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NASA NTRS1994

The Arctic Boundary Layer Expedition (ABLE-3B): July - August 1990

The Arctic Boundary Layer Expedition (ABLE) 3B used data from ground-based, aircraft, and satellite platforms to characterize the chemistry and dynamics of the troposphere in subarctic and Arctic regions of midcontinent and eastern Canada during July - August 1990. This paper reports the experimental design for ABLE 3B and a brief overview of results. The detailed results are presented in a series of papers in this issue. The chemical composition of the atmospheric mixed layer over remote tundra, boreal wetland, and forested environments was influenced by emissions of CH4 and nonmethane hydrocarbons from biogenic sources, emissions of gases and aerosols from local biomass burning, and transport of pollutants into the study areas from urban/industrial sources. Minimum concentrations of both trace gas and aerosol species in boundary layer air were associated with Arctic source areas. In the free troposphere the biospheric influence was undetectable, and major sources of chemical variability were related to long-range transport of pollutants into the study areas from biomass burning and industrial sources in Alaska and the Great Lakes regions, respectively. Minimum concentrations of both trace gas and aerosol species in the free troposphere were associated with a persistent, widespread air mass which both chemistry and air mass trajectory analyses suggested had originated in the tropical Pacific. Subsidence of air from the upper troposphere and lower stratosphere frequently enhanced ozone and influenced other trace gas and aerosol species at midtropospheric altitudes. The North American Arctic is a complex dynamical and chemical environment with considerable spatial and temporal variability in aerosol and trace gas concentrations. The use of atmospheric chemical indicators for climate change detection will require a much more comprehensive Arctic monitoring program than currently exists.

Harriss, R. C.↗
NASA NTRS1994

Stratospheric/tropospheric exchange affecting the northern wetlands regions of Canada during summer 1990

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted over the northern wetlands region of Canada during July and August 1990. Several stratospheric/tropospheric exchange events were noted by zenith-looking airborne lidar and in situ measurements of ozone and other trace gas species. Isentropic trajectories and potential vorticity analyses are utilized to determine the frequency of stratospheric inputs which would have affected the tropospheric column over the Moosonee and Schefferville regions and to describe the favored pathways of transport of stratospheric air arriving at these locations. At the 310 K potential temperature level (middle troposphere), trajectories having 'aged stratospheric' values of potential vorticity at some point in their 5-day history arrived at Moosonee or Schefferville roughly 40% of the time during the ABLE 3B study period, most often via large-scale subsidence enroute from 'stratospheric input regions' over the Arctic Ocean or northern and central Canada. At 325 K (upper troposphere), 'fresh' stratospheric input was evident on about 80% of the trajectories, most often associated with jet streaks within the polar and Arctic jet streams. A case study is presented which illustrates both of these general stratospheric input processes.

Bachmeier, A. Scott↗
NASA NTRS1994

Coolness in the tropical Pacific during an El Nino episode

The response of radiation budgets to changes in water vapor and clouds in an El Nino episode is investigated using the analyzed sea surface temperature (SST) and satellite-derived clouds and the earth radiation budgets for the tropical Pacific (30 deg N-30 deg S, 100 deg E-100 deg W). Analyses are performed for April 1985 and April 1987. The former is a non-El Nino year and the latter is an El Nino year. Compared to April 1985, when the SST over the central and eastern equatorial Pacific is approximately 2 C lower, the high-level cloudiness in April 1987 increases in the central and eastern equatorial Pacific. Corresponding to the increase in cloudiness, the outgoing longwave radiation and the net downward solar radiation at the top of the atmosphere decrease. The patterns of these changes are reversed in the western tropical Pacific and the Northern Hemispheric (NH) subsidence region centered at approximately 20 deg N, indicating an eastward shift of the convection center from the maritime continents to the central equatorial Pacific and a strengthened NH Hadley circulation. The earth-atmosphere system in the region receives less radiative energy by 4 W/sq m in the warmer month of April 1987 than in the month of April 1985, which is primarily caused by a reduced atmospheric clear sky greenhouse effect in the NH tropical Pacific in April 1987. Clouds have strong effects on both the IR and solar radiation, but the net effect on the radiation budget at the top of the atmopshere changes only slightly between April 1985 and April 1987. The results are consistent with Lindzen's hypothesis that reduced upper-tropospheric water vapor in the vicinity of the enhanced convection region produces cooling that counteracts warming in the Tropics.

