bluuebunny/crossref_metadata_2025 huggingface dataset:
Response of the West Florida Shelf circulation to climatological wind stress forcing
Authors: Yang Huijun , Weisberg Robert H.
Published: September 2002

The response of the west Florida continental shelf circulation to monthly mean climatological wind forcing is investigated by using the three‐dimensional, primitive equation Princeton Ocean Model. Two basic seasonal patterns of circulation and sea surface elevation occur under a barotropic setting: a winter pattern from October to March and a summer pattern from April to September. An interesting finding in winter is an anticyclonic gyre over the northeastern (Florida Big Bend) region that merges with a northwestward flow from the south. The Big Bend Gyre is caused by a convergence of two separate flows: a nearshore along‐shelf southeastward flow and an offshore northwestward flow. Winter circulation characteristics also include offshore surface transport, coastal upwelling, and relatively low coastal sea level. The summer pattern features a continuous northwestward directed flow, onshore surface transport, coastal downwelling, and relatively high coastal sea level. Transitions between the two seasonal patterns show either a development or relaxation of the Big Bend Gyre. These three‐dimensional circulation patterns result from wind‐driven Ekman transports and their resulting surface‐slope‐induced geostrophic flows. Qualitative agreement exists in some respects with recent in situ observations, historical drift bottle retrievals, and tide gauge data. However, climatological wind stress forcing alone does not account for the seasonally varying southeastward currents observed at midshelf. Thus it may be concluded that for depths less than 50 m the seasonal winds may play a dominant role in the seasonal variation of the shelf circulation; and beyond this limit seasonal density related effect must also be a factor in the seasonally varying circulation on the West Florida Shelf.
Ocean model validation of the NASA scatterometer winds
Authors: Verschell M. A. , Bourassa M. A. , Weissman D. E. , O'Brien J. J.
Published: September 2002

The suitability of basin‐scale, satellite‐based scatterometer winds for forcing of numerical ocean models is examined using a reduced gravity, primitive equation model of the tropical Pacific Ocean. Three surface forcing fields are validated in a comparison of upper layer thickness (ULT) from the ocean model with observed sea level data. The forcing fields are the Florida State University observed winds, winds derived from the NASA Scatterometer (NSCAT), and stresses derived directly from NSCAT. The sea level data sets are the World Ocean Circulation Experiment “fast” sea level data set from island measurements and sea level anomalies from TOPEX/POSEIDON. Results of this comparison demonstrate that while the three model results are qualitatively similar, the results are quantitatively better when forcing with the NSCAT derived stresses. This is particularly true in the eastern tropical Pacific and in convergent zones where forcing with the NSCAT stresses can lead to large differences in ULT (>40 m) compared with results from the other two wind products.
A third‐generation wave model for coastal regions: 2. Verification
Authors: Ris R. C. , Holthuijsen L. H. , Booij N.
Published: September 2002

A third‐generation spectral wave model (Simulating Waves Nearshore (SWAN)) for small‐scale, coastal regions with shallow water, (barrier) islands, tidal flats, local wind, and ambient currents is verified in stationary mode with measurements in five real field cases. These verification cases represent an increasing complexity in two‐dimensional bathymetry and added presence of currents. In the most complex of these cases, the waves propagate through a tidal gap between two barrier islands into a bathymetry of channels and shoals with tidal currents where the waves are regenerated by a local wind. The wave fields were highly variable with up to 3 orders of magnitude difference in energy scale in individual cases. The model accounts for shoaling, refraction, generation by wind, whitecapping, triad and quadruplet wave‐wave interactions, and bottom and depth‐induced wave breaking. The effect of alternative formulations of these processes is shown. In all cases a relatively large number of wave observations is available, including observations of wave directions. The average rms error in the computed significant wave height and mean wave period is 0.30 m and 0.7 s, respectively, which is 10% of the incident values for both.
Estimates of bottom roughness length and bottom shear stress in South San Francisco Bay, California
Authors: Cheng Ralph T. , Ling Chi‐Hai , Gartner Jeffrey W. , Wang P. F.
Published: September 2002

A field investigation of the hydrodynamics and the resuspension and transport of particulate matter in a bottom boundary layer was carried out in South San Francisco Bay (South Bay), California, during March‐April 1995. Using broadband acoustic Doppler current profilers, detailed measurements of turbulent mean velocity distribution within 1.5 m above bed have been obtained. A global method of data analysis was used for estimating bottom roughness length zo and bottom shear stress (or friction velocities u*). Field data have been examined by dividing the time series of velocity profiles into 24‐hour periods and independently analyzing the velocity profile time series by flooding and ebbing periods. The global method of solution gives consistent properties of bottom roughness length zo and bottom shear stress values (or friction velocities u*) in South Bay. Estimated mean values of zo and u* for flooding and ebbing cycles are different. The differences in mean zo and u* are shown to be caused by tidal current flood‐ebb inequality, rather than the flooding or ebbing of tidal currents. The bed shear stress correlates well with a reference velocity; the slope of the correlation defines a drag coefficient. Forty‐three days of field data in South Bay show two regimes of zo (and drag coefficient) as a function of a reference velocity. When the mean velocity is >25–30 cm s−1, the ln zo (and thus the drag coefficient) is inversely proportional to the reference velocity. The cause for the reduction of roughness length is hypothesized as sediment erosion due to intensifying tidal currents thereby reducing bed roughness. When the mean velocity is <25–30 cm s−1, the correlation between zo and the reference velocity is less clear. A plausible explanation of scattered values of zo under this condition may be sediment deposition. Measured sediment data were inadequate to support this hypothesis, but the proposed hypothesis warrants further field investigation.
An improved fast‐response vacuum‐UV resonance fluorescence CO instrument
Authors: Gerbig Christoph , Schmitgen Sandra , Kley Dieter , Volz‐Thomas Andreas , Dewey Ken , Haaks Dieter
Published: September 2002

