Air temperature problem creating FOX from EPW

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Long
Posts: 9
Joined: Tue Jan 28, 2020 10:44 am

Air temperature problem creating FOX from EPW

Postby Long » Fri Feb 07, 2020 3:11 pm

Hello ENVI-Met team,

I'm trying to creat FOX file from EPW file for full forcing (documents seen in the attachments). And I have 2 questions:

Question 1: But I found that the air temperature in FOX (atomatically generated by the software with EPW file) sometimes can be different from EPW. As an example, for Jan 28, the air temperature of the FOX file is around 2 °C lower than EPW. But for August 9, they are almost exacly the same. I attached the files (see pictures 3 and 4). Could you please clarify this?
0128EPW.jpg
Jan 28 of EPW
0128FOX.jpg
Jan 28 of FOX


Qustion 2: In the FOX file, for each hour, there are five colomes (in the red box in picture 1 and 2), the first is height, second is air temperature, fifth is specifc humidity (I soppose?); what are third and fourth? Why they are negative?

Qusetion 3: I used Version 4.4.4 and 4.4.3 genertaed the FOX file, and I found the third and fourth columes are different for 2500 m height. Did you change the method for this calculation?
Fox file-4.4.4.JPG
fox 4.4.4
Fox file-4.4.3.JPG
fox 4.4.3

Thanks a lot for your kind reply.

Best regards.
Long
Attachments
FOX.zip
FOX files and EPW
(1.51 MiB) Downloaded 1 time

Tim
Posts: 1203
Joined: Mon May 15, 2017 3:46 pm

Re: Air temperature problem creating FOX from EPW

Postby Tim » Sun Feb 09, 2020 1:25 pm

Hi Long,

1. EPW files contain bulb temperature. We recalculate it to potential temperature. That is why they differ.
2. First column is height, second column is potential temperature, third and fourth are U and V components of wind and thus negative sometimes and fifth is specific humidity.
3. Yes, we corrected the calculation of wind for the 2500 m height. It was calculated much too high and thus caused instabilities in the simulation.

Best regards,
Tim

Long
Posts: 9
Joined: Tue Jan 28, 2020 10:44 am

Re: Air temperature problem creating FOX from EPW

Postby Long » Tue Feb 11, 2020 4:05 pm

Hi Tim,
Thanks for your reply. It still confuses me:

1. Why you use potential temperature instead of directly using the recorded dry bulb temperature in EPW file? If you want to convert the temperature recorded at the meteo station's pressure to a reference pressure (e.g. 1000 hPa), the pressure of the simulated area might be different from this reference pressure, meaning this conversion also can not reflect the temperature at the site's pressure. Is this conversion really meaningful?

2. I read somewhere that Envi-met cannot deal with the air temperature below 0 ℃, could you please clarify this point, please?

Thanks a lot for your help.

Best regards,
Long

Tim
Posts: 1203
Joined: Mon May 15, 2017 3:46 pm

Re: Air temperature problem creating FOX from EPW

Postby Tim » Tue Feb 11, 2020 6:05 pm

Hi Long,

1. We are recalculating it because we rather take the potential temperature as input for the model than the bulb temperature. EPW files additionally contain pressure values as well and thus enable us to recalculate the temperature values with the real pressure instead of reference pressure of 1013 hPa.
In general, we are indeed convinced that it would be even more accurate to allow the user to change the atmospheric pressure from 1013 hPa to user-defined values, e.g. from measurements and take that as additional input in an new column of the FullForcing file. We are already programming and testing that. It may be released within the next few versions if testing will be successful.

2. All water related calculations do not take freezing of water (and all additional features coming with that) into account. Since more or less everything is connected to everything in microclimate, those inaccuracies in water handling in air and soil will influence the simulation results for temperatures below 0 °C in general.

Best regards,
Tim

Long
Posts: 9
Joined: Tue Jan 28, 2020 10:44 am

Re: Air temperature problem creating FOX from EPW

Postby Long » Thu Feb 13, 2020 4:47 pm

Hi Tim,

Thanks for your answer and have a nice evening!

Long


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