Quantification of Clustering in Warm (RICO) Cumuli

Stratton Park Engineering Co., Inc., Boulder CO

Investigators

Abstract

While the most common modes of precipitation formation in the earth's atmosphere rely on the presence of ice crystals, in some instances clouds confined totally below the freezing level--so-called "warm clouds"--undergo surprisingly rapid (and often poorly anticipated) development of large precipitation-sized particles. Key to such developments is the collision-coalescence process in which cloud-sized droplets interact so as to form these larger particles capable of falling out of the cloud. Traditionally, observations of the small component droplets at smaller size ranges (extending to sizes < 100 micrometers) have been non-existent or at best unreliable. During the past ~5 years, hardware development of an aircraft-mounted 2D-S (two-dimensional stereo) particle imaging probe by SPEC Inc. was completed in conjunction with support from the Office of Naval Research, with an initial phase of evaluation in conjunction with the Rain in Cumulus over the Ocean (RICO) field campaign previously supported by NSF. This initial work showed considerable promise in illuminating water droplet interactions in this key size range. Of particular interest is the phenomenon of "clustering," in which otherwise uniformly distributed droplets become concentrated in small spatial zones where collision-coalescence type interactions may be greatly accelerated, thus producing precipitation at rates normally confined to mixed-phase clouds incorporating ice crystals. Such conditions have accompanied several notable warm-season flooding events in the midlatitudes, including the Fort Collins, Colorado flood of 1997. This single event killed five people, injured scores of others, and caused in excess of $200 million in damage. Moreover, comparatively shallow/warm clouds (e.g., marine stratocumulus) in which such processes may occur are exceedingly common at subtropical latitudes, where they appear to play a key role in regulating earth's climate. This work represents a continuation of analysis of 2D-S probe data collected during RICO aboard the NSF/NCAR (National Science Foundation/National Center for Atmospheric Research) C-130 aircraft. This instrument provides novel information on comparatively small water droplets and their spatially-varying concentrations, and will be used to more reliably demonstrate and quantify the occurrence of droplet "clustering" in warm cumulus clouds than has been possible in past studies. The investigators will focus on refinement of statistical/data-processing techniques (and associated computer software, much of which will be made available to benefit future users of the 2D-S instrument) to eliminate contamination of cloud droplet measurements by unrepresentative artifacts such as caused by splashing of raindrops as the aircraft and instruments fly through clouds. The presence and degree of droplet clustering will be evaluated with respect to location relative to cloud boundaries, cloud age, intensity of turbulence (thought capable of swirling droplets into locally-concentrated zones) and other cloud properties across the breadth of the entire RICO dataset, which encompassed multiple long-track C-130 flights over a two-month long experimental period. In later stages of the proposed project, summary information re: this clustering behavior is to be shared with a collaborators at the University of Utah, for direct comparison with output from a high-resolution computer model that seeks to accurately replicate details of small-scale processes within clouds. The RICO observations will thus be used to guide design and calibration of the model so as to accurately reproduce the degree and preferred scale of droplet clustering observed. The model can subsequently be improved and more fully exploited to study the persistence and impacts of such clusters upon relevant cloud microphysical processes that control development of warm clouds and precipitation falling from them. The "Broader Impacts" of this work may ultimately have benefits to society ranging from more accurate anticipation of certain flash-flood events to improved understanding of shallow marine clouds that play a key role in earth's radiative balance and hence global climate. In addition, this research will continue to contribute to broader impacts through improved graduate and undergraduate student education (both at the Univ. of Utah and in the Boulder, Colorado area where SPEC Incorporated is located), and is also slated for potential inclusion in an upcoming year-long Discovery Channel series highlighting processes related to global climate change.