International Research Fellowship Program: New Measurements of Atmospheric Ice Crystals and Their Fall Velocities

Fugal Jacob P, Boulder CO

Investigators

Abstract

1002568 Fugal The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award will support a twenty-four-month research fellowship by Dr. Jacob P. Fugal to work with Dr. Stephan Borrmann at the Max Planck Institute for Chemistry and the Johannes Gutenberg University at Mainz, Germany. Ice crystal sedimentation-rates or fall-velocities influence important ice-particle processes and cloud properties and thereby the Earth's energy budget and its climate. These fall-velocities have been extensively modeled, but few actual measurements have been made. This is largely because of the difficulty in making simultaneous measurements of the ice crystal's terminal velocity, area projected in the fall direction, and mass (and thereby particle density). Better measurements are clearly needed to test the validity of these simulated fall velocities. Employed is a new method to greatly expand data set of the aforementioned simultaneous measurements on thousands of ice crystals of various crystal habits, masses, and modes of aggregating, and falling in environments of various temperatures and pressures. This is done using an instrument that enables semi-automated measurement of natural- and laboratory-grown ice crystals. The instrument employs vertically-aligned digital in-line holography to classify, size, and track ice crystals which provides crystal habit, projected area, velocity, and mass measurements. Using this instrument, measurements are made using laboratory generated ice crystals and natural particles such as ice fog. These measurements are the beginning of a particle-property database that can validate the fall velocities that have been simulated. Beside fall-velocities, the population characteristics of ice crystals in atmospheric clouds (such as particle size distributions) is critical to accurately model Earth's climate. The new HOLODEC 2 (Holographic Detector for Clouds 2) instrument, that also uses in-line holography as the measurement method, is able to measure small ice-crystal size distributions more accurately than legacy probes. This probe's advantages include a well-known sample-volume size and additional information available to detect shattered ice particles and remove them from the sample volume. The new small ice-crystal size distribution measurements together with the fall velocity measurements will provide a more accurate bulk fall-velocity of natural ice particles which is critical to accurately model clouds and global climate.