2012 — 2015 |
Marchand, Roger |
N/AActivity Code Description: No activity code was retrieved: click on the grant title for more information |
Extratropical Cyclone Hydrometeor Vertical and Horizontal Spatial Correlation Structure @ University of Washington
This project will investigate the vertical and horizontal spatial structure of hydrometeor (cloud and precipitation) fields using statistical correlation analysis applied to observed radar reflectivity as evidenced in satellite data e.g. CloudSat, derived surface rain-rates and other satellite data (including AMSR-E, MODIS, and CERES retrievals). Preliminary analysis incorporating all observations regardless of synoptic conditions, has indicated that southern hemisphere extratropics have longer correlation lengths of hydrometeor occurrence in the horizontal and vertical than the northern hemisphere, however there are shorter correlation lengths in precipitation occurrence. To gain insight into such hemispheric differences, the plan is to draw out factors controlling precipitation and cloud structure in cyclones applying compositing techniques in the South Pacific region. This project will advance discovery and understanding while at the same time promote teaching and learning by supporting directly a graduate student researcher, who will give local seminars on this research, as well as disseminate research results through conference presentations and peer-review publications.
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0.955 |
2017 — 2020 |
Bretherton, Christopher (co-PI) [⬀] Wood, Robert [⬀] Marchand, Roger |
N/AActivity Code Description: No activity code was retrieved: click on the grant title for more information |
Analysis of G-V Aircraft and Complementary Socrates Observations to Understand Clouds, Aerosols and Their Interactions Over the Southern Ocean @ University of Washington
The Southern Ocean (SO), meaning the global ocean of the high latitude Southern Hemisphere, has a well-deserved reputation as the stormiest place on earth. The remoteness of the SO and its unforgiving conditions have severely limited observations of atmospheric processes occurring above it, including cloud processes in the cyclones traveling along the South Polar front. Yet these processes are of interest for a variety of reasons, including the fact that SO clouds are relatively free from the effects of continental and anthropogenic aerosols, and the region is thus a natural laboratory for the study of cloud behavior under pristine conditions. SO clouds also play a significant cooling role in the energy balance of the planet by reflecting incoming sunlight back to space. There is evidence to suggest that this cooling has a long-range effect on the distribution of the low-latitude rainfall associated with the intertropical convergence zone, and that changes in SO cloudiness due to global climate change will affect the location and strength of the Southern Hemisphere jet stream. One indicator of our lack of understanding of SO cloud processes is the inadequate SO cloud cover found in climate model simulations, which is accompanied by excessive absorption of sunlight by the ocean surface which may in turn cause errors in estimates of climate sensitivity. The deficiency in simulated cloud cover is most pronounced in boundary layer and lower-tropospheric clouds (tops below 3km) in the cold, dry sectors of frontal weather systems traveling along the SO storm track.
This project is part of a larger field campaign titled Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES). The primary activity of the campaign is the deployment of a Gulfstream V (GV) research aircraft maintained by the Earth Observing Laboratory of the National Center for Atmospheric Research. The GV will be based in Hobart, Australia and make multiple flights across the South Polar front collecting data on SO clouds and the meteorological conditions in which they occur. The GV is equipped with dropsondes to record ambient meteorological conditions, radar and lidar to observe the clouds, and instruments mounted on the wings or positioned behind inlets to to sample, collect and analyze aerosols and cloud particles (liquid droplets and ice crystals). The SOCRATES campaign is complementary to SO activities planned internationally and by other US agencies, including surface observations taken on ships and on MacQuarie Island, a small uninhabited island at 54 degrees South.
Work supported under this award specifically addresses the relatively warm, shallow clouds found at the top of the atmospheric boundary layer, with a specific focus on cloud droplet number concentration, or the number of droplets per unit volume in a cloud. Droplet number concentration is a key parameter for understanding cloud properties and evolution, and in a pristine region like the SO it can be limited by the availability of cloud condensation nuclei (CCN), aerosol particles which absorb water vapor from the air to form droplets. The PIs seek to determine if droplet number concentration can be related to CCN concentration in the local boundary layer or the overlying free troposphere, or if droplet concentrations are more strongly influenced by turbulent cloud motions, removal through precipitation, or other meteorological factors. Previous studies suggest that low clouds in the colder air to the south of the South Polar front have higher mean droplet concentrations than the warmer clouds to the north, a difference which is not presently understood. Possible explanations include increased CCN production in the colder ocean waters to the south (meaning closer to the South Pole), and higher precipitation rates with increased CCN removal to the north.
