Keywords: cloud microphysics, cirrus clouds, cloud optical properties, remote sensing, climate engineering, global climate modeling, climate change, North American monsoon
Dr. Mitchell's research has focused on the following areas: (1) theoretical understanding and modeling of the microphysical evolution within cirrus and frontal clouds, especially with regard to particle size spectra and crystal concentrations; (2) understanding and modeling the radiative properties of ice clouds; (3) remote sensing of cloud properties; (4) understanding and predicting the onset, strength and extent of the North American monsoon; (5) modification of cirrus clouds to reduce global warming.
Accomplishments regarding (1) include the development of two models successfully predicting the evolution of ice particle size spectra. The input for one model consists of the ice water content and temperature profiles, while the other is driven by changes in super-saturation. These models are computationally efficient, utilizing analytical solutions for ice particle growth by vapor diffusion and aggregation, and can be easily used to improve radar estimates of precipitation at ground level.
Regarding (2), the optical properties of ice clouds have been successfully described by parameterizing the absorption and scattering processes and rigorously treating their dependence on cloud microphysics. This treatment, the Modified Anomalous Diffraction Approximation (MADA), was formulated in terms of the size distribution and ice particle shape, and agrees with explicit electrodynamic solutions of ice crystal single scattering properties within 15%. These developments, along with parameterizing the asymmetry parameter for various crystal shapes, have lead to a new treatment of ice cloud radiative properties which is used in (i) the GCM and operational forecast model at the Hadley Centre/U.K. Meteorological Office, (ii) in the new NCAR GCM (CCSM4), (iii) in the Colorado State University GCM, (iv) in the Regional Atmospheric Modeling System (RAMS) at CIRES and (v) in the Rapid Radiation Transfer Model (RRTM) and the Paleoclimate version of RRTM at Atmospheric and Environmental Research (AER), Inc.
Regarding (3), a new method for estimating the amount of ice contained in clouds (i.e. the ice water path, or IWP) from satellite- or ground-based platforms has been developed, based on the heat emitted by the earth at discrete wavelengths in the infrared “window” region. The method considers the details of the size distribution and ice particle shape. In addition, two new satellite retrieval algorithms have been developed that estimate (1) the ice particle size distribution including the number concentration of small (D < 60 μm) ice crystals and (2) the percentage of liquid water relative to the total (ice + liquid) condensate in cold clouds. Method (1) is of value due to the difficulty in measuring small ice crystal concentrations from aircraft (which help determine cirrus cloud optical properties) and method (2) is important since ice cloud optical properties strongly depend on the fraction of liquid water when present. Both methods were the first to retrieve the indicated cloud properties.
Regarding (4), a new approach to understanding the Mexican monsoon has been pursued in terms of sea surface temperatures (SSTs) in the eastern tropical Pacific and the Gulf of California. Results from six monsoon seasons show that relatively heavy rainfall in Arizona commences once the SST in the northern Gulf of California exceeds 29 oC. Moreover, ten years of satellite altimeter observations of sea surface height in the eastern tropical Pacific indicate this threshold SST can be predicted 1-2 months in advance. Together these relationships provide a potential means of predicting the Arizona onset of the North American monsoon 1-2 months in advance.
Regarding (5), it is generally accepted that the mean increase in global surface temperatures (relative to pre-industrial times) should not exceed 2°C if mankind is to avoid “unacceptable” consequences of climate change. Recent research has led some scientists to conclude that this threshold may be unavoidable unless some type of climate engineering is invoked to remove CO2 from the atmosphere and/or temporarily cool the planet (e.g. by reflecting more sunlight) while simultaneously very rapidly converting to non-carbon based energy systems. A new type of climate engineering has been proposed, based on the aircraft seeding of the coldest cirrus clouds to reduce their global coverage, resulting in a global cooling effect. This has some potential advantages over the most studied climate engineering approach; the injection of sulfur compounds into the stratosphere to reflect more sunlight.
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