Question about origin and meaning of legacy static desert-fraction values in lus_data_module.F (CMAQ v5.3.1)

CMAQ
1

Hello,

I am using the CMAQ v5.3.1 windblown dust physics in an offline framework at much higher spatial resolution than a traditional CMAQ application, and I am trying to better understand the origin and intended interpretation of the legacy static desert-fraction values in lus_data_module.F.

In the v5.3.1 source I am using, the BELD3 dust classes are assigned the following static desert-fraction values:

  • shrubland = 0.50

  • shrubgrass = 0.25

  • barrenland = 0.75

Other land use schemes forego the shrubgrass category and generally use the shrubland (0.50) and barrenland (0.75) fractions.

My questions are:

  1. Where did these legacy values come from originally?
    Were they based on a paper, dataset, expert judgment, calibration exercise, or inherited from an earlier code base?

  2. What were they intended to represent physically?
    Were they meant to approximate unresolved subgrid variation in source availability/erodibility within a CMAQ grid cell, or were they mainly an empirical/tuning factor?

  3. In the original CMAQ/FENGSHA context, were these values intended to account for things like vegetation patchiness, crusted or non-erodible fractions, rocky/armored terrain, wet/ponded surfaces, subgrid soil heterogeneity, or some combination of these?

  4. Has there been any guidance on whether these values should be revisited when the model is applied at much finer spatial resolution with more explicit surface characterization?

I am asking because, in my current workflow, I explicitly resolve several surface controls that may have previously been folded into these factors, including time-specific remote-sensing-derived masks/fields for standing water, salt crust, and vegetation fractions, along with finer wind fields. Because of that, I am trying to determine whether applying the legacy static desert-fraction values unchanged could lead to double counting of reduced source availability.

At the same time, I recognize that unresolved supply limitations may still remain, especially for rocky/armored/bedrock terrain, cobbles, crusts, and unresolved soil variability. So I am trying to understand whether these values were originally intended as a coarse-grid source-availability correction, a geomorphic/supply-related mask, an empirical calibration factor, or some mixture of those ideas.

Any clarification on the historical origin and intended physical meaning of these values would be greatly appreciated.