Changes between Version 106 and Version 107 of Documentation/TrunkFunctionality4


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Timestamp:
09/28/21 17:43:33 (4 months ago)
Author:
luyssaert
Comment:

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  • Documentation/TrunkFunctionality4

    v106 v107  
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    424424=== Pgap === 
    425 the Pgap calculation in r7278 was checked line by line by three persons. No obvious problems were found. Why was it checked in the first place? Pgap based veget is higher than Lambert-Beer used veget. Given the differences in definitions/approach this is not really a suprise but it seemed worth to check. This line by line check revealed two possible issues: (a) The Pgap that goes into the calculation of veget assumes zenith solar angle. This might be problematic for radiation-related uses of Pgap. Should we introduce two vegets? one for zenith angle to be used for interception and one for the real solar angle to be used for shading and temperature related processes? (b) It needs to be checked how overlap is dealt with in Pgap. The approach is very insensitive to the canopy porosity this might be cause by the fact that overlap between canopies in not properly accounted for (thus the cumulative crown volume may exceed the entire space of the layer for which we calculate the crown volume). A ticket was made for these issues. 
     425The Pgap calculation in r7278 was checked line by line by three persons. No obvious problems were found. Why was it checked in the first place? Pgap based veget is higher than Lambert-Beer based veget. Given the differences in definitions/approach this is not really a surprise but it seemed worth to check. This line by line check revealed that the Pgap that goes into the calculation of veget assumes zenith solar angle. We have two options but both might be problematic: (a)assume zenith angle equals to 0 degrees when calculating Pgap. This woul be OK for interception and turbulence but problematic for radiation-processes, and (b) use the zenith angle at noon. This is an OK for radiative processes (but will always overestimate the gap fraction but it is underestimates the gap fraction for interception. A third solution could be to introduce two or more vegets based on different solar angles? Easy to calculate but then the problem might be shifted to sechiba: which veget to use when? 
     426 
     427Since r789X the calculation of Pgap was changed such that the total crown volume cannot exceed the entire canopy space. In earlier version this happened quite often but the impact on porosity was rather small. The same version calculates Pgap for the local zenith angle at noon. Canopy shape was made to vary between the tropics, temperate and boreal PFTs. 
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