Maar ik weet ook dat juist de speed shear voor vorticiteit zorgt en als deze lucht met vorticiteit opgetild wordt krijg je twee roterende kolommen, die kunnen splitsen en tot een links/rechts bewegende supercell kunnen leiden.
Uit: THE SEVERE WEATHER EVENT OF 18 JUNE 1997:
AN EXAMPLE OF SPLITTING SUPERCELLS
Door: Randall Graham - NWSFO Salt Lake City, UT
and Mike Staudenmaier, Jr. - NWSO Flagstaff
(Formerly NWSFO SLC/ WRH-SSD)
The Theory Behind Splitting Supercells
"Forecasters have known for some time that supercells tend to be deviant movers, that is, they generally move in a direction other than along the mean shear vector (surface - 6 km depth). In the Northern Hemisphere, supercells tend to move to the right of the mean shear vector, and tend to move slower than the magnitude of the vector would suggest. Numerical work by Wilhelmson and Klemp (1978), Klemp and Wilhelmson (1978), and Weisman and Klemp (1982) have helped forecasters understand the dependence of supercells on vertical wind shear and buoyancy. Their results suggest a spectrum of storm types ranging from short-lived single cells, through multicellular thunderstorms, to the longer-lived supercell. It was found that the environmental wind shear associated with the storm environment combined with the amount of buoyancy in the atmosphere determined the type of storms which will most likely occur. Of particular interest for this TA, is the concept of splitting supercells and what storm environment supports this type of phenomena.
Using one dimensional wind profiles, Klemp and Wilhelmson (1978) found that the tendency of an initial storm to split into two self-sustaining storms was strongly dependent on the intensity and distribution of the low-level wind shear. When the hodograph is more or less a straight line, the most likely development due to this shear profile is splitting storms that form mirror images of one another. In this environment, neither of the pair is favored, so both storms tend to persist. When splitting occurs, a cyclonically rotating updraft propagates to the right of the mean wind, while an anticyclonically rotating one moves to the left. However, Klemp and Wilhelmson (1978) along with Weisman and Klemp (1982) suggest that when the lowest several kilometers of the hodograph are curved, one member of the split pair is favored over the other. This means that the favored one persists while the unfavored one dies out rapidly, if it even forms. It turns out that a clockwise turning of the hodograph favors the cyclonic, right-moving member of the split, while counterclockwise turning favors the anticyclonic, left-moving member (in the Northern Hemisphere). The sort of storm most typical, and which has been seen most often by forecasters, is a cyclonically-rotating, right-moving storm; this sort of storm becomes likely when the hodograph's lowest several kilometers are characterized by clockwise turning of the hodograph. In this situation, the left-mover never forms, or if it does, quickly dies away, leaving just the right-mover to continue. This hodograph curvature arises from both dynamical and frictional effects. In the Northern Hemisphere, the Ekman Spiral dictates that, because of frictional effects, the wind generally will veer with height through the boundary layer, creating a clock-wise turning of the lower atmosphere. This also favors right-moving supercells.
How the rotation develops in splitting cells has been widely debated. The most widely accepted theory was discussed by Wilhelmson and Klemp (1978). As shown in Figure 2a, during the initial stages of updraft development, there is a tilting of the vorticity associated with the mean shear flow, which gives rise to a vortex pair straddling the updraft. Due to the updraft tilting this mean shear flow, there is cyclonic rotation on the southern flank of the storm and anticyclonic rotation on the northern flank (for a storm moving to the east). Waterloading in the updraft which creates a downdraft (Wilhelmson and Klemp 1978), along with forced ascent along the gust front (Brown 1992) and lifting of low-level air by the mid-level vortices along the storms flanks through dynamical vertical pressure gradients (Rotunno and Klemp 1982) all act to split the original pair of vortices into two separate pairs (Fig. 2b). The lifting of low-level air by the mid-level vortices was found to be a significant contributor to storm splitting, and can even cause storm splitting without precipitation and the associated downdraft (Rotunno and Klemp 1982). This was verified by observations of splitting towers occurring without any observable precipitation by Bluestein et al. (1990). Thus, the right-moving storm has a cyclonically rotating updraft and an anticyclonically rotating downdraft, while the opposite is true for the left-mover. These new updrafts strengthen the vorticity further through stretching, which in turn enhances the dynamic vertical pressure gradients.
By this time, the downdraft of the original storm has produced a cool outflow boundary which is moving away from the thunderstorms. For the cyclonically-rotating right-moving storm, the downdraft is located along the rear and left flank, and outflow near the ground spreads out underneath the updraft, forcing continuous uplifting of the moist low-level inflow along the right flank (Klemp and Wilhelmson 1978). In this manner, the storm maintains its moisture source and tends to propagate to the right. Remember that storm movement is the sum of two contributions: advection and propagation. Thus, even though the advection term is along the mean shear vector, the propagation term is to the right of the storm. Therefore, the actual storm movement is to the right of the mean shear vector. For the anticyclonically-rotating left-moving system, the convergence would be along the left portion of the outflow boundary, creating a storm movement to the left of the mean shear vector. This deviant propagation alters the storm-relative helicity values and may encourage supercell development (Vasiloff et al. 1984).
If the low, middle, and upper-level winds lie along a one-directional shear line, i.e. a straight line hodograph, both right- and left-moving storms formed through splitting will have similar opportunities to establish a self-sustaining structure. However, in the real world, and especially in the western United States, topography and influences from nearby storms can modify the storm environment and cause one storm or another to be preferred, even with straight line hodographs. Curvature of the wind hodograph causes a relative enhancement of the downdraft associated with one of the storms, which in turn increases the gustfront-induced convergence beneath the storm. Thus, with a hodograph that turns clockwise with height, development of the right-moving storm is favored, while if it turns counterclockwise, the left-moving storm is favored."
Tekst bij onderstaand plaatje:
"Depiction of how a vortex tube contained within westerly environmental shear is deformed as it interacts with a convective cell (viewed from the southeast). Cylindrical arrows show the direction of cloud-relative airflow, and bold solid lines represent vortex lines with the sense of rotation indicated by circular arrows. Shaded arrows show the forcing influences that promote new updraft and downdraft growth. Vertical dashed lines represent regions of precipitation. (a) Initial stage: Vortex tube loops into the vertical as it is pulled into the updraft. (b) Splitting stage: Downdraft forming between the splitting updraft cells tilts the vortex tubes downward, resulting in two vortex pairs. The barbed line at the surface marks the boundary of the cold air speading out beneath the storm. (Adapted from Rotuno 1981)"