The fast‐response resonance fluorescence instrument for the airborne measurement of carbon monoxide described by Gerbig et al. [1996] was modified by implementing an improved optical filter with more efficient optics and an optimized resonance lamp. Besides reductions in size and weight, the new instrument achieves a sensitivity 10 times higher, a lower background (65 ppb compared with 250 ppb), and a faster time response (<0.1 s) than the original instrument. The precision is ±1.5 ppb at an atmospheric mixing ratio of 100 ppb CO, and the detection limit is 3 ppb (2σ) for an integration time of 1 s. First results from the North Atlantic Regional Aerosol Characterization Experiment (ACE‐2) campaign during July 1997, when the new instrument was deployed aboard the U.K. Meteorological Office C‐130 aircraft, are used to demonstrate the performance of the new instrument.
Widespread solid particle formation by mountain waves in the Arctic stratosphere
Authors: Carslaw Kenneth S. , Peter Thomas , Bacmeister Julio T. , Eckermann Stephen D.
Published: September 2002

Observations of polar stratospheric clouds (PSCs) by lidar show that the clouds often contain solid particles, which are most likely composed of nitric acid hydrates. However, laboratory experiments indicate that such hydrate particles are not easily formed under Arctic synoptic scale conditions, suggesting that solid PSC particles should be rather rare. Here we show results from a model study indicating that mountain‐induced mesoscale temperature perturbations may be an important source of nitric acid hydrate particles in the Arctic. Multiple Arctic vortex trajectories were combined with a global mountain wave forecast model to calculate the potential for solid particle formation during December and January 1994/1995. The mountain wave model was used to calculate adiabatic cooling over several thousand ridge elements. Nitric acid hydrate particles were assumed to form in the mountain waves according to several microphysical mechanisms, and were then advected using polar vortex‐filling synoptic trajectories to generate maps of solid particle occurrence. The calculations show that mountain waves may be a significant source of PSCs containing solid particles that are observed on the synoptic scale. In particular, the east coast of Greenland, the Norwegian mountains, and the Urals are found to be solid particle sources, with the PSCs often predicted to survive several thousand kilometers downstream.
An evaluation of chemistry's role in the winter‐spring ozone maximum found in the northern midlatitude free troposphere
Authors: Yienger J. J. , Klonecki A. A. , Levy H. , Moxim W. J. , Carmichael G. R.
Published: September 2002

We employ the Geophysical Fluid Dynamics Laboratory global chemical transport model to investigate the contribution of photochemistry to the winter‐spring ozone maximum in the northern hemisphere (NH) midlatitude free troposphere (770–240 mbar; 30°N–60°N). Free tropospheric ozone mass slowly builds up in the winter and early spring, with net chemistry and transport playing comparable roles. Winter and early spring conditions are favorable to net ozone production for two reasons: (1) Winter conditions (cold, low Sun angle, and dry) reduce HOx and lower the level of NOx needed for chemical production to exceed destruction (balance point); and (2) throughout the winter and early spring, NOx, because of its longer chemical lifetime, increases above normally net‐destructive levels in the remote atmosphere. Interestingly, net production in the midlatitude NH free troposphere maximizes in early spring because relatively high NOx and low balance point conditions are present at a time when increasing insolation is speeding up photochemistry. Conceptually, the net ozone production is associated with an annual atmospheric “spring cleaning” in which high levels of NOx are removed via OH oxidation. Further, we find that human activity has a major impact on both the levels of tropospheric ozone and the role of chemistry in the NH midlatitude, where anthropogenic NOx emissions dominate. In that region, modern ozone levels have increased by ∼20% in the winter and ∼45% in the spring, winter‐spring chemistry has switched from net destructive to net productive, the winter‐spring balance between transport and chemistry has switched from transport dominance in preindustrial times to the present parity, and the preindustrial February maximum has progressed to March‐April. Estimated 2020 levels of NOx emissions were found to lead to even greater net production and to push the O3 spring maximum later into April‐May.
The influence of the OH + NO2 + M reaction on the NOy, partitioning in the late Arctic winter 1992/1993 as studied with KASIMA
Authors: Ruhnke Roland , Kouker Wolfgang , Reddmann Thomas
Published: September 2002