A variety of observations, collected in collaboration with other SOCRATES projects, are used to examine the controls on space and time variability of droplet number concentration in SO clouds:
* size-resolved aerosol and CCN concentrations, both above and within the boundary layer, using probes and inlets on the GV at multiple flight levels;
* precipitation rates, cloud updraft and downdraft strength, and cloud base and top heights determined from onboard radar and lidar.
* near-surface aerosol and CCN concentrations from low-level flight legs, planned to sample as low as 150m above surface;
* profiles of ambient temperature, moisture, and wind from dropsondes released from the aircraft.
These data are combined to produce integrated datasets including a north-south "SOCRATES curtain" dataset which facilitates comparison with numerical model output. Modeling activities based on these datasets are also planned.
The work has broader impacts due to the potentially significant role of SO clouds in determining the sensitivity of global climate to external forcing from greenhouse gas increases and other factors. Data from the campaign will be used to develop better representations of clouds in models used for weather prediction and climate impacts assessments. The data will be made available to the worldwide scientific community, thus the campaign has broader impacts by creating a community resource for basic science research. Outreach to K-12 students and the general public is conducted during the campaign through regular newsletters and blogs, and an interactive "Ask SOCRATES" website. This project provides support to two graduate students, thereby providing for the future workforce in this research area.
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0.955 |
2021 — 2024 |
Wood, Robert (co-PI) [⬀] Marchand, Roger |
N/AActivity Code Description: No activity code was retrieved: click on the grant title for more information |
Precipitation and Coalescence Scavenging in Shallow Southern Ocean Clouds @ University of Washington
Clouds in the marine boundary layer (say the kilometer of the atmosphere just above the ocean surface) have a global cooling effect as they reflect sunlight back to space but are too low to effectively trap outgoing infrared radiation. The climatic effect of these clouds has become an important issue in climate change research, as warmer temperatures may alter either the reflectivity of the clouds or their typical lifetimes, the factors which determine the strength of their cooling effect. The reflectivity of marine boundary layer clouds is determined in large part by the abundance of cloud condensation nuclei (CCN), the tiny aerosol particles which absorb moisture from the atmosphere to seed the growth of cloud droplets. When CCN are more abundant the available cloud water is spread over a larger number of smaller droplets, leading to a more reflective cloud than would occur with fewer CCN and hence a smaller number of larger droplets. Clouds made of smaller droplets also last longer, as the process of collision and coalescence that combines cloud droplets (perhaps a million or so) to form a raindrop takes longer when the droplets are smaller and thus more are needed to make a raindrop, meaning a drop big enough to fall from the cloud.
Previous work by the PIs developed a simplified budget equation for CCN relating changes in CCN abundance to sources, including the generation of aerosols by biological processes in the surface ocean, and sinks, including precipitation. Precipitation is quite effective in removing CCN from liquid clouds as each one of the million or so droplets in a raindrop contains a CCN. The PIs used their budget analysis to show that CCN removal by precipitation, referred to as precipitation scavenging or coalescence scavenging, is the dominant mechanism for CCN removal and accounts for much of the geographic variability of CCN abundance over the oceans. They also used their budget to derive a formula for the droplet number concentration (Nd) in marine boundary layer clouds under the assumption that the CCN sources and sinks are balanced and all the CCN in a cloud have seeded droplets.
Work performed here applies the budget equation and the Nd formula to the boundary layer clouds in the storm track over the Southern Ocean using observations collected during the 2018 Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES), a field campaign that used a research aircraft to sample clouds on flights over the ocean south of Tasmania (see AGS-1660609). The aircraft measured CCN concentrations in, above, and below the clouds, and used radar and lidar to observe cloud droplets and raindrops, information which can be combined with satellite data and meteorological analysis to determine CCN concentrations and measure CCN sources and sinks. Results of the budget study are compared with output from a Large Eddy Simulation (LES) model which generates detailed clouds, CCN, and precipitation simulations based on large-scale meteorological inputs and measurements of above-cloud CCN concentrations. The project also processes and analyzes data from the aircraft radar and lidar to examine the precipitation produced by the SO clouds, looking at the amount of precipitation produced, the size distribution of raindrops, and the extent to which ice phase precipitation is also produced.
The work is of societal as well as scientific interest as change in the extent of cooling provided by low clouds is among the largest uncertainties in estimates of the sensitivity of global temperature to greenhouse gas increases. The SOCRATES campaign was largely motivated by concern that the SO clouds are poorly represented in climate models used to make projections of future climate change. The project also produces datasets on cloud, precipitation, and CCN properties that can be used by the worldwide community of climate researchers. In addition, the award provides support and training for a graduate student, thereby promoting the future workforce in this research area.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
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0.955 |