Dr. David L. Mitchell
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 led to a new treatment of ice cloud radiative properties which is used in (i) the Community Atmosphere Model version 5 (CAM5) global climate model (GCM), (ii) 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 developed at Atmospheric and Environmental Research (AER).
Regarding (3), the MADA scheme (noted above) resolves the two main processes responsible for the absorption of thermal radiation in ice particles; Beers law and wave resonance absorption. This led to two satellite retrieval algorithms that have been developed that estimate (1) the ice particle size distribution including the number concentration of small (D < 60 m) ice crystals and (2) the percentage of liquid water relative to the total (ice + liquid) condensate in mixed phase 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. This improved understanding of thermal radiation absorption by ice has also enabled the satellite retrieval of the N/IWC ratio, where N is ice particle number concentration and IWC is the cloud ice water content. This retrieval may allow us to identify whether cirrus clouds have been primarily formed through homo- or heterogeneous ice nucleation.
Regarding (4), a new approach to understanding the North American monsoon (NAM) has been pursued in terms of sea surface temperatures (SSTs) in the eastern tropical Pacific and the Gulf of California (GC). Results from nine monsoon seasons show that relatively heavy rainfall in Arizona commences once the SST in the northern GC exceeds 29oC. Both observational and modeling research indicates that humid air overlying the GC is trapped by an inversion that breaks when SSTs exceed 29C. Once the marine inversion is removed, the warm SSTs humidify a deep layer of free tropospheric air that can be advected over land to often produce thunderstorms. In addition, this mechanism along with climatological and reanalysis data suggest that NAM convection is initiated in central Mexico through the poleward propagation of warm tropical surface water along the Pacific coast, and that this convection contributes to the poleward propagation of the NAM anticyclone that eventually steers mid-level moisture into the NAM region, augmenting the NAM rainfall.
Regarding (5), it is generally accepted that the mean increase in global surface temperatures (relative to pre-industrial times) should not exceed 2C if mankind is to avoid unacceptable consequences of climate change. Recent research has led some scientists to conclude that exceeding this threshold may be unavoidable unless some type of climate intervention (CI) is invoked to remove CO2 from the atmosphere and/or cool the planet (e.g. by reflecting more sunlight) while simultaneously and very rapidly converting to non-carbon based energy systems. A new type of CI has been proposed, based on the aircraft seeding of the coldest cirrus clouds to reduce their coverage and optical thickness, resulting in a significant global cooling by releasing more thermal radiation to space (instead of reflecting more sunlight). GCM simulations of this approach by others show that it has reduced or no side-effects (e.g. the hydrological cycle and ozone perturbation) relative to the most studied solar radiation management (SRM) CI approach (the injection of sulfate aerosols into the stratosphere to reflect more sunlight). Moreover, this cirrus CI method preferentially cools the Polar Regions where climate change is most severe, which is less easily achieved by the SRM methods.
- Cloud Microphysics
- Cloud Radiative Properties (especially ice clouds)
- Remote Sensing of Cloud Physical Properties
- Climate Dynamics
- Large-Scale and Mesoscale Dynamic Meteorology
- Precipitation Scavenging
- North American Monsoon Research
|Ph.D..||1995||University of Nevada, Reno||Atmospheric Science|
|M.S.||1986||University of Nevada, Reno||Atmospheric Science|
|B.Sc.||1981||California Polytechnical State, University, San Luis Obispo||Chemistry
Erfani, E. and D. L. Mitchell, 2015: Developing and bounding ice particle mass- and area-dimension expressions for use in atmospheric models and remote sensing. Atmos. Chem. Phys. Discuss., 15, 2851728573, doi:10.5194/acpd-15-28517-2015.
Erfani, E., and D. L. Mitchell, 2014: A Partial Mechanistic Understanding of the North American Monsoon. J. Geophys. Res. Atmos., 119, 13,09613,115, doi:10.1002/2014JD022038.
Mishra, S., D. L. Mitchell, D. D. Turner and R. P. Lawson, 2014: Parameterization of ice fall speeds in mid-latitude cirrus: Results from SPartICus. J. Geophys. Res. Atmospheres, 119, 3857-3876, doi:10.1002/2013JD020602.
Mitchell, D. L., S. Mishra, and R. P. Lawson, 2011: Representing the ice fall speed in climate models: Results from Tropical Composition, Cloud and Climate Coupling (TC4) and the Indirect and Semi-Direct Aerosol Campaign (ISDAC), J. Geophys. Res., 116, D00T03, doi:10.1029/2010JD015433.
Mitchell, D.L., R.P. Lawson, and B. Baker, 2011: Understanding effective diameter and its application to terrestrial radiation in ice clouds. Atmos. Chem. Phys., 11, 3417-3429, doi:10.5194/acp-11-3417-2011.
Mitchell, D. L., and R. P. dEntremont, 2012: Satellite retrieval of the liquid water fraction in tropical clouds between -20 and -38C. Atmos. Meas. Tech. Discuss., 4, 1-42, doi:10.5194/amtd-4-1-2011.
