754 Relationships Between Cloud Liquid Water Content, Cloud Droplet Number Concentration, and Cloud Droplet Distribution for Summertime Convective Clouds in the Northern Plains

Wednesday, 9 January 2013
Exhibit Hall 3 (Austin Convention Center)
Jeffrey S. Tilley, Univ. of North Dakota, Grand Forks, ND ; and D. Delene

Deeper understanding of cloud systems is essential to improving our ability to model the Earth's climate. Since aerosols that act as cloud condensation nuclei are known to change cloud properties in climatically significant ways, it is thereby critical to be able to accurately model aerosol/cloud interactions. Of particular interest is the representation of the cloud droplet size distribution (DSD), which is often assumed to follow a generalized gamma distribution in model bulk microphysical schemes. Typically, in bulk schemes the shape and breadth parameters of the gamma distribution are fixed as constants over the course of a simulation, thus bounding the simulated autoconversion processes within a limited parameter space. However, it may be more appropriate to allow either the shape or breadth parameters to vary in time based upon evolving relationships with other variables such as the liquid water content (LWC) or cloud droplet number concentration (Nd).

We utilize field observations of midlatitude and subtropical continental clouds obtained using University of North Dakota and Weather Modification, Inc. research aircraft to examine the degree to which observed droplets in continental summertime convective clouds follow the oft-assumed gamma distribution. We examine data not only for clouds as a whole but also at high temporal resolution (1-5 sec) to determine how appropriate the gamma distribution formulation is in totality or in a piecewise fashion for the observed DSDs. Examining the observations at high temporal resolution allows us to determine the degree to which the assumptions of constant shape and breadth parameters within the convective clouds are appropriate. Analysis results will be used to determine if robust relationships between LWC, Nd, and distribution parameters exist that are appropriate for use in bulk microphysical schemes.

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