A holistic microclimate model


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UTCI

The calculation of UTCI is based on a 6th order polynomial regression function estimating UTCI from wind speed, air temperature, mean radiant temperature and air vapour pressure (see COST 730 Webpage). Due to that approach, there is almost nothing that can be changed in the personal settings such as clothing, activity etc.

Limits:

  • Wind Speed: UTCI as used in the software is limited to a wind speed range of 0.5 to 17 m/s related to 10 m Height
  • Height reference: UTCI always related to wind speed at 10 m height while the biometeorological parameters are normally define between 1.5 and 2 m…

As the UTCI calculation expects the wind given at the 10 m level, ENVI-met BioMet uses the calculated local wind speed $Wind_{zlevel}$ in the model domain related to the pedestrian level ($zlevel$) and scales it up to the 10 m level equivalent using the logarithmic wind power profile $$ Wind_{10m} = \frac{ln(10 / z0)}{ln(zlevel / z0)} \cdot Wind_{zlevel}$$

In other words, if you calculate UTCI at some point for zlevel= 1.6 m, all ENVI-met data such as air temperature or radiative temperature will be used from this level, except of the wind speed which will be extrapolated to the 10 m level value.

Obviously, that doesn't make much sense as the $z0$ roughness value is unknown for most sites, but ENVI-met provides detailed and calculated wind flow data for all levels.

Final Remarks

We do not recommend to use UTCI in the regression-based version based on using 2m (1.6m) level wind speeds extrapolated to 10m. In a complex urban environment, wind speeds at pedestrian level are unique and cannot be related to some above-roof general quantity. Instead, we propose the application of a pure physically-based approach such as PET. PET is a much more open platform and the final outcomes are more or less the same for all static indicators.

Please Note: Due to these restrictions, areas outside the official bounds of UTCI will not be calculated by BioMet. These areas are marked with “No Data”. Especially in urban areas, the lower limit of 0.5 m/s can create a number of white “No Data” areas in the maps. In future versions of BioMet, will add an option to ignore these UTCI bounds. BUT: Remember that UTCI is not a part of ENVI-met. Using this option, your maps might look better and more complete. But scientifically, you are leaving the “official” and pre-calcuated UTCI range.

References

Link to the original source code and to the EU COST Action 730 Homepage: http://www.utci.org/utci_doku.php

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