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Pinar Kus
,
Christian M. Carrico
,
Mark J. Rood
, and
Allen Williams

Abstract

Closure experiments were completed to compare measured and modeled aerosol optical properties and their dependence on controlled relative humidity (RH) and wavelength of light. NaCl, (NH4)2SO4, and NH4NO3 aerosol particles with approximate geometric mass mean diameters of 0.2 μm and geometric standard deviations of 1.7 were tested as part of this study. High evaporative losses (up to 40%) were observed for NH4NO3 aerosol at this particle size range due to heating, and the results from these tests have been excluded from the closure analysis. Aerosol optical properties were measured with a RH-scanning nephelometry system (humidograph) and modeled with a Mie–Lorentz light scattering model. Particle size distributions were measured with a scanning differential mobility analyzer. Closure between the measured and modeled values of the total light scattering coefficient (σ sp), backscatter ratio (b), and Ångström exponent (å) for dry (low RH) aerosols was achieved within 0.0%–5%, 4%–15%, and 3%–17%, respectively. The values of f(RH), hemispheric b, and å at 80% RH agreed within 2%–27%, 1%–27%, and 1%–28%, respectively. Correcting for nephelometer nonidealities, including a heating artifact, improved the agreement between the measured and predicted σ sp values at RH = 80% from 35% to 13% for the TSI nephelometer at the maximum heating condition, and from 18% to 11% for the Radiance Research, Inc. (RR), nephelometer. Accurate quantification of the closure for these optical properties is important when establishing visibility standards, and assessing the progress toward meeting those standards, as well as reducing the uncertainties in estimating radiative forcing due to those aerosols.

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Zhidong Li
,
Allen L. Williams
, and
Mark J. Rood

Abstract

Atmospheric aerosol particles consisting of ammonium sulfate [(NH4)2SO4] or sodium chloride (NaCl) have reasonably well-defined hygroscopic properties compared to other materials in aerosol particles, such as organic material. The effect of internally mixing organic compounds with these salts is not clear when considering the hygroscopic properties of the resulting particles, including activation of particles in clouds. This research describes the activation of aerosol particles consisting of sodium dodecyl sulfate (SDS) and NaCl solute. SDS is used as a surrogate for soluble atmospheric surfactants. Köhler theory is used to model droplet activation while considering droplet properties such as surface tension (σ), surface excess surfactant concentration, and critical micelle concentration (CMC).

Reduction in critical supersaturation (S c ) caused by the reduction in σ (Kelvin effect) associated with the surfactant is dominated by the increase in S c with the decreasing number of moles of solute in the droplet (Raoult effect) as surfactant displaces NaCl solute mass. For an initially dry 0.1-μm diameter particle, S c increases from 0.10 to 0.25 as NaCl solute mass changes from 100% (0% SDS solute) to 0% (100% SDS solute). Such dependence of cloud droplet activation on mixed solute composition is important when considering atmospheric chemistry and physics. The partitioning of materials between aerosol particles and cloud drops are influenced by mixing the surfactant with NaCl. Also, inhibition of droplet activation when displacing NaCl solute with a high molecular weight soluble surfactant could significantly influence the indirect effects aerosols have on climate change.

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Paul E. Johnston
,
Christopher R. Williams
, and
Allen B. White

Abstract

Using NOAA’s S-band High-Power Snow-Level Radar (HPSLR), a technique for estimating the rain drop size distribution (DSD) above the radar is presented. This technique assumes the DSD can be described by a four parameter, generalized gamma distribution (GGD). Using the radar’s measured average Doppler velocity spectrum and a value (assumed, measured, or estimated) of the vertical air motion w, an estimate of the GGD is obtained. Four different methods can be used to obtain w. One method that estimates a mean mass-weighted raindrop diameter Dm from the measured reflectivity Z produces realistic DSDs compared to prior literature examples. These estimated DSDs provide evidence that the radar can retrieve the smaller drop sizes constituting the “drizzle” mode part of the DSD. This estimation technique was applied to 19 h of observations from Hankins, North Carolina. Results support the concept that DSDs can be modeled using GGDs with a limited range of parameters. Further work is needed to validate the described technique for estimating DSDs in more varied precipitation types and to verify the vertical air motion estimates.

