The intent of the Oklahoma Weather Modification Demonstration Program was to be an evaluation the potential effects and benefits of an operational program rather than an experiment attempting to "prove the concept" of weather modification. Since the operational aspect of this program did not allow for a statistical determination of any impact on the amount of precipitation, this report provides some general analysis and case studies. Although this is an excerpt of the 1997 evaluation, the results of the 1996 and 1998 evaluations were similar.
Key findings can be summarized as follows:
May 15 Case Study (Precipitation Enhancement)
As part of the 1997 OKWMDP a cloud physics research plane made several flights in May. The research plane made initial passes through suitable seeding candidates which were then seeded or not seeded on a random basis. On May 15 the research plane made several passes through a cloud located over southwestern Canadian county. Looking at the first page of the flight log for this flight you can see that the first pass was begun at 1648Z (11:48 AM CDT), the cloud was seeded at 1705Z (12:05 PM CDT) and the last pass was begun at 1717Z (12:17 PM CDT). The supercooled liquid water (SLW) content for this cloud decreases after seeding, and the SLW distributions show a similar trend. These facts are consistent with the static phase seeding hypothesis but they are not conclusive because the SLW content in clouds will naturally decrease as the cloud matures.
Archived products from the Frederick (KFDR) and Twin Lakes (KTLX) WSR-88D radar sites show the reflectivity (BREF2) and storm total precipitation (PRETX) before the reasearch plane's first pass, during the first pass, during the last pass, and after the last pass:
The reflectivity images show that the cloud did grow in intensity during the course of the research/seeding activites, which is to be expected if the seeding is having the desired effect. The precipitation images show that this cloud produced 0.3 inches of rain. From this it is easy to jump to the conclusion that the seeding worked--the cloud was seeded and it rained. However, the images show that the reflectivity was increasing and it was raining before the cloud was even seeded. So, does this mean that the seeding did not work? We can't make that conclusion either because it's possible that the seeding accelerated the growth of the cloud and enhanced the amount and/or duration of the rain. This case illustrates the main difficulty of evaluating cloud seeding: it is impossible to know with certainty what would have happend if the cloud had not been seeded.
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