Cooked boundaries: results from numerical experiments

A cooked boundary is a preexisting outflow boundary for which the mesoscale airmass on the dense side of the boundary has a higher CAPE than the synoptic-scale airmass in which it is embedded. Preexisting outflows are typically more stable than the synoptic-scale environment that they are propagating through. Preexisting outflows however can be modified by solar insolation and moisture pooling along the boundary. This allows the dense airmass to become more unstable than the surrounding environment. The supercells that produced the largest hailstone and the strongest observed mesocyclone on 22 June 2003 in Nebraska occurred in association with a cooked boundary. Another example occurred on 2 June 1995 near Dimmitt, Texas. Only the supercells that crossed the cooked boundary into the dense airmass produced tornadoes.

Cooked boundaries are hypothesized to occur when insolation or moisture pooling is present. Incident radiation acting in the vicinity of a preexisting outflow airmass will begin to mix the lowest portion of the atmosphere, which begins to dry and warm the airmass. Due to the rate of boundary layer thermal growth, the preexisting outflow airmass warms and dries significantly slower than the synoptic-scale airmass in which it is embedded. After a period of time the preexisting outflow airmass is cooler but more moist, which leads to a cooked boundary. Moisture pooling produces a cooked boundary when an excess of moisture is transported near the boundary by converging flow. This localized area of high moisture creates high CAPE values in the preexisting outflow airmass.

The aforementioned hypotheses are being tested through sensitivity studies conducted using the two-dimensional Advanced Regional Prediction System (ARPS) model. The parameter space used to test the ability of solar insolation to form a cooked boundary includes the coldness of the outflow boundary, low-level shear, incident solar radiation, and soil moisture. The same parameters except incident radiation were used to test the moisture pooling hypothesis. Results will be presented at the conference.