The northern hemispheric winter 1992/1993 is simulated with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA). The model is a combination of an off‐line model using analyses from the European Centre for Medium Range Weather Forecasts up to a pressure altitude of 10 hPa and a mechanistic prognostic model on top with an entire altitude range from 10 up to 120 km. The chemistry scheme included in the model represents the gas phase chemistry as well as heterogeneous reactions on polar stratospheric clouds and on liquid sulfate aerosols. We compare model results with global measurements performed by the instruments of the Upper Atmosphere Research Satellite. The model is able to simulate the global distribution of source gases and long‐lived species as well as reactive species in good agreement with the observations. The most significant discrepancies occur inside the vortex where the calculated ozone mixing ratio is overestimated. Below the pressure altitude of 30 hPa inside the vortex the calculated NO2 mixing ratio is underestimated compared to the measurement. A sensitivity study with a new recommendation of the OH + NO2 + M rate constant using the full form of the falloff function for the pressure dependence has been performed. The mixing ratios of the NOy species differ by only about 5% due to the new recommended rate constant inside the vortex with no significant effect on the denoxification.
Verification of German methane emission inventories and their recent changes based on atmospheric observations
Authors: Levin Ingeborg , Glatzel‐Mattheier Holger , Marik Thomas , Cuntz Matthias , Schmidt Martina , Worthy Douglas E.
Published: September 2002

Continuous methane concentration records and stable isotope observations measured in the suburbs of Heidelberg, Germany, are presented. While δ13C‐CH4 shows a significant trend of −0.14‰ per year, toward more depleted values, no trend is observed in the concentration data. Comparison of the Heidelberg records with clean air observations in the North Atlantic at Izaña station (Tenerife) allows the determination of the continental methane excess at Heidelberg, decreasing by 20% from 190 ppb in 1992 to 150 ppb in 1997. The isotope ratio which is associated with this continental methane pileup in the Heidelberg catchment area shows a significant trend to more depleted values from δ13Csource = −47.4 ± 1.2‰ in 1992 to −52.9 ± 0.4‰ in 1995/1996, pointing to a significant change in the methane source mix. Total methane emissions in the Heidelberg catchment area are estimated using the 222radon (222Rn) tracer method: from the correlations of half‐hourly 222Rn and CH4 mixing ratios from 1995 to 1997, and the mean 222Rn exhalation rate from typical soils in the Rhine valley, a mean methane flux of 0.24 ± 0.5 g CH4 km−2 s−1 is derived. For the Heidelberg catchment area with an estimated radius of approximately 150 km, Core Inventories Air 1990 (CORINAIR90) emission estimates yield a flux of 0.47 g CH4 km−2 s−1, which is about 40% higher than the 222Rn‐derived number if extrapolated to 1990. The discrepancy can be explained by overestimated emissions from waste management in the CORINAIR90 statistical assessment. The observed decrease in total emissions can be accounted for by decreasing contributions from fossil sources (mainly coal mining) and from cattle breeding. This finding is also supported by the observed decrease in mean source isotopic signatures.
Heterogeneous nucleation of the efflorescence of (NH4)2SO4 particles internally mixed with Al2O3, TiO2, and ZrO2
Authors: Han Jeong‐Ho , Martin Scot T.
Published: September 2002

Accurate models of atmospheric processes are currently hindered by poor microphysical models of particle phase transitions, including efflorescence. For aqueous particles, it is possible that insoluble crustal inclusions behave as heterogeneous nuclei to induce efflorescence. This paper reports on the results of laboratory experiments concerning the efflorescence of (NH4)2SO4 particles that are internally mixed with TiO2, Al2O3, or ZrO2 particles. The particles are prepared by condensing SO3 and H2O vapors in the presence of insoluble metal oxide particles, followed by neutralization with NH3. A Fourier transform infrared spectrometer is employed to observe the aqueous‐to‐solid phase change as relative humidity decreases. Efflorescence occurs at 35% relative humidity (±2%) in the absence of heterogeneous nuclei. In contrast, internally mixed particles effloresce at 57% (Al2O3), 59% (ZrO2), and 65% (TiO2) at 298 K.
Pacific Exploratory Mission in the tropical Pacific: PEM‐Tropics A, August‐September 1996
Authors: Hoell J. M. , Davis D. D. , Jacob D. J. , Rodgers M. O. , Newell R. E. , Fuelberg H. E. , McNeal R. J. , Raper J. L. , Bendura R. J.
Published: September 2002

The NASA Pacific Exploratory Mission to the Pacific tropics (PEM‐Tropics) is the third major field campaign of NASA's Global Tropospheric Experiment (GTE) to study the impact of human and natural processes on the chemistry of the troposphere over the Pacific basin. The first two campaigns, PEM‐West A and B were conducted over the northwestern regions of the Pacific and focused on the impact of emissions from the Asian continent. The broad objectives of PEM‐Tropics included improving our understanding of the oxidizing power of the tropical atmosphere as well as investigating oceanic sulfur compounds and their conversion to aerosols. Phase A of the PEM‐Tropics program, conducted between August‐September 1996, involved the NASA DC‐8 and P‐3B aircraft. Phase B of this program is scheduled for March/April 1999. During PEM‐Tropics A, the flight tracks of the two aircraft extended zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand. Both aircraft were instrumented for airborne measurements of trace gases and aerosols and meteorological parameters. The DC‐8, given its long‐range and high‐altitude capabilities coupled with the lidar instrument in its payload, focused on transport issues and ozone photochemistry, while the P‐3B, with its sulfur‐oriented instrument payload and more limited range, focused on detailed sulfur process studies. Among its accomplishments, the PEM‐Tropics A field campaign has provided a unique set of atmospheric measurements in a heretofore data sparse region; demonstrated the capability of several new or improved instruments for measuring OH, H2SO4, NO, NO2, and actinic fluxes; and conducted experiments which tested our understanding of HOx and NOx photochemistry, as well as sulfur oxidation and aerosol formation processes. In addition, PEM‐Tropics A documented for the first time the considerable and widespread influence of biomass burning pollution over the South Pacific, and identified the South Pacific Convergence Zone as a major barrier for atmospheric transport in the southern hemisphere.
Six years of UARS Microwave Limb Sounder HNO3 observations: Seasonal, interhemispheric, and interannual variations in the lower stratosphere
Authors: Santee M. L. , Manney G. L. , Froidevaux L. , Read W. G. , Waters J. W.
Published: September 2002

