From: Jim Coakley
Organization: Oregon State University
Research Area: AEROSOLS
Mission Scenario: Indirect Radiative Forcing in Urban Plumes
Radiative forcing due to man-made aerosols renders estimates of the net
radiative forcing of the climate due to human activity highly uncertain.
The radiative forcing due to aerosols has two components. The direct
radiative forcing is due to the sunlight scattered and absorbed by the
particles. The indirect radiative forcing is due to the effect of the
particles on the sizes and numbers of cloud droplets, the subsequent
evolution of the affected clouds, and the consequent changes in cloud
radiative properties. Strategies for measuring the direct radiative
forcing due to aerosols have been tested and have led to useful
estimates in a few cases. One method for measuring the indirect
radiative forcing due to aerosols is to use remote sensing techniques to
link cloud properties, such as cloud liquid water amount and droplet
effective radius, to aerosol burden for broken cloud systems imbedded in
an urban plume. The links are established by using the aerosol burden
inferred for the breaks in the clouds to the properties of the nearby
clouds. For such work, instrumented aircraft observations have distinct
advantages over satellite observations: 1) Aircraft observations allow
active sensors, such as lidars and radars, and higher spatial resolution
thereby reducing ambiguities in identifying scenes as either cloud-free,
suitable for aerosol retrievals, or overcast, suitable for retrievals of
cloud properties. 2) Aircraft flight paths can be aligned to transect
individual plumes over regions where the aerosol loading is distinctly
different, thereby obtaining measurements that span a range of aerosol-
cloud effects. 3) Two coordinated aircraft can observe the effect of
the aerosol and clouds on the radiation field simultaneously with
measurements of the chemical and physical properties of the aerosol and
the cloud.
HIAPER Mission: Two coordinated aircraft are required. A high-flying
aircraft uses lidar, multispectral imagery, mm-cloud radar, and imaging
microwave radiometer to map aerosol and cloud properties, such as
retrieved droplet effective radius, cloud liquid water, cloud top and
base altitudes, over a domain that transects an urban plume in which a
broken, layered cloud deck is imbedded. Transects would be made from
plume edge, where aerosol burden is low, to plume center, where aerosol
burden is high. While the upper-level aircraft is mapping the domain, a
low-flying aircraft is flying legs beneath the high-flying aircraft
observing the physical and chemical properties of the plume and the
clouds affected by the plume. The low-level plane will fly legs beneath
the cloud, in the cloud, and above the cloud, as well as obtain profiles
of winds, thermodynamic variables, gas concentrations, and particle
concentrations from the free-troposphere, well above the clouds, to near
the surface. Summertime fair weather Cu in the midwest, during lulls in
thunderstorm activity, would be appropriate for study. Plumes off the
east coast of the U.S., the east coast of Asia, and over the North Sea
east of the U.K. would also be appropriate.
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