...maar ik las recent een wat ouder artikel van Rasmussen waarin staat dat bij 70 % van de tornado's die zij hebben onderzocht er een aanwezigheid was van een 'oude outflow'. Kon zijn van een bui van tot 6 uur terug! Dit was een aanjager voor het ontstaan van rotatie en tornado's.
Is dat nog steeds geldende informatie? Hou je er rekening mee met het voorspellen en de kansen waar zich een tornado kan vormen?
Bedankt Wilco en nee ik ben niet op pad, vanwege momenteel duidelijke redenen waarschijnlijk...
Zogenaamde outflow boundaries zijn erg belangrijk, niet alleen voor het ontstaan van tornado's, maar vooral ook voor het ontwikkelen van nieuwe convectie. Deze boundaries (grenzen/frontjes zo gezegd) kunnen niet alleen 6 uur oud zijn, soms wel een heel etmaal!
Nabij de outflow boundaries heerst een grote temperatuurgradiënt/vochtgradiënt en er is doorgaans sprake van convergentie nabij het aardoppervlak. Dit alles kan bijdragen aan een vergrote horizontale vorticiteit en daarmee een vergrote SRH.
Enkele stukjes theorie;
Erik Rasmussen:
"From theoretical arguments, SRH (for certain storm motions) increases when horizontal vorticity increases. This happens whenever a buoyancy (temperature) gradient exists, and parcels spend time in the gradient instead of passing quickly through. It takes relatively little time to generate horizontal vorticity large enough to support a tornadic supercell. For typical temperature gradients associated with outflow boundaries, parcel residence of about 20 minutes leads to horizontal vorticity of this magnitude. Low-level outflow boundaries are probably the dominant source of narrow zones of enhanced SRH.
Although we do not yet have the sensors we need in order to detect enhanced SRH (a mesonet of lower-tropospheric profilers would be nice!), it may be possible to infer its presence. Areas with earlier outflow, wetted soil, and satellite-indicated boundaries are suspect. And assuming that the flow is much more uniform at the 1-3 km level than it is at the ground (assuming the temperature contrasts are largest at the ground), then monitoring the surface winds for local backing is also an important tool for detecting augmented SRH.
We do not know what the details of the low-level horizontal distribution of SRH look like. We suspect that on days after rain has fallen, or days with active convection, there could be a spaghetti-like distribution of enhanced SRH, and predicting which storm will encounter one of these spaghetti is nearly impossible. Sometimes, it is simpler, with one dominant outflow boundary associated with most of the tornadic storms. It is one of our goals to measure the SRH distribution in the pre-storm environment using dual mobile Doppler radar during this first decade of 2000.
In summary, we cannot say how much SRH is required for a tornadic supercell to occur. This threshold may vary with updraft strength, persistence, measurement scale, and other factors. We can say that if synoptic-scale SRH is augmented, significant tornadoes are more likely, probably because the additional augmentation is more likely and less needs to occur. But because low-level baroclinic zones, if parcels reside in them for sufficient lengths of time, are such strong generators of SRH, forecasters must be cognizant of these zones even when background SRH is small."
Jeff Snyder:
"Outflow boundaries are low-level features; as such, you cannot find them on upper-air charts. Your best bet for locating any possible OFB is to examine surface observations and radar and satellite data. OFBs, nearly by definition, mark the leading edge of surface cold pools produced by thunderstorms (and just showers too, I suppose), so you should be able to pick them out by looking at surface temperatures, dewpoints, and winds. For example, you'll often see lower Ts, lower Tds, and more backed surface winds in the relatively cool air behind the OFB. You may also be able to corroborate your suspicion of the OFB location by looking at radar and visible satellite data, as OFBs may be marked by a radar "fine-line" or a difference in the character of low-level clouds across the OFB.
Since the surface thermodynamic conditions are usually different across the OFB, there tends to be enhanced near-surface vorticity along the OFB (the baroclinic generation term of the vorticity generation / destruction equation). In terms of forecasting for tornadoes, supercells, etc, we often look for the "cool side" of the OFB to "cook" during the day. In other words, many chasers and mets closely monitor how much insolation is occurring on the cool side of the boundary. Sometimes, strong surface heating can lead to significant destabilization on the cool side of the OFB, leading to very favorable shear-instability conditions. Note that enhanced, baroclinically-generated vorticity may still be present even if the temperature or theta-e gradient across the OFB is removed by the afternoon! The presence of a buoyancy gradient or baroclinity means that vorticity is being generated, so the removal of said gradient merely means that vorticity is no longer being generated, even though vorticity may still be very significant since it has been accumulating through the preceeding hours. Eventually, friction / diffusion / viscous forces will "spin down" this vorticity, but it's important to remember nonetheless."
Case study:
Bericht laatst bijgewerkt: 27-05-2008 13:10
