DOE ATMOSPHERIC SCIENCE PROGRAM SUMMER 2005 FIELD CAMPAIGN
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MARINE STRATUS EXPERIMENT (MASE)
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Project Overview

MASE PROJECT NOW IN THE DATA ANALYSIS PHASE

The following is the tentative schedule for analysis and interpretation of field measurements and conveying findings to the scientific public.

Most of the data from the field campaign are now available to ASP investigators as a CD ROM. Interested investigators should contact Stephen Springston (srs@bnl.gov). [2006-05-26]

MASE EXPERIMENT PAGE

For links to data, photos of operations, and the like click on the MASE Experiment Home Page.

RATIONALE

Atmospheric aerosols affect the climate by scattering and absorbing sunlight (direct effect) and by changing the microphysical structure, lifetime, and amount of clouds (indirect effects). At least two indirect aerosol effects have been identified. The first, commonly known as the Twomey effect, results from an increase in the number concentration and decrease in the size of cloud droplets due to increases in aerosol loading. If cloud macroscopic features such as liquid water path, physical thickness and similar properties are fixed, clouds with liquid water distributed among larger numbers of small particles will have a larger optical depth and reflectivity than the same cloud with liquid water distributed among smaller numbers of larger particles. Alterations of droplet size distributions in polluted clouds are also thought to inhibit precipitation formation, increasing cloud lifetime and coverage, and thereby enhancing the planetary albedo. This phenomenon is known as the second indirect aerosol effect. Both the first and second indirect aerosol effects result in a cooling influence on climate, but the magnitude of this forcing is highly uncertain.

While there is a good conceptual understanding of the first indirect aerosol effect, and ample evidence from in-situ, and remote sensing studies that aerosol loading has an effect on cloud microphysics and alters cloud reflectivity, adequate representation of the connection between aerosol loading and cloud reflectivity in climate models remains elusive. Present understanding of the second indirect aerosol effect is rudimentary. While it is generally accepted that the smaller droplets in clouds influenced by anthropogenic aerosols will inhibit precipitation formation processes, thereby increasing cloud lifetime and coverage, quantitative estimates of the magnitude of this effect are highly uncertain. Much work needs to be done on the basic physics of the initiation of precipitation and on factors that control its rate and duration.

Marine stratus is an important, yet under-sampled, component of the Earth's climate system. These clouds are known to be susceptible to infusions of anthropogenic aerosols that likely alter the in-cloud microphysical processes and are known to have at least two stable modes: one with relatively large cloud droplets and relatively large drizzle rates and another with relatively smaller cloud droplets and little or no drizzle. Cloud condensation nuclei (CCN) apparently play a critical role in determining which stable mode is observed. Marine stratus clouds also exhibit a strong diurnal cycle due to a pronounced cloud and radiation feedback involving changes in the net radiative flux at cloud top.

Extensive sheets of stratus and stratocumulus clouds of varying radiative impact lie above the eastern boundary current upwelling regions commonly present over the world's oceans in summer. These clouds are known to exert a large scale cooling effect on the ocean surface and are thought to be an important component in the global cloud forcing, although they are characterized by considerable mesoscale variability. The large-scale cloud structure often fluctuates between solid stratus sheets with a high albedo to thin, broken patches with low albedo (sometimes termed rift zones). These fluctuations are often observed over regions where the large-scale subsidence rate is relatively constant. This situation implicates microphysical processes or local thermodynamic processes as a possible cause of the large observed variations in albedo. However, despite a rich theoretical underpinning, relatively few actual measurements of marine stratus exist. This lack of relevant observational data has limited research progress over the past decade.

For these reasons ASP has decided to conduct a study of marine stratus over the eastern Pacific Ocean during the Summer of 2005. This month-long program will be conducted in conjunction with the first deployment of the DOE Atmospheric Radiation Measurements (ARM) program's ARM mobile facility with its array of in-situ and remote sensing instruments. The mobile facility will be located at Pt. Reyes National seashore just north of San Francisco, a site that is frequently covered by marine stratus clouds. ASP will use the DOE G-1 Research aircraft as the primary measurement platform, but will also provide measurements at the Point Reyes site to supplement those provided by ARM.

