations for up to 9 years. This study shows that the gradual impact of monsoon precipitation affects the spatial variation of δ18O‐T relationship along the transect. Strong monsoon activity in the southern Tibetan Plateau results in high precipitation rates and more depleted heavy isotopes. This depletion mechanism is described as a precipitation “amount effect” and results in a poor δ18O‐T relationship at both seasonal and annual scales. In the middle of the Tibetan Plateau, the effects of the monsoon are diminished but continue to cause a reduced correlation of δ18O and temperature at the annual scale. At the monthly scale, however, a significant δ18O‐T relationship does exist. To the north of the Tibetan Plateau beyond the extent of the effects of monsoon precipitation, δ18O in precipitation shows a strong temperature dependence. δ18O records from two shallow ice cores and historic air temperature data were compared to verify the modern δ18O‐T relationship. δ18O in Dunde ice core was positively correlated with air temperature from a nearby meteorological station in the north of the plateau. The δ18O variation in an ice core from the southern Plateau, however, was inversely correlated with precipitation amount at a nearby meteorological station and also the accumulation record in the ice core. The long‐term variation of δ18O in the ice core record in the monsoon regions of the southern Tibetan Plateau suggest past monsoon seasons were probably more expansive. It is still unclear, however, how changes in large‐scale atmosphere circulation might influence summer monsoon precipitation on the Tibetan Plateau.
Dependence of diffuse photosynthetically active solar irradiance on total optical depth
Authors: Cho Hi Ku , Jeong Myeong J. , Kim Jhoon , Kim Young J.
Published: May 2003

Photosynthetically active solar irradiance (PAR, 400–700 nm) data measured with a rotating shadowband radiometer at Yonsei University, Seoul (37.57°N, 126.97°E) from 1996 through 1997 have been analyzed to investigate statistical features in components of the horizontal PAR and the dependence of the diffuse PAR on total optical depth (TOD) for visible range. Based on our measurements, it is found that the seasonal and sky conditional percentages of the global PAR for the direct and diffuse components depend considerably on the magnitude of the TOD. Annual mean diffuse PAR at Seoul appears to be 51% of the global‐horizontal PAR (5.12 ± 1.27 M J m−2 d−1) for all‐sky conditions with a TOD of 1.02 ± 0.32. However, the diffuse PAR are 39% of the global PAR (6.10 ± 2.27 M J m−2 d−1) under clear‐sky conditions (cloud amount ≤ 2/10) with a TOD of 0.47 ± 0.23 and 73% of the global PAR (2.99 ± 2.07 M J m−2 d−1) under cloudy sky conditions (cloud amount ≥ 8/10) with a TOD of 1.80 ± 0.45, respectively. Simple statistical models are presented for the determination of direct and diffuse‐horizontal PAR as a function of TOD, which is fitted by a curvilinear regression at a given solar zenith angle (SZA). Overall, there was reasonable agreement between predicted and measured values of two PAR components. On the basis of the statistical models for direct and diffuse PAR, we found that the four parameters of the maximum diffuse PAR, the TOD corresponding to the maximum diffuse PAR, the PAR and TOD when direct and diffuse PAR have the same value, decrease all with increasing SZA. It is of interest to note that these all four parameters can be expressed as simple linear functions of SZA.
Determination of land surface temperature and soil moisture from Tropical Rainfall Measuring Mission/Microwave Imager remote sensing data
Authors: Wen Jun , Su Zhongbo , Ma Yaoming
Published: May 2003

