Laserfiche WebLink
<br />A relationship between LWC and the two parameters CTT and U700 has <br />been found to exist. Three different types of observation of LWC all <br />yield similar results. LWC increases as U700 increases, provided the <br />CTT is warmer than a critical value. The LWC data were collected by <br />aircraft using a JW device, by extensive aircraft icing observations, and <br />by dual frequency microwave radiometry. <br /> <br />Furthermore, the quantity LWC/P is separated into two distinct <br />regimes, high values when the cloud top temperature is warmer than a <br />critical value, and low values when the CTT is colder than the critical <br />value. The critical value of cloud top temperature separating the two <br />regimes decreases from approximately -180C at U700 equal to 2 m s-l <br />to -240C at 15 m s-1. These limits are derived from the various data <br />sets. However, during a specific episode, these limits may vary by <br />several degrees. <br /> <br />Cloud Seeding Delivery Detailed analysis of ice nuceli measure- <br />ments have led to the conclusion that delivery of ice nuclei into super- <br />cooled clouds remains as the greatest obstacle to attaining a successful <br />weather modification technology for increasing winter orographic pre- <br />cipitation. Airborne measurements of ice nuclei released from an air- <br />craft in northern Utah during two winter seasons (20 flights) clearly <br />showed diffusion of ice nuclei in clouds is such that the outward spread <br />veloc ity of an AgI plume is about 1 m s-l or less. Thus, airborne <br />delivery is made rather inefficient concerning the length of a barrier <br />that can be effectively treated. Also, it takes one to three hours to <br />have an aircraft arrive at the seeding location. The aircraft can remain <br />there only for about four hours. <br /> <br />With ground seed ing, the three main prob lems are that inversions <br />often trap ice nuclei during winter, the horizontal movement is often <br />unpredictable, and placement of ice nuclei into the clouds far enough <br />upwind to initiate precipitation is characterized as uncertain at best. <br />Extensive airborne and ground based measurements made in Utah, and <br />findings elsewhere lead to this assessment. <br /> <br />As a direct attempt to overcome these difficul ties with either <br />airborne or ground based seeding, an entirely new delivery system was <br />envisioned in 1981. Since that time the primary research effort at USU <br />has been directed toward development of the new system. This system uses <br />remotely released balloons from an array of balloon launching sites. <br />Each balloon released carries a mini acetone-based seeding generator. <br />Balloons can be released by remote control so that in principle a volume <br />of cloud can be treated continuously both in space (along the barrier> <br />and with time. <br /> <br />The balloon system of seeding overcomes the difficulties of the <br />airborne or ground-based systems. The response time is much faster than <br />either of the others; that is, balloons can be released and attain cloud <br />altitudes within about 15 minutes of the decision to seed. The length of <br />barrier and duration of seeding can be made as desired and there is <br />little if any hazard. Ice nuclei can be placed in the upwind portion of <br /> <br />46 <br />