Inconsistencies in the UTCI calculation

1

Hello,

I am trying to model outdoor comfort using vegetation, grass and trees, using Honeybee Legacy. I got very useful information from the example files available in Hydra posts as Trees in Outdoor Thermal Comfort and Green Roof in Energy model and from all the information and files available here Outdoor comfort UTCI microclimat map || Vegetation limitations. Thank you all for these resources.

The shading effect and MRT blockage of trees is straightforward. Also the use of grass reduces consistently the ground surface temperature. Nevertheless, I have inconsistent results for the UTCI calculation. The UTCI values are higher above the portion of ground with grass than above the ground with asphalt (I use the word grass to define the area with lower surface temperatures rather than a ground with specific characteristics). Hope someone can help to understand the issue and suggest how it can be fixed.

I show some images and the GH and epw files are attached.

The model is simple, three buildings surrounding the paved area. The grey area is asphalt and the green area is grass.

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This figure presents the surface temperatures using asphalt for all the square area. (The ground volume is extruded toward negative Z).

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This figure presents the surface temperatures using asphalt for most of the area and grass for the smaller rectangular area shown in the first figure. The surface temperatures of the grass portion are about 12 °C lower than the asphalt.

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The following figures shows the UTCI calculation at 1.5 m of height using the Honeybee Legacy workflow. This figure presents the UTCI calculation using asphalt for all the square area.

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And this figure below presents the UTCI calculation using asphalt for most of the area and grass for the smaller rectangular area shown in the first figure. It is possible to see that the UTCI right above the grass area that yields lower surface temperatures (highlighted rectangle) are slightly higher (up to +0.7 °C) than the UTCI calculated using asphalt also for the pavement of the small rectangular area (that yields higher surface temperatures than grass). Actually above the grass area the UTCI are the highest of the area.

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Thank you in advance.
Francesco

UTCI-grass.zip (1007.2 KB)

2

Sorry for the very late response, @FrancescoDeLuca .

If I had to guess, the reason why the UTCI is higher in the case of grass is that grass tends to have a much higher albedo than asphalt and so it will reflect more solar radiation upwards to increase the shortwave MRT delta. So I recommend checking the albedos of your materials and seeing if that could be the culprit.

FYI, the comfort maps in legacy honeybee generally don’t take shortwave solar reflections into account except for the ground reflection, which is done simply by taking the reflectance of the material directly below a given analysis point and multiplying it by the overhead shortwave radiation.

I hope that helps.

3

Hello @chris,

Thank you for the reply and for the suggestion about the possible cause of having an higher UTCI in correspondance of a low surface temperature area, that being grass indeed has an higher albedo than asphalt.
Thank you also for the clarification about the comfort maps in legacy honeybee. I knew about that. I guess the new comfort analysis tools in Ladybug Tools (not legacy) can take into account shortwave solar reflections from all the environment. Didn’t have time to study the new tools yet. Hope soon.

Best,
Francesco

4

@FrancescoDeLuca ,

The new LBT plugin doesn’t have thermal mapping capabilities yet but I know exactly how I will implement them and they will take into account multi-directional shortwave solar reflections (using Radiance). All of the core infrastructure for the thermal maps in the LBT plugin is done so I should hopefully be able to get them into the next stable release of the LBT plugin.

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Thank you for the information @chris. Looking forward for new LBT thermal mapping capabilities.