Wind Damage >> Tornado Chasers Causing Damage

Mesovortices While storm downdrafts likely account for the greater proportion of damaging nontornadic winds in a derecho-producing convective system, observational evidence and numerical simulations show that corridors of enhanced wind damage in derecho-producing squall lines often are associated with Tornado Chasers Causing Damage embedded circulations or"vortices." 

These features may occur in various sizes and generally are referred to as mesovortices. Mesovortices can arise in many ways. For example, Tornado Chasers Causing Damage the bookend vortices mentioned in Derecho-producing storms are counter-rotating circulations that arise as the rear-inflow jet of a developing convective system impinges on the system's gust front (see Derecho development). 

This forces horizontally-rotating "tubes" of air along the gust front to tilt vertically in storm updrafts on either side of the jet. Once formed, the pair of resulting vortices can enhance the rear-inflow jet and thereby initiate or accelerate the bowing process, Tornado Chasers Causing Damage as shown here. Bookend circulations may reach dimensions of several tens of miles. 

Counter-rotating vortices also occasionally arise from the downward tilting of horizontal vorticity by storm downdrafts, Tornado Chasers Causing Damage as shown in this study. Smaller circulations on the order of a mile or two in diameter sometimes develop along gusts fronts as a result of what is known as horizontal shear instability. 

This form of instability is derived from the very sharp changes in wind direction and speed that exist across such boundaries. When the background wind shear and/or buoyancy are great, Tornado Chasers Causing Damage shear-induced vortices sometimes sufficiently strengthen to produce tornado-like damage. Many such features accompanied the "Super Derecho" of May 8, 2009 (see Figure 4). 

Mesocirculations in some derecho-producing convective systems also may be traced to supercell storms that were present during the early stages of system development and Tornado Chasers Causing Damage subsequently were absorbed within it (e.g., see the "People Chaser Derecho" of 2001). 

Similar circulations appear when an accelerating line or bow echo overtakes a more slowly-moving supercell storm. The illustration below shows several mesovortices along a derecho-producing squall line as viewed by Doppler radar. The left panel shows radar reflectivity, the right, radar velocity, Tornado Chasers Causing Damage with the radar location depicted by the black circles at the top left in both panels. 

Vortices 1 and 2 are small-scale circulations that may have arisen due to the presence of strong horizontal shear along the gust front. Vortex 3 is larger, Tornado Chasers Causing Damage but not as well-organized. Animated reflectivity data (not shown) suggest that this feature most likely is the residual circulation associated with a supercell storm that was overtaken by the convective system's accelerating gust front. 

When a derecho-producing convective system becomes particularly large and long-lasting, Tornado Chasers Causing Damage a larger-scale circulation known as a mesoscale convective vortex or "MCV" may develop. These vortices arise from localized pressure falls that occur in response to the concentrated release of latent heat (i.e., warming) that accompanies the development of deep cumulonimbus clouds. 

On occasion MCVs may grow in size to occupy an area as large as that of a typical Midwestern state; Tornado Chasers Causing Damage such circulations may last for more than a day and enhance the convective system's strength and longevity (e.g., the "Super Derecho" of 2009 previously mentioned). Smaller MCVs commonly form in weaker wind-producing systems. 

They often appear as swirls of residual mid-level clouds, or as bands of showers in radar imagery, Tornado Chasers Causing Damage long after the parent convection has dissipated. The radar reflectivity image below shows the very well-developed MCV associated with the "Super Derecho" of May 2009 as the circulation entered southwest Missouri from southeast Kansas. 

Once developed, Tornado Chasers Causing Damage it is the superpositioning of vortex-induced winds with those of the convective system's larger-scale outflow that can lead to corridors of enhanced damage in a derecho. 

Typically, with convective systems that form in the mid-latitudes where the mean tropospheric flow is westerly, Tornado Chasers Causing Damage this superpositioning yields the strongest winds on the equatorward side (i.e., the southern side in the Northern Hemisphere) of the vortices, where the wind fields are additive. 

An analogous additive effect accounts for the fact that the highest wind Tornado Chasers Causing Damage speeds in a hurricane generally occur in the storm's right front quadrant (when facing in the direction of storm motion). DERECHO CLIMATOLOGY 

Where and when derechos are most frequent in the United States Derechos in the United States are most common in the late spring and summer (May through August), with more than 75% occurring between April and August (see graph below). As might be expected, Tornado Chasers Causing Damage the seasonal variation of derechoes corresponds rather closely with the incidence of thunderstorms. 

Derechos in the United States most commonly occur along two axes. One extends along the "Corn Belt" from the upper Mississippi Valley southeast into the Ohio Valley, and Tornado Chasers Causing Damage the other from the southern Plains northeast into the mid Mississippi Valley (figure below). 

During the cool season (September through April), derechos are relatively infrequent but are most likely to occur from east Texas into the southeastern states. Although derechos are extremely rare west of the Great Plains, Tornado Chasers Causing Damage isolated derechos have occurred over interior portions of the western United States, especially during spring and early summer. 

Additional climatological information on United States derechos is available here. Derechos outside North America Derechos likely occur in other areas of the world where meteorological conditions are favorable for their development. However, Tornado Chasers Causing Damage only one such event has been formally documented in recent years. 

On July 10, 2002, a serial derecho occurred over eastern Germany and adjacent portions of neighboring European countries. In Berlin and surrounding areas, 8 people were killed and 39 were injured, mainly from falling trees. In Bangladesh and Tornado Chasers Causing Damage adjacent parts of India, a type of storm known as a "Nor'wester" occasionally occurs in the spring. 

From various descriptions and knowledge of the meteorological environments involved, it appears that some of these Tornado Chasers Causing Damage storms may be progressive derechos. DERECHOS IN 2004 AND 2005 

The Storm Prediction Center's severe weather report data base was examined to identify those severe weather events that involved widespread damaging winds associated with Tornado Chasers Causing Damage convective storms in the years 2004 and 2005. 

These events include all the derechos that occurred over the continental United States during that two-year period. Hyperlink maps were created Tornado Chasers Causing Damage to show the approximate path for each event.

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