Topic: Surface Lifted Index and CAPE comparisons

[Jimmy Deguara]

This topic came about when I (and even Nick Moir) noted that surface lifted index values over the inland today 6th February 2008. With 500hPa level temperatures hovering at down to -17C, it certainly reflected a favourable and unstable surface lifted index value. Now the rule of thumb on these models seem to reflect that the CAPE (Convective Available Potential Energy) would also be high typically of the order of about 1500J/kg or higher. The models this morning only progged a value lucky to near 1000J/kg.

Such has been the frustration of virtually wasted systems this season for violent thunderstorm outbreaks. I guess at least we have broken the drought in many areas.

Regards,

Jimmy Deguara

[David Brodrick]

Well as you know, CAPE is an integral over the whole atmosphere, so just because 500mb temperatures look good doesn't tell us anything about the rest of the atmosphere and therefore what CAPEs one might expect.

Interestingly, however, I read a paper, could probably dig up the reference if you want it, where some Dutch meteorologists compared something like 30 indices to see which was statistically the best at predicting thunderstorms, and at least in Holland, with whatever models they used, surface lifted index performed better than CAPE.

It would be very interesting to see the results of some similar research in Australia if it has been done.

Regards,
  Dave

[Jimmy Deguara]

David,

Interestingly, however, I read a paper, could probably dig up the reference if you want it, where some Dutch meteorologists compared something like 30 indices to see which was statistically the best at predicting thunderstorms, and at least in Holland, with whatever models they used, surface lifted index performed better than CAPE.

Funny that - in 1998 when I learned of surface lifted index as a parameter for forecasting basically from the experimental NOAA NDEC website, I immediately took an interest in the reliability of the index. Over the years i tested its strengths and weaknesses including sunburnt chases:) Many in the past tried to use other means such as the soundings as a guide to their forecasting but in the end, using this index was the most simple technique one could follow as an amateur.

Now it seems sometimes even a quick look and one can sometimes determine the event unfolding.

Regards,

Jimmy Deguara

[Macca]

Hey Jimmy,

I agree.  Surface lifted index (SLI) is the simplest "quick look" tool in my opinion.  I look at an MSLP chart and an SLI chart and within 1 minute I can have a basic idea of what is happening.  Funnily enough, I start with the SLI and look for something of interest (anything below zero).  I'll then look at the MSLP and if the SLI does not make sense based on the MSLP chart, I'll investigate further.  As the SLI is heavily dependent on the 500mb temps and the surface DP, it is easy to see it end up way off (usually over-estimated).  Once I find a "day of interest", I then move onto the other storm parametres to see whether it is going to be worth investing my time in.

Though when I'm chasing in the US, I tend to overlook the instability and go straight to searching for shear and boundaries.  The instability tends to take care of itself much of the time.

Macca