Explicit radiation balance

[johannbjorn]

Dear forum,

I am simulating a roof with 30°slope in WUFI 2D, the roofing felt under the metal deck (the metal deck and air space is not simulated, rain turned off).

I have created a weather file with data from a meteorological station.

I did not receive any data on counterradiation from the station.

In the figure below you can see the variables in my .WAC file and the difference in simulations.

Case 1: gypsum elements under roofing felt simulated with explicit radiation balance.
Case 2: gypsum elements under roofing felt simulated without explicit radiation balance.

Guidelines:
"Therefore, if night-time overcooling and its consequences for dew deposition or reduced drying potential (for flat roofs in particular) shall be simulated, WUFI must be switched to "explicit radiation balance" mode."

WUFI 2D menu: "Be sure to have appropriate values for the atmospheric counterradiation in the climate file you use before enabling the explicit radiation balance. Otherwise unrealistic temperatures may result at this surface."

My question is:
1. If I do not have the data for counterradiation, does WUFI approximate values from my weather data to calculate its effect?
2. Should I be using explicit radiation balance in general if I am not simulating a flat roof?
3. Is it neccessary to have the explicit radiation balance if simulating a flat roof?
4. What explains the difference (small) in relative humidity and temperature seen on the plots below?
5. If I turn the explicit radiation balance off, WUFI shows a warning saying your roof is 30°, are you sure you want to turn the explicit radiation balance off?, why is that?



Many thanks and warm regards, Jóhann Björn

[Manfred Kehrer]

Hello JohannBjorn
1. If I do not have the data for counterradiation, does WUFI approximate values from my weather data to calculate its effect?
Yes using a annual constant cloud index which you can change in your inputs

2. Should I be using explicit radiation balance in general if I am not simulating a flat roof?
yes

3. Is it neccessary to have the explicit radiation balance if simulating a flat roof?
probably yes, but it depends what question(s) you want to answer

4. What explains the difference (small) in relative humidity and temperature seen on the plots below?
The loss of energy due to overcooling

5. If I turn the explicit radiation balance off, WUFI shows a warning saying your roof is 30°, are you sure you want to turn the explicit radiation balance off?, why is that?
See answer 2.

[johannbjorn]

Dear Manfred,

Thank you for your answer.

1. Yes using a annual constant cloud index which you can change in your inputs

I have hourly cloud index in octans in my weather data for Iceland (as seen in the figure before).

i. Does the annual constant cloud index, (which I presume is the 0.66 from the figure below), overwrite the hourly octans cloud data?
Cloud index:
Describes the fraction of sky which is covered with clouds. Allows estimating the atmospheric counterradiation if no measured data are available.


ii. Is the value 0.66 an average for most locations in northern Europe e.g.? Can I find this value for Iceland?

[Manfred Kehrer]

i. Sorry I have overseen your cloud index input data. In case the the hourly input will be used and the constant value (0.66) will be ignored

ii. Not sure, I think I went through a bunch of climate files back in 2007 or so and figured that 0.66 is reasonable number.

[johannbjorn]

Hi,

i. So my understanding is that the counterradiation is therefore estimated from the cloud octans of every hour from my weather data (even though 0.66 is the input there).

Thank you very much.

Jóhann