Feasibility Study for Rainfall Enhancement - UAE

  • Project Description
  • Project Objectives

Project Description

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:

  • Collating existing data and collecting specific data on clouds and rainfall
  • Establishing the natural background and variability of aerosols in the region
  • Adapting and developing numerical models for simulating UAE clouds
  • 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?

This website presents the results of numerous efforts related to the objectives of the feasibility study. A significant part of the study involved field work: the intensive collection of observations during four field project periods (winter and summer of 2001 and 2002). Numerous people and institutions contributed to the field efforts, and many opportunities arose to train and educate UAE personnel on aspects of the field projects. Aircraft measurements were an essential component to the program. To assist in collecting and archiving quantitative data from the radar network being established through DWRS, NCAR installed TITAN/CIDD software on computer systems at several locations and provided training and guidance on the operation of these systems. Substantial effort was expended in integrating and networking these systems with the radars and DWRS, and in evaluating the capabilities of the radars themselves. Over the past two years, NCAR radar engineers have identified several problems with the operation of the radars, some of which remain unsolved. The Radar Echo Classifier (REC) routine used in the USA radar network was tuned for the UAE environment, and largely removed ground returns or "clutter" in the radar data.

A brief climatology of conditions in the UAE describes the rationale for selecting two field project periods: winter and summer. The summary of flights by the research aircraft shows that a total of 200 flights were made, equally split between summer and winter as well as between 2001 and 2002. About half of the flights collected microphysical data in clouds and precipitation, with seeding trials having been performed on about half of those flights. Descriptions and analyses of airborne aerosol and microphysical observations and of radar data comprise a major portion of the website. Microphysical observations of cloud droplets and aerosols show continental conditions in both the UAE and Oman during the summer. More varying conditions exist 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 show that the vast majority of convective storms occurred over the Oman Mountains, southeast of Al Ain and northward, though they were relatively short-lived. The short lifetimes of the thunderstorms act to minimize the window of opportunity for cloud seeding to enhance rainfall, emphasizing the need for accurate prediction of these situations in planning seeding operations and for basing aircraft near the mountains for quick response.

Seeding assessments include the development and characterization of an improved hygroscopic flare, a summary of cloud seeding experiments during the field projects, and a cursory analysis of a several seeding events. Summaries of the 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. Part of this difference is likely due to year-to-year variations, but part may also be due to different radar operating characteristics. Storm duration and initiation time further define optimal operating parameters for potential seeding operations.

The hydrological studies concentrate on characterizing processes important in the UAE, such as rainfall to groundwater recharge, that may be impacted by a rainfall enhancement program. The major near-surface unconfined aquifers that receive recharge from precipitation in a timeframe compatible with rainfall enhancement activities are those near the Oman Mountains. Understanding the spatial characteristics of recharge zones in relation to a precipitation climatology in both a spatial and temporal sense is important. Therefore, the development of a radar-derived rainfall climatology of the UAE and the correlation of this climatology with regional groundwater recharge zones is essential.

An assessment of the MM5 real-time forecast model demonstrates its strengths and weaknesses as a forecasting tool. Further work on improving and using the forecasts from the MM5 could benefit the operational community, specifically including: Better initialization data to more accurately capture and resolve phenomena such as the land/sea breeze interactions; better sea surface temperature data for initialization of the model, especially over the Arabian Gulf; and improved soil moisture and temperature parameterization and associated physical parameterizations. The MM5 model output is also used to initialize the background environment and to update boundary conditions for the Clark-Hall model simulations of a winter and a summer case. These cases will be used to further investigate cloud and precipitation development in UAE clouds and a multitude of variables that may have implications for cloud seeding using hygroscopic flares. Recently, a new microphysical parameterization was developed for the Clark-Hall model that will provide improved opportunity to conduct enhanced seeding simulations.

In summary, the results presented in this website have mostly answered the two fundamental questions and thus support proceeding with Phase II of the feasibility study. 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. Several activities or components are required in order to quantitatively assess any increases in rainfall from cloud seeding and to initially estimate the overall economic benefit of such a program.

Project Objectives

A scientific evaluation of cloud seeding for rainfall enhancement requires several efforts designed to systematically characterize clouds and precipitation in order to determine their potential response to seeding. The two fundamental questions that needed to be answered during the first phase of the UAE program were:

  • Is the frequency of cloud occurrence sufficient to warrant the investment in a cloud seeding program?
  • Are the clouds that do occur amenable to hygroscopic seeding?
In order to answer these questions, more specific objectives were defined, including:
  1. A climatology to assess cloud characteristics and rainfall distribution. These studies were important to determine if suitable clouds existed and their frequency of occurrence.
  2. Field programs, using an instrumented cloud physics aircraft and weather radars with data-recording capability, during the months with the largest occurrence of clouds and rainfall in the UAE (winter) and during the months with a high frequency of convective clouds over the Oman Mountains (summer). The field project objectives were to document: a) the impact of natural aerosols and pollution on cloud processes, and b) the microphysics and dynamics of natural clouds. Perturbations of these processes were attempted during several seeding trials.
  3. Analysis of the collected aircraft, radar, and chemistry data to determine the natural precipitation processes and the effect of seeding on these processes.
  4. Assessment and improvements in the data collection procedures for the weather radar network, including implementation of the NCAR Thunderstorm Identification, Tracking, and Analyses (TITAN) and Central Integrated Data Display (CIDD) software systems for archival, display, and analysis of quantitative radar data.
  5. Provision of real-time high-resolution numerical model forecasts with the MM5 model during the field project periods.
  6. Atmospheric modeling studies of the general meteorological conditions and the formation of clouds and rain for evaluating and assessing the potential for cloud seeding.
  7. Hydrological studies to determine the impact of possible rainfall increases on surface- and groundwater.