SPARTICUS

Small Particles in Cirrus (SPartICus)

1 January 2010 - 30 June 2010

Lead Scientist: Gerald Mace

Observatory: AAF , SGP

One of the overarching goals of ACRF is to characterize the properties of clouds so that their representation can be improved in Global Climate Models (GCMs). Toward this end, ACRF has compiled remote sensing data from ground sites since the early 1990’s. Cirrus are a critical link in this problem because they exert significant controls on the earth’s radiation budget. The remote sensing data from ACRF has the potential to contribute to emerging parameterizations of cirrus. However, the present uncertainty regarding retrievals of the microphysical properties of cirrus is a significant hindrance to full utilization of the ACRF data for its intended purpose. This project addressed the present state of uncertainty and advanced our understanding of midlatitude cirrus. The Small Particles in Cirrus (SPartICus) project targeted three basic science questions. These questions were 1. To what degree do small particles (i.e., < 50 ìm diameter) contribute to the mass and radiative properties of midlatitude cirrus? 2. How do cloud-scale dynamical processes control the evolution of cirrus properties through nucleation, particle growth, and sublimation? 3. What degree of complexity is required in cloud property retrieval algorithms, and what minimal set of algorithms can be used to rigorously describe cirrus microphysical properties using ground-based ACRF data? A persistent theme of this project was quantifying what controls the concentrations of small particles in cirrus. Because of the uncertainty regarding the role of shattering of large ice crystals artificially amplifying these concentrations in past data sets, questions have persisted in the cirrus community that has hindered progress on the questions raised above. With a new generation of probes designed to minimize artifacts due to shattering, we were able to address these questions with rigor and precision. We estimated that 150 hours of in situ data were necessary for this purpose although some substantial fraction of this amount would still enable us to address many of the questions posed. In order to produce this extensive data set and address the science goals of SPartICus, we proposed a multi-phased approach where a minimal but sufficient set of robust probes were flown routinely using a moderate-payload jet aircraft over a period of 6-10 months spanning the occurrence of jet stream cirrus over the Southern Great Plains site that begins in autumn and extended to the period when convective cirrus are frequent during spring. An intensive phase of the project focused more on microphysical processes and fielded a more extensive set of experimental probes that observe the aerosol and ice nuclei properties of the upper troposphere. While this project was far reaching, the science objectives overlaped significantly with that of the NASA Radiation Sciences Program, and Dr. Hal Maring offered his support of the program by sharing cost and providing airborne resources. This partnership made SPartICus possible and a valuable contribution to the knowledge needed to advance the goals of ACRF.