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1. To measure the CCN concentration and the aerosol concentration below cloud base level of growing towering cumulus. 2. To measure the cloud droplet concentrations and size distributions of droplets from the cloud bases to the cloud tops of young and growing convective towers. 3. To measure the onset of precipitation and identify the predominant precipitation forming mechanism within young and growing convective towers. In order to achieve these tasks, and to increase the measurement capability of the region, a new Texas based cloud physics aircraft was equipped specifically as part of this project. The Seeding Operations & Atmospheric Research (SOAR) program Piper Cheyenne II was equipped with instrumentation purchased by the Texas Department of Licensing and Regulation (TDLR) through a grant administered by the Weather Damage and Modification Program (WDMP) of the Bureau of Reclamation (BOR). The WDMP funded the aircraft operations and the scientific crew required to conduct the SPECTRA field research program. 1.3 Cloud physics aircraft measurements in Texas The importance of aircraft measurements in Texas, southeastern New Mexico and Oklahoma is well demonstrated by the progress that these data helped to achieve in the cloud physics community over the last 20 years. Advances were quite spectacular over this period both in aircraft capabilities and in the instrumentation carried by them. During this period, the first field research program within the state consortium was developed in Oklahoma as part of the continuing effort to develop the basis for a scientifically sound weather modification program. The Bureau of Reclamation and the Oklahoma Water Resources Board supported this research effort. The University of North Dakota Citation cloud physics aircraft research team collected the microphysical data for analysis. Data were collected during the spring of 1986 and the summer of 1987. The analyses indicate that Oklahoma convective clouds appear to develop precipitation initially through a warm rain process. As continued vertical development occurs and the clouds reach lower temperatures, the drops freeze, subsequently evolving into graupel. The data also indicated that supercooled liquid water does not persist long enough to enable static seeding to work. However, an ample supply of supercooled liquid water exists initially, suggesting that a dynamic seeding approach might be more appropriate (Pflaum, 1989). The testing of cold-cloud seeding concepts for the enhancement of rainfall has been conducted in west Texas on an intermittent, as-funds-permitted basis since the summer of 1986. Attention was focused initially on the revision of the dynamic seeding conceptual model that has been guiding the Texas seeding programs ever since. This research effort has the acronym TEXARC (Texas Exercise in Augmenting Rainfall through Cloud-Seeding). During TEXARC, the cloud physics aircraft were the T-28 of the South Dakota School of Mines and Technology in 1994 and the Piper Cheyenne of Weather Modification, Inc. in 1995. Temperature (Reverse-Flow and/or Rosemount), dew point, cloud drafts, cloud water using either the Johnson-Williams hot wire (on the T-28) or the King hot wire (on the Cheyenne), GPS position, and particle sizes (PMS FSSP and 2D-C) were measured. Seeding was accomplished from a Cessna 340 in 1994 and from the Piper Cheyenne in 1995. The research plan was to document the physical processes in vigorous supercooled cloud towers before and after randomized treatment. In 1994, the T-28 cloud physics aircraft made a pass through the subject tower immediately prior to its treatment from the Cessna 340 and made repetitive passes through the tower afterwards. In 1995, the Cheyenne made measurements during the treatment run and thereafter. During TEXARC 1995, flights of the Piper Cheyenne were conducted on all days on which suitable clouds were expected. Fifteen clouds received simulated treatment and 19 clouds received AgI treatment during TEXARC 1995. 11