April 7, 2003— In addition to its arid climate (i.e., extreme heat, low humidity and little precipitation) the most notable natural hazard in Iraq (and surrounding areas) are dust and sand storms. These storms are most prevalent in the spring and summer when a prevailing northwesterly wind — known locally as the “shamal” — kicks up the fine desert sand and the silt along the Tigris and Euphrates river basins. So what does this have to do with NOAA? Actually, quite a bit. Did you know that the NOAA Air Resources Laboratory has conducted research on large scale dust and sand storms in the Middle East and continues to conduct research on physics of dust emissions in the United States and that NOAA Satellites and Information uses it’s polar-orbiting satellites to detect and track these storms in Iraq and surrounding regions?

Dust and Sand Storm Formation
Dust and sand storms are a persistent problem in Iraq and other areas in the Middle East, but they are most prevalent in the spring and summer months due to the strong (northwesterly shamal) winds that characterize the weather during the winter-spring seasonal transition. Specifically, dust and sand storms occur when the strong (mostly dry) storms — that often accompany well-defined cold fronts — stir up these particles. Dust and sand lift both ahead of and (even more so) behind cold fronts (since winds tend to be stronger behind the front than ahead of it). This seasonal trend can best be characterized as a combination of two separate weather systems: the sub-tropical jet stream pushing up from south of the Arabian Peninsula and a polar front jet stream pushing down from the European continent. When these two systems come into close proximity, they create much more dynamic weather than is usually found in this region, especially the strong northwesterly “shamal” winds (shamal winds at several Southwest Asia international airports have been recorded as high as 43 knots or 49 mph).

The larger the particles, the stronger the wind required to lift them into the air. But for there to be any long-range transport, there also needs to be considerable vertical motion. The vertical speed determines how much the particulate matter is lifted into the air. Another factor that influences the impact of the shamal is the dampness of the sand. Even a very small amount of precipitation can keep a tremendous amount of sand from entering the air (although the very dry conditions that predominate in Iraq mean that there is a high probability that strong sandstorms can arise — as recent events have shown).

The unique topography and human intervention within the region also contribute to the frequency and intensity of dust and sand storms in this area. The natural funneling of large air masses by the high mountains in Turkey and Iran, combined with the high plateaus in Saudi Arabia, help to funnel air across the Mediterranean into the Persian Gulf. Furthermore, many Iraqi wetlands have been drained for agriculture or seriously deprived of water by reservoirs upstream. This exacerbates dust as wind lifts dry silt from exposed lake and marsh beds.

Depending on location, it is not unusual for Iraq to encounter 20 to 50 days of blowing sand and dust each year. Dust and sand storms can persist for days, however, because the air is so dry in this region, there are wide diurnal temperature differences that can influence dust and sand storms (especially during the summer months). In other words, rapid heat loss at night lowers the temperature inversion, helping to settle the dust and sand. Therefore, dust and sand storms generally subside at the source soon after sunset and are strongest in the late morning and afternoons.

Sand Versus Dust Storms
Technically speaking there are subtle, yet distinct differences between dust and sand storms.

• Sand Storms: A “sandstorm” is basically a wind storm that carries sand through the air, forming a relatively low cloud near the ground. Typical sandstorms only reach heights of up to 15 meters (49 feet), contain sand particles with average sizes between 0.15 to 0.30 millimeters, have wind speeds exceeding 10 miles per hour and last as long as wind speeds persist. When wind reaches a critical velocity, grains of sand begin to roll forward along the ground surface. For higher wind speeds, sand particles in a sand storm move by “saltation,” a process under which particles are temporarily lifted and then bounced along the surface in a hopping/jumping motion. When one saltating grain collides with another, the impact may lift either particle into the air. Once aloft, these particles are subjected to the forces of gravity (pulling them down) and horizontal wind velocity... and the process starts all over again. Once a dust storm starts, it roughly increases with the cube of the wind speed.
• Dust Storms: Dust storms are a similar phenomena but have distinctly different characteristics. Dust storms form in semi-arid and arid regions where small dust (and sand) particles are literally blown into the air. Unlike in pure sand storms, dust particles are small enough to be lifted aloft by currents of turbulent air and carried into suspension. Ironically, however, research has shown that wind does not usually pick up dust-sized particles less than 0.05 mm in diameter along many completely “smooth” surfaces because each individual dust particle either: 1) lies within a zone of air that is protected by larger particles or aggregates (right along the surface of the desert) or 2) is aggregated onto the surface of a larger particle or larger aggregate. Under these circumstances, sufficient energy to liberate the small particles from the surface is delivered by saltating sand-sized particles. Irregularities in the surface or the presence of sand grains may create sufficient turbulence, so that sand grains gain sufficient wind energy to initiate saltation. Saltation-free dust emissions are possible, but rare, found in fine material where sand grains or sand-sized aggregates are not found. Vertical downdrafts of chilled air during thunderstorms may locally strike the ground with velocities of 40 to 80 km/hour (25 to 50 miles/hour). Under such conditions, fine particles may also be swept upwards hundreds or thousands of feet into the air. The average height of a dust storm is 3,000-6,000 feet and stronger storms have dust to 8,000-10,000 feet. Haze and dust with extreme storms have been documented as high as 35,000-40,000 feet.

