Seongsoo Yum

Atmospheric Sciences Center
Desert Research Institute (DRI)
P.O. Box 60220
Reno, Nevada 89506-0220, USA
Tel: (702) 677-3118
E-Mail: seongsoo@sage.dri.edu

Education and Professional Career

Ph.D. in Atmospheric Sciences, University of Nevada, Reno, USA (August 1998)
M.S. in Meteorology, Yonsei University, Korea (February 1986)
B.S. in Meteorology, Yonsei University, Korea (February 1984)

Post-Doctoral Research Associate, Desert Research Institute (September 1998 - Present)
Graduate Research Assistant, University of Nevada, Reno (September 1993 - August 1998)
Graduate Research Assistant, University of Oklahoma, (September 1993 - August 1998)
Research Meteorologist, Meteorological Research Institute, Korea (August 1987 - July 1990)
Research Assistant, Yonsei University (March 1987 - August 1987)
Graduate Teaching Assistant, Yonsei University (March 1985 - July 1985)

Dissertation Research

Title: Cloud Droplet Spectral Broadening in Warm Clouds - An Observational and Model Study
Advisors: Dr. James G. Hudson

ABSTRACT

The crucial topic of cloud droplet spectral broadening was investigated with numerical models for condensational droplet growth and gravitational collectional growth, and observational data taken from the stratocumulus clouds over the Southern Ocean and the cumulus clouds in eastern Florida. The condensation model calculation showed that droplet spectral width (i.e., standard deviation of diameter) was closely related to the supersaturations, and thus updrafts, and the input CCN spectrum that a cloud parcel was associated with. When cloud parcels with different updrafts were mixed together, the standard deviation of diameter of the internally mixed parcel was larger than any of the parcels involved in mixing, and more importantly, the standard deviation of diameter increased with mean diameter, contrary to a single parcel calculation.

The collection model calculation showed that it was critical to have droplets of diameter greater than 40 $\mu m$ for efficient gravitational collection. Another important finding was that the mean diameter and standard deviation of diameter for droplet size ranges (diameter < 50 $\mu m$ ) decreased, as gravitational collection became active. Comparisons of CCN and cloud microphysics revealed that the first and second Twomey effects were evident for the extensive observational data set presented here. One of the important findings from the analyses of cloud microphysical data was that the correlation between cloud droplet concentration and mean diameter was predominantly positive on a flight scale, and even more so on a sub-flight scale, especially in clouds with small drizzle (diameter > 50 $\mu m$ ) liquid water content. Based on detailed analyses of several cloud segments, it was suggested that significant contribution to the positive correlation between droplet concentration and mean diameter came from entrainment and mixing that was proceeded as Broadwell and Breidenthal (1982) envisaged. Superadiabatic droplet growth in diluted cloud parcels seemed not to have occurred in the clouds presented here. Nevertheless, droplet spectral broadening was evident, especially in the clouds with large drizzle liquid water content. The most strong droplet spectral broadening mechanism seemed to be internal mixing of cloud parcels that is caused by turbulence inside clouds.