Tornadic Supercell Definition

Tornadic Supercell Definition

Nelson (1983) calculated hail growth in flow fields derived by multiple Doppler radar in a supercell storm. Like Browning and Foote, he calculated that strong hail growth occurred on a single upward and downward trajectory. His calculations suggest that a more important factor for hail growth than the maximum speed of the updraft is the presence of a large region with a moderate updraft (20-40 m s−1), because hail cannot grow in the strongest part of the updraft. He concludes that the storm`s flow field, not the absence of embryos, is an important modulating factor in hail growth. FIGURE 8.61. (a) and (b) Schematic model of hailstone trajectories in a supercell storm based on the derived airflow model of Browning and Foote (1976). (a) hailstorm tracks in a vertical section along the direction of travel; (b) the same trajectories in plane view. Trajectories 1, 2 and 3 represent the three growth stages of large hailstones discussed in the text. The transition from stage 2 to stage 3 corresponds to the re-entry of a hailstone embryo into the main updraft, before a final upward and downward trajectory in which the hailstone can grow, especially if it develops near the edge of the vault. Other hailstones, a little less preferred, push a little further from the edge of the vault and follow trajectories that resemble the dotted trajectory. Cloud particles that grow “from zero” in the ascending core are rapidly transported upwards and anvils along path 0 before they can reach precipitation size. The fact that hot advection occurs in relation to wind deviation with altitude is a consequence of the thermal wind equation.

(7.20)), in which the Coriolis parameter f = 2Ω sinφ appears as a linear factor. Wind deviation with height in the boundary layer due to frictional resistance, as explained in Section 7.2.5, also contributes to the prevalence of anticyclonic curved hodographs. Therefore, it can be said that the Coriolis force is indirectly responsible for the prevalence of cyclonic rotating supercell storms. FIGURE 16. CD in vertical cross-section (see Figure 15) by an idealized storm resembling a supercell. The contours are of Ze. The presence of a very strong updraft is indicated by the notch of the limited weak echo region (BWER) in the reflection profile. The bold spiked line shows the gust front.

The weak ambient air approaching from the right is pushed upwards by the gust front. [According to Chisholm, A. J., & Renick, J. H. (1972). In “Hail Studies Report 72-2,” pp. 24-31, Alberta Research Council, Edmonton.] A third supercell affected the north-east region of England. The storm hit the Tyneside area directly and without warning during the evening rush hour, causing extensive damage and travel chaos as people left cars and were trapped due to lack of public transport. Flooded shopping centres were evacuated, Newcastle train station was closed, as was the Tyne & Wear tube, and main roads were flooded, causing massive congestion. 999 fixed telephone services were closed in some areas and the damage amounted to huge amounts that were only visible the next day after the water was cleaned. Parts of Durham County and Northumberland were also affected, with thousands of homes in the northeast without power due to lightning.

Lightning was seen striking Tyne Bridge (Newcastle). Figure 8.53 shows a composite hodograph formed by averaging the hodographs of borehographs from nearly 62 tornadic supercell thunderstorms over the central United States. The mean motion of storms is east-northeast (ENE), far to the right of the vertically averaged control current. Wind speed increases rapidly with altitude, especially in the lower troposphere, and wind direction continuously changes from ground ESS to WSW in the upper troposphere, a change in direction of nearly 90°. The lower part of the hodograph has a strong curvature in the same direction as the idealized profile in Fig. 8.44b. The updraft of the mature supercell (near the intersection of the AB and CD lines in Figure 15) has a dominant influence on the radar reflectivity structure shown in Figures 15 and 16. The updraft is so strong that cloud droplets don`t have time to turn into radar-detectable hydrometeors until they`re halfway through the storm. The resulting limited low echo region (BWER) extends to the average altitudes of the storm. The BWER is covered by an area of high reflectivity, as the largest hydrometeors form in the upper regions of the updraft.

Small hydrometeors are carried in the wind by strong environmental currents and form an anvil-shaped cloud. Larger hydrometeors immediately fall downwind of the updraft. In response to ambient winds as they fall, their trajectories curve cyclonically (counterclockwise) around the updraft, forming a leeward area on the left and front left flanks of the storm. The large hail, whose growth is limited to the primary updraft of the supercell by accretion of supercooled water droplets, is located next to the updraft along the left and rear storm perimeter. The resulting curvature of the reflection pattern – which often takes the form of a “hook” – around the back of the low-altitude updraft suggests the presence of a cyclonic rotation, which can be easily confirmed by time-lapse photography and Doppler radar observations. Figure 5 shows the significant surface characteristics commonly observed during the maturity phase of a supercell. The main area of updraft is above the hook or notch in the rainfield, with two primary downdraft regions, referring to the downdraft of the front flank and the downdraft of the rear flank, located respectively on the descending and ascending sides of the updraft. A surface gust front separates cool, rainy air from warm ambient air, with the gust front often wrapping around the southern flank of the storm due to circulation associated with a surface mesocyclone. This front rear gust may exceed the gusty frontal boundary associated with the downdraft of the front sidewall, creating an occlusion of these frontal features. A tornado, if any, often forms at the end of this occlusion (at the edge of the hook echo) on the gradient between the updraft and the downdraft (but inside the updraft).

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