multiple platforms (with different instruments, view volumes, times, etc.) may also be intensive and difficult, and require significant staff time.

To inform the sponsor about specific types of data to present at the September workshop, individual workshop participants discussed a number of ways that understanding of clouds, aerosols, and their interactions could be advanced with additional data from classified assets. These concepts and issues should not be seen as conclusions of the workshop or as consensus statements of the workshop participants or organizing committee.

• Improve understanding of aerosol-cloud interactions.
  • These interactions could be elucidated with better observations of changes in cloud brightness (sometimes), droplet size, cloud height, and cloud fraction.
  • A significant challenge is that the greatest microphysical effects are in the cleanest environments, while the greatest radiative effects are in environments with the heaviest aerosol loading.
  • Aerosol-cloud interactions span many scales.
    • The spatial scales range 13 orders of magnitude from the different classes of aerosols and droplets (fine mode [100 nm]/coarse mode/cloud drop/drizzle drop/rain drop, with a factor of 10 between each); to the macroscales of turbulence (10 m) updrafts, cumulus, thunderstorms, convective systems (also factor of 10 between each). In some cases the spatial resolution of the instrument will determine the values calculated for the fields, especially in the case of cloud fraction.
    • The timescale ranges from seconds to days.
  • Some participants said periodic snapshots are sufficient, even though observations of how processes evolve over time provide more information. Some participants said available satellite observations provide enough constraints to retrieve process rates, while others said vertically-resolved measurements would be needed.
• Improve understanding of cloud physical processes and their representation in models.
  • Important variables to measure include:
    • Liquid cloud: water content or liquid water path (LWP), optical depth, particle size, and number concentration.
    • Ice variables: water content, particle size, extinction or optical depth, number concentration and crystal shape.
• Improve discrimination between liquid and ice phases.
  • Important variables to measure include:
    • A combination of particle size and backscatter (lidar and radar).
• Improve understanding of long-term processes.
  • Targeted observations of processes that are stable year-to-year might advance understanding.
• Improve observations of aerosols located over snow or ice.

Throughout the workshop, individual workshop participants identified numerous types of data and instruments that would potentially be useful to advance understanding of cloud, aerosols, and their interactions. These concepts and issues should not be seen as conclusions of the workshop or as consensus statements of the workshop participants or the organizing committee.

• Measurements (e.g., air quality or meteorological monitoring) from under-observed areas (e.g., embassies or military bases) to estimate emissions.
• Aerosol sources (e.g., injection site, frequency of emissions, plume height, etc.) would be useful to initialize models.
• Aerosol type (e.g., chemical composition of aerosols, measurements of local visibility) to validate remote sensing retrievals.
• Vertical velocity measurements (e.g., vertical profiles of temperature, humidity) as many cloud and aerosol processes have strong dependencies on the meteorology.
• Observations from a radiative forcing perspective over the “pristine” Southern Hemisphere ocean.
• Measurements of microphysical properties rather than bulk or column properties (aerosol optical depth, cloud condensation nuclei, aerosol index).
  • Aerosol optical depth, for example, is taken to be a proxy for number of particles, which limits process-level understanding. Cloud effective radius does not represent droplet size distribution, which varies throughout the cloud. These measurements are important for comparing model outputs, which give different parameters.
• High-resolution ground- or aircraft-based radar or lidar data (e.g., radar with more or different wavelengths or high sensitivity, even if in one location, to either co-locate or get a general climatology).
  • HSRL (High Spectral Resolution Lidar) can measure aerosol optical properties and derive measures of size, composition, aerosol backscatter, extinction, and optical depth. HSRL can also measure cloud properties including: fraction, optical depth, extinction, and cloud droplet number concentration, particularly in difficult retrieval conditions (high latitudes, nighttime, above clouds).
  • HSRL measurements of aerosol extensive and intensive parameters provide additional constraints for developing and assessing models. The aerosol profiles are used to assess and improve aerosol data assimilation systems.
• Hyperspectral infrared (IR) provides good horizontal measurements to tell us something about cloud height and depth. IR can also be utilized over ice and snow.
• High-resolution (~1 km) microwave observations.
• Computed tomography to leverage full information content of various sensors.
• Observations from satellites with unique orbits (e.g., highly elliptical) that could “dwell” on specific areas. One example is the proposed Canadian PCW (Polar Communications and Weather) system focused on Arctic.
• Very high spatial resolution (e.g., Landsat) combined with time information. A system that could stare and be pointed (versus the typical nadir-view of satellites), as it moves over the variable of interest. It could look at the variable from different angles: multi-angle, stereo, visible and infrared (IR)/thermal IR.
• In situ data sources and observations (e.g., in situ atmospheric ice crystals, buoys that make atmospheric measurements above the ocean).
• A sensor similar to VIIRS (Visible/Infrared Imager/Radiometer Suite) that could be used for nighttime aerosol retrievals.