Gettelman, A., X. Liu, S.J. Ghan, H. Morrison, S. Park, A.J. Conley, S.A. Klein, J. Boyle, D.L. Mitchell, and J.-L.F. Li, 2010: Global simulations of ice nucleation and ice supersaturation with an improved cloud scheme in the Community Atmosphere Model. J. Geophys. Res., 115, D18216, doi:10.1029/2009JD013797.
Mitchell, D.L., R.P. D'Entremont, and R.P. Lawson, 2010: Inferring cirrus size distributions through satellite remote sensing and microphysical databases. J. Atmos. Sci., 67, 1106-1125, doi:10.1175/2009JAS3150.1.
Mitchell, D.L. and W. Finnegan, 2009: Modification of cirrus clouds to reduce global warming. Environ. Res. Lett., 4, 045102 (8 pp), doi:10.1088/1748-9326/4/4/045102.
Mitchell, D.L., P.J. Rasch, D. Ivanova, G.M. McFarquhar, T. Nousiainen, 2008: Impact of small ice crystal assumptions on ice sedimentation rates in cirrus clouds and GCM simulations.Geophys. Res. Lett.,35,doi:10.1029/2008GL033552.
Mishra, S., D.L. Mitchell, and D. DeSlover, 2008: Ground based retrievals of small ice crystals and water phase in Arctic cirrus. American Institute of Physics (AIP), Proceedings of theInternational Radiation Symposium,Foz do Iguassu, Brazil, 3-8 August 2008.
Mitchell, D.L., and R.P. dEntremont, 2008: Satellite remote sensing of small ice crystal concentrations in cirrus clouds. American Institute of Physics (AIP), Proceedings of theInternational Radiation Symposium, Foz do Iguassu, Brazil, 3-8 August 2008.
Mitchell, D.L., A.J. Brown, W.P. Arnott, and C. Schmitt, 2006: Testing and comparing the modified anomalous diffraction approximation. J. Atmos. Sci., 63, 2948-2962.
Mitchell, D.L., A. Huggins and V. Grubisic, 2006: A new snow growth model with application to radar precipitation estimates. Atmos. Res., 82, 2-18.
Mitchell, D.L. and A.J. Heymsfield, 2005: Refinements in the treatment of ice particle terminal velocities, highlighting aggregates. J. Atmos. Sci., 62, 1637-1644.
Mitchell, D.L., R.P. d'Entremont, D.H. DeSlover, and W.P. Arnott, 2003: Multispectral thermal retrievals of size distribution shape, effective size, ice water path, optical depth and photon tunneling contribution. 12th Conf. on Satellite Meteorology and Oceanography, AMS Annual Meeting, Long Beach , California, 9-13 Feb. 2003.
Mitchell, D.L., 2002: Effective diameter in radiation transfer: General definition, applications and limitations. J. Atmos. Sci., 59, 2330-2346.
Mitchell, D.L., D. Ivanova, R. Rabin, K. Redmond, and T.J. Brown, 2002: Gulf of California sea surface temperatures and the North American monsoon: Mechanistic implications from observations. J. Climate, 15, 2261-2281.
Ivanova, D.C., D.L. Mitchell, W. Patrick Arnott and M. Poellot, 2001: A GCM parameterization for bimodal size spectra and ice mass removal rates in mid-latitude cirrus clouds.Atmos. Res., 59, 89-113.
Mitchell, D.L., 2000: Parameterization of the Mie extinction and absorption coefficients for water clouds. J. Atmos. Sci., 57, 1311-1326.
Mitchell, D.L., 1996: Use of mass- and area-dimensional power laws for determining precipitation particle terminal velocities. J. Atmos. Sci., 53, 1710-1723.
Mitchell, D.L., A. Macke, and Y. Liu, 1996: Modeling cirrus clouds. Part II: Treatment of radiative properties. J. Atmos. Sci., 53, 2967-2988.
Mitchell, D.L., 1994: A model predicting the evolution of ice particle size spectra and the radiative properties of cirrus clouds. Part I: Microphysics. J. Atmos. Sci., 51, 797-816.
Mitchell, D.L. and W.P. Arnott, 1994: A model predicting the evolution of ice particle size spectra and the radiative properties of cirrus clouds. Part II: Dependence of absorption and extinction on ice crystal morphology. J. Atmos. Sci., 51, 817-832.
Book Reviews and Book Chapters
Mitchell, D. L., 2011: Book review of Geo-engineering Climate Change: Environmental Necessity or Pandoras Box?, Brian Launder and J. Michael T. Thompson (Eds.), 2010, 332 pp., Cambridge Univ. Press, ISBN 978-0-521-19803-5. Bull. Amer. Meteorol. Soc., 92, 1503-1504.
Mitchell, D. L., S. Mishra and R. P. Lawson, 2011:Cirrus clouds and climate engineering: New findings on ice nucleation and theoretical basis.In: Planet Earth 2011 - Global Warming Challenges and Opportunities for Policy and Practice, Prof. Elias Carayannis (Ed.), ISBN 978-953-307-733-8, InTech, Available from