An analytical algorithm for the determination of land surface temperature and soil moisture from the Tropical Rainfall Measuring Mission/Microwave Imager (TRMM/TMI) remote sensing data has been developed in this study. The error analyses indicate that the uncertainties of the enrolled parameters will not cause serious errors in the proposed algorithm. By applying the proposed algorithm to TRMM/TMI remote sensing data collected during the Global Energy and Water Experiment (GEWEX) Asian Monsoon Experiment (GAME)/Tibet Intensive Observation Period field campaign in 1998 (IOP'98), the temporal and regional distributions of land surface temperature and volumetric soil moisture are evaluated over the central Tibetan plateau area. To validate the proposed method, the ground‐measured surface temperature and volumetric soil moisture are compared to TRMM/TMI‐derived land surface temperature and soil Fresnel reflectivity respectively. The results show that the estimated surface temperature is in good agreement with ground measurements; their difference and correlation coefficient are 0.52 ± 2.41 K and 0.80, respectively. A quasi‐linear relationship exists between estimated Fresnel reflectivity and ground‐measured volumetric soil moisture with a correlation coefficient 0.82. The land surface thermal status can also be clearly identified from the regional distribution of the estimated land surface temperature; the mountainous area and water bodies have a very lower surface temperature, while the river basin shows a higher surface temperature compared to the mountainous area. The southeastern part of the selected area has lower soil moisture, while the river basin exhibits high soil moisture. It is therefore concluded that the proposed algorithm is successful for the retrieval of land surface temperature and soil moisture using TRMM/TMI data over the study area.
Preprocessing of total ozone content as an input parameter to UV Index forecast calculations
Authors: Schmalwieser Alois W. , Schauberger Günther , Weihs Philipp , Stubi Rene , Janouch Michal , Coetzee Gerrie J. R. , Simic Stana
Published: May 2003

In this paper we present a simple method to correct satellite based measurements of the total ozone content of the atmosphere (TOC) to be valid without any spatial restriction. The method delivers also a global data set of TOC, which can be used for the operational forecast of the erythemally effective ultraviolet (UV) irradiance, in the form of the UV Index. The method is based on TOC measurements made by NASA's Earth Probe TOMS. Corrections are applied to gain TOC values that are in good agreement with ground‐based measurements. The algorithm of Kriging is applied for interpolation and for filling up gaps in grids. This method is validated by a comparison to measurements of TOC from the ground made at 5 observatories between 50°N and 30°S and between 285 m and 3106 m asl. From an auto‐correlation analysis with different time lags it is shown that the assumption of TOC persistency of a 2‐day period guarantees a quality that is convenient with the requirements of UV Index forecast procedures. For a wide zonal range covering 34% of the Earth's surface only a small improvement can be made by more sophisticated TOC forecast models, since uncertainties of our scheme are already on the order of the intrinsic uncertainties of the measuring devices.
Seasonal changes in the transport of pollutants into the Arctic troposphere‐model study
Authors: Klonecki A. , Hess P. , Emmons L. , Smith L. , Orlando J. , Blake D.
Published: May 2003

In association with Tropospheric Ozone Production about the Spring Equinox (TOPSE) measurement campaign a regional episodic chemical transport model is used to study the seasonal mechanisms of transport of pollutants from their Northern Hemisphere emission regions into the remainder of the troposphere. The model simulates the strong seasonal cycle for CO and hydrocarbons that agrees well with TOPSE measurements. In this study we use the isentropic perspective to analyze transport during different seasons and from different emission regions. Simulations with diagnostic tracers are conducted to quantify (1) the contribution of different emission regions to tracer distribution and (2) the importance of cross‐isentropic transport. In the high latitudes during winter, in agreement with previous studies, the European and Siberian emission sources are the largest contributors to the diagnostic tracer distributions in the lower troposphere due to large‐scale circulation patterns, low temperatures at the source and presence of a cold stable boundary layer that facilitates diabatic cooling. North American emissions are located further south, often south of the polar front, and are on average emitted at higher potential temperatures. Owing to their predominant transport over the relatively warm Atlantic, they experience strong diabatic heating due to the general instability in the air column and heavy precipitation in the storm track. During the summer months the pollution from all emission regions is more likely to be diabatically transported to higher potential temperatures and diluted. In addition, direct transport of pollutants into the Arctic is less frequent during summer due to the differences in the large‐scale circulation patterns.
Measured size distributions of aerosols over the Indian Ocean during INDOEX
Authors: Kamra A. K. , Murugavel P. , Pawar S. D.
Published: May 2003

