Publications Published in JOURNAL OF GEOPHYSICAL RESEARCH

1. An overview of aircraft observations from the Pacific Dust Experiment campaign. J. L. Stith, V. Ramanathan, W. A. Cooper, G. C. Roberts, P. J. DeMott, G. Carmichael, C. D. Hatch, B. Adhikary, C. H. Twohy, D. C. Rogers, D. Baumgardner, A. J. Prenni, T. Campos, RuShan Gao, J. Anderson, Y. Feng.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2008
   DOI: 10.1029/2008JD010924
   Notes
   Fourteen research flights were conducted in the Pacific Dust Experiment (PACDEX) during April and May 2007 to sample pollution and dust outbreaks from east Asia as they traveled across the northern Pacific Ocean into North America and interacted with maritime storms. Significant concentrations of black carbon (BC, consisting of soot and other light-absorbing particles measured with a soot photometer 2 instrument) and dust were observed both in the west and east Pacific Ocean from Asian plumes of dust and pollution. BC particles were observed through much of the troposphere, but the major finding is that the percentage of these particles compared with the total number of accumulation mode particles increased significantly (by a factor of 2–4) with increasing altitude, with peak values occurring between 5 and 10 km. Dust plumes had only a small impact on total cloud condensation nuclei at the sampling supersaturations but did exhibit high concentrations of ice nuclei (IN). IN concentrations in dust plumes exce ded typical tropospheric values by 4–20 times and were similar to previous studies in the Saharan aerosol layer when differences in the number concentrations of dust are accounted for. Enhanced IN concentrations were found in the upper troposphere off the coast of North America, providing a first direct validation of the transport of high-IN-containing dust layers near the tropopause entering the North American continent from distant sources. A source-specific chemical transport model was used to predict dust and other aerosols during PACDEX. The model was able to predict several features of the in situ observations, including the general altitudes where BC was found and a peak in the ratio of BC to sulfate between 5 and 10 km.
   
   
   
2. A simple statistical-dynamical downscaling scheme based on weather types and conditional resampling. J. Boe´, L. Terray, F. Habets, and E. Martin.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2006
   DOI: 10.1029/2005JD006889
   Notes
   A multivariate statistical downscaling methodology is implemented to generate local precipitation and temperature series at different sites based on the results from a variable resolution general circulation model. It starts from regional climate properties to establish discriminating weather types for the chosen local variable, precipitation in this case. Intratype variations of the relevant forcing parameters are then taken into account by multivariate regression using the distances of a given day to the different weather types as predictors. The final step consists of conditional resampling. The methodology is evaluated in the Seine basin in France. Using reanalysis fields as predictors, satisfying results are obtained at daily timescale and concerning low-frequency variations, both for temperature and precipitation. The use of model results as predictors gives a realistic representation of regional climate properties. Nevertheless, as the validation of a statistical downscaling algorithm for present day
   
   
   
3. Cloud condensation nucleation activity of biomass burning aerosol. Markus D. Petters, Christian M. Carrico, Sonia M. Kreidenweis, Anthony J. Prenni,Paul J. DeMott, Jeffrey L. Collett Jr., Hans Moosmu¨ller.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2009
   DOI: 10.1029/2009JD012353
   Notes
   We examine the hygroscopic properties of particles freshly emitted from laboratory biomass burning experiments conducted during the second Fire Lab At Missoula Experiment (FLAME-II). Values of the hygroscopicity parameter, kappa, were derived from both hygroscopic growth measurements and size-resolved (30–300 nm in diameter) cloud condensation nuclei (CCN) measurements for smokes emitted by the open combustion of 24 biomass fuels from the United States and Asia. To analyze the complex cloud condensation nuclei response curves we propose a new inversion scheme that corrects for multiple charge effects without the necessity of prior assumptions about the chemical composition and mixing state of the particles. Kappa varied between 0.02 (weakly hygroscopic) and 0.8 (highly hygroscopic). For individual smokes, kappa was a function of particle size, with 250 nm particles being generally weakly hygroscopic and sub-100 nm particles being more hygroscopic. At any given size the emissions were often externally mixed, howing more and less hygroscopic growth modes and bimodal CCN activation spectra. Comparisons between growth factor-derived and CCN-derived hygroscopicities were consistent when taking this heterogeneity into account. A conceptual model of biomass burning emissions suggests that most particles are CCN active at the point of emission and do not require conversion in the atmosphere to more hygroscopic compositions before they can participate in cloud formation and undergo wet deposition.
   