We present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO3 in the lower stratosphere based on measurements of gas‐phase HNO3 made by the UARS Microwave Limb Sounder (MLS) through six complete annual cycles in both hemispheres. Outside of the winter polar regions, zonal‐mean HNO3 mixing ratios on the 465‐K potential temperature surface are comparable in the two hemispheres in all latitude bands and in all years examined. Except at high latitudes, interannual variability is minimal, and there is no significant hemispheric asymmetry in the overall HNO3 distribution or its seasonal cycle. Although the Antarctic experiences widespread severe denitrification, the MLS data indicate that the denitrification is not complete; that is, not all polar stratospheric cloud (PSC) particles sediment out of the lower stratosphere. Replenishment of HNO3 at 465 K during the mid‐ to late‐winter period (when temperatures, though still low, are generally rising) is most likely achieved through a combination of PSC evaporation and continuing weak diabatic descent. Despite large interhemispheric and interannual differences in the extent and duration of PSC activity and denitrification, HNO3 recovers to similar values at the end of every winter in both the Arctic and the Antarctic. Zonal‐mean HNO3 values for the two hemispheres are virtually indistinguishable for the latitudes equatorward of 65°, even during the winter months. Thus the effects of severe denitrification are confined in both space and time to the regions poleward of 65°S during the winter and early spring.
Simulation of ozone and other chemical species using a Center for Climate System Research/National Institute for Environmental Studies atmospheric GCM with coupled stratospheric chemistry
Authors: Takigawa Masayuki , Takahashi Masaaki , Akiyoshi Hideharu
Published: September 2002

A new middle‐atmosphere general circulation model that includes the photochemistry for ozone and other chemical species (19 photolysis and 52 chemical reactions) has been constructed. The horizontal spectral resolution is T21 (about a 600 km horizontal grid spacing) with 30 layers in the vertical. Preliminary results from over 10 years of model integration are presented. The distributions of long‐lived species, such as N2O, are rather similar to those of satellite observations in a climatological sense, although the sharp meridional gradient around 30° latitude is not well simulated in the model stratosphere. Neither is the double peak structure that occurs during equinox periods well reproduced. This result is consistent with the fact that the westerly phase of the semiannual oscillation is weak in this model. This may be due to the coarse resolution of the model. The seasonal evolution of the ozone column abundance is quite realistic, although the model slightly underestimates total tropical ozone. The model also underestimates ozone amounts around the equatorial tropopause. The February midlatitude number density of OH in the model upper stratosphere is about 1.8 × 107 cm−3, which is slightly less than that observed. The horizontal distributions of short‐lived species, such as NO, suggest a reasonable model diurnal variation. The model has a cold bias of about 25 K in the lower stratospheric Northern Hemisphere winter and 5 K in the Southern Hemisphere winter. The model residual mean vertical velocity in the equatorial lower stratosphere is too weak (about 0.1 mm/s) during the Northern Hemisphere winter, compared with the observed (about 0.4 mm/s), while the model temperature around the equatorial tropopause is cooler than that observed.
Tidal signatures in temperature data from CRISTA 1 mission
Authors: Ward W. E. , Oberheide J. , Riese M. , Preusse P. , Offermann D.
Published: September 2002

Temperature measurements in the stratosphere and mesosphere were taken during the first Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA 1) mission using CO2 emissions. These measurements range from 13 to close to 100 km, and individual temperature measurements have a precision of 1 K. The CRISTA orbit was circular at an inclination of 57°, so local time variations during the 7 day mission were small for a given latitude and orbit leg. Zonal averages of the data show significant structure in the vertical and as a function of latitude. Temperature differences between the zonal mean data from the ascending and descending portions of the orbit are of the form expected from the diurnal tide. The maximum zonal mean difference is ≈20 K and occurs over the equator at an altitude of 75 km. Zonal variations in the temperature difference indicate that the tidal amplitude is not uniform at all longitudes. At the equator the maximum amplitude (30 K) appears over the African sector and the minimum (10 K) over the Pacific sector. This variation is most likely a nonmigrating wavenumber 2 diurnal tide although other less plausible possibilities exist. To facilitate comparisons with model results the temperature variations are converted to equivalent vertical displacements, assuming the tidal motions are adiabatic. Such an approach is appropriate in the mesosphere where diurnal variations due to radiative effects are small and reduces the dependence of the measured temperature amplitude on the background temperature profile. Equivalent vertical displacements are also calculated using results from the global scale wave model. For the most part the model results and observations are in excellent qualitative and good quantitative agreement from the tropopause to the mesopause. However, the observed vertical wavelength is smaller, and its amplitude in terms of equivalent displacement is smaller in the upper mesosphere than that in the model. The decrease in amplitude is consistent with that expected from the transition from equinox to solstice conditions.
Chlorine activation and ozone destruction in the northern lowermost stratosphere
Authors: Lelieveld J. , Bregman A. , Scheeren H. A. , Ström J. , Carslaw K. S. , Fischer H. , Siegmund P. C. , Arnold F.
Published: September 2002

