Seasonal configurations of the WRF EMS

This is a general post, related to previous CSTAR findings and implementation into operations (R2O type stuff).

During a past CSTAR project on warm season precipitation forecasting, Dr. Gary Lackmann and his research team noticed some interesting characteristics of the WRF model, specifically the behavior of atmospheric mixing by convective (CP) and planetary boundary layer (PBL) parameterization schemes.   The details were presented in a CSTAR progress report, but to summarize, they found redundant shallow mixing at the top of the boundary layer by the BMJ CP scheme and the YSU PBL scheme together.    That is, the combined mixing appeared to be too strong.  Applying this directly to cold air damming (CAD) situations, this could have significant impacts on the erosion of CAD airmasses by mixing dry air into the top of the near surface, often stratus laden cold dome.  Of course, this mainly applies when other erosion mechanisms are not present (cold advection aloft, for example).  They did some alternate testing with the MYJ boundary layer scheme and found much less of this shallow mixing.  Thus, it was suggested that an optimal model configuration for forecasting CAD could be a combinations of the BMJ CP scheme and MYJ PBL scheme.

We (RAH) were considering changing our WRF ARW configuration to run with the MYJ PBL scheme during the upcoming cool season, based on the above results, and then changing back to the “default” parameterizations next Spring.  However, like most WFO’s, we are running our WRF at grid spacing of 4km, without an active CP scheme.  After some personal communication with Dr. Lackmann on the issue, it is suggested that use of the MYJ PBL scheme with no CP scheme may under represent mixing, which could result in too slow erosion of CAD.  Thus, we are leaving our WRF ARW running with the YSU PBL scheme, but we hope to do some parallel testing of the two configurations (MYJ vs YSU) during CAD events this winter.  If and when we are able to, we will follow-up with some results.

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One Response to Seasonal configurations of the WRF EMS

  1. Jonathan Blaes @ WFO RAH says:

    The portion of the previous CSTAR study that Barrett is referring to can be found on pages 11 and 12 of the PDF available at: http://www.erh.noaa.gov/rah/science/NCSU_CSTAR_III_Final_Report.pdf

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