Thermodynamics- Lifted Index & Low Level Lapse Rates

[Eric Matthews]

I was looking at the Mesoscale Analysis at SPC and found two readings that I seemed to conflict. I obviously don't understand Surface Based Lifted Index and Low Level Lapse Rates like I thought I did.

I noticed that at the same area in southeast SD today, the Lifted Index was -9 but the Low-Level Lapse Rates were only 5.5. Those two readings seem to conflict with each other based on my understanding of them. The LI suggests a very unstable atmosphere while the LLLR suggests a very stable lower atmosphere.

What am I looking at incorrectly? Which would be more appropriate to consider when trying to fine tune a chase area?

Heres what my little pea-sized brain thought about each of these.

SBLI= This gives an estimate of the instability. Found by comparing the temp a parcel of air can reach once it reaches its LCL compared to the temp of a parcel of air would reach if raised from surface to 500mb . (In my example above, the Surface Based CAPE was 2000 and SBLI was -9)

LLLR= The steeper the lapse rate (faster temp drop with height), the more unstable the atmosphere. The reading was 5.5 above, suggesting a pretty stable lower atmosphere. Is this because of Conv Inhibition?

Thanks.

 

[brice.coffer]

They're both important. LIs are indicative of cold air aloft (~6km) while low-level lapse rates tell you how fast the temperature decreases with height near the surface (0-3km).

You might find this paper interesting:

http://www.jondavies.net/LLthermo.PDF

Today, there is a prominent EML between 1-3km and the environment is very capped. This is why you are seeing those kind of low-level lapse rates. However, there is still cold air aloft yielding highly negative LI values.

You can still get tornadoes in these types of environments, like yesterday. Not as favorable though.


Edit: Looking at the sounding from Omaha today at 00Z:

http://www.spc.noaa.gov/exper/soundings/

You can see that the temperatures between 0-3km don't decrease rapidly with height. However at 500mb, there is quite a bit of difference between the parcel path and the environmental temperature.