   
   
4. Correlations of small cumuli droplet and drizzle drop concentrations with cloud condensation nuclei concentrations. James G. Hudson, Stephen Noble, Vandana Jha, Subhashree Mishra.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2009
   DOI: 10.1029/2008JD010581
   Notes
   Aircraft field measurements of cloud condensation nuclei (CCN) and cloud microphysics in maritime air masses showed ubiquitous influence of CCN. Flight averages of CCN concentrations and cloud droplet and drizzle drop concentrations were examined for as many as 17 flights during the Rain in Cumulus over the Ocean (RICO) project. CCN concentrations at only one supersaturation (S) of 1% measured at 100-m altitude were compared with cloud droplet and drizzle drop concentrations at six altitude bands between 600 and 3000 m. High positive correlations (R) between these CCN concentrations and the small size threshold of the cumulative cloud droplet concentrations (i.e., total activated cloud droplets) were found at all altitudes. These high R values also persisted for cloud parcels with a wide span of liquid water contents (LWCs), most of which were far below adiabatic (unmixed) values. For all but the lowest LWC parcels, R was essentially constant. There was an even more consistent negative R between CCN and large cloud droplet and drizzle drop concentrations. There was a sharp transition from positive to negative R over a small size range. The size at which this R transition occurred increased with altitude and LWC as overall droplet sizes increased with altitude or LWC. Entrainment seemed to show an opposite effect on R, but this was only apparent at the highest altitudes where entrainment was greatest and only for the smallest droplet sizes. These results indicate that the effect of CCN concentrations on cloud microphysics was pervasive with altitude, LWC, cloud droplet, and drizzle drop concentrations. This indicates greater impact of the indirect aerosol effect (IAE) in both of its manifestations, first IAE cloud radiation and second IAE precipitation.
   
   
   
5. First observation‐based estimates of cloud‐free aerosol radiative forcing across China. Zhanqing Li, Kwon‐Ho Lee,Yuesi Wang, Jinyuan Xin, Wei‐Min Hao.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2010
   DOI: 10.1029/2009JD013306
   Notes
   Heavy loading of aerosols in China is widely known, but little is known about their impact on regional radiation budgets, which is often expressed as aerosol radiative forcing (ARF). Cloud‐free direct ARF has either been estimated by models across the region or determined at a handful of locations with aerosol and/or radiation measurements. In this study, ARF is determined at 25 stations distributed across China where aerosol optical thickness has been measured since 2004. In combination with the single‐scattering albedo retrieved from ground and satellite measurements, ARF was determined at all the stations at the surface, inside the atmosphere, and at the top of atmosphere (TOA). Nationwide annual and diurnal mean ARF is found to be −15.7 ± 8.9 at the surface, 0.3 ± 1.6 at the TOA, and 16.0 ± 9.2Wm−2 inside the atmosphere. These values imply that aerosols have very little impact on the atmosphere‐surface system but substantially warm up the atmosphere at the expense of cooling the surface. The strong atmos heric absorption is likely to alter atmospheric thermodynamic conditions and thus affects circulation considerably.
   
   
   
6. Future change in wintertime atmospheric blocking simulated using a 20-km-mesh atmospheric global circulation model. Mio Matsueda, Ryo Mizuta, Shoji Kusunoki.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2009
   DOI: 10.1029/2009JD011919
   Notes
   Future change in the frequency of atmospheric blocking is investigated through present-day (1979–2003) and future (2075–2099) simulations using 20-, 60-, 120-, and 180-km-mesh atmospheric general circulation models (AGCMs) under the Intergovernmental Panel on Climate Change Special Reports on Emission Scenarios A1B emission scenario, focusing on the Northern Hemisphere winter (December–February). The results of present-day climate simulations reveal that the AGCM with the highest horizontal resolution is required to accurately simulate Euro-Atlantic blocking, whereas the AGCM with the lowest horizontal resolution is in good agreement with reanalysis data regarding the frequency of Pacific blocking. While the lower-resolution models accurately reproduce long-lived Pacific blocking, they are unable to accurately simulate long-lived Euro-Atlantic blocking. This result suggests that the maintenance mechanism of Euro-Atlantic blocking is different from that of Pacific blocking. In the future climate simulations, oth frequencies of Euro-Atlantic and Pacific blockings are predicted to show a significant decrease, mainly in the western part of each peak in present-day blocking frequency, where the westerly jet is predicted to increase in strength; no significant change is predicted in the eastern part of each peak. The number of Euro-Atlantic blocking events is predicted to decrease for almost all blocking durations, whereas the decrease in the number of Pacific blockings is remarkable for long-duration events. It is possible that long-lived (>25 days) Euro-Atlantic and Pacific blockings will disappear altogether in the future.
   
   
   
7. Future change in wintertime atmospheric blocking simulated using a 20-km-mesh atmospheric global circulation model. Mio Matsueda, Ryo Mizuta, Shoji Kusunoki.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2009
   DOI: 10.1029/2009JD011919
   Notes
   Future change in the frequency of atmospheric blocking is investigated through present-day (1979–2003) and future (2075–2099) simulations using 20-, 60-, 120-, and 180-km-mesh atmospheric general circulation models (AGCMs) under the Intergovernmental Panel on Climate Change Special Reports on Emission Scenarios A1B emission scenario, focusing on the Northern Hemisphere winter (December–February). The results of present-day climate simulations reveal that the AGCM with the highest horizontal resolution is required to accurately simulate Euro-Atlantic blocking, whereas the AGCM with the lowest horizontal resolution is in good agreement with reanalysis data regarding the frequency of Pacific blocking. While the lower-resolution models accurately reproduce long-lived Pacific blocking, they are unable to accurately simulate long-lived Euro-Atlantic blocking. This result suggests that the maintenance mechanism of Euro-Atlantic blocking is different from that of Pacific blocking. In the future climate simulations, oth frequencies of Euro-Atlantic and Pacific blockings are predicted to show a significant decrease, mainly in the western part of each peak in present-day blocking frequency, where the westerly jet is predicted to increase in strength; no significant change is predicted in the eastern part of each peak. The number of Euro-Atlantic blocking events is predicted to decrease for almost all blocking durations, whereas the decrease in the number of Pacific blockings is remarkable for long-duration events. It is possible that long-lived (>25 days) Euro-Atlantic and Pacific blockings will disappear altogether in the future.
   