Disruptions to Human Activity
In addition to the already harsh desert conditions (often accompanied by the risk of heat exhaustion and dehydration), dust and sandstorms disrupt human activity. They reduce visibility, layer on skin and cloths, infiltrate buildings and find their way into food and drinking water — leaving a permanent sandy feeling in your mouth. Pounding sand and dust storms also wear away textile materials, such as protective outer wear and shoes.

These storms also wreak havoc on machinery, electronics and buildings. Blowing sand and dust scour surfaces and wear away protective coverings (i.e., glass becomes frosted, wire wrap wears away and electric circuits ground out). Unfortunately, the more sophisticated an electrical system is, the more dust affects it. Dust compacts easily, solidifies with little added moisture and combines with lubricants — often resulting in clogged and/or jammed equipment and machinery. Dust and sand storms also set up electrostatic discharges that, while not typically fatal, can have negative consequences in fueling operations, computer or electrical systems.

NOAA’s Sand Storm Research and Monitoring Activities
The NOAA Air Resources Laboratory has conducted research on large scale dust and sand storms in the Middle East and continues dust emission work in the United States. NOAA Satellites and Information uses it’s polar-orbiting satellites to detect and track these storms in Iraq and surrounding regions.

• Large Scale Dust Storm Models: Scientists from the NOAA Air Resources Laboratory adapted a method used to predict dust injections from the Sahara desert to dust emissions over southwestern Asia (i.e., Iraq, Kuwait and Saudi Arabia). Specifically, NOAA scientists used a regional transport and dispersion model to compute ground-level air concentrations of PM10 (particles with a diameter of 10 µm or less) . These air concentration data were actually used for health assessments in the area occupied by U.S. troops during the Gulf War period (August 1990 through April 1991) — prior to the onset of surface based sampling in May of 1991. The data needed as input into the model included the threshold friction velocities for initiation of dust emission, the aerodynamic roughness length of the surface, and a coefficient that relates surface soil texture to PM10 dust emissions. Using gridded meteorological data for the region and maps of larger-scale soil features (in Kuwait and Saudi Arabia), these local parameters were extended to estimate the dust emission potential over the entire study area (i.e., Kuwait, Iraq, part of Syria, Saudi Arabia, the United Arab Emirates and Oman). A dust emission rate was then computed from each cell when the local wind velocity exceeded the threshold velocity for the soil characteristics of that emission cell. Computations were made for the period of August 1990 through August 1991. During this time there was extensive soil disturbance in the region due to military maneuvers that culminated in the Gulf War during January and February of 1991. The model calculated air concentrations from mid-May through mid-July, the period of the most frequent and intense dust storms. These calculations were then compared with the measured data (i.e., ground-based PM10 sampling). Spatial patterns of the model predictions were also compared with the aerosol index parameter derived from NASA’s TOMS satellite instrument.

  Results indicate that the model accurately predicted each of the major dust storm events over a two month period. However, the model consistently over-predicted the PM10 air concentrations in many coastal areas by a small margin. The over-prediction of PM10 in coastal areas may be attributed to the development of diurnal flows, such as a sea-breeze (i.e., the entrainment of cleaner air from offshore) — features not well represented by the input data. Despite this, the study demonstrated that it is possible to incorporate an inventory of complex soil characteristics and meteorological data into a model to produce reasonable estimates of dust storm frequency and their spatial extent.

• Dust and Sand Storm Detection and Monitoring by NOAA Polar-orbiting Satellites: Dust and sand storms are easily seen by instruments on NOAA satellites (i.e., NOAA-16 and NOAA-17), such as NOAA's Advanced Very High Resolution Radiometer (AVHRR). Because the satellites provide global coverage twice a day, NOAA is able to monitor the dust and sand storm sources, distribution and movement of the particle plumes, particle transport routes, and areas of dust and sandfall over specific time periods and geographic regions. It is important to note, however, that this type of satellite imagery is not able to distinguish between cloud cover versus dust and sand storms. Therefore, this type of technology works best on clear days.

Relevant Web Sites
NOAA SAND STORM DOCUMENTS: Modeling Large Scale Duststorms

Draxler, R.R, Gillette, D.A., Kirkpatrick, J.S., Heller, J., 2001, Estimating PM10 Air Concentrations from Dust Storms in Iraq, Kuwait, and Saudi Arabia, Atmospheric Environment, Vol. 35: 4315-4330

NOAA SATELLITE CAPTURES IRAQ SAND AND DUST STORMS

NOAA SATELLITE SHOWS MASSIVE DUST STORM IN IRAQ MOVING SOUTHWARD

MOTORISTS BEWARE !!

Media Contact:
Jana Goldman, NOAA Research, (301) 713-2483