We report aircraft measurements from the Stratosphere‐Troposphere Experiments by Aircraft Measurements (STREAM) II campaign, performed during February 1995 from Kiruna, northern Sweden, near 67°N latitude. We have measured trace species, e.g., O3, nitrogen compounds, HCl, hydrocarbons, CO, ice particles, and aerosols, to characterize the chemical conditions of the lowermost stratosphere at altitudes up to ∼12.5 km. From the observation of anomalously high CO/C2H6 ratios, caused by enhanced C2H6 breakdown, we derive that heterogeneous chlorine activation has occurred. This has liberated a diurnal mean Cl concentration up to 1.0 × 104 atoms cm−3, which efficiently destroys ozone. We also infer that much of the Cl activation has taken place on ice crystals above the tropopause. The measured crystal number densities were typical of thin cirrus. Model calculations suggest that heterogeneous chlorine conversion on ice crystals, in addition to that on liquid aerosols, may contribute significantly to ozone destruction in the middle‐ and high‐latitude lowermost stratosphere.
A high‐resolution air chemistry record from an Alpine ice core: Fiescherhorn glacier, Swiss Alps
Authors: Schwikowski M. , Brütsch S. , Gäggeler H. W. , Schotterer U.
Published: September 2002

Glaciochemical studies at midlatitudes promise to contribute significantly to the understanding of the atmospheric cycling of species with short atmospheric lifetimes. Here we present results of chemical analyses of environmentally relevant species performed on an ice core from Fiescherhorn glacier, Swiss Alps (3890 m above sea level). This glacier site is unique since it is located near the high‐alpine research station Jungfraujoch. There long‐term meteorological and air quality measurements exist, which were used to calibrate the paleodata. The 77‐m‐long ice core was dated by annual layer counting using the seasonally varying signals of tritium and δ18O. It covers the time period 1946–1988 and shows a high net accumulation of water of 1.4 m yr−1 allowing for the reconstruction of high‐resolution environmental records. Chemical composition was dominated by secondary aerosol constituents as well as mineral dust components, characterizing the Fiescherhorn site as a relatively unpolluted continental site. Concentrations of species like ammonium, nitrate, and sulfate showed an increasing trend from 1946 until about 1975, reflecting anthropogenic emission trends in western Europe. For mineral dust tracers, no trends were obvious, whereas chloride and sodium showed slightly higher levels from 1965 until 1988, indicating a change in the strength of sea‐salt transport. Good agreement between the sulfate paleorecord with direct atmospheric measurements was found (correlation coefficient r2 = 0.41). Thus a “calibration” of the paleorecord over a significant period of time could be conducted, revealing an average scavenging ratio of 180 for sulfate.
Annual cycle of energy balance of Zongo Glacier, Cordillera Real, Bolivia
Authors: Wagnon Patrick , Ribstein Pierre , Francou Bernard , Pouyaud Bernard
Published: September 2002

An 18‐month meteorological data set recorded at 5150 m above sea level (asl) on Zongo Glacier, in the tropical Andes of Bolivia, is used to derive the annual cycle of the local energy balance and to compare it to the local mass balance. The roughness parameters needed to calculate the turbulent fluxes over the surface are deduced from direct sublimation measurements performed regularly on the field site and serve as calibration parameters. For the hydrological year September 1996 to August 1997, net all‐wave radiation (16.5 W m−2) is the main source of energy at the glacier surface and shows strong fluctuations in relation to the highly variable albedo. An important peculiarity of tropical glaciers is the negative latent heat flux (−17.7 W m−2) indicating strong sublimation, particularly during the dry season. The latent heat flux is reduced during the wet season because of a lower vertical gradient of humidity. The sensible heat flux (6.0 W m−2), continuously positive throughout the year, and the conductive heat flux in the snow/ice (2.8 W m−2) also bring energy to the surface. There is a good agreement between the monthly ablation calculated by the energy balance and the ablation evaluated from stake measurements. The seasonality of the proglacial stream runoff is controlled by the specific humidity, responsible for the sharing of the energy between sublimation and melting.
Application of the NCAR Regional Climate Model to eastern Africa: 2. Simulation of interannual variability of short rains
Authors: Sun Liqiang , Semazzi Fredrick H. M. , Giorgi Filippo , Ogallo Laban
Published: September 2002

