• Aerosol & Cloud Studies
  • Hydrologic Studies
  • Modeling Studies

Aerosol & Cloud Studies

Weather modification research requires the involvement of a wide range of expertise due to the multifaceted nature of the problem and the large range of scales that are addressed. Large-scale and mesoscale dynamics, which determine the characteristics of cloud systems, and small-scale microphysics, which determine the nucleation and growth characteristics of water droplets and ice particles, form part of the chain-of-events leading to precipitation development. The large-scale and mesoscale dynamics are addressed in the Climatology and Modeling Studies. Here, we address the smaller-scale influences.

Instruments were flown that measured cloud microphysical parameters, aerosol characteristics, and trace gas concentrations. This suite of instruments combined to provide information about natural conditions important to cloud and precipitation development. For example, emissions of SO2 into the atmosphere result in the production of aerosols through oxidation. Highest concentrations of these particles were expected to occur in the vicinity of the highest emissions of SO2 in the UAE region. This is due to the fact that the synoptic conditions over the UAE are dominated by anticyclonic circulation, with low wind speeds and stagnant air. In addition, the afternoon and evening sea breeze could bring many of these pollutants onshore. CN aerosol concentrations were indeed found to be highest at identified SO2 sources. These aerosols can be very effective CCN and therefore have the potential to influence cloud microphysical processes and rainfall production efficiencies.


Measurements of CCN showed that background levels were enhanced due to local pollution sources in the region, which resulted in higher cloud droplet concentrations in clouds, making them more continental in nature. These are conditions generally thought to be more amenable to seeding with hygroscopic flares to enhance rainfall, since the polluted background particles generally work to suppress precipitation development. When mineral (desert) dust exists in conjunction with sulfates (and possibly other pollutants), the dust particles can become coated with sulfate, making them more active as cloud condensation nuclei. This is especially important in the UAE region where both sources exist and likely have an impact on the precipitation processes in clouds. In 2002, filters for detailed examination under electron microscopes were collected to determine the composition of the aerosols and assess whether sulfate coating of dust particles occurred and was significant. Analysis is still ongoing, but preliminary results suggest that sulfate coating is not a significant process and therefore may not be a complicating mechanism in determining the natural variability of CCN.

Vertical profiles of the aerosols show that the top of the boundary layer was at about 1500 m for the experimental periods in the winter and much higher (to 3500 m or more) in the summer. This is the layer that is influenced by surface sources and processes. Concentrations of aerosols in all size ranges as well as most trace gases decreased rapidly above the boundary layer. The large difference between the CN and aerosol (PCASP) concentrations indicates that the UAE region is a large source of nucleation mode aerosols. This is most obvious near specific source regions within the UAE. The aerosol measurements above the boundary layer also suggest that nucleation mode aerosols are as important a source of CCN as the accumulation mode aerosols.

Cloud Microphysics

Microphysical measurements collected in the UAE showed that cloud droplet effective radii were typically less than 10 mm indicating that clouds are continental in nature. Although low droplet concentrations were evident when high cloud bases occurred in the winter, the available water vapor at higher levels in the atmosphere is also less and the depth of the cloud between the base and the 0°C level is minimal or non-existent, inhibiting the growth of droplets to larger sizes. Clouds with bases much warmer than 0oC are more amenable to the hygroscopic seeding method. In the winter, favorable conditions seem to be the situations where the cloud base is lower than about 3000 m MSL. However, as stated above, these conditions for convective clouds occurred infrequently. During the summer, bases as high as 4500 m were still warm enough to provide sufficient depth and time for coalescence to begin. We are continuing with data analysis at this time. One area that needs further investigation is determining the existence of drizzle-sized drops and conditions in which they occur. Drizzle drops were occasionally measured in some clouds, which has implications on the effectiveness of seeding in those cases.


Precipitation processes in the UAE were characterized by using both radar and in-situ measurements of precipitation sized particles. The radar data were essential in developing a climatology to understand precipitation distribution, to assist in hydrological analyses, and to assess and quantify the potential effects of seeding.

Radar indicates periods of cloudiness and rain occured sporadically during the winter and was associated with passage of troughs and frontal systems. Many times, there were weak convective cores embedded within a more stratiform precipitation signature. These systems generally took 1-2 days to pass through the UAE. The summer was dominated storms over the Oman mountains east and southeast of Al Ain, in Omani airspace, consistently developed earlier and more often than over areas in UAE airspace to the north. On many of the days, storms initiated in Omani airspace 40-60 min prior to formation in the UAE.

In-situ precipitation measurements show an abundance of ice and mixed-phase precipitation in the winter clouds. This finding is supported by noting that many of the clouds sampled during the winter were more altostratus or altocumulus in nature. Summer clouds showed signs of an active ice process in clouds reaching -3 C. Drizzle and rain formation was limited in the shallower clouds (less than 1.5 km deep). Analysis of the precipitation characteristics of the natural clouds are ongoing at this time.

Seeding Trials

Seeding experiments took place on a total of fifty days during the four field campaigns. There were twenty-four days with seeding during the winter, but amenable clouds developed on only six of the seeding days over the two seasons. The summer clouds appear to be more amenable to seeding with at least 25 flight days when good candidate clouds were found. During the Summer 2002 season, flights into Omani airspace became possible and allowed for a broader investigation of the clouds over the Oman Mountains. The results of the seeding studies are still being analyzed but the initial scruitiny suggests follow-on work in the summer time, including a randomized seeding experiments, is warranted.