Chou, Ming-Dah↗
NASA NTRS1995

Comparison between global latent heat flux computed from multisensor (SSM/I and AVHRR) and from in situ data

Comprehensive Ocean-Atmosphere Data Set (COADS) and satellite-derived parameters are input to a similarity theory-based model and treated in completely equivalent ways to compute global latent heat flux (LHF). In order to compute LHF exclusively from satellite measurements, an empirical relationship (Q-W relationship) is used to compute the air mixing ratio from Special Sensor Microwave/Imager (SSM/I) precipitable water W and a new one is derived to compute the air temperature also from retrieved W(T-W relationship). First analyses indicate that in situ and satellite LHF computations compare within 40%, but systematic errors increase the differences up to 100% in some regions. By investigating more closely the origin of the discrepancies, the spatial sampling of ship reports has been found to be an important source of error in the observed differences. When the number of in situ data records increases (more than 20 per month), the agreement is about 50 W/sq m rms (40 W/sq m rms for multiyear averages). Limitations of both empirical relationships and W retrieval errors strongly affect the LHF computation. Systematic LHF overestimation occurs in strong subsidence regions and LHF underestimation occurs within surface convergence zones and over oceanic upwelling areas. The analysis of time series of the different parameters in these regions confirms that systematic LHF discrepancies are negatively correlated with the differences between COADS and satellite-derived values of the air mixing ratio and air temperature. To reduce the systematic differences in satellite-derived LHF, a preliminary ship-satellite blending procedure has been developed for the air mixing ratio and air temperature.

Jourdan, Didier↗
NASA NTRS1993

Airborne Arctic Stratospheric Expedition 2: An overview

The sudden onset of ozone depletion in the antarctic vortex set a precedent for both the time scale and the severity of global change. The Airborne Antarctic Ozone Experiment (AAOE), stages from Punta Arenas, Chile, in 1987, established that CFCs, halons, and methyl bromide, the dominant sources of chlorine and bromide radicals in the stratosphere, control the rate of ozone destruction over the Antarctic; that the vortex is depleted in reactive nitrogen and water vapor; and that diabatic cooling during the antarctic winter leads to subsidence within the vortex core, importing air from higher altitudes and lower latitudes. This last conclusion is based on observed dramatic distortion in the tracer fields, most notably N2O. In 1989, the first Airborne Arctic Stratospheric Expedition (AASE-1), staged from Stavanger, Norway, and using the same aircraft employed for AAOE (the NASA ER-2 and the NASA DC-8), discovered that while NO(x) and to some degree NO(y) were perturbed within the arctic vortex, there was little evidence for desiccation. Under these (in contrast to the antarctic) marginally perturbed conditions, however, ClO was found to be dramatically enhanced such that a large fraction of the available (inorganic) chlorine resided in the form of ClO and its dimer ClOOCl. This leaves two abiding issues for the northern hemisphere and the mission of the second Airborne Arctic Stratospheric Expedition (AASE-2): (1) Will significant ozone erosion occur within the arctic vortex in the next ten years as chlorine loading in the stratosphere exceeds four parts per billion by volume? (2) Which mechanisms are responsible for the observed ozone erosion poleward of 30 deg N in the winter/spring northern hemisphere reported in satellite observations?

Anderson, James G.↗
NASA NTRS1993

AASE-2 in-situ tracer correlations of methane, nitrous oxide, and ozone as observed aboard the DC-8

We report in situ stratospheric measurements of CH4, N2O, and O3 obtained aboard the NASA DC-8 during the January-March 1992 Airborne Arctic Stratospheric Expedition 2 field campaign. These data demonstrate a strong linear correlation between N2O and CH4 in the lower stratosphere thus indicating that both species are effective tracers of stratospheric air motion. Measurements of both species on constant geometric height surfaces indicate that significant subsidence of the arctic stratospheric air mass occurred at DC-8 altitudes over the course of the AASE-2 expedition. In addition, a widespread reduction in O3 mixing ratios (up to 20%) relative to these conserved tracers was also observed in the lower stratosphere in March as compared to January and February results.

Collins, J. E., Jr.↗
NASA NTRS1993

Active tectonics in southern Xinjiang, China: Analysis of terrace riser and normal fault scarp degradation along the Hotan-Qira fault system

The northern piedmont of the western Kunlun mountains (Xinjiang, China) is marked at its easternmost extremity, south of the Hotan-Qira oases, by a set of normal faults trending N50E for nearly 70 km. Conspicuous on Landsat and SPOT images, these faults follow the southeastern border of a deep flexural basin and may be related to the subsidence of the Tarim platform loaded by the western Kunlun northward overthrust. The Hotan-Qira normal fault system vertically offsets the piedmont slope by 70 m. Highest fault scarps reach 20 m and often display evidence for recent reactivations about 2 m high. Successive stream entrenchments in uplifted footwallls have formed inset terraces. We have leveled topographic profiles across fault scarps and transverse abandoned terrace risers. The state of degradation of each terrace edge has been characterized by a degradation coefficient tau, derived by comparison with analytical erosion models. Edges of highest abandoned terraces yield a degradation coefficient of 33 +/- 4 sq.m. Profiles of cumulative fault scarps have been analyzed in a similar way using synthetic profiles generated with a simple incremental fault scarp model.