The submicron aerosol number size distributions are measured over the Indian Ocean (15°N, 77°E to 20°S, 58°E) during the Indian Ocean Experiment (INDOEX) in winter monsoon seasons of 1998 and 1999. Large concentrations of 4 × 102 and 104 cm−3 are observed with comparatively much higher values toward north than south of the Intertropical Convergence Zone (ITCZ). The nucleation mode in the aerosol size distributions shows large variability, especially north of the ITCZ. The monomodal size distributions of particles observed close to coastline gradually become more and more organized into bimodal size distributions as one proceeds from north to south. The shift of the position of the minimum in size distributions to larger particle size as one proceeds southward suggests the growth of nucleation mode particles by homogeneous or heterogeneous processes. Observation of the north‐to‐south increase in the accumulation mode particles indicates the removal of these particles by sedimentation or cloud droplet nucleation and precipitation scavenging as the aerosol particles are advected southward. The bimodal size distributions mostly show a maximum at 133 nm and a minimum at 42 nm and are generally open‐ended at the small‐size end, indicating a maximum for ultrafine particles of <13 nm diameter. There are strong indications that the nucleation mode particles formed in the free troposphere by the gas‐to‐particle conversion processes in the outflow regions of clouds of the convective cells are transported down to the marine boundary layer in the descending leg of Hadley cell in the Northern Hemisphere and then advected southward along with the continental aerosols with the persistent northeasterly surface winds in this season. A high peak of 13 nm particles exceeding 103 particles cm−3 is associated with the mesoscale subsidence of a depression.
Diffuse fraction of UV radiation under partly cloudy skies as defined by the Automated Surface Observation System (ASOS)
Authors: Grant Richard H. , Gao W.
Published: May 2003

A major limitation in predicting the ultraviolet‐B (UVB) irradiance on organisms is the difficulty in determining the UVB under partly cloudy sky conditions. This study developed models for estimating the diffuse fraction of ultraviolet‐A (UVA) and UVB radiation under partly cloudy skies based on National Weather Service (NWS) cloud cover measurements and Department of Agriculture (USDA) UVB Radiation Monitoring and Research Network UV radiation measurements made from 1997 through 1999 at nine locations in the United States. The UVB diffuse fraction and the difference between clear and partly cloudy sky diffuse fraction were empirically modeled as a function of solar zenith angle and cloud fraction with a resulting mean bias error (MBE) of 0.019 and 0.014, respectively, and a root mean squared error (RMSE) of 0.069 and 0.079, respectively. A semiempirical model was also evaluated where the diffuse fraction was treated as the summation of clear and overcast sky diffuse fractions, respectively, weighted by the probability of the Sun's direct beam being obstructed or not for a given cloud cover fraction. This model MBE was less than 0.01 of both the UVA and UVB wave bands, while the RMSE was 0.035 within the UVB wave band and −0.030 in the UVA wave band. Model errors were greatest for low solar zenith angles and high cloud fractions in the UVB. The cloud fraction measurements used in these models only include clouds up to 3.6 km. Consequently, they should only be used when the cloud information has the same characteristics.
Tunable diode laser measurements of formaldehyde during the TOPSE 2000 study: Distributions, trends, and model comparisons
Authors: Fried Alan , Wang Yuhang , Cantrell Chris , Wert Bryan , Walega James , Ridley Brian , Atlas Elliot , Shetter Rick , Lefer Barry , Coffey M. T. , Hannigan Jim , Blake Donald , Blake Nicola , Meinardi Simone , Talbot Bob , Dibb Jack , Scheuer Eric , Wingenter Oliver , Snow Julie , Heikes Brian , Ehhalt Dieter
Published: May 2003

