Dust and the Shamal Winds

Dust2Dust aerosols are one of the greatest sources of uncertainty in atmospheric modeling and solar energy forecasting.  Aerosols vary in time and space, and can lead to meso-scale variations in cloud cover, temperature, and ground-level radiation.  

In the GCC countries, dust storms are most evident during the change in seasons when the Shamal wind is at its strongest.  This was evidenced on the evening of October 29, when Doha experienced accelerated winds just prior to sunset, combined with limited visibility over a 2 hour period as shown in the picture below:

The ground level image is a familiar site to those in Doha.  What is less familiar is the fact these events can be very large, facilitating operational forecasting.  For example, just after noon on October 29, and well before rush-hour traffic,  an airborne dust cloud was visible over the Arab Gulf that was captured in this image (tan haze) from MODIS (the Moderate Resolution Imaging Spectroradiameter) on the Aqua satellite:

Source: NASA Goddard Space Flight Center

The surface area of the aerosol disturbance was roughly 220 miles long in the direction of the prevailing wind and 90 miles wide.  Analysis by NOAA/NASA estimated airborne concentrations of dust exceeded 8,000 micrograms per cubic meter.  To put this in perspective, brushfires typically generate aerosol concentrations of 500 micrograms.  Analysis also estimated that the storm carried over 2 millions tons of dust from Iraq and Kuwait.  In practice, storms like this reduce Direct Normal Irradiance from average values of 1800 w/m^2 to as low as 200 W/m^2.

Anticipating large dust storms allows solar operators to initiate defensive actions with long-lead times: to take health and safety precautions, to preposition equipment cleaning crews, and to feather modules out of the wind to minimize mechanical loads.  The result can be improved system reliability, lower operating costs, and faster cycle times to return solar energy output to maximum production.

MODIS is an imaging instrument flying on two satellites in NASA’s Earth Observing System; Terra and Aqua. Launched in 1999, Terra orbits on a south to north trajectory with a morning equatorial crossing. Launched in 2002, Aqua orbits on north to south flight path with an afternoon equatorial crossing. The two satellites allow MODIS to image the same area at two different times of the day, and to cover the entire earth in less than 2 days.

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