| NSF Org: |
AGS Division of Atmospheric and Geospace Sciences |
| Recipient: |
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| Initial Amendment Date: | February 3, 2004 |
| Latest Amendment Date: | June 25, 2010 |
| Award Number: | 0342618 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Bradley F. Smull
AGS Division of Atmospheric and Geospace Sciences GEO Directorate for Geosciences |
| Start Date: | July 1, 2004 |
| End Date: | June 30, 2011 (Estimated) |
| Total Intended Award Amount: | $400,368.00 |
| Total Awarded Amount to Date: | $446,597.00 |
| Funds Obligated to Date: |
FY 2005 = $95,068.00 FY 2006 = $97,896.00 FY 2007 = $58,429.00 FY 2010 = $46,229.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
2215 RAGGIO PKWY RENO NV US 89512-1095 (775)673-7300 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
2215 RAGGIO PKWY RENO NV US 89512-1095 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): |
PHYSICAL METEOROLOGY, Physical & Dynamic Meteorology |
| Primary Program Source: |
app-0105 app-0106 app-0107 01001011DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
The goals of this project are to measure cloud condensation nuclei (CCN) in air ingested into trade wind Cumulus clouds and use those measurements to consider how the CCN population influences the development of rain in those clouds. New features of the instrumentation will allow the measurements to extend to low supersaturation (large particle size), an important extension because such particles are thought to be important in warm-rain initiation. The measurements will also be made by different instruments providing different growth times for the droplets that form on the CCN to investigate if the limited growth time available in the instrument biases the measurements. The investigators will also study the possible role of occasional Saharan-dust episodes in accelerating rain formation. The project is part of the "Rain in Cumulus over the Ocean" (RICO) experiment, a multi-investigator study of warm rain formation and the nature of trade wind Cumulus clouds. The CCN measurements are a critical component of the broader experiment because, as the key input determining the initial droplet size distribution, they are needed by many of the other investigators in their studies of specific hypotheses regarding the warm-rain process. Other broad impacts of the project include likely resolution of an instrumentation controversy affecting many other microphysical studies, better understanding of rain formation as it influences weather and climate, and graduate student training in this area of research.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Certain very small atmospheric particles are important for the formation of clouds. These cloud condensation nuclei (CCN) are measured with very special instruments that simulate the atmospheric conditions of cloud formation, namely carefully controlled relative humidities slightly above 100%, supersaturated. The highly variable concentrations of CCN throughout the atmosphere determine two important properties of clouds: 1) how much sunlight they reflect, and 2) likelihood of precipitation. Low altitude clouds cool the planet by preventing sunlight from heating the surface. Higher CCN concentrations cause more reflection of sunlight, which increases this cooling by low clouds. Higher CCN concentrations are also usually thought to reduce precipitation, which allows clouds to persist and thus reflect even more sunlight. Although many CCN come from natural sources such as wind blown sea salt and geothermal activities such as volcanoes, air pollution is a significant source. Air pollution, therefore, affects clouds but we do know the extent that it perturbs natural clouds largely because of inadequate knowledge of natural CCN. The air pollution effect on clouds is called the indirect aerosol effect (IAE), which is the largest climate uncertainty.
The first paper of this grant showed that there is too much variability in the sizes of CCN to be able to estimate CCN concentrations from much easier particle size measurements. Our results showed that this shortcut method advocated by a paper by a German group cannot be used for determining CCN concentrations.
Our CCN measurements in the Rain in Cumulus over the Ocean (RICO) project showed that CCN are very well anticorrelated with the concentrations of large cloud droplets that can lead to rain. This confirms the hypothesis that higher CCN concentrations inhibit precipitation. When this is due to CCN from air pollution it is referred to as the second part of IAE, which would cause clouds to persist and thus reflect more sunlight and thus produce even more planetary cooling.
The main goal of the RICO project was to determine whether relatively large atmospheric particles produced by wind at the ocean surface affect precipitation. Early RICO analysis by other scientists indicated that these giant nuclei (GN) did not cause precipitation. But our reexamination of that data indicated positive results, which means that GN do cause precipitation. Overall our RICO results and analysis indicate that the entire CCN and GN spectra need to be considered with regard to aerosol impact on precipitation. This has important implications for IAE.
Other results from RICO indicated a contrary effect of higher CCN concentrations. The smaller cloud droplets produced by higher CCN concentrations seemed to more easily evaporated at cloud edges. This effect produces decreased cloud droplet concentrations when CCN concentrations are higher, which is opposite of the usual increase of droplet concentrations with increased CCN concentrations. Although this effect seems to be limited to the smallest droplet sizes it may prove to reduce the magnitude of IAE.
Wind blown salt from the sea produces not only GN but also CCN, but there is considerable controversy about the sea salt contribution to global CCN concentrations. Further RICO analysis quantified the sea salt contribution to CCN. This was determined to be considerable, but only for very large CCN. The sea salt contribution to total CCN concentrations was found to be small, except when the wind speed over the ocean was extremely high. On the other hand we found that clouds also affect CCN concentrations. Since clouds tend to reduce CCN concentrations we deduced that they have more of an effect on particles that are transported from distant sources such as air pollution. This tends to decrease the influence of air pollution compared to the sea salt CCN that are continually produced in maritime regions. Analysis of these results provided more accurate estimates of the impact of this natural source of CCN so that it can be compared with the effect of air pollution CCN to then provide more accurate estimates of IAE.
The results of this study provide fundamental knowledge of the effects of the aerosol that produces clouds. This is important in the overall understanding of clouds and precipitation, especially regarding IAE the largest climate uncertainty. The results have practical implications because it might be possible to predict cloud and precipitation properties from relatively easy and inexpensive surface aerosol measurements.
Last Modified: 09/27/2011
Modified by: James G Hudson
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