We have applied the NCAR RegCM2 to the simulation of the interannual variability of precipitation over eastern Africa for the short‐rains season by performing a set of experiments for the years 1982 to 1993. The model reproduced the observed interannual variability of precipitation in most of the years. The results show that remote factors play a dominant role in determining the precipitation anomalies. Interannual variability of precipitation over Tanzania is closely related to El Niño events in their mature phase and sea surface temperature (SST) anomalies over the Indian and Atlantic Oceans. The southward shift of the Arabian High results in a southward shift of the zonal component of the Intertropical Convergence Zone (ITCZ), which is responsible for early onset of the rainy season (e.g., 1982 and 1986). The enhanced St. Helena High and weaker Mascarene High lead to the eastward shift of the meridional branch of the ITCZ for the wet years. Model simulations confirmed a strong positive correlation between precipitation anomalies over Lake Victoria and the warm El Niño‐Southern Oscillation events, by which enhanced moist westerly flow from the Atlantic Ocean and the mainly easterly flow from the Indian Ocean converge over Lake Victoria during wet years. The interannual variability of precipitation over Lake Victoria and the western Kenya Highlands (WKH) are strongly coupled. Positive precipitation anomalies over the WKH region are usually associated with weaker Arabian High and Mascarene High, which weaken the large‐scale divergence over the WKH region and favor the development of convection. The interannual variability of precipitation over eastern Kenya Highlands (EKH) is not directly related to the El Niño events, but the association with a warm SST anomaly pattern over the western Indian Ocean is evident during wet years. An El Niño signal is, however, evident for wet years over the Turkana Channel, warm SST anomalies over the northern Indian Ocean contribute enhanced water vapor transport over the region.
Improved application of exponential sum fitting transmissions to inhomogeneous atmosphere
Authors: Sun Zhian , Rikus Lawrie
Published: September 2002

An improved method of applying exponential sum fitting transmissions (ESFT) to inhomogeneous atmospheres (referred to as inhomogeneous ESFT or IESFT) is presented, and its use in a broadband radiative transfer model is documented. Instead of using a scaling function to represent the pressure and temperature dependence of the absorption coefficient we apply the ESFT to all pressures and temperatures based on weight terms determined at reference conditions. The exponential terms corresponding to all pressures and temperatures form a look‐up table which can be used to interpolate to the corresponding value at the real pressure and temperature. The IESFT is performed using a nonlinear least squares routine with some constraints to ensure that the ranking of the exponential terms is maintained. The method is tested against line‐by‐line calculations, and the results show that the use of IESFT in the two stream approximation of radiative transfer can reduce the cooling rate error by a factor of 2 relative to the use of a scaling function, particularly in the upper atmosphere. An application of the treatment of overlapping gaseous absorption developed by Mlawer et al. [1997] to ESFT is described. The basic idea of this method is to treat the mixture of two absorbing gases as a single gas by introducing a binary interpolation parameter which reflects the relative abundance of the two designated gases. Tests against line‐by‐line calculations show that this method is not only accurate but also computationally efficient.
On the use of long‐term global data of land reflectances and vegetation indices derived from the advanced very high resolution radiometer
Authors: Gutman Garik G.
Published: September 2002

The availability of advanced very high resolution radiometer (AVHRR) time series of global shortwave data for the past two decades motivated many scientists to investigate interannual variability and trends in land surface conditions. For these studies the observed change in radiances due to two varying factors, namely, sensor responsivity and illumination conditions, must be known a priori because of the degradation of AVHRR shortwave channels and the orbit drift of afternoon spacecraft. The current work analyzes the behavior of global land AVHRR shortwave time series data for the last 12 years, processed using postlaunch calibration, and investigates their usefulness for the monitoring of global land surface processes. Its focus is on verifying the postlaunch calibrations for the AVHRR sensors on board NOAA 11 and 14. It is assumed that the NOAA 9 AVHRR calibration is correct so that the changing illumination effects can be parameterized based on its data. After accounting for the illumination effects, the residual trends in data, averaged over global deserts and rain forests, are attributed to calibration discrepancies. In particular, NOAA 11 calibration was found to yield only small residuals, whereas NOAA 14 calibration produced significant unrealistic global increase in both reflectances and vegetation indices. The artificial trends caused by the combination of calibration residuals and satellite‐orbit drift should be removed to alleviate their misidentification as real trends in Earth's climate system and to make statistical studies of anomalies more reliable. This study draws attention to the above aspects of time series analysis with the available global AVHRR data and suggests ways to improve these data for interannual variability studies.
Combining afternoon and morning NOAA satellites for thermal inertia estimation: 2. Methodology and application
Authors: Sobrino J. A. , El Kharraz M. H.
Published: September 2002

This is the second in a series of two papers on thermal inertia estimation from satellite data. In this paper we present the methodology that permits thermal inertia mapping using NOAA‐advanced very high resolution radiometer data from the four temperatures algorithm developed in the first paper. Four NOAA images are used to assess thermal inertia: an image at 14:30 (nominal hour) is used to estimate the surface albedo; others at 2:30 and 14:30 are used to estimate the diurnal surface temperature range, while images at 7:30, 14:30, and 19:30 are used to assess the phase difference. The main advantage of the methodology is that the phase difference is obtained from satellite data without the need to know the value of the effective surface emissivity and total water vapor content of the atmosphere. Following the proposed algorithm and the developed methodology, maps of thermal inertia are presented, two for the Iberian Peninsula (on two consecutive days to check consistency of the results) and one for Morocco (to check behavior in different landscapes). The thermal inertia values obtained are consistent with the known properties of these zones.
Complex craters: Relationship of stratigraphy and rings to impact conditions
Authors: O'Keefe John D. , Ahrens Thomas J.
Published: September 2002

