How much of an inversion layer at around 700mb can different types of lift overcome?

[Andrew Clope]

I know it's probably too simplistic if a question and more goes into it but broadly speaking, is there a rough estimate for things like cold fronts/warm fronts/OFB/Drylines/Convergence at the surface level/Divergence aloft/Orographic?

 

[Jeff Duda]

Yes, the question is too simplistic.

 

[Mark Blue]

Here is where most people start when trying to understand the 700 mb temperatures and convection. I hope it helps.

 

[Rob H]

I think there's something to this topic, and hopefully it's what the OP intended. Maybe it would help if we used some real world examples, like yesterday's (6/18/14) setup:

@18z 700mb temps were 14°. Jon Davies says that's too much of a cap, and if you look at the small number of cells that popped 6/16-6/17 in 12°-13° that seems about right. So you're not going to have storms in 14° temps / LSI of whatever / CINH of whatever.

Now what's this though? @23z temps near tail end charlie are only 11°! Something changed:

I've been told 700-400 differential vorticity advection can help figure out where impulses are that can break down the cap. I honestly have no idea how an impulse breaks down a cap, or how to read this specific chart, so any help from you smart weather guys would be appreciated

So back to the point of the OP... in this case something in the mid-upper levels caused the 700mb temps to drop at least 3 degrees per RAP mesoanalysis. The RAP forecast several hours out showed it dropping a degree or two in a very narrow window, so it looked like storms might not be able to go up. What caused this 3 degree change? Is 3 degrees common for the "strength" of whatever process caused this? How do you evaluate that process strength and breadth?

 

[Andrew Clope]

Thanks guys for the helpful links. So looking at the 700 Mb models and then pulling the soundings up it looks like the cap if there is one generally lies between 800mb and 700mb?

Also, Rob, I am very interested in more explanation on the different factors that can/do break the cap down like you're saying. I know that just hoping for a bunch of daytime heating at the surface isn't best due to the Td depressions being too large and lcls rising.

 

[Jeff Duda]

I (sort of) apologize for my previous post, as it probably came across as whatever (insert some negative adjective here). However, the issue still is quite general. There are three main ways to break a "cap" (I put cap in quotation marks because there are two types of stratifications that prevent or make difficult, convective initiation: 1) a capping inversion in which the temperature increases with height at some layer - this is probably the type you're asking about - and 2) a warm layer above the surface containing lapse rates too shallow for a parcel to reach its LFC, and thus contributing to CIN, but with no actual inversion):

1) Warm the parcel (i.e., the surface or PBL). This increases theta-e and moves the parcel path to the right relative to the environmental temperature profile in the region of the cap.
2) Moisten the parcel. This increases theta-e and moves the saturated portion of the parcel path to the right relative to the environmental temperature profile in the region of the cap.
3) Lift the cap. This is what Rob's post was alluding to when talking about 700-400 DVA. When you lift a layer that has potential instability (i.e., high RH at the bottom of the layer and low RH at the top of the layer), you steepen the lapse rate. Lift also cools the air by expansion and first law of thermodynamics. These will both serve to weaken the cap, again, mainly by reducing the magnitude of the inversion or by completely removing it.

There is an additonal method by which CI can occur in the presence of an early-day cap, but it's not as useful when chasing severe storms. Given strong enough heating and sensible heat flux, a cap can be mixed out from below by PBL processes. However, this generally requires, or will be accompanied by, significant dryness in the PBL. When this type of cap removal occurs, you are usually left with a very deep and very dry PBL with neutral lapse rates. This occurs frequently on the high plains and desert regions. The only type of severe weather you can expect in such situations is strong straight-line wind.

When it really comes down to it, the most widely applicable way of analyzing the cap strength is by convective inhibition (the area between the environment and parcel path between the parcel source layer and its LFC). This is an integrated measure of the cap strength at all relevant layers. To break a cap, you need to either reduce CIN to 0 or you need to give a parcel enough lift so that it can reach its LFC in spite of the CIN. From a PE to KE point of view, there is a vertical motion value you can give a parcel to get it to its LFC based on the formula W = sqrt(2*CIN), where W is the vertical motion in m/s. This is idealized and assumes no entrainment, and assumes large-scale vertical motion does not contribute. For example, if there is 100 J/kg CIN, then you need to give a parcel 14.14 m/s of upward motion. That's very significant (basically doesn't happen unless 50 m/s surface winds are flying up the side of a steep mountain or something like that). Typically, vertical motions created by even the strongest divergence/frontogenesis bands only approach 1 or 2 m/s. However, if all the CIN is located in a fairly thin layer and you have large-scale lift throughout the depth of that layer, that can also get a parcel to its LFC without having to obtain a ridiculous vertical motion (assuming it doesn't take too long for the forced lift to get a parcel to its LFC).