Avouac, Jean-Philippe↗
NASA NTRS1994

Chemical change in the arctic vortex during AASE 2

We measured column abundances of HF, HCl, O3, HNO3, and H2O on the NASA DC-8 during the AASE II campaign, using thermal emission spectroscopy. We made multiple traversals of the Arctic vortex and surroundings. Using HF as a tracer, we remove the effects of subsidence from the measured column abundances; perturbations in the resulting column abundances are attributed to chemical processing. We find that by January 1992 the stratospheric column in the vortex had been chemically depleted by about (55+/-10)% in HCl and (35+/-10)% in O3, and enhanced by about (15+/-10)% in HNO3 and (0+/-10)% in H2O.

Traub, Wesley A.↗
NASA NTRS1995

Uplift of the Kenai Peninsula, Alaska, since the 1964 Prince William Sound earthquake

Using Global Positioning System (GPS) receivers, we reoccupied several leveling benchmarks on the Kenai Peninsula of Alaska which had been surveyed by conventional leveling immediately following the March 27, 1964, Prince William Sound earthquake (M(sub w) = 9.3). By combining the two sets of measurements with a new, high-resolution model of the geoid in the region, we were able to determine the cumulative 1993-1964 postseismic vertical displacement. We find uplift at all of our benchmarks, relative to Seward, Alaska, a point that is stable according to tide gauge data. The maximum uplift of about 1 m occurs near the middle of the peninsula. The region of maximum uplift appears to be shifted northwest relative to the point of maximum coseismic subsidence. If we use tide gauge data at Nikishka and Seward to constrain the vertical motion, then the observed uplift has a trenchward tilt (down to the southeast) as well as an arching component. To explain the observations, we use creep-at-depth models. Most acceptable models require a fault slip of about 2.75 m, although this result is not unique. If the slip has been continuous since the 1964 earthquake, then the average slip rate is nearly 100 mm/yr, twice the plate convergence rate. Comparing the net uplift achieved in 29 years with that observed over 11 years in an adjacent region southeast of Anchorage, Alaska, we conclude that the rate of uplift is decreasing. A further decrease in the uplift rate is expected as the 29-year averaged displacement rate is about twice the plate convergence rate and therefore cannot be sustained over the entire earthquake cycle.

Cohen, Steven↗
NASA NTRS1995

Variability of moisture in the upper troposphere as inferred from TOVS satellite observations and the ECMWF model analyses in 1989

Large-scale variability of moisture in the upper troposphere is examined using TIROS (Television Infrared Observation Satellite) Operational Vertical Sounder (TOVS) satellite observations and European Centre for Medium-Range Weather Forecasts (ECMWF) model analyses from 1989 in the latitude band from 40 deg N to 40 S. To compare these dissimilar datasets, upwelling radiances were computed for the 6-7 micrometer water vapor band from the ECMWF temperature and moisture analyses, and these computed radiances were compared to the corresponding TOVS satellite observations. The ECMWF-based radiances reproduce the general locations and seasonal cycle of the TOVS-observed moisture features, particularly after an improved convective parameterization scheme was adopted by ECMWF in May 1989. However, the ECMWF analysis scheme still results in much milder lateral moisture gradients and seasonal contrasts than indicated by the TOVS observations. Seasonally, the upper troposphere in each hemisphere dries in winter and moistens in summer, but there are regions in each hemisphere that run counter to this seasonal trend, apparently depending on continental- and monsoon-scale dynamics. Dynamically, the TOVS-observed regions of significant subtropical dryness are correlated with persistent subsidence indicated by ECMWF 300-mb vertical velocity analyses. The TOVS radiance observations indicate large variations in space and time of the upper-tropospheric moisture field, which are not fully captured by the ECMWF analyses.

