David L. Mitchell

Associate Research Professor
Division of Atmospheric Sciences

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 ice production; (2) understanding and modeling the radiative properties of ice clouds; (3) satellite remote sensing of cloud properties; (4) understanding and predicting the onset, strength and extent of the Mexican monsoon and (5) climate engineering by seeding cirrus clouds using commercial aircraft.

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 to predict the ice particle size distribution through ice crystal nucleation and ice particle growth by vapor diffusion and aggregation.

Regarding (2), the optical properties of ice clouds have been successfully described by 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 electrodynamic solutions 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the asymmetry parameter for various crystal shapes, have lead to a new treatment of ice cloud radiative properties (outdated link) which is used in (i) the new NCAR global climate model (Community Climate Systems Model version 4, or CCSM4), (ii) the Colorado State University GCM, (iii) the Regional Atmospheric Modeling System (RAMS) at CIRES, and (iv) in the Rapid Radiation Transfer Model (RRTM) at Atmospheric and Environmental Research (AER), Inc.

Regarding (3), a satellite remote sensing method for estimating the concentrations of small ice crystals in cirrus clouds, as well as the ice water path and effective size, has been developed.  It is based on the heat emitted by the earth at discrete wavelengths (e.g. 11 and 12 μm) and a new theoretical explanation for the cirrus emissivity difference between these wavelengths.

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°C. 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.

RESEARCH AREAS:

• Cloud Microphysics, Cloud Radiative Properties (especially ice clouds), Remote Sensing of Cloud Physical Properties, Climate Dynamics, Large-Scale and Mesoscale Dynamic Meteorology, Precipitation Scavenging .

SELECTED PUBLICATIONS:

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 the International Radiation Symposium, Foz do Iguassu, Brazil, 3-8 August 2008.

Mitchell, D.L., and R.P. d’Entremont, 2008:

Satellite remote sensing of small ice crystal concentrations in cirrus clouds. American Institute of Physics (AIP), Proceedings of the International 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. Accepted for publication in 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.