Comments on: Godowitch et al 1985 on Urban Boundary Layers

By: JP

JP — Fri, 22 Jun 2007 14:52:02 +0000

Operational weather forecaster have always been interested in the Boundary Layer. Temp inversons can play havoc when forecating cieling and visibilities. The art of forcasting hourly temperatures relies heavily on when the surface temp warms to an amount equal to that of the top of the boundary layer. Even being off by an hour can mean the difference between VFR conditions and IFR. This is particulary so in Central Europe. Forecaster who pay attention to the local geography and land use which surround the reporting site, realize that these micro scale changes have a large impact on fog and low level cloud dispersion; these changes in turn have a direct impact of surface temps.

There were many occaisons when I forecasted in Germany that our airfiled remained in IFR conditions (0/0), while 2km down the road it was sunny and clear.

By: jmrSudbury

jmrSudbury — Tue, 19 Jun 2007 15:42:25 +0000

Speaking of old ideas, has everyone already seen or have an explanation of the effect of global dimming, and subsequent reduction thereof, on global temperatures?

http://www.co2science.org/scripts/CO2ScienceB2C/articles/V10/N24/EDIT.jsp

John M Reynolds

By: steven mosher

steven mosher — Mon, 18 Jun 2007 20:31:44 +0000

SteveS Thanks.

By: Steve Sadlov

Steve Sadlov — Mon, 18 Jun 2007 19:56:51 +0000

Another note, Hans Erren had photos of fog over the Benelux region of Europe. It was apparent in them that the urbanization patterns were affecting the radiation pattern (and thereby, the pattern of the radiation fog).

By: Steve Sadlov

Steve Sadlov — Mon, 18 Jun 2007 19:54:24 +0000

RE: #8 – Out in the truly rural country the overall pattern of heat flux is more uniform as you move from one location to another and there are fewer rigid flat structures jutting up into the air flow. As a result, radiative flux dominates at night. In the city, heat flux varies widely – a hot spot here (e.g. a large building, mall, biz park, data center, etc) a heat sink there (a park, pond, lake, etc). As a result you end up with a mini “sea breeze” effect. Lots more chaotic flow of heat. Radiation, convection and conduction all happening at the same time based on solely the anthropgenic dissipation effects. Add a bit of wind, and the difference between city and country is even more pronounced.

By: steven mosher

steven mosher — Mon, 18 Jun 2007 19:41:49 +0000

SteveS.

I need to buy a vowel. Can I get the 5th grade version of your comment.

By: Steve Sadlov

Steve Sadlov — Mon, 18 Jun 2007 17:48:45 +0000

In rural settings, radiative upward cooling is less inhibited by local issues such as short distance thermal gradients and the turbulent flows arising from man made structures. The above results vis a vis nocturnal inversion behavior do not surprise me.

By: Julian Williams

Julian Williams — Sun, 17 Jun 2007 17:20:51 +0000

There is a massive body of work on inversions and boundary layers, mainly concerned with their impact on the dispersion of atmospheric pollution.

As a rule, true inversions occur on calm days (wind less than 3 m/s). During the day the boundary layer can be up to 1500 m high, and at night it can get as low as 50 m. At dusk and dawn, the temperature gradient starts to reverse, and there is no inversion for a while, although a mixing height of around 800 m is often employed. The ground roughness (buildings, trees, hills) will also tend to distort the inversion.

By: gb

gb — Sun, 17 Jun 2007 15:20:29 +0000

‘Maybe someone can comment on whether these nonurban inversions are characteristic in other environments (e.g. oceans, tropics)’

I don’t think inversions will appear so often above the ocean. Land has a low heat capacity and can cool down relatively quickly after sunset. The sea surface doesn’t cool down so quickly.

I haven’t read the article yet but I would expect that inversions mostly happen on calm days. If the wind is too strong the turbulence in the atmospheric boundary layer will be intense and this leads to strong turbulent mixing and a more uniform temperature. Also obstacles (buildings) can create perhaps more mixing and a more uniform temperature.

By: steven mosher

steven mosher — Sat, 16 Jun 2007 23:18:43 +0000

Ok, when can we get back to the simple cartoon diagrams where sunlight comes in,
and watts stay trapped in the air forever and burn us up eventually? That story is much
easier to understand than this.

Just kidding. Thanks SteveM. My head is hurting so I think I’m having fun.

The boundary layer/heating issue remined me of this odd experiment/demostration I had the
pleasure to watch.

Drag Reduction with Boundary Layer Heating

The benefits of reducing the drag of either a new or existing aircraft configuration are obvious. An aircraft’s endurance is directly proportional to the lift to drag ratio. Decreased drag also translates into faster top speed, quicker acceleration, shorter take-off distances and lower direct operating costs in the form of fuel savings. In order to project military air power, or on the commercial side, receive better range and fuel economy, reducing drag during the cruise portion of a flight is the most critical. During cruise, the drag of the aircraft primarily comes from profile drag (skin friction), induced drag (drag due to lift), compressibility drag, separation drag and interference drag. Of these, skin friction (from the “wetted” elements of the aircraft) typically accounts for more than 50% of the total. By applying active surface heating in the turbulent regions of the aircraft’s boundary layer, the skin friction is reduced as a function of the ratio of the skin temperature to the ambient temperature. The result is an effective drag reduction method that can be retrofitted to existing aircraft.

basically, We had a surface ( say wing leading edge) and the flow was turbulant. By HEATING THE SURFACE
you could reduced drag. Kinda cool. Roger might like it.

Also, note this neat application of surface roughness/groving to reduce drag

http://home1.gte.net/pjbemail/RibletFlow.html#222
http://www.biolbull.org/cgi/reprint/192/3/341.pdf

I wonder if Roger P has looked at agricultural groving of the land surface and its effect on boundary
layer?.