Lysis
(User)
Thu Jul 14 2005 04:02 PM
convective diurnal maximum

To anyone who knows the answer:
I was looking at the 24 hour visible loop of Emily and noticed how different she looked before and after nightfall. If I didnít know better, I would say that I was looking at two different systems.

Now, I am sure Keith probably asked this question in his on-going learning stage,Ö but could you explain in detail about the concept of overnight convective diurnal maximum. I did a Google search because the question was is so vague, but all I came up with was NHC discussions using it in context. Thanks.


Keith234
(Storm Chaser)
Thu Jul 14 2005 04:34 PM
Re: convective diurnal maximum

Actually I did and it's a few threads beneath this one.

Here it is

http://flhurricane.com/cyclone/showflat.php?Cat=0&Number=25541&an=0&page=1#25541


Lysis
(User)
Thu Jul 14 2005 05:34 PM
Re: convective diurnal maximum

I was unable to find a definitive answer to what exactly it is

His answer makes perfect sense, I suppose... but I was supprised Clark didn't know exactly what it was. The term is used alot in discusions. Regardless... thanks, buddy.


Terra
(Storm Tracker)
Thu Jul 14 2005 05:58 PM
Re: convective diurnal maximum

I wasn't around when this topic was initially discussed (or else, I missed it), but anyway, I want to take a stab at the explanation... Not sure if I can do better than Clark, but since our perspectives are different, it might work.

Solar radiation maximizes at solar noon (forget about daylight savings time and the like) due to the sun being highest in the sky. There is a shorter pathlength (i.e. less chance for scattering) and the rays are more concentrated at the surface. Temperature lags radiation and maximizes around 3-4PM. This has to do with net radiation at the surface... if this is positive, which it is from right after sunrise until mid afternoon. During these times, temperature rises. When net radiation at the surface is negative, temperature falls. This is true during evening, night, and early morning (before the sun rises too high in the sky). So, at night, temperature decreases because more terrestrial radiation is leaving the Earth's surface than is being absorbed. The atmosphere still exhibits vertical mixing, even though no more radiation is being absorbed at the surface. The warm suface layer mixes convectively upward at night, being replaced by cooler air at the surface. The warming of the surface starts again the next morning when the net incoming radiation again becomes positive.

Does this help at all?


Clark
(Meteorologist)
Thu Jul 14 2005 06:32 PM
Re: convective diurnal maximum

I think I can take a better stab at it, but apologies ahead of time if it seems jumbled.

At night, the atmospheric profile -- including the near-surface profile -- is relatively moist due to the lack of daytime heating. At night, generally temperatures fall to the dewpoint; this is no different in the tropics. Tropical convection inherently does not depend upon CAPE (convective available potential energy), which is at a maximum during the day due to daytime heating; this is why you see a convective maximum late in the afternoon over land. With a moister profile through the atmosphere and given sufficient forcing for rising motion -- in the tropics, generally from converging winds at low levels in association with a tropical wave or the ITCZ...or even a tropical cyclone -- convection is more likely to be at its maximum just before the sun comes up. It also helps, particularly for tropical cyclones, that at or just after nightfall, SSTs are at a relative maximum for the day. At that time, convection/tropical systems can tap into the increased surface heat and moisture energy, something that continues through the night in conjunction with the aforementioned moisture profile, allowing for increased development.


Lysis
(User)
Thu Jul 14 2005 06:40 PM
Re: convective diurnal maximum

.... Ok, is that why, statistically, you are just as likely to experience a tornado in South Florida at midnight, then you are during mid afternoon ? ...or is that something different?


EDIT: Forgive my ignorance, but you mentioned converging winds in association with a storm system... so this process does not occur unless there is a tropical cyclone in the area?


Terra
(Storm Tracker)
Thu Jul 14 2005 07:30 PM
Re: convective diurnal maximum

That made a lot of sense Clark, actually... I am definitely an atmosphere person... when you add oceans and storms, the details are kind of there and that makes me think I understand something, just sometimes I'm a little incomplete.... but, eventually, I'll catch on!

Clark
(Meteorologist)
Thu Jul 14 2005 07:42 PM
Re: convective diurnal maximum

Lysis -- South Florida tornadoes have a number of origins, almost all of them equally likely: weak tornadoes from daytime convection, tornadoes with squall lines/cold fronts in the overnight hours, tornadoes from tropical cyclones, and so on. I imagine it's the relative equality of the first two factors that plays the largest role and not really tropical convection, which isn't generally associated with tornadoes (tropical cyclones being the exception, but only as they make landfall and interact with the midlatitude weather pattern).

Converging winds are found in a lot of places in the tropics, not just tropical cyclones. As mentioned, you can find them along the ITCZ or with tropical waves, but two other examples. There exist others on smaller scales.


Keith234
(Storm Chaser)
Thu Jul 14 2005 07:58 PM
Re: convective diurnal maximum

...Such as slantwise convection which goes into that whole vorticity arguement.

Lysis
(User)
Thu Jul 14 2005 09:27 PM
Re: convective diurnal maximum

slantwise convection?... enlighten me.

Keith234
(Storm Chaser)
Thu Jul 14 2005 09:37 PM
Re: convective diurnal maximum

ooh this is a toughy and extremely complex.


Here's an presentation
http://meted.ucar.edu/norlat/slant/


Common in the winter time in area's of low vorticity, little lift and some light to moderate shear. It is very unique mesoscale banding feature, and can result in very heavy snowfall in the winter due to the weak steering regime.


Terra
(Storm Tracker)
Thu Jul 14 2005 09:47 PM
Re: convective diurnal maximum

Give me 30 minutes to go through the tutorial... (at least, as the only thing on the table of contents that I know about is atmospheric instability, so I may need more time).

Yea, ok... I'm not ready for that one yet... Let's see what else meted.ucar.edu has to offer before I try understanding that one. I'm not quick enough with the met jargon yet....


Keith234
(Storm Chaser)
Thu Jul 14 2005 09:55 PM
Re: convective diurnal maximum

It's much more mathematical then geo-fluid physic's.

Clark
(Meteorologist)
Thu Jul 14 2005 10:56 PM
Re: convective diurnal maximum

Keith -- conditional slantwise instability isn't really found within tropical cyclones. It is true that the system is neutrally stable to moist slantwise convection; however, that does not imply conditional symmetric instability is present within the storms. Instead, CSI is found north of warm fronts in relatively stable environments (not a tropical cyclone!) during the wintertime.

It is possible, however, to have conditional symmetric instability within the convective structures found north of extratropically transitioning tropical cyclones, way up there in the North Atlantic. Usually that is dominated by other, upright instabilities, but CSI may play a role. A similar slantwise instability was found to play a role, for instance, with Floyd at landfall, a system that was interacting pretty heavily with the midlatitudes and on its way to transition.

I'll be presenting a poster on this very topic in about 2 weeks at a conference in Washington D.C. -- if anyone is around there and wants to pay the $250 or so registration fee, feel free to stop by!

I was thinking more along the lines of topographically-induced convergence and things like that, not so much instability or convective-types. My mistake...


Keith234
(Storm Chaser)
Fri Jul 15 2005 09:38 AM
Re: convective diurnal maximum

I thought you were talking more about small scale convection features not necessarily associated with the tropics. Best of luck with the presentation....

Clark
(Meteorologist)
Fri Jul 15 2005 10:41 AM
Re: convective diurnal maximum

Nah, my mistake if that wasn't clear -- I was looking specifically at the tropics.


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