• Instruments which employ the A-Train model: multiple simultaneous measurements from active and passive sensors.
• Observations that have overlap with existing observation systems to improve utility and to avoid to the extent possible difficult geo-rectification efforts. Ideally, the observations would be completely coincident or slightly offset in time to observe the development of systems (e.g., between the A-train satellites or Terra MISR/Landsat offset).
• Pre-A-Train observations, in similar orbit, to help extend the record.
• Miniaturization of current technologies to place on commercial or military aircraft, ships, satellites to improve coverage.
• Multi-angle, stereoscopic observations and high capability geosynchronous observations.
• Observations from broadband radiometers such as CERES (Clouds and the Earth's Radiant Energy System) to better constrain the global radiation budget.
• Polarimeter and polarized data could offer insights on droplet size distributions, for example, and in understanding the full 3D character of the atmosphere.
  • Retrievals of fine and coarse mode aerosol: optical depth, size distribution, complex refractive indices for each mode, cloud phase, droplet size distribution, shape, roughness, and asymmetry parameter retrievals for ice clouds, aerosols retrievals under cirrus. These instruments are not sensitive to absorption.
    • Ideally has wide spectral range from blue to NIR; perhaps also UV. Wide angular range to capture a wide range of scattering angles. High accuracy: 0.5% or smaller, 0.1-0.2% would be ideal. Dense angular sampling to resolve structures in rainbow peak to calculate droplet size distribution while not being sensitive to 3D cloud effects or cloud shadows. This allows for aerosol retrievals over oceans, over land, and under (thin) clouds.
  • Retrievals of water and ice clouds: dense angular sampling (~50 viewing angles; aerosol could be done with ~5-6 viewing angles).
• Observing systems that could track the evolution of cloud systems to compare with cloud models.
  • This could be achieved by a few satellites flying in unison in low orbit, providing views of a cloud system with temporal resolution on the order of 5 minutes and recording at least 15-30 minutes of evolution.
  • It would be difficult to record a cloud system from birth to senescence except via geo-orbit. The definition of a single cloud becomes vague as cloud systems evolve. However, a series of snapshots of the system over a brief period of time (e.g., on the order of 30 minutes) would catch a range of systems at different stages of their evolution and thus it would be useful to test the dynamics of aerosol-cloud models if the swath of observations covered a range of aerosol-cloud systems.
  • To the extent that some of these cloud systems exhibit self-similar properties, much could be deduced about system evolution from composites of a series of snapshots.
  • Commercial satellite imagery such as the Quickbird or Digital Globe systems would provide a zoomed in view of cloud structure for detailed process inference. These instruments have a resolution of 0.6 m panchromatic, and 2.4 m in 4 bands (RGB [red, green, blue] + NIR) over a 16 km footprint. With 30-sec retargeting, it would be possible to get multi-angle views (-45°, -30°, nadir, +30°, +45°) but this only covers 2 minutes.
• Suborbital sampling (even if scientists do not know the location) to provide verification, albeit at poorer spatial sampling.
  • Examples include ER-2 and Global Hawk.

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