Longwave radiation emitted by green wall

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Wanlu
Posts: 23
Joined: Wed May 02, 2018 1:24 pm

Longwave radiation emitted by green wall

Post by Wanlu »

Dear ENVI-met team,

I am testing two INX, one with green walls while one with bare walls.
After that, microclimate parameters were extracted for the same location and compared between green-bare pairs.

Based on the results, although the green wall had a lower surface temperature and near-surface air temp, the building emitted longwave radiation was higher than that of the bare wall.
This confuses me, in my understanding, the longwave radiation is mainly based on the surface temperature, the higher surface temp, the higher longwave radiation.

Could you please give me a hint about this?

Attached figs are for your reference.
Thanks in advance.

Best,
Wanlu
SurfT.png
SurfT.png (8.64 KiB) Viewed 155 times
Temp.png
LongwaveRad.png
LongwaveRad.png (9.81 KiB) Viewed 155 times

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Hi Wanlu,

I would suggest that the total amount of emitted longwave radiation is higher because there are leaves at the wall which additionally store and release heat compared to the bare wall building.

Best regards,
Tim

Wanlu
Posts: 23
Joined: Wed May 02, 2018 1:24 pm

Re: Longwave radiation emitted by green wall

Post by Wanlu »

Hi Tim,

Thanks for getting back. Noted.

I found there are three types of longwave radiation emission, i.e., wall emitted, building emitted, and substrate emitted.
I know building emitted longwave radiation includes wall emitted and vegetation emitted. Is substrate emitted radiations also included?
Besides, what is the exact proportions of these elements for the building emitted radiation?

Thanks,
Wanlu
Tim wrote: Fri Nov 27, 2020 8:14 am Hi Wanlu,

I would suggest that the total amount of emitted longwave radiation is higher because there are leaves at the wall which additionally store and release heat compared to the bare wall building.

Best regards,
Tim

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Hi Wanlu,

In addition, there Greening: Longwave Radiation Emitted as well. Building emitted longwave radiation takes greening and substrate into account if there is any. If there is no substrate, a portion of the wall is taken into account. The portion depends on the greenings' LAI (it gets higher if the greenery is rather sparse because more of the wall and less of the greening is seen / contributes to the longwave radiation emission).

Best regards,
Tim

Wanlu
Posts: 23
Joined: Wed May 02, 2018 1:24 pm

Re: Longwave radiation emitted by green wall

Post by Wanlu »

Hi Tim,

Thanks for your explanation.
Please allow me to ask further whether there is a general contribution portion for green, substrate, wall to building emitted longwave radiation? i.e., 0.3*green + 0.2*substrate + 0.5*wall = building?
I checked the 2012 and 2016 thesis and found limited clues about it. Kindly correct me if I am in the wrong direction.

Best,
Wanlu
Tim wrote: Fri Nov 27, 2020 4:55 pm Hi Wanlu,

In addition, there Greening: Longwave Radiation Emitted as well. Building emitted longwave radiation takes greening and substrate into account if there is any. If there is no substrate, a portion of the wall is taken into account. The portion depends on the greenings' LAI (it gets higher if the greenery is rather sparse because more of the wall and less of the greening is seen / contributes to the longwave radiation emission).

Best regards,
Tim

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Hi Wanlu,

No, there is no general contribution of each components. Firstly, it depends on whether there is a greening or not and whether there is a substrate or not.
Additionally, the LAI of the greening is checked. LAI * 0.7 is the weight of the greening portion (which is limited to 1 of course). 1 - greening portion is the weight for the substrate if there is any. If there is no substrate it is the weight of the wall.

We checked the longwave emission in a small scale model and found that the roof showed lower longwave emissions for greened parts as one would expect. Walls however showed it the other way round as you reported it as well. We will have closer look at that now.

Best regards,
Tim

Wanlu
Posts: 23
Joined: Wed May 02, 2018 1:24 pm

Re: Longwave radiation emitted by green wall

Post by Wanlu »

Hi Tim,

Thanks for the response.
Okay. If I understand correctly, the green wall has a contribution in building longwave radiation by (LAI * 0.7)? Then the resting contribution comes from either the substrate or the wall?
But if the LAI is 2, the portion will be 1.4, which is larger than 1. Maybe this is why the longwave emission of a green wall is higher than that of a bare wall?

