Clark J. Weaver

Mineralogy of aeolian dust reaching the North Pacific Ocean: 2. Relationship of mineral assemblages to atmospheric transport patterns

Atmospheric dust particles actually comprise a small number of mineral assemblages. We determined the composition of these assemblages and their geographic distribution by statistical analysis of mineralogy data. Air mass trajectory analysis was used to identify transport paths and possible source regions for these samples. Three types of trajectories are most common, and each displays different proportions of four mineral assemblages. The first indicates outflow of air from central Asia and is associated with an illite-rich mineralogy. The second type comprises trajectories which include low-altitude transport over the Japanese islands, for mixed mineralogies which contained a small amount of the illite-rich end-member. The third type is made up of trajectories crossing the North American continent and is present exclusively in the eastern North Pacific area. There is a mixed mineralogy associated with these trajectories. Our results suggest that both source area mineralogy and transport pathway contribute to the compositional variability of the mineral aerosol at a given location. The results presented here show that event-specific meteorological data can be useful in understanding spatial patterns of eolian transport.

A three‐dimensional simulation of the ozone annual cycle using winds from a data assimilation system

The wind fields from the NASA Goddard stratospheric data assimilation procedure are used in a three-dimensional chemistry and transport model to produce an ozone simulation for the year September 11, 1991 to September 10, 1992. Photochemical production and loss are taken from the Goddard two-dimensional model. The calculated ozone is compared with observations from the total ozone mapping spectrometer (TOMS) onboard Nimbus 7 and the microwave limb sounder on the upper atmospheric research satellite. Although the model total ozone is about 50 Dobson units (DU; =2.69 × 10−16 molecules cm−2) lower than TOMS in the tropics and up to 70 DU higher than TOMS in middle to high latitudes, the simulated ozone fields reproduce many of the features in the observations. Even at the end of this integration, the synoptic features in the modeled total ozone are very similar to TOMS observations, indicating that the model maintains realistic values for the horizontal and vertical gradients, at least in the lower stratosphere. From this good comparison between model and observations on timescales ranging from days to months, we infer that the transport driven by the assimilated wind fields closely approximates the actual atmospheric transport. Therefore the assimilated winds are useful for applications which may be sensitive to the lower stratospheric transport.

Aviation Fuel Tracer Simulation: Model Intercomparison and Implications

An upper limit for aircraft-produced perturbations to aerosols and gaseous exhaust products in the upper troposphere and lower stratosphere (UT/LS) is derived using the 1992 aviation fuel tracer simulation performed by eleven global atmospheric models. Key findings are that subsonic aircraft emissions: 1) have not be responsible for the observed water vapor trends at 40°N; 2) could be a significant source of soot mass near 12 km, but not at 20 km, 3) might cause a noticeable increase in the background sulfate aerosol surface area and number densities (but not mass density) near the northern mid-latitude tropopause, and 4) could provide a global, annual mean top of the atmosphere radiative forcing up to +0.006 W/m² and −0.013 W/m² due to emitted soot and sulfur, respectively.

Thermodynamic Balance of Three-Dimensional Stratospheric Winds Derived from a Data Assimilation Procedure

Abstract The NASA/Goddard three-dimensional chemistry and transport model is driven by winds from a stratospheric data assimilation system. Synoptic- and planetary-scale patterns, apparent in satellite observations of trace constituents, are successfully reproduced for seasonal integrations. As model integrations proceed, however, the quality of simulations decreases, and systematic differences between calculation and measurement appear. The differences are explained by examining the zonal-mean residual circulation. The vertical residual velocity w* is calculated two ways: (i) from the diabatic heating rates and temperature tendency and (ii) from the Eulerian vertical velocity and the horizontal eddy heat flux convergence. The results from these calculations differ substantially. Periodic insertion of observational data during the assimilation process continually shocks the general circulation model and produces these differences, which leads to an overestimate of the mean vertical heat and constituent t...

Radiative forcing of Saharan dust: GOCART model simulations compared with ERBE Data

Abstract This study uses information on Saharan aerosol from a dust transport model to calculate radiative forcing values. The transport model is driven by assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System. The model produces global three-dimensional dust spatial information for four different mineral aerosol sizes. These dust fields are input to an offline radiative transfer calculation to obtain the direct radiative forcing due to the dust fields. These estimates of the shortwave reduction of radiation at the top of the atmosphere (TOA) compare reasonably well with the TOA reductions derived from Earth Radiation Budget Experiment (ERBE) and Total Ozone Mapping Spectrometer (TOMS) satellite data. The longwave radiation also agrees with the observations; however, potential errors in the assimilated temperatures complicate the comparison. Depending on the assumptions used in the calculation and the dust loading, the summertime forcing ranges from 0 to −18 W ...

Airborne Measurements of CO2 Column Concentration and Range Using a Pulsed Direct-Detection IPDA Lidar

We have previously demonstrated a pulsed direct detection IPDA lidar to measure range and the column concentration of atmospheric CO2. The lidar measures the atmospheric backscatter profiles and samples the shape of the 1,572.33 nm CO2 absorption line. We participated in the ASCENDS science flights on the NASA DC-8 aircraft during August 2011 and report here lidar measurements made on four flights over a variety of surface and cloud conditions near the US. These included over a stratus cloud deck over the Pacific Ocean, to a dry lake bed surrounded by mountains in Nevada, to a desert area with a coal-fired power plant, and from the Rocky Mountains to Iowa, with segments with both cumulus and cirrus clouds. Most flights were to altitudes >12 km and had 5–6 altitude steps. Analyses show the retrievals of lidar range, CO2 column absorption, and CO2 mixing ratio worked well when measuring over topography with rapidly changing height and reflectivity, through thin clouds, between cumulus clouds, and to stratus cloud tops. The retrievals shows the decrease in column CO2 due to growing vegetation when flying over Iowa cropland as well as a sudden increase in CO2 concentration near a coal-fired power plant. For regions where the CO2 concentration was relatively constant, the measured CO2 absorption lineshape (averaged for 50 s) matched the predicted shapes to better than 1% RMS error. For 10 s averaging, the scatter in the retrievals was typically 2–3 ppm and was limited by the received signal photon count. Retrievals were made using atmospheric parameters from both an atmospheric model and from in situ temperature and pressure from the aircraft. The retrievals had no free parameters and did not use empirical adjustments, and >70% of the measurements passed screening and were used in analysis. The differences between the lidar-measured retrievals and in situ measured average CO2 column concentrations were 6 km.