Salathe, Eric P., Jr.↗
NASA NTRS1994

Climatic implications of the seasonal variation of upper troposphere water vapor

Satellite observations indicate that the humidity of the upper troposphere is higher in summer than in winter. We use general circulation model (GCM) simulations to explore the processes that maintain upper troposphere water vapor and determine its seasonal cycle. In the subtropics, drying by Hadley cell subsidence and stratiform condensation is offset primarily by moistening by eddies, with moist convection playing a minor role. Elsewhere, both mean meridional circulation and eddies moisten the upper troposphere and are balanced primarily by stratiform condensation drying. The effect of the seasonal shift of the Hadley cell is limited to latitudes equatorward of 30 deg. At higher latitudes where the largest observed summer moistening occurs, eddy moisture fluxes are primarily responsible despite the eddies being weaker in summer than winter. The same mechanism causes upper level humidity to increase in GCM climate warming simulations. The observed seasonal variation may thus be a good proxy for decadal climate change. This suggests that upper troposphere water vapor feedback is positive at all latitudes, consistent with GCM predictions.

Del Genio, Anthony D.↗
NASA NTRS1993

A regional simulation of marine boundary-layer clouds

A regional version of a two-layer model is used to simulate marine boundary-layer clouds over an area (20 deg - 42 deg and 117 - 145 deg W) of the eastern North Pacific. The large-scale conditions requried for the model are provided by European Centre for Medium-Range Weather Forecasts analyses and the Comprehensive Ocean-Atmosphere Data Set. The simulated cloud-top height and fractional cloudiness are compared with satellite data. The model predicts a realistic pattern of cloud-top height, although the heights appear to be overestimated in the southwest and underestimated along the eastern border of the domain. The overall simulated cloudiness is qualitatively comparable to the satellite derived, but the model predicts excessive cloudiness over cold ocean surfaces and insufficient cloudiness over an area centered at 30 deg N, 130 deg W. The possible causes for these discrepancies are discussed. Budget studies show that horizontal advection contributes substantially to the cloud-top height and cloud-layer moisture budgets. Thus, one may need to consider the effects of horizontal advection when validating steady-state solutions of a one-dimensional model. The simulated cloudiness is shown to be sensitive to large-scale subsidence, moisture above the clouds, and drizzle. An increase in the large-scale divergence by 2 x 10(exp -6)/sec decreases average cloudiness for the area by 30%. An increase in the moisture at 850 mb by 2 g/kg increases the cloudiness by 20%. The suppression of drizzle in the model increases the cloudiness by 34% and significantly changes its pattern.

Wang, Shouping↗
NASA NTRS1993

Use of stratospheric aerosol properties as diagnostics of Antarctic vortex processes

Physical properties of the stratospheric aerosol population are inferred from cloud-free SAGE II multiwavelength extinction measurements in the Antarctic during late summer (February/March) and spring (September/October, November). Seasonal changes in these properties are used to infer physical processes occurring in the Antarctic stratosphere over the course of the winter. The analysis suggests that the apparent springtime cleansing of the Antarctic stratosphere is the result of aerosol redistribution through subsidence of the polar vortex air mass and sedimentation of large polar stratospheric cloud particles. The analysis also suggests that vortex processes are responsible for a significant downward transport of aerosol through the tropopause.

Thomason, Larry W.↗
NASA NTRS1993

CITE 3 meteorological highlights

Meteorological highlights from the third NASA Global Tropospheric Experiment Chemical Instrumentation Test and Evaluation (GTE/CITE 3) are presented. During August and September 1989, research flights were conducted from Wallops Island, Virginia, and Natal, Brazil, and included airborne sampling of air masses over adjacent regions of the Atlantic Ocean. Isentropic backward trajectory calculations, wind vector/streamline fields, rawinsonde data, and GOES and METEOSAT satellite imagery are utilized to examine the meteorological conditions for each flight and to determine the transport paths of the sampled air masses. Some aspects of the chemical signatures of the sampled air are also discussed. During the series of flights based at Wallops Island, Virginia, the flow into the experiment area was governed primarily by the position of the North Atlantic subtropical anticyclone. The large-scale tropospheric circulation switched from primarily a marine flow during flights 1-4, to a predominantly offshore mid-latitude continental flow during flights 5-10. During these later flights, the regional influences of large eastern U.S. cities along with vertical mixing by typical summertime convective activity strongly influenced the chemical characteristics of the sampled air. During the series of flights based at Natal, Brazil, the dominant synoptic feature was the South Atlantic subtropical anticyclone which generally transported air across the tropical Atlantic toward eastern Brazil. Pronounced subsidence and a well-defined trade wind inversion often characterized the lower and middle troposphere over the Natal region. Some high-altitude recirculation of air from South America was observed, as was cross-equatorial transport which had come from northern Africa. Biomass burning plumes were observed on segments of all of the flights, the source region being the central and southern savannah regions of Africa.