One of the key issues associated with the understanding of large scale impacts is how the observable complex crater structural features (e.g., central peaks and pits, flat floors, ring shaped ridges and depressions, stratigraphic modifications, and faults) relate to the impactor's parameters (e.g., radius, velocity, and density) and the nonobservable transient crater measures (e.g., depth of penetration and diameter at maximum penetration). We have numerically modeled large‐scale impacts on planets for a range of impactor parameters, gravity and planetary material strengths. From these we found that the collapse of the transient cavity results in the development of a tall, transient central peak that oscillates and drives surface waves that are arrested by the balance between gravitational forces and planetary strength to produce a wide range of the observed surface features. In addition, we found that the underlying stratigraphy is inverted outside of the transient cavity diameter (overturned flap region), but not inside. This change in stratigraphy is observable by remote sensing, drilling, seismic imaging and gravity mapping techniques. We used the above results to develop scaling laws and to make estimates of the impact parameters for the Chicxulub impact and also compared the calculated stratigraphic profile with the internal structure model developed by Hildebrand et. al. [1998], using gravity, seismic and other field data. For a stratigraphy rotation diameter of 90 km, the maximum depth of penetration is ∼43 km. The impactor diameter was also calculated. From the scaling relationships we get for a 2.7 g/cm3 asteroid impacting at 20 km/s, or a 1.0 g/cm3 comet impacting at 40 km/s, an impactor diameter of ∼13 km, and for a comet impacting at 60 km/s, an impactor diameter of ∼10 km.
Soil‐like deposits observed by Sojourner, the Pathfinder rover
Authors: Moore Henry J. , Bickler Donald B. , Crisp Joy A. , Eisen Howard J. , Gensler Jeffrey A. , Haldemann Albert F. C. , Matijevic Jacob R. , Reid Lisa K. , Pavlics Ferenc
Published: September 2002

Most of the soil‐like materials at the Pathfinder landing site behave like moderately dense soils on Earth with friction angles near 34°–39° and are called cloddy deposits. Cloddy deposits appear to be poorly sorted with dust‐sized to granule‐sized mineral or rock grains; they may contain pebbles, small rock fragments, and clods. Thin deposits of porous, compressible drifts with friction angles near 26°–28° are also present. Drifts are fine grained. Cohesions of both types of deposits are small. There may be indurated soil‐like deposits and/or coated or crusted rocks. Cloddy deposits may be fluvial sediments of the Ares‐Tiu floods, but other origins, such as ejecta from nearby impact craters, should be considered. Drifts are probably dusts that settled from the Martian atmosphere. Remote‐sensing signatures of the deposits inferred from rover observations are consistent with those observed from orbit and Earth.
The magnetic properties experiments on Mars Pathfinder
Authors: Madsen M. B. , Hviid S. F. , Gunnlaugsson H. P. , Knudsen J. M. , Goetz W. , Pedersen C. T. , Dinesen A. R. , Mogensen C. T. , Olsen M. , Hargraves R. B.
Published: September 2002

The Mars Pathfinder lander carried two magnet arrays, each containing five small permanent magnets of varying strength. The magnet arrays were passively exposed to the wind borne dust on Mars. By the end of the Mars Pathfinder mission a bull's‐eye pattern was visible on the four strongest magnets of the arrays showing the presence of magnetic dust particles. From the images we conclude that the dust suspended in the atmosphere is not solely single phase particles of hematite (α‐Fe2O3) and that single phase particles of the ferrimagnetic minerals maghemite (γ‐Fe2O3) or magnetite (Fe3O4) are not present as free particles in any appreciable amount. The material on the strongest magnets seems to be indistinguishable from the bright surface material around the lander. From X‐ray fluorescence it is known that the soil consists mainly of silicates. The element iron constitutes about 13% of the soil. The particles in the airborne dust seem to be composite, containing a few percent of a strongly magnetic component. We conclude that the magnetic phase present in the airborne dust particles is most likely maghemite. The particles thus appear to consist of silicate aggregates stained or cemented by ferric oxides, some of the stain and cement being maghemite. These results imply that Fe2+ ions were leached from the bedrock, and after passing through a state as free Fe2+ ions in liquid water, the Fe2+ was oxidized to Fe3+ and then precipitated. It cannot, however, be ruled out that the magnetic particles are titanomagnetite (or titanomaghemite) occurring in palagonite, having been inherited directly from the bedrock.
Imager for Mars Pathfinder (IMP) image calibration
Authors: Reid R. J. , Smith P. H. , Lemmon M. , Tanner R. , Burkland M. , Wegryn E. , Weinberg J. , Marcialis R. , Britt D. T. , Thomas N. , Kramm R. , Dummel A. , Crowe D. , Bos B. J. , Bell J. F. , Rueffer P. , Gliem F. , Johnson J. R. , Maki J. N. , Herkenhoff K. E. , Singer R. B.
Published: September 2002