Airborne measurements of formaldehyde (CH2O) were acquired employing tunable diode laser absorption spectroscopy (TDLAS) during the 2000 Tropospheric Ozone Production About the Spring Equinox (TOPSE) study. This study consisted of seven deployments spanning the time period from 4 February to 23 May 2000 and covered a wide latitudinal band from 40°N to 85°N. The median measured CH2O concentrations, with a few exceptions, did not show any clear temporal trends from February to May in each of five altitude and three latitude bins examined. Detailed measurement–model comparisons were carried out using a variety of approaches employing two different steady state models. Because recent emissions of CH2O and/or its precursors often result in model underpredictions, background conditions were identified using a number of chemical tracers. For background conditions at temperatures warmer than −45°C, the measurement–model agreement on average ranged between −13% and +5% (measurement–model/measurement), which corresponded to mean and median (measurement–model) differences of 3 ± 69 and −6 parts per trillion by volume (pptv), respectively. At very low temperatures starting at around −45°C, significant and persistent (measurement–model) differences were observed from February to early April from southern Canada to the Arctic Ocean in the 6–8 km altitude range. In such cases, measured CH2O was as much as 392 pptv higher than modeled, and the median difference was 132 pptv (83%). Low light conditions as well as cold temperatures may be important in this effect. A number of possible mechanisms involving the reaction of CH3O2 with HO2 to produce CH2O directly were investigated, but in each case the discrepancy was only minimally reduced. Other possibilities were also considered but in each case there was no compelling evidence to support any of the hypotheses. Whatever the cause, the elevated CH2O concentrations significantly impact upper tropospheric HOx levels at high latitudes (>57°N) in the February–April time frame.
Calculation of chemical ozone loss in the Arctic winter 1996–1997 using ozone‐tracer correlations: Comparison of Improved Limb Atmospheric Spectrometer (ILAS) and Halogen Occultation Experiment (HALOE) results
Authors: Tilmes Simone , Müller Rolf , Grooß Jens‐Uwe , McKenna Daniel S. , Russell James M. , Sasano Yasuhiro
Published: May 2003

The ozone‐tracer correlation method is used to deduce the stratospheric ozone loss in the Arctic winter 1996–1997. Improvements of the technique are applied, such as a new calculation of the vortex edge [Nash et al., 1996] and an improved early vortex reference function. Winter 1996–1997 is characterized by a late formation and an unusually long lifetime of the polar vortex. Remnants of vortex air were found until May. Chemical ozone losses deduced from two satellite data sets, namely Improved Limb Atmospheric Spectrometer (ILAS) and Halogen Occultation Experiment (HALOE), are discussed. The ILAS observations allow a detailed analysis of the temporal evolution of the ozone‐tracer correlation inside the polar vortex and, in particular, of the development of the early vortex. For November and December 1996, it is shown that horizontal mixing still influences the ozone‐tracer relation. Significant PSC related chemical ozone loss occurred beginning at mid‐February, and the averaged column ozone loss is increasing toward the middle of May. From April onwards, ozone profiles in the vortex became more uniform. The decrease of ozone in the vortex remnants in April and May occurred due to chemistry. HALOE observations are available for March to May 1997. In the period 4–16 March 1997, the calculated ozone loss deduced from HALOE and ILAS is in good agreement. The average of the result from the two instruments is 15 ± 7 Dobson units (DU) inside the vortex core, in the altitude range of 450–550 K. At the end of March, a discrepancy between HALOE and ILAS ozone loss arises due to a significant difference (0.6 ppmv) between the two data sets in the relatively low ozone minimum measured at 475 K. Nonetheless, both data sets consistently show an inhomogeneity in ozone loss inside the vortex core at the end of March. The vortex is separated in two parts, one with a large ozone loss (HALOE 40–45 DU, ILAS 30–35 DU) and one with a moderate ozone loss (HALOE 15–30 DU, ILAS 5–25 DU) for 450–550 K. The ozone loss from HALOE in 380–550 K at that time was calculated to be 90–110 DU for the large ozone loss and 20–80 DU for the moderate ozone loss. The vortex average of column ozone loss from HALOE inside the vortex core at the end of March is 61 ± 20 DU, which is an increase of about 20% compared to the earlier study by Müller et al. [1997b] brought about by the improvement of the technique.
Dispersion and chemical evolution of ship plumes in the marine boundary layer: Investigation of O3/NOy/HOx chemistry
Authors: Song C. H. , Chen G. , Hanna S. R. , Crawford J. , Davis D. D.
Published: May 2003