Additionally, it's not uncommon to see convection develop even in the presence of high CIN. However, when that occurs, that pretty much means the convection is not rooted in the layer that has the high CIN. In other words, if convection develops in an area where SBCIN is high, then the convection is probably elevated.

 

[Mark Blue]

I was going to say earlier, parcel theory here we come! Thanks for righting yourself Jeff and the dynamics lesson you provided. I appreciate it.

I thought Skip's post in the TA in late April was very enlightening as to the LSI, CIN, his experiences with using this data and the cap in general. If you're interested AD it can be found here.

 

[Jeff Duda]

The lid-strength index is an intriguing product. I'd love to know how it's calculated on Earl Barker's site. I have incorporated it into my own forecast graphics site (www.meteor.iastate.edu/~jdduda/forecast), but the values look quite a bit different than those on wxcaster.com. I used a formula I found in an old NWA Digest article, but I guess that might not be the only way to calculate it? Does anyone have any inside information on that?

 

[Mark Blue]

I found this on Andy Revering's website. Not sure if it's what Earl uses though.

Cap Strength (Lid Strength Index)= Saturated wet bulb potential temperature (Theta-E) between the surface and 500 mb MINUS the maximum saturated wet bulb potential temperature (Theta-E) in the lowest 100 mb of the atmosphere. Note in the formulas below M = Mixing Ratio & WBc = Wet Bulb Temperature in °C.
MB = Surface Level Pressure
Do until MB <= 500
Q = (WBc + 273.15) * ( 1000 / Mb ) ^ 0.286 + (3 * M)
If Q > Qsw then
Qsw = Q
End If
MB = MB - 25
Loop


SFC100 = Surface Level Pressure - 100
MB = Surface Level Pressure
Do until MB <= SFC100
Q = (WBc + 273.15) * ( 1000 / Mb ) ^ 0.286 + (3 * M)
If Q > Qwmax then
Qwmax = Q
End If
MB = MB - 25
Loop


LSI = Qsw - Qwmax

 

[Jeff Duda]

I've seen that code before, and it's horrible. So much important information neglected. And that's similar, but not the same, as what I read in the paper.

 

[Jeff Duda]

@18z 700mb temps were 14°. Jon Davies says that's too much of a cap, and if you look at the small number of cells that popped 6/16-6/17 in 12°-13° that seems about right. So you're not going to have storms in 14° temps / LSI of whatever / CINH of whatever.

Now what's this though? @23z temps near tail end charlie are only 11°! Something changed.

This has sort of boggled my mind, too. Temperature advection will move that 14 C contour around, but won't shrink it. Large-scale lift would shrink it, but there were 12-hour height rises of 30-60 m at 500 mb during the same time, so I have a hard time believing large-scale lift removed that cap. My other guess is that the mesoanalyses/RAP forecasts were adjusting to 700 mb temps that were initially too warm in previous forecast cycles.

The RAP forecast several hours out showed it dropping a degree or two in a very narrow window.

This is most likely in response to the convection parameterization scheme in the RAP activating, or possibly just the shallow component to it. The deep convection portion of the scheme should redistribute the thermal/moisture profile to try to return the column to saturated and neutral lapse rate state (if there is sufficient moisture in the low levels to do so). Regarding the shallow component of the scheme, since potential temperature pretty much increases with height above the PBL, vertical mixing would reduce it just above the PBL, which would mean a reduction in temperature, too.

 

[Mark Blue]

I guess it was the formula versus the code I hoped was relavent. I guess not! Is the paper with the formula you incorporated from Graziano and Carlson circa 1987?

 

[Rob H]

FWIW in my own personal experiences, I've found LSI to be very reliable, 700mb good for a quick glance, and CINH difficult to use. Most maps will show a pseudo-arbitrary CINH amount like -50 J/Kg and I've seen storms seem to struggle with the cap in 25 CINH, while thriving in 100 CINH. Now was there *actually* 100 CINH, probably not, but mesoanalysis said there was. CINH faces the same problem as CAPE in that it can be contaminated by bad surface data.

