ATPL Met question

[Woodend1]

Gday,

I'm having trouble with this and was hoping someone will show me the light. Something to do with mixing and turbulence inversions. Here goes....

Air which had a surface temp of 10'C and lapse rate of 2'C/1000ft is subjected to frictional turbulence from surface to 3000ft. After mixing and assuming the air remains dry, the surface temp is likely to be:

A) 11.5'C
B) 16'C
C) 7'C
D) 10'C

Cheers all:ok:

[Arm out the window]

Don't know if this is showing you the light, mate, but how does this sound:

They tell you that the air was 10 deg at the surface and has an actual lapse rate of 2 deg per 1000 ft.
That means it must be getting continuously colder the higher you go.

Any mixing must therefore mix up the colder air from up above with the 10 deg air at the surface. Ignoring any other factors, that would mean that it must get colder at the surface, because that's where the air was warmest when you started.

So I reckon the only possible answer is C, because that's the only one that's colder than 10 deg.

[Howard Hughes]

Now this is just a guess! So feel free to dispute it!

Ground temp=10c
Temp at 1000ft=8c
Temp at 2000ft=6c
Temp at 3000ft=4c

Average temp would be 7c, or in other words the temp at 1500 ft midway between the ground and 3000.

Well what do you know I came up with the same answer as Arm out the Window. We must be getting close!!

Cheers, HH.

:ok:

[Tinstaafl]

I have feeling that using the ELR isn't the whole story. During the mixing the unsaturated cool air above would warm at the DALR, not the ELR.

When it descends it will warm to be 13 deg and then mix with the 10 deg surface air.

Using a simple average puts the surface temp at 11.5 deg.

[Arm out the window]

Good point, Tinstaafl!

[Woodend1]

Thanks for that. I thought the mixing was something to do with the DALR seeing as it stayed dry. Hopefully the exam will be Met orientated not an English test too cause my england no good eh...:8

[*Lancer*]

I'd take [C]. Regardless about the differences between SALR, ELR and DALR, the actual lapse rate is stated in the question (just so happens it equals the standard ELR). RTFQ! ;)

Tinstaafl... If you want to use the DALR you can't forget that the 10deg surface air should rise and cool to 1deg at 3000. The 'mixed' average is still 7deg.

[Tinstaafl]

Environmental lapse rate is not the same as DALR. It *is* needed however to find the 3000' temp. Once a parcel of air at any particular level starts to ascend or descend then it will cool or warm at the DALR (or SALR if applicable).


Surface air rising is irrelevent. The Q. asked for surface temp. so the temp drop as air rises doesn't count.

[centurian]

Have sat this paper twice now (got the bugger now) and both times this question came up.

This is straight from my notes

Turbulence inversion occurs when turb. mixing near the ground steepens the lapse rate in the friction layer. Air brought adiabatically from the upper part of the layer is warmed to a temp higher than that at the surface. Similarly air carried aloft is cooled below the temp of the upper layers when the ascending air cools adiabatically. As mixing proceeds the effect is to warm the air at the bottom of the friction layer and cool the air at the top of the layer.

I struggled with this for ages but it seems this is the only question they ask about turb inversions, so just remember the effect of a turb inversion is to increase the surface temp.

I answered A and got it right and thats really all i care about :D

[Counter-rotation]

Could I please extend this thread by asking another (possibly dumb) question...
It sounds as though Centurian has answered the original question, so we have warmer surface air, cooler air aloft, and an increased lapse rate - doesn't this create instability? I thought inversions were all about stability in the lower atmosphere? Isn't that why they're called "inversions"?

l
l \
l \
l \
l \ ('scuse the crude graphic - temp v's alt...)
l \
l \
l /
O_________/___

:confused: CR...

Well the graph didn\'t work but I\'m sure you\'ve all seen it...

CR.

[centurian]

CR

Remember this is only occuring in the friction layer (to about 3000'), and for it to occur there must be a stable lapse rate above the friction layer otherwise you will get convective developments. You also need a bit of wind to stir things up a bit.

[Thump & Go]

Tinstaffl is correct, the answer is A.
If you can't be bothered doing the maths, pick the temp that is just slightly warmer than the original - works every time.

The stability that you ask about occurs on top of the mixing layer - that's where the temp is rising with height. Because the new surface temp(11.5 C - A) is higher than originally and the 3000ft temp cooler you have in effect steepened the ELR in that 3000ft block of air. Above that though the ELR has continued as "normal", so at some point air temp must increase with height to "catch up." If I could draw on here it might make sense :{ but if you draw a temp/height graph with 1 linear lapse rate through out(2 C/1000ft for arguments sake),a straight horizontal line representing 3000ft, then steepen the ELR below that line you may see what I'm getting at???At some point the ELR will have to reverse (ie / not \) to meet back up. That potentially thin block of air is your inversion - temp incr. with height, prevents further ascent of air(stable), possible formation of strata-form cloud (all Trevor Thom inversion characteristics) - not the friction layer below 3000ft which must be unstable in order for all of this to occur.

Hope this helps more than it hinders.:ok:

Thump

[Counter-rotation]

Thankyou both it is now clear(er)...

Only 10001 more things to refresh/remember and I might be able to move on from G.A.:(

Ha HA HAAAAA!:ok:
CR.