Coherent PV anomalies associated with (extreme) deep moist convective cells

The WEX-MOP project aims at a next generation ensemble prediction system for the mesoscale. One goal of WEX-MOP is to quantify the role of conserved quantities during extreme convective weather. Conserved variables might offer new insight in the predictability of those events. An important conserved quantity is potential vorticity (PV), a fundamental property of the atmospheric flow on synoptic and planetary scales. However, investigations thereof on the atmospheric mesoscale are relatively new. PV has a close relation to rotation and balance, which is important in storm dynamics. Here we characterise the evolution of storm cells in terms of PV to provide new insights into storm dynamics.

Tracking of storm cells has been frequently performed using radar and/or satellite data. It received less attention using model data. We present storm cell tracks for 6 cases of severe convection in 2011, 2012 and 2013 simulated using the non-hydrostatic COSMO-DE weather model. The cases are selected with a different synoptic background, i.e. different background shear and CAPE. For each of the cases vertical velocity maxima are tracked. Composites of the typical evolution of a storm cell are made.

There is a relatively large variability between the characteristics of individual cells. However, the PV dipole associated with the storm updraft is clearly visible in the composites. When we cluster on the strength of CAPE, the differences between the composites with large and small CAPE values are small. However, clustering on other intensity measures like PV, vertical velocity or precipitation rates gives, for intense cells, composites with a supercell structure i.e., a relatively strong rotating updraft. The results show that PV, unlike CAPE, might be a good predictor for intense convective cells, and motivates the use of PV on smaller scales.