View more detailed results in our publications and documentation

Hydrological Studies

An important issue in any rainfall enhancement program is the determination of the potential benefiting sectors. These would include direct benefits to rain fed agriculture, the forestry sector, and possible social and ecosystem service benefits, with irrigated agriculture and municipal and industrial (M&I) water users being secondary benefiting sectors. In the UAE there is little to no surface water, which forces agriculture to be solely sustained by supplemental irrigation through groundwater withdrawals or desalinated water. An exception is the historic Falaja's, which are man-made 'springs' that take advantage of local topography and groundwater gradients to transport water to end-user points via underground viaducts. Therefore, if rainfall enhancement is shown to be viable in the UAE, it would likely benefit only the secondary sectors (irrigated agriculture and M&I water supplies) through enhanced groundwater recharge.

Ground Water Recharge

Groundwater recharge is the replenishment of an aquifer with water from the land surface. It is usually expressed as an average rate of millimeters of water per year, similar to precipitation. Thus, the volume of recharge is the rate times the land area under consideration, and is typically expressed in millions of cubic meters per year.

The quantity of recharge to an aquifer has been considered equivalent to the "safe yield" or quantity of water that could be removed from an aquifer on a sustainable basis. It is believed that the "sustainable yield" of an aquifer is almost always appreciably less than recharge. Nevertheless, a sustainable yield figure is derived from a recharge determination, and any sustainable yield study will usually involve the determination of recharge as a necessary first step. However, recharge is not well understood, so it is difficult to estimate aquifer sustainability unless the recharge-related processes are carefully studied.

There are two fundamentally different rainfall mechanisms that lead to groundwater recharge. The majority of recharge most likely occurs during winter frontal rainfall episodes on the synoptic scale. However, hygroscopic seeding is amenable to convective and not synoptic weather systems, so the focus of enhancement analysis has focused on the summer convective season. Some summer convective rainfall likely leads to recharge, but on a smaller scale in terms of both volume and lateral extent when compared to winter recharging events.

Early evidence from the rainfall and cloud physics experiments conducted during the summer of 2001 and 2002 suggests that the southeastern region of the Oman Mountains, a region east to southeast of Al Ain, seems to be a triggering point for significant convective rainfall events. There appears to be a preference for convective storms to form over this region and migrate west to northwest across the Oman Mountains and onto the desert plains. Convective storms that formed over the Oman Mountains to the north of Al Ain tended to be smaller and shorter lived.

Read more about Ground Water Recharge in the UAE

Benefit/Cost Analysis - Water Resources Modeling

Ultimately, results from randomized rainfall enhancement experiments should be used to suggest potential costs and benefits associated with a long-term enhancement program. To do this, the following should be know of the water resource infrastructure in the UAE, including:

  • Detailing the current water resource supplies, their allocation and their costs of production and delivery.
  • Quantifying current water resource demands.
  • Developing forecasts (perhaps 10 years) of future water demands, how those demands will be met, and the costs associated with those demands.
  • Developing costs associated with enhancement based on the outcome of the randomized experiment.

A Decision Support tool, such as the Water Evaluation and Planning Model (WEAP) can be used for performing such an analysis. This tool allows for the detailed description of the water supply and demand components for each sector (desalinization and groundwater supplies; municipal, industrial, and agricultural demands, etc.), and allows the user to project those demands and supplies into the future at varying rates of growth and associate costs of delivering that water to the various demand sectors.

The WEAP model has been updated as a part of this project to include dynamic hydrology within the simulation framework (Yates, 1996). This marks a significant methodological contribution in water resource modeling, as it will now be possible to incorporate the direct effects of hydrologic changes within the water distribution system and to capture the propagating and non-linear effects of water withdrawals for different uses on the water system.

Read more about Cost/Benefit Analysis through Hydrologic Modeling

Modeling Studies

The MM5 model was provided a daily real-time forecast of weather conditions over the UAE and the Clark-Hall model was used as a tool to understand, in detail, the mechanisms for precipitation formation in specific case studies.

MM5 Realtime Forecasts

Prior to 2001, only very coarse (50-110 km and greater) resolution general forecast models were run for the UAE and surrounding region. These global models, such as the ECMWF, the United Kingdom Meteorological Office Bracknell model, and the U.S. National Center for Environmental Prediction (NCEP) Medium Range Forecast (MRF) model capture very large, continental-scale meteorological features. A modeling system with factors of ten greater resolution than the global models was necessary to study cloud development and local meteorology of the region as well as forecast for the operational field program.

Read more about ongoing MM5 Realtime Forecasting and Validation

Clark-Hall Research Simulations

The three-dimensional, non-hydrostatic anelastic meteorological model described by Clark (1977), Clark and Hall (1991), and Clark et al. (1996), exploits features such as two-way interactive grid nesting and vertically-stretched terrain-following coordinates for fine-scale atmospheric simulations. The dynamic factors and precipitation formation mechanisms involved in clouds in the UAE have been numerically investigated using this model. We have selected two cases, 10 January 2001 and 31 July 2001 for detailed study. The first is an example of the synoptic event characteristic of days that bring rain to the UAE as part of a passing synoptic system, and the second explores the flow interactions that complicate summer precipitation forecasting.

Read more about Clark-Hall high-resolution modeling studies