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20030729: converting lcl to meters from the plcl function

Kevin,

PLCL (pres, tmpc, dwpc) is a functional interface I added to
the PR_PLCL subroutine. I will add a similar TLCL ( tmpc, dwpc)
interface. 

As for conversion from millibars to meters, recall that from the
hypsometric equation, that this is an integrated quantity, and
the Tavg for the layer is only strictly valid in the limit.
When you use an avarage temperature for a layer, and that layer becomes 
deeper, then your Tbar approximation may be less valid.
To see that, just use your equation to calculate the average
temparature of the layer between 1000 and 500mb with a thickness
of 5400 meters (a common synoptic lab exercise). You will find that
assuming a single Tbar for that deep of a layer will not be adequate
(if you are expecting snow and a Tbar below freezing).

As for cape, it is the integration of positive areas of the sounding,
so yes, the LFC and equilibrium level are found and a trapezoidal
integration of areas between the environment and parcel temperature
are computed. Thus, the integration requiures the model levels and
not just the initial parces conditions. The primary assumption
being made is that you have enough data levels to make the integration
reasonable. Morte levels, a better calculation.

Steve Chiswell


 
>From: "Kevin Polston" <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200307292143.h6TLhhLd012435

>Hey Chiz,
>
>I am trying to convert the lcl height which I can get from the PLCL
>function into meters. Currently it displays in millibars.  I am
>relatively sure I have the gfunc correct but the output seems just a
>little bit off.   Here is the gfunc I am using:
>
>gfunc = mul(ln(quo(pmsl, plcl(pmsl,tmpc,dwpc))), mul(avg(tmpc,
>tmpc@850%pres), 29.3))
>
>I have 850 millibars shown in the equation as a proxy for the
>temperature at the LCL.  Unfortunately  I tried using the TLCL function
>in the equation but I got an error back saying TLCL was not recognized
>(I am using nawips 5.6j).   I am a little confused as to why the TLCL
>would not work when the PLCL works.  In any case, I thought I would
>check with you to see if this is the correct way to get the LCL
>converted into meters.  The output from just this function here
>....gfunc = plcl(pmsl,tmpc,dwpc).....does create the LCL but it is in
>millibars.   I have been using as this other function as a substitute
>for the LCL in meters  (mul(150,sub(tmpcdwpc)) and it is a good
approximation but I wanted to see if I could get it "better"  by using
>the actual LCL functions available.  So, could you tell me if I am doing
>this right and why the TLCL does not seem to work (even by itself).
>
>As a follow on to this question.....how would you go about caclulating
>the LFC height in meters (in gempak format)?  Lets make the assumption
>that the TLCL function works.....couldn't you use the TLCL and PLCL
>together to find your LCL point....then lift the parcel using the TMST
>function?  The only thing I am not sure of is how to compare that
>temperature to your ambient air temperature to determine where your LFC
>is and how it would look in a gempak format.  Have you done anything
>like this? And if so could you share it please.  I have been trying for
>awhile now to figure the LFC height out....but everything I have tried
>has ended up not right.   I have even tried the LFCT function but that
>doesn't seem to work on model data or objective analysis data.
>
>This leads me into my last question.....I was searching through the
>archives and I saw where you have incoporated a "cape function" into
>this version of nawips.  I believe you said the function was  --  cape =
>(P_bottom,H_bottom, temp, dwpt)  where P is the pressure and H is the
>height at the level you start from.  My question is how do you know what
>the cape is?  Wouldn't you have to have an LFC and an EL to figure this
>out?   I suppose if I was going to try and do a cape field from
>objective analysis data then I would have to blend the surface obs
>objective analysis with the latest data from the RUC to get the
>different pressure levels higher up. So I guess I would have to add
>those RUC model grids to the grid file I have with the surface
>data...right?
>
>That's all for now.  As always, I appreciate your help and am looking
>forard to hearing from you.
>
>
>Kevin Polston
>Senior Instructor, NWSTC
>Kansas City MO 64152
>
>816-880-9314 ext 273
>
>