Microscale Convective Vortices

1. Microscale Convective Vortices Low Pressure, Warm Core Vortices DustDevils Waterspouts A Dust Devil is a strong, well-formed, and relatively long-lived whirlwind occurring in desert regions. On average dust devils are 500 to 1000 feet tall and can be 3 to 300 feet wide. Their wind speeds range from 12 and 30 mph. Dust Devil Formation Dust devils form when small surface “hot spots” cause air parcels to rise and accelerate rapidly upward, and horizontal wind shear sets the rising air spinning. The wind shear may be caused by the overturning of the air in the heated layer that brings higher winds to the surface or by objects on the desert floor disturbing the surface wind flow. Once the air begins rising, air rushes in from all sides to replace it, concentrating the rotation of the inflowing air in the rising column (Figure 3). Found Dust devils can be found in flat barren terrains, deserts or tarmacs (Figure 4). They have also been seen to roam the surface Mars. A waterspout is a funnel which contains an intense vortex, sometimes destructive, of small horizontal extent which occurs over a body of water. On average , the height of a waterspout ranges from 30 to 300 feet and as wide as 2000 feet. Their wind speeds range from 3 to 80 mph. Waterspout Formation Waterspouts receive their vorticity from local wind shear. They rotate either cyclonically or anticyclonically. Waterspouts form under convective clouds and in a region of horizontal wind shear between updrafts and downdrafts. They need warm, humid air, as well as the cooler air provided by a cloud system. As a patch of warm air on the surface of the water rises, the cooler air around it begins to rotate because of the horizontal wind shear causing it to rotate. Humid air over the water forms an ample amount of water vapor and the water vapor swirls up into the air, cooling and condensing, becoming a waterfall around the vortex. As the water vapor condenses into droplets, it releases more heat, increasing the vortex effect as it rises skyward (Figure 1). After a short time, the waterspout dissipates as the air within it cools. Types There are two types of waterspouts: Tornadic and Fair Weather. Tornadic waterspouts begin as true tornadoes over land in association with a thunderstorm, and then move out over the water. These are large and capable of considerable destruction. Fair Weather waterspouts are the most common and they form only over open water. They are small, brief and less dangerous. Figure 1: Waterspout formation over the Florida Keys region, a warm water region. There are five keys to the creation of a waterspout: 1. The temperature of the water is warm; 2. Warm rising air forms lines toward the cumulus cloud; 3. The horizontal winds rotate the rising warm air; 4. A dark spot is visible on the ocean surface; 5. Surface winds can produce sea spray. (http://www.ourkeywest.net/tag/waterspouts) Figure 3: Formation of a dust devil when the surface is strongly heated. The warm air moving across a strongly heated surface causes the air to do convective rolls. The rolls begin to tilt upward because the warm air begins to rise. Horizontal winds start to swirl around the rising air parcel causing a rotation creating a dust devil. (http://www.weatherquestions.com/What_are_dust_devils.htm) Figure 2: The annual average waterspouts days in the Ionian, Aegean , and Mediterranean Seas around Greece. The island of Crete experiences more waterspout days because of the warm water of the Mediterranean surrounding the island. (http://www.sciencedirect.com/science/article/pii/S016980951000205X) Sources: - Bluestein, Howard and NiltonRenno. “A Simple Theory of Waterspouts.” Journal of the Atmospheric Sciences. Vol. 58 Issue 8. - Heidorn, K. 2002. Water Twisters. The Weather Doctor’s Weather Almanac. - Smith, Bruce. “Waterspouts.” National Weather Service. - Woodcock, A. “Dust Devils in the Desert.” Weatherwise. Vol. 44 Issue 4. Figure 4: Dust Devil sighting in Lake Xau, Botswana. This dust devil formed over a dry lake bed. (http://birdlifebotswana.blogspot.com/2010/10/water-reaches-lake-xau.html) Found Waterspouts are found, but not limited to the Florida Keys, the Great Lakes, and areas around the Ionian, Aegean and Mediterranean Seas (Figure 2). Jeremy Manny Georgia College and State University Geography 4112 Spring 2012