SCIENCE OBJECTIVES AND DELIVERABLES

The overall goal of this field program is to enhance understanding of the effects of anthropogenic aerosols on the properties of marine stratus clouds so that the relevant processes can be more accurately represented in global climate models.

Specific objectives for the ASP MASE study are to:

1. Examine relationships between aerosol properties (particle size distribution and composition) and CCN spectra (number concentration of CCN as a function of supersaturation), and between CCN spectra, updraft velocity, and the number and size distribution of cloud droplets

2. Compare composition of particles forming droplets in ambient clouds to the composition of those remaining as aerosol particles in cloud interstitial air.

3. Characterize the effects of turbulence and cloud microphysical properties such as liquid water content, droplet number concentration and droplet dispersion on the initiation of drizzle.

4. Gather in-situ cloud data suitable for examination of the local (as opposed to area mean) dependence of the autoconversion rate (rate of conversion of cloudwater to precipitation) on droplet size distribution and number concentration.

Deliverables from this project will include:

1. A publicly available comprehensive data set of aerosol and cloud properties, including aerosol composition, size distribution, CCN spectra, optical properties, cloud droplet and drizzle number concentrations and size distributions, cloud updraft velocities and turbulence.

2. An evaluation of parameterizations of warm rain initiation and rate suitable for inclusion in models of various scales.

3. An assessment of the role of turbulence in the initiation of warm rain.

4. An evaluation of the role of turbulence, size, and chemical composition in determining the activation behavior of ambient aerosols.

COLLABORATIONS

This study will be conducted in collaboration with an ARM Intensive Observational Period (IOP) called MArine Stratus Radiation, Aerosol, and Drizzle (MASRAD) and with flights of the CIRPAS (Center for Interdisciplinary Remotely Piloted Aircraft Studies) Twin Otter. As part of MASRAD, ARM will deploy the ARM Mobile Facility (AMF) that will be described below.

MASRAD has two principal scientific objectives:

(1) To investigate the general relationship between cloud mesoscale structure, aerosols, cloud microphysics, drizzle, and radiation in marine stratus clouds, and

(2) To examine the specific effects of aerosols on the discrepancy between the measured and modeled amount of solar radiation absorbed by these clouds.

These scientific issues will be investigated by combining detailed remote sensing measurements of cloud, drizzle, and radiation from the AMF 95-GHz radar and other sensors with detailed comprehensive aerosol measurements obtained by in-situ aircraft profiles. The effects of aerosols on the amount of solar radiation absorbed by marine stratus will be quantified using similar methods supplemented by state-of-the-art in-cloud radiation measurements made from aircraft. These combined measurements will improve on previous studies addressing the discrepancy between models and observations of the solar absorption of clouds by direct and accurate observation of the ambient aerosols, surface albedo, and downwelling solar flux. In particular, highly accurate solar flux measurements will be obtained from a state of- the-art, stabilized platform for making radiometric measurements from an aircraft.

MASRAD will deploy the AMF at Point Reyes National Seashore for the period March 1 - September 1, 2005. A satellite image of the Point Reyes area and its location relative to San Francisco is shown in Figure 1; clouds such as those shown are a common occurrence at this site.

Figure 2 shows a map of Pt. Reyes National Seashore and indicates the former AT&T site where the AMF is located. This site is about 0.75 miles from the coast.

Figure 3 shows the proposed location of the instruments associated with the experiment at the former AT&T site at Point Reyes.

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AMF instrumentation will include a 95 GHz cloud radar, microwave radiometers for measurement of liquid water path, a complete radiometer package, lidars for measuring cloud base, a balloon borne sounding system, a surface meteorology package, an aerosol package, and other instruments. A complete listing can be found at http://www.arm.gov/sites/amf.stm/. In addition ASP investigators and others will be providing instruments for deployment at the site during the month of July. A list of these instruments will be provided when it becomes available.

Much of the emphasis of the present project is examination of the chemical and microphysical properties of aerosol particles which have, or have not, been activated to become cloud droplets. The Point Reyes site is well situated to such a study, being shrouded in fog much of the time. A key sampling device in the present study is the Cloudwater Virtual Impactor (CVI), which inertially separates cloud droplets from the surrounding air, and then evaporates these droplets so that the residual aerosol particles may be studied. A schematic of the CVI sampling system and downstream instruments is shown in Figure 4. Several of the instruments are either duplicated in the aerosol sampling system, or are switched between the two systems, to permit comparison of the properties of activated and unactivated aerosol particles.