The dispersion and chemical evolution of NOx in ship plumes has been investigated for marine boundary layer (MBL) conditions. This effort has involved combining a plume dispersion algorithm with a time‐dependent photochemical box model. The analysis has considered several factors, all of which can influence the net impact of NOx on the background environment. These include the following: season of the year, latitude of point of release, meteorological setting, and ship NOx emission rate. Reaction rates within a plume were shown to be a nonlinear function of the levels of NOx, leading to relative estimates of ship plume NOx lifetimes that were factors of 2.5–10 times shorter than for ambient marine conditions. The shortened ship‐plume NOx lifetime reflects both elevated daytime levels of OH and nighttime levels of NO3 and N2O5, all of which were estimated to be several times larger than those typical of ambient marine conditions. During daylight hours, elevated ship plume OH resulted in the net photochemical production of O3, with peak concentrations being 5–65% higher than background values, depending on latitude. The areal integrated O3 effect, however, is estimated to be quite small due to further plume dilution. In addition, because of the shorter estimated lifetime for NOx, it would seem reasonable that the integrated O3 production from the current Lagrangian modeling effort would be significantly lower than that predicted by global 3‐D grid models. The current predicted shortened lifetime for NOx is quite significant in terms of assessing a ship plume's impact on background marine levels of NOx. In fact, these results would seem to explain a significant fraction of the overprediction of NOx levels in and near shipping lanes recently estimated using 3‐D Eulerian global models.
Springtime photochemistry at northern mid and high latitudes
Authors: Wang Yuhang , Ridley Brian , Fried Alan , Cantrell Christopher , Davis Douglas , Chen Gao , Snow Julie , Heikes Brian , Talbot Robert , Dibb Jack , Flocke Frank , Weinheimer Andrew , Blake Nicola , Blake Donald , Shetter Richard , Lefer Barry , Atlas Elliot , Coffey Michael , Walega Jim , Wert Brian
Published: May 2003

Physical and chemical properties of the atmosphere at 0–8 km were measured during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiments from February to May 2000 at mid (40°–60°N) and high latitudes (60°–80°N). The observations were analyzed using a diel steady state box model to examine HOx and O3 photochemistry during the spring transition period. The radical chemistry is driven primarily by photolysis of O3 and the subsequent reaction of O(1D) and H2O, the rate of which increases rapidly during spring. Unlike in other tropospheric experiments, observed H2O2 concentrations are a factor of 2–10 lower than those simulated by the model. The required scavenging timescale to reconcile the model overestimates shows a rapid seasonal decrease down to 0.5–1 day in May, which cannot be explained by known mechanisms. This loss of H2O2 implies a large loss of HOx resulting in decreases in O3 production (10–20%) and OH concentrations (20–30%). Photolysis of CH2O, either transported into the region or produced by unknown chemical pathways, appears to provide a significant HOx source at 6–8 km at high latitudes. The rapid increase of in situ O3 production in spring is fueled by concurrent increases of the primary HOx production and NO concentrations. Long‐lived reactive nitrogen species continue to accumulate at mid and high latitudes in spring. There is a net loss of NOx to HNO3 and PAN throughout the spring, suggesting that these long‐term NOx reservoirs do not provide a net source for NOx in the region. In situ O3 chemical loss is dominated by the reaction of O3 and HO2, and not that of O(1D) and H2O. At midlatitudes, there is net in situ chemical production of O3 from February to May. The lower free troposphere (1–4 km) is a region of significant net O3 production. The net production peaks in April coinciding with the observed peak of column O3 (0–8 km). The net in situ O3 production at midlatitudes can explain much of the observed column O3 increase, although it alone cannot explain the observed April maximum. In contrast, there is a net in situ O3 loss from February to April at high latitudes. Only in May is the in situ O3 production larger than loss. The observed continuous increase of column O3 at high latitudes throughout the spring is due to transport from other tropospheric regions or the stratosphere not in situ photochemistry.
Microphysical and radiative evolution of aerosol plumes over the tropical North Atlantic Ocean
Authors: Garrett Timothy J. , Russell Lynn M. , Ramaswamy V. , Maria Steven F. , Huebert Barry J.
Published: May 2003