The Imager for Mars Pathfinder returned over 16,000 high‐quality images from the surface of Mars. The camera was well‐calibrated in the laboratory, with <5% radiometric uncertainty. The photometric properties of two radiometric targets were also measured with 3% uncertainty. Several data sets acquired during the cruise and on Mars confirm that the system operated nominally throughout the course of the mission. Image calibration algorithms were developed for landed operations to correct instrumental sources of noise and to calibrate images relative to observations of the radiometric targets. The uncertainties associated with these algorithms as well as current improvements to image calibration are discussed.
Assessment of Mars Pathfinder landing site predictions
Authors: Golombek M. P. , Moore H. J. , Haldemann A. F. C. , Parker T. J. , Schofield J. T.
Published: September 2002

Remote sensing data at scales of kilometers and an Earth analog were used to accurately predict the characteristics of the Mars Pathfinder landing site at a scale of meters. The surface surrounding the Mars Pathfinder lander in Ares Vallis appears consistent with orbital interpretations, namely, that it would be a rocky plain composed of materials deposited by catastrophic floods. The surface and observed maximum clast size appears similar to predictions based on an analogous surface of the Ephrata Fan in the Channeled Scabland of Washington state. The elevation of the site measured by relatively small footprint delay‐Doppler radar is within 100 m of that determined by two‐way ranging and Doppler tracking of the spacecraft. The nearly equal elevations of the Mars Pathfinder and Viking Lander 1 sites allowed a prediction of the atmospheric conditions with altitude (pressure, temperature, and winds) that were well within the entry, descent, and landing design margins. High‐resolution (∼38 m/pixel) Viking Orbiter 1 images showed a sparsely cratered surface with small knobs with relatively low slopes, consistent with observations of these features from the lander. Measured rock abundance is within 10% of that expected from Viking orbiter thermal observations and models. The fractional area covered by large, potentially hazardous rocks observed is similar to that estimated from model rock distributions based on data from the Viking landing sites, Earth analog sites, and total rock abundance. The bulk and fine‐component thermal inertias measured from orbit are similar to those calculated from the observed rock size‐frequency distribution. A simple radar echo model based on the reflectivity of the soil (estimated from its bulk density), and the measured fraction of area covered by rocks was used to approximate the quasi‐specular and diffuse components of the Earth‐based radar echos. Color and albedo orbiter data were used to predict the relatively dust free or unweathered surface around the Pathfinder lander compared to the Viking landing sites. Comparisons with the experiences of selecting the Viking landing sites demonstrate the enormous benefit the Viking data and its analyses and models had on the successful predictions of the Pathfinder site. The Pathfinder experience demonstrates that, in certain locations, geologic processes observed in orbiter data can be used to infer surface characteristics where those processes dominate over other processes affecting the Martian surface layer.
Formation of Beta Regio, Venus: Results from measuring strain
Authors: Rathbun Julie A. , Janes Daniel M. , Squyres Steven W.
Published: September 2002

Beta Regio is an area of rifting and volcanism on Venus, constituting a topographic rise. A shield volcano, Theia Mons, lies near the center of the region and is surrounded by several radially oriented rifts. We use Magellan altimetry, gravity, and synthetic aperture radar data of the area to constrain some subsurface parameters. First, we derive hoop strain. Using altimetry data and a fault dip angle derived from the split crater Somerville, we determine the extension in the rifts surrounding Beta Regio. We then derive the hoop strain accommodated by the rifts from the extension in these rifts. Except near Theia Mons, the hoop strain follows the shape expected from a mantle upwelling. The difference near the volcano, we believe, is due to volcanic infilling. We then model three observable quantities, the newly derived strain along with gravity and uplift, using two separate modeling techniques, one for the strain and uplift and another for the gravity. The model results show that the data are consistent with the view that a relatively low density contrast region now exists below Beta and has caused the uplift and rifting in the region.
Admissible height of local roughness of Titan's landscape
Authors: Dimitrov Vasili I. , Bar‐Nun Akiva
Published: September 2002

Titan's landscape has been formed by both short‐term phenomena, such as internal tectonic processes and atmospheric activity, and long‐term factors of planetary scale, such as global stress and gravity forces. Short‐term phenomena can be ignored since the timescale of any relaxation process 5 × 1014 s is shorter than Titan's lifetime 1.4 × 1017 s. Global stress can be ignored as well, because Titan's figure is practically a right sphere. Thus both Titan's body as a whole and the crust in particular are free from any stress, except the gravity force. For the cold, stable, incompressible Titan, the only significant relief‐forming factor is the counteraction between the gravity force and the crust's ability to resist this force. This results in a simple estimation of the maximal height of any admissible feature located above ocean level (RAOL) Hmax = 3σ/ρg. The issue is the proper estimation of the crust density and real shear stress. Chemically, Titan's crust is an ice‐rock medium, while physically, it is a frozen two‐phase system, the density of the crust being equal to 1.81 × 103 kg m−3. Titan's crust is considered here to be similar to Earth's permafrost, corrected for a lower temperature. This medium was treated as a quasi‐isotropic entity that is subjected to a slow viscoelastic deformation, the ultimate shear stress being equal to 2.15 MPa. The estimation of the admissible height of Titan's RAOL results eventually in Hmax ∼ 1900 m. The validity of the gravitational approach was verified by the calculation of known Hmax for the rocky inner planets, Earth, Venus, Mars, and the icy Jovian satellites, Ganymede and Callisto, and it appears to be reliable within ∼50%.