   
   
8. Global observations of aerosol impacts on precipitation occurrence in warm maritime clouds. Tristan S. L’Ecuyer, Wesley Berg, John Haynes, Matthew Lebsock, Toshihiko Takemura.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2009
   DOI: 10.1029/2008JD011273
   Notes
   The impact of aerosols on precipitation occurrence in warm clouds is assessed using a combination of multisensor satellite cloud and precipitation data sets and aerosol information from both satellite and a global transport model. Aerosols are found to suppress the formation of precipitation in polluted regions, evidenced by a trend toward higher liquid water path prior to the onset of light rainfall. Polluted clouds are also found to be more vertically developed than those in more pristine environments. Coupled with an apparent reduction in the size of the raindrops that subsequently form in these clouds, these findings indicate that pollution inhibits precipitation processes by redistributing water among a greater number of smaller cloud droplets. Evidence is also provided that sea-salt aerosols have the opposite effect on precipitation development. Maritime clouds that form in regions of enhanced sea-salt concentrations tend to precipitate more frequently, form larger raindrops, and be less vertically dev
   
   
   
9. Global observations of aerosol impacts on precipitation occurrence in warm maritime clouds. Tristan S., L’Ecuyer, Wesley Berg, John Haynes, Matthew Lebsock, Toshihiko Takemura.
   JOURNAL OF GEOPHYSICAL RESEARCH: 2009
   DOI: 10.1029/2008JD011273
   Notes
   The impact of aerosols on precipitation occurrence in warm clouds is assessed using a combination of multisensor satellite cloud and precipitation data sets and aerosol information from both satellite and a global transport model. Aerosols are found to suppress the formation of precipitation in polluted regions, evidenced by a trend toward higher liquid water path prior to the onset of light rainfall. Polluted clouds are also found to be more vertically developed than those in more pristine environments. Coupled with an apparent reduction in the size of the raindrops that subsequently form in these clouds, these findings indicate that pollution inhibits precipitation processes by redistributing water among a greater number of smaller cloud droplets. Evidence is also provided that sea-salt aerosols have the opposite effect on precipitation development. Maritime clouds that form in regions of enhanced sea-salt concentrations tend to precipitate more frequently, form larger raindrops, and be less vertically deloped. This suggests that the nucleation of sea-salt particles may provide a source of embryonic raindrops in maritime clouds accelerating precipitation processes and ultimately reducing cloud lifetime. The net effect of aerosols on the onset of precipitation in any given region is, therefore, defined by the relative magnitudes of the competing effects of sulfate aerosols and sea-salt particles, the strengths of which depend strongly on both cloud liquid water path and the thermodynamic properties of the local environment.
   
   
   
10. Impacts of climate change from 2000 to 2050 on wildfire activity and carbonaceous aerosol concentrations in the western United States. D. V. Spracklen, L. J. Mickley, J. A. Logan, R. C. Hudman,R. Yevich, M. D. Flannigan, A. L. Westerling..
    JOURNAL OF GEOPHYSICAL RESEARCH: 2009
    DOI: 10.1029/2008JD010966
    Notes
    We investigate the impact of climate change on wildfire activity and carbonaceous aerosol concentrations in the western United States. We regress observed area burned onto observed meteorological fields and fire indices from the Canadian Fire Weather Index system and find that May–October mean temperature and fuel moisture explain 24–57% of the variance in annual area burned in this region. Applying meteorological fields calculated by a general circulation model (GCM) to our regression model, we show that increases in temperature cause annual mean area burned in the western United States to increase by 54% by the 2050s relative to the present day. Changes in area burned are ecosystem dependent, with the forests of the Pacific Northwest and Rocky Mountains experiencing the greatest increases of 78 and 175%, respectively. Increased area burned results in near doubling of wildfire carbonaceous aerosol emissions by midcentury. Using a chemical transport model driven by meteorology from the same GCM, we calculate that climate change will increase summertime organic carbon (OC) aerosol concentrations over the western United States by 40% and elemental carbon (EC) concentrations by 20% from 2000 to 2050. Most of this increase (75% for OC and 95% for EC) is caused by larger wildfire emissions with the rest caused by changes in meteorology and for OC by increased monoterpene emissions in a warmer climate. Such an increase in carbonaceous aerosol would have important consequences for western U.S. air quality and visibility.