Best,
Wanlu
Tim wrote: Mon Nov 30, 2020 9:36 am Hi Wanlu,

No, there is no general contribution of each components. Firstly, it depends on whether there is a greening or not and whether there is a substrate or not.
Additionally, the LAI of the greening is checked. LAI * 0.7 is the weight of the greening portion (which is limited to 1 of course). 1 - greening portion is the weight for the substrate if there is any. If there is no substrate it is the weight of the wall.

We checked the longwave emission in a small scale model and found that the roof showed lower longwave emissions for greened parts as one would expect. Walls however showed it the other way round as you reported it as well. We will have closer look at that now.

Best regards,
Tim

Wanlu
Posts: 23
Joined: Wed May 02, 2018 1:24 pm

Re: Longwave radiation emitted by green wall

Post by Wanlu »

Hi Tim,

Thank you for checking this closely.

An update for another finding.
At some points, leaf surface temperature is higher than the bare wall in the same location.
Moreover, canopy air temperature is higher than the air temperature in front of facade (both bare and green).
I am not sure whether this makes sense.

The attached figure is for your reference.

Thanks,
Wanlu
LeafTemp.png

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Wanlu wrote: Mon Nov 30, 2020 10:14 am Hi Tim,

Thanks for the response.
Okay. If I understand correctly, the green wall has a contribution in building longwave radiation by (LAI * 0.7)? Then the resting contribution comes from either the substrate or the wall?
But if the LAI is 2, the portion will be 1.4, which is larger than 1. Maybe this is why the longwave emission of a green wall is higher than that of a bare wall?

Best,
Wanlu
Tim wrote: Mon Nov 30, 2020 9:36 am Hi Wanlu,

No, there is no general contribution of each components. Firstly, it depends on whether there is a greening or not and whether there is a substrate or not.
Additionally, the LAI of the greening is checked. LAI * 0.7 is the weight of the greening portion (which is limited to 1 of course). 1 - greening portion is the weight for the substrate if there is any. If there is no substrate it is the weight of the wall.

We checked the longwave emission in a small scale model and found that the roof showed lower longwave emissions for greened parts as one would expect. Walls however showed it the other way round as you reported it as well. We will have closer look at that now.

Best regards,
Tim
Hi Wanlu,

No, as I said it is of course limited to 1. Maximum greening portion is thus 1 even if the calculation would mean it should be 1.4.

Best regards,
Tim

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Wanlu wrote: Mon Nov 30, 2020 10:30 am Hi Tim,

Thank you for checking this closely.

An update for another finding.
At some points, leaf surface temperature is higher than the bare wall in the same location.
Moreover, canopy air temperature is higher than the air temperature in front of facade (both bare and green).
I am not sure whether this makes sense.

The attached figure is for your reference.

Thanks,
Wanlu

LeafTemp.png
Thanks, we will check that as well.

Best regards,
Tim

Wanlu
Posts: 23
Joined: Wed May 02, 2018 1:24 pm

Re: Longwave radiation emitted by green wall

Post by Wanlu »

Hi Tim,

I am waiting for your update to continue the following experiments.
Any progress or follow-ups?

Thanks,
Wanlu
Tim wrote: Mon Nov 30, 2020 10:47 am
Wanlu wrote: Mon Nov 30, 2020 10:30 am Hi Tim,

Thank you for checking this closely.

An update for another finding.
At some points, leaf surface temperature is higher than the bare wall in the same location.
Moreover, canopy air temperature is higher than the air temperature in front of facade (both bare and green).
I am not sure whether this makes sense.

The attached figure is for your reference.

Thanks,
Wanlu

LeafTemp.png
Thanks, we will check that as well.

Best regards,
Tim

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Hi Wanlu,

No, we don't have any updates on it, yet. We will get back to this as soon as we find something.

Best regards,
Tim

Tim
Posts: 1989
Joined: Mon May 15, 2017 3:46 pm

Re: Longwave radiation emitted by green wall

Post by Tim »

Hi Wanlu,

We recreated your findings in some test simulations and based on these results we changed, among other things, the calculation of longwave radiation within greenery. The updated version adresses the high leaf temperatures as wells as the humidity and wind flow patterns at facades. Since these updates yield considerable changes in the model results, we decided not to release them right now, as discontinuity within the same version would result.
The updates are thus going to be released in the next version coming around May.

Best regards,
Tim

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