Severe Weather Parameters

We've added several plots that may aid forecasting of thunderstorms and severe weather. At current resolutions, MM5 cannot resolve individual thunderstorms, but their prediction may be possible using derived fields. The following fields will be tested.

• CAPE - Convectively available potential energy in J/kg or m2/s2 (Weisman and Klemp 1982). CAPE is calculated using the most unstable parcel in the lowest third of the model atmosphere. While Renno and Williams (1995) have shown that the source layer is often the surface parcel, we wish to consider the possibility of elevated convection which commonly occurs over the central plains in late spring. Problems - Just exactly what is the source layer in the real atmosphere? How much mixing and dilution occurs in the source parcel?
• SREH - Storm-relative helicity (or helicity). This is the integrated streamwise component of horizontal vorticity flux. Graphically, it is minus twice the area swept out by the hodograph with vertices at the storm motion. Helicity has been found to correlate strongly with the development of rotating updrafts. The correlation with tornadoes is less clear. We calculate the SREH using the 0-3 km winds above ground level based on an assumed storm motion which is 30 degrees to the right and 75% of the 3-10 km mean wind. Problems - The 0-3 km layer may not be the inflow layer to the storm (e.g. elevated convection). The profile of the horizontal winds within the updraft may differ from the environment. Helicity is very sensitive to the storm motion. Storms which encounter boundaries or slow down can have radically different helicities than the general environment. It is possible to get a high-helicity, low-shear environment. Helicity is a conditional variable - it is only meaningful when there is a storm.
• 0-6 km shear vector - The "original" shear vector as used by Weisman and Klemp (1982), is the average wind over the lowest 6 km minus the average wind over the lowest 500 m. A general rule is that 50 kts of shear are needed for supercells. It might be possible to have supercell tornadoes with little helicity, but shear is always necessary. The shear vector also has the advantage that it is Galilean invariant - it doesn't depend on the storm motion. Problems - The proper heights at which to calculate the shear is unknown. The lower threshold for supercells is unclear. I've seen them with less than 50 kts of shear. The environment may change locally leading to adequate shear in a region which was not forecast.

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  The shear and helicity have been combined on one plot so it will be simple to make comparisons with each other and reality. CAPE values greater than 500 J/kg are color-filled.

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• CIN - Convective Inhibition (J/kg) is the negative area or energy that must be supplied to a parcel in order for it to reach its level of free convection. For our plots, it is calculated for the most unstable parcel. Areas with more than 200 J/kg of inhibition aren't contoured since those areas should be absolutely capped. Problems - the CIN may be smaller for source layers other than the one we're considering. As for CAPE calculations, what are the effects of mixing or sub-grid perturbations on CIN? What is the threshold for convective initiation? Usually it is about -50 J/kg, but convection doesn't break out in all areas with less inhibition.
• Boundary layer vertical velocity - Lifting tends to reduce CIN while subsidence enhances it. Overlayed on the CIN plot, it provides another clue where convection may be triggered. Problems - Again, the level of the plot may not be the proper one in every case or location.
• Storm-relative flow - This plot follows the work of Rich Thompson of NOAA/NCEP/SPC on supercell tornadoes. Supercell tornadoes tend to occur near where the low level inflow is greater than 10 m/s and the mid-level storm relative flow is between 10 and 25 m/s. Problems - Who knows? This is a new field to look at.
• VGP - Vorticity Generation Potential. Another experimental field used to predict supercell tornadoes. Based on the work of Rasmussen and Wilhelmson (1982) it relates CAPE and shear. It is defined as the sqrt(CAPE) times the 0-3 km mean shear. The units are m/s2 (an acceleration) or m/s times 1/s which is the product of w times vorticity - a tilting term. It is possible to have high values of VGP when helicity (and the EHI) is low. The VGP climatology shows that supercell tornadoes rarely happen with VGP less than 0.3, and go from unlikely to likely as VGP goes from 0.5 to 0.6, and are likely above 0.6. It's also possible that the larger the VGP the stronger the tornado. The VGP was developed by Erik Rasmussen for use over the central plains and may or may not be valid over other regions. Problems - Unknown. But, since it depends on the assumed CAPE and depth of the inflow wind layer, it might inherit their problems.
• CELL - Supercell type and motion plot. The supercell motion vector is based on a large climatology compiled by Rasmussen et al. (in press). The motion is computed by taking 60% of the magnitude of the shear vector between the 4 km AGL wind and the 0-1 km mean wind and then going 8 m/s orthogonal to the right. (This motion is based on low-level inflow to a storm rather than a deep-layer mean wind). The supercell type (HP - green, CL - blue, LP - orange, and none - white) is determined by the 9-10 km (anvil-level) storm-relative flow (Straka and Rasmussen, 1997). It's valid for isolated supercells which have not been seeded by other storms. The basic idea is that the precip production is determined by where the anvil hydrometeors fall relative to the updraft. Preliminary results have been quite good.

Questions and Comments: Please direct any comments or questions about the real-time model to Jim Bresch (bresch@ucar.edu)