Hygroscopic Cloud Seeding
- UAE Climatology
- Hygroscopic Cloud Seeding
Understanding the impact of hygroscopic seeding requires knowledge of the climatology of the area, especially with an emphasis on cloud forcing.
Past climatological studies have identified the winter season (Dec-March) as accounting for the bulk of rain in the UAE. These studies were based on reporting stations located primarily along the northern coast or in the Gulf. Troughs, depressions and the occasional front, moving into the region from the west and northwest, result in large-scale systems that provide significant rainfall. During strong frontal conditions, rainfall can be experienced throughout the country but may reach a peak over the mountains due to additional uplift as airflow is forced over the mountains.
Convective rainfall over the Oman Mountains during the summer season is a phenomenon that is widely known to local meteorologists but is not described adequately in climatological studies. Although there is some indication of the existence of a summer convective component in UAE climatology, the frequency and importance of mountain rainfall is not well documented. During the summer, the UAE region is under the influence of upper level easterly to northeasterly flow associated with the tropical easterly belt, enhanced by the thermal low over the sub-Asian continent.
Example of Winter Westerlies
Example of Summer Easterlies
The seeding technique evaluated in this study is performed at convective cloud base, preferably at warm temperatures. In contrast to silver-iodide seeding performed at -10 C to enhance ice nucleation, this technique is based on an enhancement of the coalescence process in the clouds as a means to increase rainfall.
In the past ten years, a new approach to hygroscopic seeding has been explored in summertime convective clouds in South Africa as part of the National Precipitation Research Programme (Mather et al., 1997).
This approach involves seeding summertime convective clouds below cloud base with pyrotechnic flares that produce small salt particles on the order of 0.5 mm diameter in an attempt to broaden the initial cloud droplet spectrum and accelerate the coalescence process. The burning flares provide larger CCN (>0.3 micron diameter) to the growing cloud, influencing the initial condensation process and allowing fewer CCN to activate to cloud droplets.
The larger artificial CCN inhibit the smaller natural CCN from nucleating, resulting in a broader droplet spectrum at cloud base. The fewer cloud droplets grow to larger sizes and are often able to start growing by collision and coalescence with other cloud droplets within 15 minutes (Cooper et al., 1997), initiating the rain process earlier within a typical cumulus cloud lifetime of 30 minutes.Read More about Hygroscopic Cloud Seeding Principles
The recent model study by Cooper et al. (1997) gives good insights into the theory behind hygroscopic cloud seeding and provides guidance on the necessary steps to optimize the cloud seeding flares. If the CCN that are introduced into the cloud from the seeding flare are larger in size than the natural CCN, the introduced CCN will activate preferentially over the natural CCN and change the character of the droplet size distribution to favor coalescence and the formation of rain. In addition, the modeling study indicated that the larger particles, if present in sufficient concentrations, would further enhance the transformation to precipitation.
Several different manufacturers have started to make hygroscopic flares, following the initial promising results from South Africa. It is important to evaluate the output particle spectra from these flares in order to understand the effect they will have on the condensation/coalescence process and precipitation development in convective clouds.
It is difficult to obtain measurements of the particle sizes produced by the flares in a field environment. This generally requires two aircraft, one to generate the seeding material, and the second to make the measurements. A further difficulty is the ability to reproduce these measurements, since the environment changes. To study hygroscopic cloud-seeding flares, NCAR/RAP designed and constructed a test facility that simulates the burning of flares from an aircraft.
Airborne in-situ and remote sensing instruments were used during the two-year project. The aircraft and crew were provided by Weather Modification Incorporated (WMI) for air chemistry, aerosol and cloud physics studies, and hygroscopic cloud seeding. Aircraft instrumentation was provided by NCAR, WMI, University of Witwatersrand, University of Arizona, and South Dakota School of Mines and Technology. A network of C-band radars developed by the DWRS, in collaboration with the UAE Air Force, Dubai's Department of Civil Aviation (DCA), and Abu Dhabi's DCA, was extensively used for operational and research endeavors.
During 2001, a Piper Cheyenne II (registration number N233PS) twin-engine turboprop aircraft was used for the research mission. However, the need for more load and electrical capabilities to conduct more in-depth cloud studes became evident during the 2001 operations. In 2002, a Beechcraft King Air 200 (N553R), also a twin-engine turboprop, was provided by WMI and performed the same aircraft sampling functions, but allowed for an extended array of instrumentation to be carried at one time:
The DWRS, in collaboration with the UAE Air Force, Dubai's Department of Civil Aviation (DCA), and Abu Dhabi's DCA, is developing a national weather radar capability with the goal of producing quantitative precipitation estimates on a much better spatial and temporal scale than can be achieved with rain gauges. To assist in collecting and archiving quantitative radar data, NCAR installed TITAN/CIDD software on computer systems at several locations and provided training and guidance on the operation of these systems.
Reliable communications, operation, and calibration of the UAE radars are essential for the continuous collection of quantitative information on natural cloud characteristics and precipitation, and hence to the evaluation of any cloud seeding operations. 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, many of which have been solved or minimized while others remain unsolved.
View Current Radar From Department of Water Resources Studies - Office of His Highness the President - UAE