There's also the problem of pretty much needing a computer to calculate it for you (I'm not doing calculus in the car). The beauty of 700mb temps is that anyone can read a constant pressure chart of isotherms. To be fair, a lot of useful information is ignored with 700mb temps yet it does a surprisingly good job. Very similar to using temp-dewpoint spreads to estimate LCL in my mind - it will get you 95% of the way there.

Would Jeff, or anyone else, mind explaining the 700-400 Diff VA chart I linked earlier? I've tried using them before and haven't had much luck unless there was a very pronounced impulse that matched up on WV loops.

My other guess is that the mesoanalyses/RAP forecasts were adjusting to 700 mb temps that were initially too warm in previous forecast cycles.

Totally an alien world to me, but I'd like to understand it more. I was always kind of under the assumption that models whether GFS/NAM/RAP/HRRR/etc. only get "corrected" for future runs when new sounding data is incorporated. So would the 18Z soundings show up from 19Z runs on? How does that work?

 

[Jeff Duda]

Would Jeff, or anyone else, mind explaining the 700-400 Diff VA chart I linked earlier? I've tried using them before and haven't had much luck unless there was a very pronounced impulse that matched up on WV loops.

What would you like explained? I did mention in one of my previous posts on this thread that DVA can be used to infer where a cap may be removed via large-scale ascent. Was there something else?

I was always kind of under the assumption that models whether GFS/NAM/RAP/HRRR/etc. only get "corrected" for future runs when new sounding data is incorporated. So would the 18Z soundings show up from 19Z runs on? How does that work?

The graphics you see on the SPC mesoanalysis page are the output from what they call the surface objective analysis (SFCOA) package, which I believe they wrote themselves and use in-house only. However, it's just their own data assimilation system that ingests surface observations (among other types of observations) and applies them to a background field. This is actually the general format of cycled data assimilation - you take a background field (usually a very young forecast field, like a 1-hr RAP forecast in the case of the SFCOA) and augment it with observations to form a final field. Then you compute diagnostics like CAPE/CIN and others based on that final field. The key here is that the SFCOA uses 1-hr RAP forecasts as a background field. Well the full blown RAP does the same thing...each new run uses the previous run's 1-hr forecast as initial conditions. This means that short term forecast errors have some tendency to get carried along as new SFCOA analyses are produced each hour.

Think of it this way: the 12Z RAP has 12Z sounding data to use for initial conditions. Because of the wide spacing and other caveats of the soundings, the temperature field aloft contains some errors (although the errors are less than if no observations were used). A 1-hour RAP forecast will certainly contain errors in 700 mb temperatures, likely larger than in the 12Z initial condition analysis. The 13Z RAP uses the 1-hr forecast from the 12Z RAP for initial conditions (with the augmentation from any other observations such as ACARS or special soundings that might have occurred in the 12Z-13Z window). But since that 1-hr forecast contains errors, then so does the initial condition for the 13Z RAP. The 1-hr forecast from the 13Z RAP will then be used as the background for the initial condition of the 14Z RAP. Extend this all the way forward to late afternoon...21Z...22Z...by this point you have RAP forecasts based on 1-hr forecasts from previous runs that were also based on 1-hr forecasts from previous runs. Suppose the first one - the 12Z forecast - had some sort of forecast temperature bias around 700 mb. That would get carried on through future cycles. However, with the other special observations that trickle in through the intermediate hours (after 12Z), maybe some of those observations indicated it was actually cooler at 700 mb than the forecasts had predicted. Therefore, the observations augment the background forecast by cooling off the temperatures somewhat, and this then carries on into future forecast cycles.

I think this may be a part of it, but you also have to consider that during the late afternoon, some of the RAP forecasts probably contained model columns where the convective scheme had activated, and thus were probably trying to cool the low levels. If this occurred during the first hour of the forecast, then that information would've been passed to the next hour's initial condition analysis. Thus you could see decreasing temperatures that way, too. I think this explanation probably has more weight or influence on what we saw in the graphics.

 

[Andrew Clope]

Thanks guys so much for all of the information and links that you've provided! It's a lot to chew on, but exactly what I was looking for! ST has been great so far.