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John C. Carstens
,
Allen Williams
, and
Joseph T. Zung

Abstract

The interaction of two growing droplets in a supersaturated atmosphere has been examined, and the temperature and vapor density profiles have been determined. It is found that the smaller droplet tends to “catch up” with the larger at a slower rate than predicted by conventional diffusion theory. Consideration of droplet fallspeeds leads to the conclusion that, under atmospheric conditions, growth interaction becomes significant only for droplet “pairs” having equal or nearly equal radii. The number of such pairs is generally small enough so that the effect on the size distribution is quite small. Of a much greater importance is the possibility of a resulting attractive diffusio-phoretic force between two growing drops which, in turn, gives rise to a net velocity of one drop toward the other. If this diffusion force of attraction becomes sufficiently strong to overcome the hydrodynamic and thermo-phoretic forces acting in the opposite direction, both collision efficiencies and coagulation of small droplets could be further enhanced, thus accounting for departures from monodispersity in actual atmospheric clouds.

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Zhining Tao
,
Allen Williams
,
Ho-Chun Huang
,
Michael Caughey
, and
Xin-Zhong Liang

Abstract

Different cumulus schemes cause significant discrepancies in simulated precipitation, cloud cover, and temperature, which in turn lead to remarkable differences in simulated biogenic volatile organic compound (BVOC) emissions and surface ozone concentrations. As part of an effort to investigate the impact (and its uncertainty) of climate changes on U.S. air quality, this study evaluates the sensitivity of BVOC emissions and surface ozone concentrations to the Grell (GR) and Kain–Fritsch (KF) cumulus parameterizations. Overall, using the KF scheme yields less cloud cover, larger incident solar radiation, warmer surface temperature, and higher boundary layer height and hence generates more BVOC emissions than those using the GR scheme. As a result, the KF (versus GR) scheme produces more than 10 ppb of summer mean daily maximum 8-h ozone concentration over broad regions, resulting in a doubling of the number of high-ozone occurrences. The contributions of meteorological conditions versus BVOC emissions on regional ozone sensitivities to the choice of the cumulus scheme largely offset each other in the California and Texas regions, but the contrast in BVOC emissions dominates over that in the meteorological conditions for ozone differences in the Midwest and Northeast regions. The result demonstrates the necessity of considering the uncertainty of future ozone projections that are identified with alternative model physics configurations.

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Ho-Chun Huang
,
Xin-Zhong Liang
,
Kenneth E. Kunkel
,
Michael Caughey
, and
Allen Williams

Abstract

The impacts of air pollution on the environment and human health could increase as a result of potential climate change. To assess such possible changes, model simulations of pollutant concentrations need to be performed at climatic (seasonal) rather than episodic (days) time scales, using future climate projections from a general circulation model. Such a modeling system was employed here, consisting of a regional climate model (RCM), an emissions model, and an air quality model. To assess overall model performance with one-way coupling, this system was used to simulate tropospheric ozone concentrations in the midwestern and northeastern United States for summer seasons between 1995 and 2000. The RCM meteorological conditions were driven by the National Centers for Environmental Prediction/Department of Energy global reanalysis (R-2) using the same procedure that integrates future climate model projections. Based on analyses for several urban and rural areas and regional domains, fairly good agreement with observations was found for the diurnal cycle and for several multiday periods of high ozone episodes. Even better agreement occurred between monthly and seasonal mean quantities of observed and model-simulated values. This is consistent with an RCM designed primarily to produce good simulations of monthly and seasonal mean statistics of weather systems.

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Christopher R. Williams
,
Allen B. White
,
Kenneth S. Gage
, and
F. Martin Ralph

Abstract

In support of the 2004 North American Monsoon Experiment (NAME) field campaign, NOAA established and maintained a field site about 100 km north of Mazatlán, Mexico, consisting of wind profilers, precipitation profilers, surface upward–downward-looking radiometers, and a 10-m meteorological tower to observe the environment within the North American monsoon. Three objectives of this NOAA project are discussed in this paper: 1) to observe the vertical structure of precipitating cloud systems as they passed over the NOAA profiler site, 2) to estimate the vertical air motion and the raindrop size distribution from near the surface to just below the melting layer, and 3) to better understand the microphysical processes associated with stratiform rain containing well-defined radar bright bands.

To provide a climatological context for the profiler observations at the field site, the profiler reflectivity distributions were compared with Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) reflectivity distributions from the 2004 season over the NAME domain as well as from the 1998–2005 seasons. This analysis places the NAME 2004 observations into the context of other monsoon seasons. It also provides a basis for evaluating the representativeness of the structure of the precipitation systems sampled at this location. The number of rain events observed by the TRMM PR is dependent on geography; the land region, which includes portions of the Sierra Madre Occidental, has more events than the coast and gulf regions. Conversely, from this study it is found that the frequencies of occurrence of stratiform rain and reflectivity profiles with radar bright bands are mostly independent of region. The analysis also revealed that the reflectivity distribution at each height has more year-to-year variability than region-to-region variability. These findings suggest that in cases with a well-defined bright band, the vertical profile of the reflectivity relative to the height of the bright band is similar over the gulf, coast, and land regions.