Shipham, Mark C.↗
NASA NTRS1994

Topography and tectonics of the central New Madrid seismic zone: Results of numerical experiements using a three-dimensional boundary element program

We present results of a series of numerical experiments designed to test hypothetical mechanisms that derive deformation in the New Madrid seismic zone. Experiments are constrained by subtle topography and the distribution of seismicity in the region. We use a new boundary element algorithm that permits calcuation of the three-dimensional deformation field. Surface displacement fields are calculated for the New Madrid zone under both far-field (plate tectonics scale) and locally derived driving strains. Results demonstrate that surface displacement fields cannot distinguish between either a far-field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults. Thus, neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present-day deformation. We have also tested hypotheses about strain accommodation within the New Madrid contractional step-over by including linking faults, two southwest dipping and one vertical, recently inferred from microearthquake data. Only those models with step-over faults are able to predict the observed topography. Surface displacement fields for long-term, relaxed deformation predict the distribution of uplift and subsidence in the contractional step-over remarkably well. Generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step-over region, with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked. These models suggest that the gently dippling central step-over fault is a reverse fault and that the steeper fault, extending to the southeast of the step-over, acts as a normal fault over the long term.

Gomberg, Joan↗
NASA NTRS1995

Evolution of the Pinatubo volcanic aerosol column above Pasadena, California observed with a mid-infrared backscatter lidar

The evolution of the volcanic debris plume originating from the June 1991 eruption of Mt. Pinatubo has been monitored since its genesis using a ground-based backscatter lidar facility sited at the Jet Propulsion Laboratory (JPL). Both absolute and relative pre- and post-Pinatubo backscatter observations are in accord with Mie scattering projections based on measured aerosol particle size distributions reported in the literature. The post-Pinatubo column-integrated backscatter coefficient peaked approximately 400 days after the eruption, and the observed upper boundary of the aerosol column subsided at a rate of approximately 200 m/mon.

Tratt, David M.↗
NASA NTRS1995

Noachian and Hesperian modification of the original Chryse impact basin topography

We prospose a new center and ring assignment for the original Chryse impact basin based upon photogeologic mapping of Noachian outcrops and re-examination of the published geology using orthographic projections. While others have centered the Chryse impact on the topographic low associated with Hesperian volcanic and fluvial deposits, we suggest that the center of the Noachian-age excavation cavity was located approximately 800 km to the north, and that the basin topography was modified significantly from the Noachian into the Hesperian. Evolution of the topographic low included structural modification by a later impact centered in Acidalia, restricted volcanic deposition and loading, localized subsidence, and restricted deposition from the circum-Chryse outlfow channels.

Stockman, S. A.↗
NASA NTRS1994

Post-Gondwana geomorphic evolution of southwestern Africa: Implications for hte controls on landscape development from observations and numerical experiments

The relationship between morphology and surficial geology is used to quantify the denudation that has occurred across southwestern Africa sicne the fragmentation of Gondwana during the Early Mesozoic. Two main points emerge. Signficant denudation, of the order of kilometers, is widespread except in the Kalahari region of the continental interior. The denudation is systematically distributed so that the continental exterior catchment, draining directly to the Cape basin, is denuded to a greater depth than the interior catchment inland of the Great Escarpment. The analysis also implies tha the majority of the denudation occurred before the beginning of the Cenozoic for both teh exerior and interior catchments. Existing models of landscape development are reviewed, and implications of the denudation chronology are incorporated into a revised conceptual model. This revision implies tha thte primary effect of rifting on the subsequent landscape evolution is that it generates two distinct drainage regimes. A marginal upwarp, or rift flank uplift, separates rejuvenated rivers that drain into the subsiding rift from rivers in the continetal interior that are deflected but not rejuvenated. The two catchments evolve independently unless they are integrated by breaching of hte marginal upwarp. If this occurs, the exterior baselevel is communicated to the interior catchment that is denuded accordingly. Denudation rates generally decrease as the margin evolves, and this decrease is reinforced by the exposure of substrate that is resistant to denudation and/or a change to a more arid climate. The observations do not reveal a particular style of smaller-scale landscape evolution, sucha s escarpment retreat, that is responsible for the differential denudation across the region. It is proposed that numerical model experiments, which reflect the observational insights at the large scale, may identify the smaller-scale controls on escarpment development if the model and natural systems are analogous. Four numerical experiments are presented in which the roles of antecedent topography, resistant substrate, climte change, and lowering the baselevel of the interior catchment are investigated for an initially high elevation margin bordered by an escarpment. The model results suggest several styles of landscape evolution that are compatible with the observations. Escarpments may retreat in a regular manner, but they also degrade and are destroyed, only to reform at the drainage divide between exterior and interior catchments.

Gilchrist, Alan R.↗