We had some further discussions on this topic within our group. Based on these discussions, here is some additional information on our understanding of the current WBD scheme’s dependency on various inputs.
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The CMAQ WBD module requires information about fractional land use. While the WRF PX LSM provides this information, in our understanding this isn’t necessarily the case for the default configurations of other WRF LSM options. The CTM_WBDUST_BELD option in CMAQ is a potential alternative to providing fractional land use information to the WBD module if this information isn’t available from the WRF LSM, but it doesn’t address the other WRF LSM related issue discussed in the next bullet
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The LSM options in WRF differ in their definition of the depth of the first soil layer. For the PX LSM, that depth is 1 cm while to our knowledge it is 10 cm for the default version of the NOAH LSM. Soil moisture in the first soil layer is used in the CMAQ WBD module calculations, and the parameterizations in the code were developed based on WRF layer 1 soil moisture provided by the PX LSM (i.e. at 1 cm). Given that the NOAH LSM has a deeper first soil layer, soil moisture would generally be expected to be higher, suppressing the generation of WBD using the current parameterization expecting soil moisture for a 1 cm deep layer.
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The CMAQ WBD scheme also relies on the vegetation fraction provided by the WRF LSM (wrfout variable VEGF_PX if the PX LSM was used and VEGFRA for other LSM). For the CMAQ WBD scheme to work, it is important for this vegetation fraction information to exhibit accurate seasonal and spatial variations. We know that this is the case for vegetation fraction information provided by the PX LSM in WRFv4.1+ but we have no experience with using vegetation fraction from other LSM.
This reiterates the point made above by @foley.kristen that at this time we can only support questions that relate to using the CMAQ WBD module with WRFv4.1+ and PX LSM.
If there are users in the community who are performing model development work to expand the WBD module to work with other WRF LSM, we would like to hear about your experiences.
@skunwar and @fliu, would you mind providing the following information about your runs in which you experienced very low or even no WBD?
- WRF version and LSM option
- MCIP version
- Did the WRF LSM provide fractional land use information (LUFRAC_## in GRIDCRO2D for MCIP < 5 or LUFRAC in LUFRAC_CRO for MCIP5), i.e. were the values of these variables all 0 and 1 or were they fractions?