Over the tropical North Atlantic Ocean in the summer, plumes of aerosol extend from Saharan Africa to the Caribbean. The microphysical and radiative evolution of such plumes is studied using a Lagrangian column model and measurements obtained near the west coast of Africa (during the second Aerosol Characterization Experiment [ACE‐2]) and the Caribbean Sea (during the Passing Efficiency of the Low Turbulence Inlet [PELTI] experiment). Mass and scattering in the plumes can be separated into two layers that overlay one another over much of the Atlantic Ocean. Mineral dust dominates in the lower free troposphere, and sea‐salt aerosol dominates in the boundary layer. Carbonaceous, sulfate, and nitrate (CSN) aerosols are a minor component of mass but contribute significantly to total column optical depth. Combined, CSN aerosols and sea‐salt contribute to more than half of total aerosol clear‐sky shortwave forcing associated with such plumes. Satellite and model data suggest that the reduction of plume forcing between the African coastline and the Caribbean is less than ∼20%. The reduction is due principally to settling of large dust particles and atmospheric subsidence; however, the reduction of forcing remains small because (a) boundary layer trade winds provide a steady source of sea‐salt, (b) dust particles are initially elevated 2.5–5.5 km from the surface and therefore have long settling distances before removal, and (c) small CSN and dust particles in the free troposphere have high specific extinction and lack significant removal processes. Measurements and climatology suggest that the CSN aerosols in the free troposphere are anthropogenic pollution from Europe.
Investigations into free tropospheric new particle formation in the central Canadian arctic during the winter/spring transition as part of TOPSE
Authors: Weber R. J. , Orsini D. , Wang B. , Scheuer E. , Talbot R. W. , Dibb J. E. , Seid G. K. , DeBell L. , Mauldin R. L. , Kosciuch E. , Cantrell C. , Eisele F.
Published: May 2003

In this paper, we investigate the role of in situ new particle production in the central Canadian sub‐Arctic and Arctic as part of the TOPSE experiment. Airborne measurements conducted primarily in the free troposphere were made from 50° to 90°W longitude and 60° to 85°N latitude during the period from February to May 2000. Data pertinent to this paper include 3–4 nm diameter (Dp) particles, ultrafine condensation nuclei (Dp > 3 nm), fine particles (0.2 < Dp < 3 μm), and the possible nucleation precursor, sulfuric acid, and its precursor, sulfur dioxide. For data averaged over this period, most species showed little evidence for a latitudinal trend. Fine aerosol number concentrations, however, showed a slight increase with latitude. The evolution of various species concentrations over the period of the study show that fine particles also had a consistent temporal trend, increasing at all altitudes from February to May, whereas sulfur dioxide at the surface tended to peak in late March. Ultrafine condensation nuclei and 3–4 nm particles showed no temporal trends. Little evidence for in situ new particle production was observed during the study, except for one atypical event where SO2 concentrations were 3.5 ppbv, 2 orders of magnitude higher than typical levels. This paper cannot address the question of whether the observed condensation nuclei were produced in situ by a low particle production rate or transported from lower latitudes.
Thermal infrared radiative transfer within three‐dimensional vegetation covers
Authors: Guillevic P. , Gastellu‐Etchegorry J. P. , Demarty J. , Prévot L.
Published: May 2003