AIRCRAFT OPERATIONS PLAN

G-1 flights will be planned on the basis of forecast or prevailing meteorological conditions, specifically the presence of cloud layers over the near coastal (100 nautical miles) Pacific Ocean. All flights will be conducted during daylight hours. Under appropriate meteorological conditions flights over the ARM site located at Point Reyes National Seashore will be emphasized. A map showing the proposed track of the aircraft over this facility is given in Figure 5. This track may be flown several times during a flight lasting between 1 -4 hrs depending on conditions. If possible, flights will include below-cloud (no lower than 500 feet above ground level), in-cloud, and above-cloud passes. Flights farther away from the coast (up to 200 nm) are also planned, both in conjunction with flights over Point Reyes and independently. Such flights will be conducted in uncontrolled airspace, or in Military Operations Areas (MOA's). Flight altitudes will range approximately between 300' above the ocean surface to no higher than 10,000'. It is anticipated that these flights will be made below-cloud, at multiple altitudes in-cloud, and just above cloud top.[This paragraph was modified 2005-05-05]

Instruments intended to be carried on the G-1 during the study are listed in Table 1. Consistent with the focus of this experiment on cloud/aerosol interactions, the only trace gas instrument that will be flown during the study is an O₃ detector.

The G-1 fights will be conducted in co-ordination with the Twin Otter that has a similar complement of instrumentation except that the Twin Otter will also carry a sophisticated complement of radiometers to measure up- and down- welling radiation fields. Further details regarding this aircraft and its payload can be found at http://www.cirpas.net/public/home.ctm.

An example of a joint flight might be to fly both aircraft over the Point Reyes site. The G-1 would sample below and in -cloud, and the Twin Otter would measure up- and downwelling radiation above clouds. Combination of the surface data with the data from the two aircraft would allow a complete closure experiment, from aerosol and CCN properties to cloud radiation fields. More detailed flight plans are being developed and will be posted as they become available.

ASP POINT REYES (SURFACE) OPERATIONS PLANS

Measurements, instruments and key personnel for surface based operations (in addition to standard operations of the ARM Mobile Facility (AMF) are listed in Table 2.

LOGISTICS

G-1 Aircraft logistics contact John Hubbe; John.hubbe@pnl.gov, (509) 372-6134

AMF logistics contact Mark Miller; miller@bnl.gov, (631) 344-2958

Aerosol Observations at Point Reyes contact Betsy Andrews; Betsy.Andrews@noaa.gov

Questions or comments on this web page contact Steve Schwartz; ses@bnl.gov

DATA MANAGEMENT

It is intended to use the NARSTO data archive for this study in order to gather all of the various researchers' data into a central archive for future use by the ASP and other researchers in the global climate research community. The researchers involved in this project will be expected to adhere to the ASP data policy that is currently being established.

CONTACTS

DOE Program Manager for ASP (Acting)
Rick Petty, US DOE Climate Change Research Division, Rick.Petty@science.doe.gov, (301) 903-5548

ASP Chief Scientist
Stephen E. Schwartz, Brookhaven National Laboratory, ses@bnl.gov, (631) 344-3100

Lead Scientist -MASE
Peter Daum, Brookhaven National Laboratory, phdaum@bnl.gov, (631) 344-7283

Aircraft Planning and Operations
Peter Daum, Brookhaven National Laboratory, phdaum@bnl.gov, (631) 344-7283

John Hubbe, Pacific Northwest National Laboratory, john.hubbe@pnl.gov (509) 372-6134
Robert Hannigan, Pacific Northwest National Laboratory, rv.hannigan@pnl.gov (509) 372-6176

Point Reyes site
Mark Miller, Brookhaven National Laboratory, miller@bnl.gov.
(631) 344-2958

CIRPAS Twin Otter
John Seinfeld, California Institute of Technology, Seinfeld@caltech.edu (626) 395-4635

ARM IOP
ARM Program Manager - Wanda Ferrell, wanda.Ferrell@science.doe.gov (301) 903-0043

Project Leader - Mark Miller, Brookhaven National Laboratory, miller@bnl.gov
(631) 344-2958

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