Dear all,
I have a doubt about the parameters to be included in the UHI magnitude:
In the User guide the UHImax is defined as "the maximum magnitude of the UHI effect for the city (or more precisely, the difference between Tair,ref and the maximum temperature observed in the city)."
I was thinking of calculating the Tair,ref temperature as the average of summer temperatures (differentiating between daytime and nighttime) over the last 3 years in the rural area.
In this case, following my logic, the UHImax, is the difference between Tair,ref (calculated as described above) and the average of summer temperatures (differentiating between daytime and nighttime) of the last 3 years in the city.
*Not therefore the maximum (absolute) value ever recorded in the city.
In your case, UHImax equals the maximum temperature observed in the city during the period of interest (the last 3 years), during day and night, respectively, MINUS summerTair,ref for day and night, respectively. I suppose you are correct in that you probably ought not subtract the maximum value ever recorded there if you are only interested in the last 3 years. Perhaps the User Guide language could be more precise here, but all input data should be relevant for the temporal period of interest (only). So if youâre interested in the last 3 years, only input data from the last 3 years. If youâre interested in summer, only use data from the summer. When investigating night versus day, be sure to differentiate your input data so that they only cover night and day, respectively. The model is time period agnostic, so itâs up to you, the user, to ensure the data cover the appropriate period. The only time-related element the model distinguishes between is related to the calculation of cooling capacity. For daytime calculations, be sure to rely on shade, albedo, and ET index values, but for nighttime, these factors are irrelevant of course, so instead be sure to use data on building intensity. But again, the model is unaware if input data cover day or night, so the user must manually adjust and select the appropriate method.
Thanks @jesseG for your reply.
Yes, it seems logical to me to use homogeneous periods to calculate the UHI max .
This would also imply to use, in addition to the homogeneous time periods, also HOMOGENEOUS METHODOLOGIES of calculation for Tair,ref (rural temperature) and urban temperature (this in my opinion is less clear in the user guide).
That is, if my Tair,ref is calculated as the AVERAGE of the last 3 years of rural temeperature, my UHI max should be calculated as the difference between the AVERAGE of the temperatures recorded in the city in the last 3 years and the AVERAGE of rural temperatures recorded in the last 3 years.
UHImax= TMEANurban-TMEANrural
Similarly if I take as Tair,ref THE MAXIMUM temperature recorded in the last 3 years in the rural area, then my UHImax becomes the difference between the MAXIMUM temperature recorded in the last 3 years in the city, minus the MAXIMUM rural temperature (Tair,ref).
UHImax=TMAXurban-TMAXrural
Obviously always comparing homogeneous time periods (night, day, summer temperatures only, etc).
In the former case we show the difference in average temperatures, in the latter the difference in maximum temperatures.
Tell me if Iâm wrong, you certainly have a much more accurate understanding of how the model works than I do, Iâm just interfacing with it for the first time and as a user.
Dear @jesseG,
Excuse me, but I have a doubt always on this issue of Tair, ref and UHI calculation.
In my study area, besides rural areas, I have natural/semi-natural areas where the average temperature is lower than both urban and rural areas.
So, also considering the equation that governs the Tair output, wouldnât it be more appropriate to set the Tair, ref as the temperature recorded in these areas?
In these natural areas in fact HMi=1 and Tair=Tair,ref; which is not the case for the rural area.
These are some great questions and I hope I can answer them to your satisfaction!
What you choose as the UHI magnitude parameter depends on what you are trying to model. If you are trying to model the largest heatwave a city has ever experienced, the UHI magnitude would be the difference between the non-urban reference temperature for the region (rural or natural) and the highest recorded urban temperature during that heatwave. You can differentiate between daytime and nighttime UHI magnitude if you wish.
If you are trying to look at a more statistical average UHI impact, you can choose the difference in long term average temperatures between the reference and urban areas (in your case over the past three years). Otherwise we recommend this source for seasonal UHI magnitudes (Global Surface UHI Explorer | Center for Earth Observation); the values are conservative but they are available globally.
I agree with your point about homogenous methodologies: you donât want to be comparing the highest temperature ever recorded in the city with a long term average non-urban baseline temperature. Either compare maximums in both or averages in both, mixing the two would raise some eyebrows.
As to your final point about reference temperature (Tref), the difference between rural and natural areas is, in the case of the user guide, semantic. Tref should be the lowest temperature you expect from your study area, which likely occurs in a more naturalized environment. âRuralâ in the users guide is a misnomer: what we really mean is the lowest non-urban temperature that the heat island will build upon. So feel free to use a temperature from the naturalized areas of your study area rather than a âruralâ reference.
Just a note on this Global Surface UHI Explorer | Center for Earth Observation
Iâve looked at it and itâs certainly useful for determining UHImax, but, it seems to me, less so for Tair,ref because the data Iâve looked at only presents the differences and not the exact temperatures measured.
Let me know if Iâm wrong or missing something, thanks.
You are definitely correct. I typically use averaged temperature data when determining non-urban reference, which you will have to search for from other sources.
Hi, I have a question linked to this thread that I posted as a new post but I was hoping someone could help me out. The User Guide says that the Reference Temperature to input in the model is âair temperature in a rural reference areaâ and the User Guide suggests the use of the UHI Global Explorer which as far as I understand measures surface urban heat island effect instead of ambient heat island effect. is it ok to compare these too? Wouldnât it be more accurate to either stick to surface reference temperature and surface UHI effect or ambient/air temperature and ambient/air UHI effect? Iâm on the final stages of my research project and this point is really confusing me and it would be really helpful to get some guidance thank you!
Hi, another question came up regarding this that relates to the InVEST User Guide. Do I understand correctly that the UHImax in Equation 111 is the same UHI that the model user places in the User Interface for âUHI Effectâ ?
Hopefully I can provide some guidance on your questions. In regards to your first post on the comparability of reference air temperature and surface temperature, you are correct in thinking that the two describe different phenomena and there may be issues in using them together. However, InVEST is designed with an eye to broad applicability, especially to data-poor regions. As you have no doubt discovered, good data on local urban heat island effects (and even reference rural air temperatures) are often unavailable and so although it predicts surface UHI, the UHI Global Explorer data is sometimes the best and only available datapoint for a given cityâs UHI. If you do have better local data, that is indeed preferable. You can also use local temperature station measurements to calibrate the model, if available, using the UHI Global Explorer data as an initial parameter value that you can adjust accordingly.
For your second question about Eq 111: âUHImaxâ is the same thing as the âUHI Effectâ parameter!