The angular distribution of thermal infrared (TIR) radiation emitted by vegetation covers can vary widely depending on environmental conditions and canopy structure. As an aid in the interpretation of TIR remotely sensed data from vegetated surface with incomplete canopies, we developed a three‐dimensional radiative transfer model in the thermal infrared domain. The model simulates the TIR radiative budget and upward spectral radiance of vegetation covers. The model is an extension to the TIR region of the DART (Discrete Anisotropic Radiative Transfer) model developed for the short wave domain. Radiative transfer simulation relies on discrete 3‐D scene representations, which include any distribution of trees and ground covers, possibly with topography. Propagation of emitted and scattered radiation is tracked with a ray‐tracing approach and the discrete ordinate method. The model was successfully tested against a physically based model for homogeneous canopies, and a partial validation was carried out with directional TIR measurements on a cotton row crop. Model simulations are presented to illustrate the influence of the canopy geometric structure on the directional apparent temperature of hypothetical vegetation covers.
Boundary layer aerosol composition over Sierra Nevada Mountains using 9.11‐ and 10.59‐μm continuous wave lidars and modeled backscatter from size distribution data
Authors: Cutten D. R. , Jarzembski M. A. , Srivastava V. , Pueschel R. F. , Howard S. D. , McCaul E. W.
Published: May 2003

An inversion technique has been developed to determine volume fractions of an atmospheric aerosol composed primarily of ammonium sulfate and ammonium nitrate and water combined with fixed concentration of elemental and organic carbon. It is based on measured aerosol backscatter obtained with 9.11‐ and 10.59‐μm wavelength continuous wave CO2 lidars and modeled backscatter from aerosol size distribution data. The technique is demonstrated during a flight of the NASA DC‐8 aircraft over the Sierra Nevada Mountain Range, California, on 19 September 1995. Volume fraction of each component and effective complex refractive index of the composite particle were determined assuming an internally mixed composite aerosol model. The volume fractions were also used to recompute aerosol backscatter, providing good agreement with the lidar‐measured data. The robustness of the technique for determining volume fractions was extended with a comparison of calculated 2.1‐μm backscatter from size distribution data with the measured lidar data converted to 2.1‐μm backscatter using an earlier derived algorithm, verifying the algorithm as well as the backscatter calculations.
Maps of polar mesospheric clouds
Authors: Carbary J. F. , Morrison D. , Romick G. J.
Published: May 2003

A middle ultraviolet imager (235–263 nm) on the Midcourse Space Experiment (MSX) has obtained the first large‐scale, two‐dimensional maps of polar mesospheric clouds (PMCs). The lower parts of individual images, which lie below the ozone horizon, are mapped onto an ellipsoidal shell at 83 km altitude and then combined to establish the transpolar PMC field in two spatial dimensions across the entire polar region. At all latitudes where they appear, the PMCs clearly evidence a “patchy” structure as opposed to a uniform layer. Among the interesting features of these cloud patches are zonal alignments, arcs, and repetitive structures characteristic of waves. Whether random or repetitive, the cloud structures exhibit scales ranging from several hundred kilometers down to tens of kilometers.
Stratosphere‐troposphere exchange: A model and method intercomparison
Authors: Meloen J. , Siegmund P. , van Velthoven P. , Kelder H. , Sprenger M. , Wernli H. , Kentarchos A. , Roelofs G. , Feichter J. , Land C. , Forster C. , James P. , Stohl A. , Collins W. , Cristofanelli P.
Published: May 2003

This paper presents one of the first extensive intercomparisons of models and methods used for estimating stratosphere‐troposphere exchange (STE). The study is part of the European Union project Influence of Stratosphere Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO). Nine different models and methods, including three trajectory methods, one Eulerian method, two Lagrangian and one Eulerian transport model, and two general circulation models applied the same initialization. Stratospheric and tropospheric tracers have been simulated, and the tracer mass fluxes have been calculated through the tropopause and the 700 hPa surface. For a 12‐day case study over Europe and the northeast Atlantic the simulated tracer mass fluxes have been intercompared. For this case the STE simulations show the same temporal evolution and the same geographical pattern of STE for most models and methods, but with generally different amplitudes (up to a factor of 4). On the other hand, for some simulations also the amplitudes are very similar.
Parameters of the fine density structure in the stratosphere obtained from spacecraft observations of stellar scintillations
Authors: Gurvich A. S. , Chunchuzov I. P.
Published: May 2003