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Xiaoyan Jiang
,
Sara A. Rauscher
,
Todd D. Ringler
,
David M. Lawrence
,
A. Park Williams
,
Craig D. Allen
,
Allison L. Steiner
,
D. Michael Cai
, and
Nate G. McDowell

Abstract

Rapid and broad-scale forest mortality associated with recent droughts, rising temperature, and insect outbreaks has been observed over western North America (NA). Climate models project additional future warming and increasing drought and water stress for this region. To assess future potential changes in vegetation distributions in western NA, the Community Earth System Model (CESM) coupled with its Dynamic Global Vegetation Model (DGVM) was used under the future A2 emissions scenario. To better span uncertainties in future climate, eight sea surface temperature (SST) projections provided by phase 3 of the Coupled Model Intercomparison Project (CMIP3) were employed as boundary conditions. There is a broad consensus among the simulations, despite differences in the simulated climate trajectories across the ensemble, that about half of the needleleaf evergreen tree coverage (from 24% to 11%) will disappear, coincident with a 14% (from 11% to 25%) increase in shrubs and grasses by the end of the twenty-first century in western NA, with most of the change occurring over the latter half of the twenty-first century. The net impact is a ~6 GtC or about 50% decrease in projected ecosystem carbon storage in this region. The findings suggest a potential for a widespread shift from tree-dominated landscapes to shrub and grass-dominated landscapes in western NA because of future warming and consequent increases in water deficits. These results highlight the need for improved process-based understanding of vegetation dynamics, particularly including mortality and the subsequent incorporation of these mechanisms into earth system models to better quantify the vulnerability of western NA forests under climate change.

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Wayne M. Angevine
,
Christoph J. Senff
,
Allen B. White
,
Eric J. Williams
,
James Koermer
,
Samuel T. K. Miller
,
Robert Talbot
,
Paul E. Johnston
,
Stuart A. McKeen
, and
Tom Downs

Abstract

Air pollution episodes in northern New England often are caused by transport of pollutants over water. Two such episodes in the summer of 2002 are examined (22–23 July and 11–14 August). In both cases, the pollutants that affected coastal New Hampshire and coastal southwest Maine were transported over coastal waters in stable layers at the surface. These layers were at least intermittently turbulent but retained their chemical constituents. The lack of deposition or deep vertical mixing on the overwater trajectories allowed pollutant concentrations to remain strong. The polluted plumes came directly from the Boston, Massachusetts, area. In the 22–23 July case, the trajectories were relatively straight and dominated by synoptic-scale effects, transporting pollution to the Maine coast. On 11–14 August, sea breezes brought polluted air from the coastal waters inland into New Hampshire.

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Sid-Ahmed Boukabara
,
Vladimir Krasnopolsky
,
Stephen G. Penny
,
Jebb Q. Stewart
,
Amy McGovern
,
David Hall
,
John E. Ten Hoeve
,
Jason Hickey
,
Hung-Lung Allen Huang
,
John K. Williams
,
Kayo Ide
,
Philippe Tissot
,
Sue Ellen Haupt
,
Kenneth S. Casey
,
Nikunj Oza
,
Alan J. Geer
,
Eric S. Maddy
, and
Ross N. Hoffman

Abstract

Promising new opportunities to apply artificial intelligence (AI) to the Earth and environmental sciences are identified, informed by an overview of current efforts in the community. Community input was collected at the first National Oceanic and Atmospheric Administration (NOAA) workshop on “Leveraging AI in the Exploitation of Satellite Earth Observations and Numerical Weather Prediction” held in April 2019. This workshop brought together over 400 scientists, program managers, and leaders from the public, academic, and private sectors in order to enable experts involved in the development and adaptation of AI tools and applications to meet and exchange experiences with NOAA experts. Paths are described to actualize the potential of AI to better exploit the massive volumes of environmental data from satellite and in situ sources that are critical for numerical weather prediction (NWP) and other Earth and environmental science applications. The main lessons communicated from community input via active workshop discussions and polling are reported. Finally, recommendations are presented for both scientists and decision-makers to address some of the challenges facing the adoption of AI across all Earth science.

Open access