MCIP V5.0 Release Notes indicate the layer collapsing has been removed with this version. Indeed you can no longer set CTMLAYS in the namelist.
I’m curious to learn more about why the developers decided to remove this feature which I believe has been widely used. I find no explanation on this. Does EPA now recommend using the full WRF layers in every CMAQ application? That might significantly increase CMAQ runrtime with little benefit in predicting surface concentrations.
I also wonder if there’s any workaround to enable the layer collapsing.
I am curious like you when I found that
Thank you for your interest in the layer collapsing in MCIP. Layer collapsing was removed starting in MCIPv5.0 for several reasons.
Layer collapsing can artificially distort the meteorological fields and introduce inconsistencies in the model simulations. The original CMAQ documentation (Byun and Ching, 1999, EPA/600/R-99/030) includes some verbiage regarding the origin of the layer collapsing in section 220.127.116.11:
"During the layer collapsing procedure, the layer description is modified. The resulting profile may have [mass] consistency problems. This option is usually used to generate meteorology data for a system test-run for code debugging and development purposes.
… Depending on the layer definitions used, the model results will be affected considerably. The implications of the layer definition are pervading across the entire system.”
The section goes on to describe that the layer structure can alter the boundary layer height, the clouds, the emissions from plume rise, among other scientific processes. While much of Chapter 12 has become outdated, the premise for layer collapsing is still the same: that it was a shortcut for testing the code.
That said, layer collapsing had become routinely used by many groups over the years–including some within the EPA–to increase the throughput of CMAQ simulations. However, the use of layer collapsing has waned over the years, particularly within the EPA. I have wanted to remove it from MCIP for years. It remained until key groups in the EPA were successfully weaned off of it.
As you mentioned, there may be little effect on surface concentrations when layer collapsing is used…depending on the length of your simulation, the spatial coverage of your domain, the atmospheric conditions, the averaging period for your concentrations, and your species of interest. BUT to the extent that you want to use a certain configuration to capture atmospheric dynamics in the meteorological model and that you want to maintain mass and dynamical consistency between your meteorological and atmospheric chemistry models, then you should not collapse layers between those models. By collapsing layers, one is redefining the atmospheric structure, which dangerously allows for substantially altering the boundary layer heights, wind shear, the stratospheric-tropospheric exchange, the representation of clouds and aqueous processes, and all vertical mixing processes. Simply put, in many cases layer collapsing is a bad idea, even if the surface concentrations are minimally affected.
In a very practical sense, it is difficult to safely program layer collapsing for the hybrid vertical coordinate. The hybrid coordinate became the default structure in WRFv4.0. Support for WRFv4.0 and the hybrid coordinate were added to MCIPv5.0. Support for layer collapsing with the hybrid coordinate was not something I wanted to tackle. This was as good of a time as any to remove layer collapsing from MCIP. It made little sense to continue to support a practice that can call into question the scientific rigor of the model results.
Hope this helps.
Thanks Tanya. I appreciate your detailed explanation.