Measurements of stellar scintillations caused by the atmosphere, conducted on board the space station Mir, are analyzed here. From scintillation spectra we obtained the parameters of spatial spectra of atmospheric density fluctuations (the altitudes 20–70 km) with vertical scales from hundreds of meters to tens of centimeters. The observed characteristic scales of atmospheric inhomogeneities are interpreted with a nonlinear model of internal wave spectrum and a theory of locally homogeneous and isotropic turbulence.
Land surface conditions over Eurasia and Indian summer monsoon rainfall
Authors: Robock Alan , Mu Mingquan , Vinnikov Konstantin , Robinson David
Published: May 2003

Using observations of snow cover, soil moisture, surface air temperature, atmospheric circulation, and Indian summer monsoon precipitation from 1870 to 2000, we examine the relationship between interannual variations of the strength of the monsoon and land surface conditions over Eurasia. For the periods 1870–1895 and 1950–1995, strong Indian summer monsoon precipitation was preceded by warmer than normal temperatures over Europe and North America in the previous winter and over western Asia in the previous spring but colder temperatures over Tibet. The European temperature anomalies were related to the positive phase of the North Atlantic Oscillation (NAO). Related negative snow cover anomalies in Europe in winter and central Asia in spring were produced by circulation and temperature anomalies. The snow‐albedo feedback is always operating, but the snow by itself did not physically control the monsoon. Anomalous snow cover impacts on temperature were not prolonged by soil moisture feedbacks because of its short time memory, and there was no obvious relationship between soil moisture and the monsoon. Strong Indian summer monsoon precipitation was actually preceded by higher than normal Tibetan snow cover in winter and spring in contrast to the suggestion of Blanford [1884] more than a century ago. The correlation between Indian summer monsoon rainfall and winter land temperatures and snow cover only exists when interannual variation of the NAO is very strong, and therefore NAO is not a robust predictor of the monsoon. Climate models show that the relationship between NAO and monsoon is random.
Daily precipitation statistics in regional climate models: Evaluation and intercomparison for the European Alps
Authors: Frei Christoph , Christensen Jens Hesselbjerg , Déqué Michel , Jacob Daniela , Jones Richard G. , Vidale Pier Luigi
Published: May 2003

An evaluation is undertaken of the statistics of daily precipitation as simulated by five regional climate models using comprehensive observations in the region of the European Alps. Four limited area models and one variable‐resolution global model are considered, all with a grid spacing of 50 km. The 15‐year integrations were forced from reanalyses and observed sea surface temperature and sea ice (global model from sea surface only). The observational reference is based on 6400 rain gauge records (10–50 stations per grid box). Evaluation statistics encompass mean precipitation, wet‐day frequency, precipitation intensity, and quantiles of the frequency distribution. For mean precipitation, the models reproduce the characteristics of the annual cycle and the spatial distribution. The domain mean bias varies between −23% and +3% in winter and between −27% and −5% in summer. Larger errors are found for other statistics. In summer, all models underestimate precipitation intensity (by 16–42%) and there is a too low frequency of heavy events. This bias reflects too dry summer mean conditions in three of the models, while it is partly compensated by too many low‐intensity events in the other two models. Similar intermodel differences are found for other European subregions. Interestingly, the model errors are very similar between the two models with the same dynamical core (but different parameterizations) and they differ considerably between the two models with similar parameterizations (but different dynamics). Despite considerable biases, the models reproduce prominent mesoscale features of heavy precipitation, which is a promising result for their use in climate change downscaling over complex topography.