(note: excerpts from: PDF of published report)
PHASE II OF THE RAINFALL ENHANCEMENT ASSESSMENT PROGRAM IN THE UNITED ARAB EMIRATES
JUNE 2003 ONWARDS
RANDOMIZED SEEDING PROJECT - EXPERIMENTAL DESIGN
In late 2000, the government of the UAE, through the newly established Department of Water Resources Studies (DWRS) of the Office of His Highness the President, approached NCAR about developing and applying the technology of cloud seeding in the UAE.
A preliminary assessment identified some key areas of study required for assessing the efficacy and potential benefits of rainfall enhancement via hygroscopic seeding, including:
a) collating existing data and collecting specific data on clouds and rainfall,
b) establishing the natural background and variability of aerosols in the region,
c) adapting and developing numerical models for simulating UAE clouds, and
d) understanding the UAE hydrology sufficiently to assess the impact of rainfall on groundwater resources.
These evolved into seven specific objectives that addressed two fundamental questions for the UAE:
1. Is the frequency of cloud occurrence sufficient to warrant the investment in a cloud seeding program?
2. Are the clouds that do occur amenable to hygroscopic seeding?
A significant part of the study involved fieldwork – the intensive collection of observations (airborne and surface) during four field project periods (winter and summer of 2001 and 2002).
Microphysical observations of cloud droplets and aerosols showed continental conditions in both the UAE and Oman (see map at left) during the summer.
More varying conditions existed during the winter, mostly due to weaker cloud conditions (higher clouds and lower updraft speeds).
During the 2001 and 2002 winter seasons, radar summaries showed that no hydrologically significant rainfall events occurred over the UAE.
For the 2001 and 2002 summer seasons, radar studies showed that the vast majority of convective storms occurred over the Oman Mountains, southeast of Al Ain and northward, though they were relatively short-lived. (image at right of Oman Mountains)
Summaries of the trial seeding cases suggest that conditions amenable to seeding occur on only a few days during the winter, typically late in the winter season.
Conversely, suitable storms developed on more than a third of the summer days, although the number of storm tracks differed considerably between 2001 and 2002.
In summary, the results have mostly answered the two fundamental questions and thus support proceeding with Phase II of the Rainfall Enhancement Assessment Program during the summer months in the UAE.
This involves designing and implementing a randomized hygroscopic cloud seeding experiment during the summer season to statistically quantify the potential for cloud seeding to enhance rainfall, specifically over the UAE and Oman Mountains.
The randomized seeding experiment will require at least two years to treat a sufficient number of cases, and requires close collaboration with Oman in operating the seeding experiment seamlessly across their border.
Based on the numerous cloud investigations in the UAE, seeding trials, and radar data analyses, we conclude that winter clouds rarely produce conditions that are sufficiently convective with warm cloud bases and identifiable updrafts to effectively seed with hygroscopic flares.
However, during the summer, suitable convective clouds develop on about a third of the days, and treating 3-4 storms on each of these days seems reasonable.
It is conceivable that a randomized seeding experiment, targeting the mountains in particular, could yield results in two to three years.