The hybrid supercell, also known as the high-precipitation (HP) supercell or modified supercell, is the class of supercells that tend to produce especially severe hail swaths and are typically characterized by bounded weak echo regions (BWER's) and pronounced hooks (or appendages) that result in a “kidney bean” radar reflectivity pattern.  The hybrid supercell has also been found to possess multiple discrete updrafts that “merge” into a quasi-singular updraft.  Additionally, hybrid supercells have been observed to form and track along preexisting boundaries.

 

Although previous simulations have produced storms with HP characteristics in the absence of horizontal heterogeneities, the near unanimity of the observations suggests that the importance of preexisting boundaries to the development and maintenance of hybrid supercells demands investigation.  Likewise, the potential dynamical significance of multiple updrafts to hybrid supercell maintenance and intensity warrants study.  Therefore, it is our intention to (1) explore the mechanisms by which a preexisting density current boundary modifies the environment to support a hybrid supercell (characterized by a quasi-singular rotating updraft modulated by periodic semi-discrete cells) and (2) examine the role multiple updrafts play in hybrid supercell maintenance and intensity.

 

Since the multicell and supercell convective modes overlap at an intermediate bulk Richardson number (BRN) of approximately 40, an environment characterized by an intermediate BRN should support hybrid supercells.  In addition, previous studies have documented the presence of discrete updrafts associated with supercells in the wings of the BRN continuum, i.e., in environments characterized by very low BRN’s as well as very high BRN’s.  Because these environments would not normally be expected to produce supercells, hybrid supercells in these regimes demand investigation.

 

The current focus of this research is on the intermediate BRN environments.  However, since the low BRN environment can be regarded as “buoyancy starved”, one of the objectives of this research must be to determine how the density current modifies the storm-scale environment so as to generate buoyant potential energy in highly sheared environments.  In addition, since the high BRN environment can be regarded as “shear starved", we must also determine the manner in which the density current allows the storm to extract kinetic energy from an environment with too little shear. 

 

Simulations will utilize the Collaborative Model for Multiscale Atmospheric Processes (COMMAS) and will be made on a high-resolution grid (200 m in the horizontal) so that we may best resolve both discrete cells and the density current interface (boundary).

 

Preliminary work for each of the three experiments (low BRN, intermediate BRN and high BRN) will include a detailed examination of the dynamics that drive updraft development and maintenance in each of the convective regimes in the absence of a preexisting boundary.  Results of this preliminary work will be used to construct sensitivity studies to identify the ways in which a preexisting density current boundary controls the development and maintenance of hybrid supercells.