200-m ARW WRF Fields
Storm A
Loop 1: Surface Divergence-Several Distinct surface convergence boundaries can be observed in this loop, all of which intersect near the center of the 0835 forecast hour image. Note the strong surface divergence present to the south of the intersection of these boundaries, which proceeds strong surface rotation. As the loop moves forward in time, convergence continues to increase at the boundary intersection, and by FHR=0845, a strongly convergent, swirling surface flow pattern is observed. By FHR=0900, the area of surface divergence, which was initially south of the incipient vortex, has been drawn north into the surface vortex. This marks a a weakening trend in the surface vortex (the simulation is concluded at FHR=0900).
Loop 2: Surface Potential Temperature-At FHR=0835, a significant surface cold pool is noted at the right center portion of the image. A strong gradient in potential temperature then separates this cold pool with warm, easterly inflow. By FHR=0845, a vortex at the surface forms at the intersection of three separate potential temperature airmasses. As the vortex moves east, it begins to draw a lower potential temperature airmass into its center. Note the wind flow (represented by the storm-relative wind vectors) is nearly parallel to the thermal gradient which extends northwest from the vortex; baroclinic generation of horizontal vorticity will be investigated as a possible source of low-level rotation.
Loop 3: Surface Pressure-No significant pressure perturbations are noted at FHR=0835, however, as time advances a notable perturbation to the pressure field develops in the center of the surface vortex, and then dissipates along with the vortex. Diagnostic fields will be generated in order to further examine the nature of this perturbation in the pressure field.
Loop 4: Surface Rain Water Mixing Ratio-At FHR=0840, the rain water mixing ratio (RWMR) field takes on a linear structure, with the highest values present to the east of a strong surface convergence zone. At FHR=0850, strong surface rotation is observed to the rear of a bowing RWMR pattern, although there is a relative minimum in RWMR to the east of the vortex, which may be correlated with a strengthening updraft. By FHR=0900, a distinctly seperate RWMR "core" has developed to the north of the surface vortex, which appears to be associated with a rapidly evolving thunderstorm. The evolution of the updraft associated with this vortex deserves further investigation.
Loop 5: Surface Vertical Vorticity-An thin, elongated region of moderate to strong surface vertical vorticity exists along the surface convergence zone at FHR=0835. This region then collapses to an intense, concentrated maximum in surface vertical vorticity, reaching a maximum value of zeta=0.11 s-1 at FHR=0850. The surface storm-relative wind vectors lend further evidence to the intense vertical vorticity at the surface, display a strong swirl surrounding the vorticity max. By FRH=0900, zeta begins to decrease, with a broadening in the swirling flow pattern.
Loop 6: 0-2 km Updraft Helicity-An experimental parameter evaluated by the SPC/NSSL 2005 Spring Program, using high resolution WRF data (4 km), was updraft helicity, which is simpily a vertical integration of zeta*dz/dt. In loop 6, updraft helicity is calculated for the 0-2 km layer in order to capture significant low-level rotation. At FHR=0815, updraft helicity is maximized in an elongated pattern along the leading edge of the surface rain water mixing ratio (RWMR). As time progresses, the elongated pattern in updraft helicity gives way to a maxima located on the northeast side of the quasi-linear RWMR pattern. This bullseye then propagates cyclonically around the RWMR field, and ends up on the back side of the field. It is somewhat correlated with the strong surface rotation observed in loop 5...the evolution/propagation of the strongly rotating low-level updraft warrants additional investigation. By way of speculation, this may be an occlusion, in which the low-level mesocyclone rotates cyclonically to the back edge of the updraft during the storms tornadic phase.
Storm B
Loop 1: Surface Rain Water Mixing Ratio-Supercell B, which is located several 10's of km's north of supercell A, displays a very different pattern in rain water mixing ratio (RWMR) when compared to supercell A. At FHR=0815, RWMR is elongated in a southwest to northeast orientation. This pattern persists until FHR=0840, at which point an appendage begins to develop on the southeastern side of the RWMR max. The storm then begins to go through an apparent transition (possibly a split?), which will be explored in more depth. By FHR=0900, a v-shape pattern develops in the RWMR field, similar to the "flying-eagle" type feature often observed on radar when examining some supercells.
Loop 2: Surface Potential Temperature-At FHR=0815, a surface cold pool is observed at the western edge of the image, with additional minima in theta present to the east and northeast of the main cold pool. By FHR=0840, lower potential temperature values begin to wrap cyclonically around a developing vortex. However, as time progresses, the surface vortex begins to propagate eastward, away from the cooler potential temperature values. Contrasting this vortex with the vortex observed with supercell A, the supercell B vortex does not develop on a strong gradient in potential temperature, and in fact, moves east into higher potential temperature air, at which point it appears to weaken quickly. Contrasting these two vortices further using additional diagnostic fields would be worthwhile, especially since vortex A (e.g., the one associated with supercell A) may be able to tap into a strong source of baroclinically generated horizontal vorticity (other modeling studies have shown that strong, tornado-like vortices are associated with strong cold pools, while weaker surface rotation occurs in association with weaker baroclinic zones...more later).
Loop 3: 0-2 km Updraft Helicity-An experimental parameter evaluated by the SPC/NSSL 2005 Spring Program, using high resolution WRF data (4 km), was updraft helicity, which is simpily a vertical integration of zeta*dz/dt. The location and evolution of updraft helicity for supercell B shows similar characteristics of observed, so called classic supercells. The strongly rotating low-level updraft is located to the south-southeast of the forward flank precipitation core, and remains so for a significant period of time. During the progression of loop 3, variations in intensity are observed in updraft helicity, and by FHR=0900, a change in supercell behavior appears to be taking place...unfortunately the simulation ends at this time. However, these interesting characteristics in morphology still may be described through additional diagnostic fields prior to simulation termination.
Loop 4: Surface Rain Water Mixing Ratio and Vertical Velocity at Sigma=9170-Morphology of rain water mixing ratio (RWMR) and low-level updraft can be observed. Note that the storm displays classical supercell struture at FHR=0800 and evolves into a storm which begins to resemble an HP supercell by FHR=0900. By FHR=0835 to 0900, a -4 to -6 m s-1 downdraft develops along the southern flank of the low-level updraft. In addition, the diameter of the low-level updraft begins to decrease, and a 2 m s-1 increase in updraft vertical velocity is observed as well...this may be due to increasing low-level convergence (due to the southern flank downdraft), or increasing low-level rotation, which would induce a vertical pressure perturbation an corresponding increase in updraft vertical velocity (will be explored further).
Loop 5: Surface Rain Water Mixing Ratio and Vertical Velocity at Sigma=9245-