Harald Richter

Did I hear coldies?
 
> On Thu, 19 Sep 2002, Andrew Burton wrote:

> > John - if you consider what the coldies diagnostic represents it becomes
> > apparent that the diagnostic, although named "coldies" doesn't care whether
> > the air is pre or post frontal ("warm" or "cold" air). We often see the
> > strongest coldies signal in the prefrontal airmass - (and we often issue
> > warnings for "coldies" in "warm" ie. prefrontal air). The coldies signal
> > represents the coincidence of instability (<-1 700hPa LI for strong
> > signal), upmotion (0.9500 sigma <-15x10-3 s-1 or 700hPa <-15hPa hr-1) and
> > low level shear (850hPa - 10m > 11ms-1). The salient characteristic
> > of  most "coldies" producing environments is an atmosphere characterised by
> > relatively low CAPE together with high shear in the low levels. These occur
> > most often in a winter environment - but I don't think the atmosphere
> > particularly cares about "warm" or "cold" air (forgive my
> > anthropomorphising).  Often the shear is stronger ahead of the front.

Thanks, Andrew, for listing the formal criteria for the coldies diagnostic
package we use.  Just for clarity,  I take "coldies" to mean "cold season tornado"
or any (?) tornado that occurs from, say, Apr-Nov.  This says nothing about
the tornadogenesis mechanism itself, it is a classification by time-of-occurence.
My (somewhat anecdotal) impression of typical coldie setups is one of short, fat
low-level CAPE embedded into fairly stout low-level shear.  Given a surface lifting
mechanism (usually some convergence line) parcels rising into such a CAPE/shear
profile can acquire some rotation, before the show ends around ~500 hPa or so.
The formal diagnostic seems to address the CAPE (700hPa LI), the near-surface
lifting mecahnism (0.9500 sigma vert.vel.) and the shear (850hPa) at three
individual reference levels.

> Thanks....  I wasn't really concerned whether the coldies occurred in cold
> air or warm air... I was wondering more why there should be much
> instability  in that situation..... You have partly answered it by
> pointing out that  they usually ocur with "relatively low CAPE"

Although Andrew mentions that the CAPE is low, the CAPE/layer depth might
not be too bad within the relatively shallow layer where CAPE>0.
I suppose dw/dz stretching zeta ain't that shabby in that shallow layer.

My 2 bobs worth,
Harald