Direct insertion of MODIS radiances in a global aerosol transport model

Abstract In this paper results are presented from a simple offline assimilation system that uses radiances from the Moderate Resolution Imaging Spectroradiometer (MODIS) channels that sense atmospheric aerosols over land and ocean. The MODIS information is directly inserted into the Goddard Chemistry and Aerosol Radiation Transport model (GOCART), which simulates the following five aerosol types: dust, sea salt, black carbon, organic carbon, and sulfate. The goal is to produce three-dimensional fields of these aerosol types for radiative forcing calculations. Products from this assimilation system are compared with ground-based measurements of aerosol optical depth (AOD) from the Aerosol Robotic Network (AERONET). Insertion of MODIS radiances draws the GOCART model closer to the AERONET AOD. However, there are still uncertainties with surface reflectivity over moderately bright surfaces and with the amount of absorbing aerosol. Also described is the assimilation cycle. The forward model takes the aerosol ...

Tracer exchange between tropics and middle latitudes

The interaction between the tropics and middle latitudes is studied using a tracer emitted at 50 hPa along a great circle route between Los Angeles, USA and Sydney, Australia. Though designed to examine the impact of stratospheric aircraft, the study more generally addresses the transport between tropics and middle latitudes for a three month period from January through March 1989. The results show that air is transported from the tropics to middle latitudes by planetary scale and tropospheric cyclonic scale waves. Except for intrusions by these wave events, the tropics are substantially isolated throughout the lower stratosphere. These waves draw material out of the tropics which ends up in the middle latitude westerly jets, with little material entering the winter polar latitudes prior to the springtime transition. The summer southern hemisphere is characterized by tracer being drawn out in streamers that extend from north and west to south and east. The material in the tropics is zonally asymmetric. The material that reaches the troposphere comes down in the synoptic scale eddies and is concentrated in the middle latitude jet stream. These characteristics are similar to those observed during the dispersion of volcanic clouds.

Implications of three-dimensional tracer studies for two-dimensional assessments of the impact of supersonic aircraft on Stratospheric ozone

A two-dimensional (2D) model, which uses a residual circulation and diffusion, and a three-dimensional (3D) model, which uses winds from a stratospheric data assimilation system, have been used to estimate the transport and dispersion of aircraft exhaust (tracer) in the lower stratosphere. Four month calculations using the 2D model with tracer injected continuously between 40°–50° north and south latitudes are compared with similar 3D calculations for the same time period. The seasonal behavior of the tracer fields in the two models is similar. The zonal mean of the 3D tracer distribution resembles the 2D distribution when the 2D calculation uses a residual circulation derived from the assimilated wind fields, but the 3D distribution indicates more rapid vertical mixing. The similarity of the 2D and 3D tracer distributions suggests the similarity of the seasonal mean mass transport in both models. However, there is a significant difference in the placement of stratosphere/troposphere exchange in the two models. In the 2D model, tracer transport to the troposphere takes place mostly at high latitudes. In the 3D model, most tracer transport takes place at middle latitudes, and is clearly associated with synoptic scale events. This may be particularly important to assessment calculations, as the pollutant source is mostly in middle latitudes. The 3D model is also used to consider the buildup of tracer in oceanic flight corridors. North Atlantic (Boston-London), north Pacific (Los Angeles - Tokyo) and tropical (Los Angeles-Sydney) corridors are considered. For northern hemisphere winter, the tracer distributions remain zonally asymmetric. The tracer from Boston-London is largely excluded from the Aleutian anti-cyclone, and the tracer from Los Angeles - Tokyo is largely contained within the anticyclone. The tropical tracer distribution is also asymmetric; the time scale for zonal mixing is long compared to the time for meridional transport processes due to weak zonal winds. Although more of the tracer injected in the tropical corridor is transported to higher altitude than for the other corridors, the transfer of mass from the stratosphere to the troposphere is nearly the same for the three corridors. There are no systematic differences that suggest that one corridor is inherently more or less polluting than another.

A 5-year simulation of supersonic aircraft emission transport using a three-dimensional model

A 5-year simulation of supersonic aircraft exhaust using a three-dimensional transport model has been completed using winds from the NASA/Goddard data assimilation system. A tracer based on emission rates of reactive nitrogen species (NOy) for all forecasted flight routes is continuously injected into the model. A parameterized upper stratospheric loss mechanism and a tropospheric sink due to rainout approximately balance the nitrogen emissions after several years of integration. Maximum values for exhaust NOy occur during the northern hemisphere (NH) summer months, and minimum values occur during winter. The pollutant is most zonally asymmetric during the NH summer. The peak values are never more than twice the zonal mean. This supports the use of zonally averaged two-dimensional models to evaluate the impact of the exhaust on the lower stratospheric composition. Budget calculations from the transport model show that most exhaust released in the NH is transported downward into the troposphere, where it is destroyed. In the model, about 15–20% of exhaust released poleward of 30°N is transported into the tropics, where it is lofted. The stratospheric residence time for the exhaust is estimated to be 13 months.