The influence of DCAPE on supercell dynamics

Abstract

The role of environmental variables such as vertical wind shear and updraft Convective Available Potential Energy (CAPE) in creating supercell thunderstorms has been well established. However, there are currently no acceptable indices that can distinguish between tornadic and non-tornadic supercell environments. Recent observational studies have shown however that non-tornadic supercells tend to be either dominated by or lacking in outflow. One factor influencing downdraft and outflow strength is the dryness of the midlevel air. Rain that falls through drier air results in greater evaporative cooling and negative bouyancy of the air. The maximum kinetic energy increase a downdraft can experience due to evaporative cooling is represented within an index called Downdraft CAPE (DCAPE). Using a non hydrostatic cloud model, supercells are simulated in environments differing only by the dryness of the midlevel air (or DCAPE). Results indicate that storms with too much DCAPE (in high CAPE and weak vertical wind shear environments) have low-level outflow that propagates faster than the midlevel mesocyclone resulting in a weaker updraft and a weak low-level mesocyclone. When the DCAPE is small, a strong lowlevel mesocyclone is produced. This study overwhelmingly suggests that an index estimating the potential downdraft strength must be included in tornado forecasts if delineating between tornadic and non-tornadic supercells is to ever be achieved. However, before it is used in an operational setting, DCA-PE needs to be studied in conjunction with other supercell indices. DCA-PE also needs further calibration with observed and modeled supercell downdrafts.