Lapse Rates

[Drew.Gardonia]

I keep seeing forecasters in here and on the SPC Convective outlooks discuss lapse rates.

Can someone either explain what the lapse rates are or provide a link that explains them and how they work?

 

[Jeff Snyder]

Ugh, I accidentally navigated away from the page right as I was finishing a more detailed reply...

The lapse rate is the change in temperature with height (-dT/dZ) and is typically used in context of how the mid-tropospheric (say, 3-6 km above ground level) or low-tropospheric (say, 0-1 or 0-2 km) temperature changes with height. For example, if the temperature at 5 km is -5 C, and the temperature at 6 km is -12 C, the lapse rate in the 5-6 km layer is 7 C / km. If the lapse rate is 7-10 C / km, you may hear people refer to it as being "steep", indicating that the temperature decreases relatively quickly with height. If unsaturated air is lifted (or sunk), it's temperature will decrease (increase) at 9.8 C / km; we refer to this as the "dry adiabatic lapse rate". If a parcel is saturated, it will cool more slowly when lifted as a result of the release of energy in the form of the latent heat of condensation (+ latent heats of deposition and fusion, technically).

In general, in context of convective instability (e.g. CAPE), we'd like to see the air in the updraft be as warm, relative to the surrounding air, as possible. When the mid-level lapse rate is very steep (say, >8 C / km), there's a better chance that the parcels rising in the updraft will remain much warmer than the environment (e.g. remain positively buoyant). Typically, I like to see mid-tropospheric lapse rates of at least 7.5 C / km to get excited. In tropical environments, it's relatively common to see mid-level lapse rates of only ~6 C / km.

Earl Barker has some 850-500 mb lapse rate graphics from the NAM at http://wxcaster2.com/central_models.htm, and the SPC mesoanalysis site has mid-level and low-level lapse rate analyses available.

 

[Drew.Gardonia]

Jeff, thanks for the reply. I'm kind of grasping what you're saying here, but I couldn't seem to find the lapse rate graphics you're talking about? I'd like to see how to identify the lapse rates on the graphics so better grasp the concept of what you're saying and how that pertains to what's going on in the atmosphere.

 

[Mark Blue]

Drew,

Jeff is linking you to Earl Barker's model output page where you have to click a forecast hour and the graphic will be produced for that time. For instance, several models are listed on that page (ETA, GFS, RUC2, Meso ETA, etc.), so within each model table you have to look for the parameter "850-500mb Lapse Rate (C/km)" then click the forecast hour to generate the graphic. What the graphic will show is an area (or geographic region) that is outlined with red dashed lines that depicts the lapse rate for that particular area. The lapse rate will be numeric value, probably around 5 - 9 assuming you select a forecast hour during peak heating in the afternoon (shoot for 3 - 6 pm or 21 - 0Z as your forecast hour). If you click the graphic it will zoom in so you can read the numbers a little easier.

As far as the SPC Mesoanalysis page, the link Jeff gave you takes you to the main page. Once you are there select a sector at the top from the choices. You can select Northern Plains, Central Plains, Southern Plains, any area you want. This will take you to another page where different parameters will be available at the top. Hover your mouse over the 5th menu over that says "Thermodynamics" then look down to the middle area where you will see Low Level and Mid Level Lapse Rates. Select one of those and it will take you to a page where the lapse rates will be displayed.

Lapse rates give you an idea of how unstable the atmosphere is. Instability is a good thing if you are a storm chaser, so as Jeff said, once you see lapse rates approaching the 7.5 or greater range, you know you can put a check mark in the instability column, then proceed with analyzing the other ingredients needed for severe thunderstorms. For a brush up on the ingredients needed for severe thunderstorms here is something you might find useful: Thunderstorm Ingredients.

Thanks,
Mark

 

[Drew.Gardonia]

Mark, I found it, didn't look far enough down the page.

but I'm not sure I understand what I'm looking at here

 

[Mark Blue]

In the above graphic, you have lapse rates from the NAM model for 12Z today (05/31/11) for the 850 - 500 mb levels of the atmosphere. If you look at Nebraska, you can see an outlined area (red dashes) in the central and western part of the state that has a lapse rate of 7. Most of the other areas on the map appear to be in the 6 range, which you can see by looking at the outlined areas and each corresponding value. Since this is a forecast hour that is very early in the morning (6 am), the lapse rates aren't that high. If you look for an area that you know is unstable (maybe the SPC said so in an outlook) you will see much higher values such as 8 or maybe even 9 in certain cases.

 

[Drew.Gardonia]

In the above graphic, you have lapse rates from the NAM model for 12Z today (05/31/11) for the 850 - 500 mb levels of the atmosphere. If you look at Nebraska, you can see an outlined area (red dashes) in the central and western part of the state that has a lapse rate of 7. Most of the other areas on the map appear to be in the 6 range, which you can see by looking at the outlined areas and each corresponding value. Since this is a forecast hour that is very early in the morning (6 am), the lapse rates aren't that high. If you look for an area that you know is unstable (maybe the SPC said so in an outlook) you will see much higher values such as 8 or maybe even 9 in certain cases.

thanks! makes perfect sense now! I'm a visual learner, so when I SEE how things work, I understand them much better than someone just telling me or trying to explain it.

 

[Mark Blue]

Hey Drew - I was dinking around on Earl Barker's charts and noticed this Power Point slideshow link about low level lapse rates put together by one of the forecasters at the SPC. I briefly looked at it since I'm out chasing but thought about this conversation we had and thought I'd share the link in case you were interested:

http://tornadocyclone.com/low_lvl_lapse_rates/low_lvl_lapse_rates.ppt