As a weather forecaster each place you forecast for in the world is different. All areas have their own unique forecasting challenges. In the country of Iraq you would think that your challenge would be; how hot is it going to be today? Although this is usually what most people ask about when you get into discussions of weather in Iraq, the real challenge is dust storms! I prefer to call them dust events because when I think of a storm I think of something that lasts only an hour or so where these dust events can last for days. As soon as you put a dust storm in your forecast, the following three questions follow: when will you get them, how often will they last, and how will it effect day to day operations especially in a military environment? There are basically two types of dust storms, the haboobs and the shamals. Haboobs are hard to forecast for due to the way they form which is from an outflow boundary of a thunderstorm. These events occur during the late fall to spring seasons and are also the most dramatic of the two because they happen so fast. This is the type of dust storm that looks like a wall of water speeding toward you only it is dust. For aviators it is a nightmare and would surely end with unfavorable consequences. For people on the ground it would go from daylight to total darkness in a matter of minutes. Haboobs move in quickly, but also move out fairly fast. The second dust event and the one that I will spend the rest of this document discussing is the shamal dust storm. This type of dust event isn't as dramatic as the haboob, but it does cause reduced visibilities. Shamal dust events are caused by persistent northwest winds blowing over a source region and picking up dust into the atmosphere. These events are not as sudden as the haboob, but it can last much longer. Shamals can last from four to fives days during one event. The first thing you need to look at when forecasting a shamal event, is source regions. Most source regions are located in the lower Tigris and Euphrates river valleys. They can also be smaller deposits located in or near dry streams or lakes in Iraq. There are also a large number of source regions in Iran, just on the eastern border of Iraq. The consistency of the dust in these areas is similar to talcum powder; this is why we call it a dust storm rather than a sand storm. Also keep in mind areas where construction is going on. These areas will cause localized dust events that can significantly reduce visibilities, but only affecting a small area like a city. The biggest reason for knowing these source regions is predicting if you forecasted winds are going to tap into these source regions and lift the dust into the air causing a dust event. Depending on the wind direction will determine where the dust will go. If the winds are from the north or northwest, then the area to the south and southeast of the source regions will be affected. If the winds are blowing from the south or southeast, then the areas to the north and northwest will be affected. These northerly winds can also tap into the source regions in Iran. During spring and summer months in Iraq the winds are mainly from the north northwest, therefore areas in the middle to southern portions of Iraq, such as Balad and Baghdad have visibilities reduced to less than 1600 meters by these dust events approximately 5% of the time. Where Tallil has visibilities reduced to less than 1600 meters 20% of the time due to is location further southeast with many source regions to the north and northwest. Areas to the North and northeast such as Mosul and Kirkuk are only reduced 1% of the time to less than 1600 meters. From late fall into early spring the winds are reversed and therefore the areas affected are reversed also. During the summer months the 300 mb pattern changes from a westerly winter flow with migratory lows moving through to a pattern dominated by high pressure light and variable winds, the result no clouds and no rain. This makes the source regions primed for dust to be lifted into the air. Closer to the surface you have winds called shamals, which mean north in Arabic, blowing from northwest. These winds last all summer long and are also called "the winds of 120 days." These winds are created from the interaction between the Pakistani Heat Low and the high pressure ridge over the Mediterranean which extends into northern Saudi Arabia. This interaction results in a pressure gradient set up and winds increasing in speed. These winds can last one to ten days and usually reduce in speed at night. Due to radiationial cooling at night an inversion sets up with surface winds less 10-15 knots. Above the inversion a low level jet develops and can get as strong as 60 knots. When the inversion is broken and the wind is mixed down to the surface you can expect dust. The wind speed needed to suspend dust is 15-20 knots. Winds of 30 knots downstream of a source region can reduce visibilities to practically nothing. Things to look at are the night skew-T sounding. If it shows 30 knots of wind at 1,000 feet you might want to think about dust in the morning. Look at models and if they are forecasting 30 knots of wind at 1,000 feet during the night from the northwest and you are southeast of a source region, you really might want to think about dust. One thing to keep in mind is that as wind mixes down the speeds reduce so don't think because you have 30 knots at 1,000 feet you are going to get 30 knots at the surface when it mixes down. As the night goes on look at upstream stations and watch there observations for winds and visibilities. They will be your first indicator of what is to come. This may be your only way of timing the dust to arrive at your area. This also may be your only way of knowing it is out there in the early morning hours while it is still dark. One of the things that make the shamal dust events so hard to forecast is the fact that they form overnight and into the early morning hours and the best way to see them is with visual satellite imagery. As everyone knows there is no visual imagery at night; therefore you feel blind when trying to see what is going on around your location. Keep in mind dust is very thin and sometimes can't even be seen on visual satellite imagery. When looking a visually imagery don't be fooled by how thin dust looks because even thin plumes of dust can reduce your visibilities significantly. Infrared imagery does not show dust very well and therefore isn't a big help when it comes to forecasting dust events. Another thing that makes it tough to forecast is that the dust is normally in a plume pattern. Because of this you might have a location to your west that is socked in with dust and your location is clear with no reductions to visibilities. These plumes start off small and narrow from the source region and the further the distance from the source region the wider they become. Of course the closer you are to the source region the thicker the dust and the lower the visibilities will be. Depending on wind speeds these plumes of dust can reach as high as 15,000 feet in the atmosphere. They average 3,000 to 8,000 feet and normally reduce visibilities between 4800 and 8000 meters. The effects of dust storms vary depending on who you talk to. If you talk to civilians in Iraq they would say that they are just a hindrance and other than having to protect their faces to keep the dust out of their eyes and lungs. For military ground troops it affects daily operations of searching for terrorists and also protecting themselves and others from terrorists. The people it affects the most is aviators. Dust storms affect all types of flying operations. When you have a dust storm you can't get fighters in the air to protect those that need it, medivac units can't take off to help those in need, and for those who are already in the air, it can prevent them from landing in a safe place or landing period. Dust also has adverse effects on any equipment that is consistently going through these storms. Extra time cleaning and protecting equipment is necessary. Areas that are prone to dust storms can also prevent people from using certain equipment. You can receive climatologically data from the 14th Weather squadron, formally known as the Air Force Combat Climatology Center. They have narratives on dust storms in Iraq. I think better night time satellite imagery would help to forecast shamal dust events better. As for haboobs being able to better forecast where thunderstorms will occur is the only improvement to finding possible areas where haboobs may occur. So as you can tell weather forecasters have a very important job. It is demanding and imperative that we give the best forecast possible. It can save lives. References: 14th Weather Squadron in Asheville, NC Michelle Moses - forecaster at Balad Air Base, Iraq 28th OWS Shaw AFB, SC |