BCSD and BCCA downscaled precipitation over the upper Mississippi River basin

In support of hydrological modeling (using SWAT) and agronomic modeling we have looked at precipitation in the BCSD and BCCA downscaled output from MIROC 3.2 over the upper Mississippi River basin over the period 1981-1999. It appears that BCCA precipitation is unrealistically low over much of the region. The following plots (prepared by Daryl Herzmann) show what we have found.

We chose MIROC 3.2 because it performs well for ENSO compared with other CMIP3 models. We have checked output from downscaled GFDL 2.1 and find that its precipitation also is too low in the BCCA downscaled output.

Raymond Arritt, Iowa State University, email rwarritt@bruce.agron.iastate.edu



Left: gridded 2 degree observations (from downscaling site). Right: PRISM climatological precipitation for 1961-1990. As expected, these two agree well. This was simply a check that the 1981-1999 period was reasonably representative of climatology.

Left: MIROC 3.2 raw precipitation. Right: MIROC 3.2 bias-corrected precipitation on the 2 degree grid. The bias correction procedure is giving realistic results.

Left: Downscaled MIROC 3.2 BCCA on the 1/8 degree grid. Right: Downscaled MIROC 3.2 BCSD. Notice that BCCA is drier than BCSD (and the observed climatology) throughout the domain.

Taking the difference between the downscaled fields (BCSD minus BCCA) shows that BCCA is lower by about 6 inches over most of the upper Mississippi River basin. There are some interesting geographical variations but no obvious trend with respect to climatological mean amount, other than very dry locations in the western U.S. having smaller differences. For example the difference is nearly as large in Nebraska as in Ohio despite observed mean precipitation in Ohio being around 50-80 percent greater.





Additional results

Following are plots for the entire continental U.S. instead of just the Mississippi River basin. A sampling of models is shown from both the CMIP3 and CMIP5 ensembles. These models were chosen in part to include members from the same "family":

Observed precipitation

Climatology for 1950-1999

Climatology for 1981-1999, regridded to 2 degrees and 1 degree.

Raw model output regridded to 2 degrees (CMIP3 models)

CCCma CGCM 3.1 and MIROC 3.2 medres

GFDL CM2.0 and CM2.1

ECHAM 5

Raw model output regridded to 1 degree (CMIP5 models)

GFDL CM3 and MIROC5

MPI ESM-LR and MPI-ESM-MR

Bias-corrected precipitaiton, CMIP3 models

CCCma CGCM 3.1 and MIROC 3.2 medres

GFDL CM2.0 and CM2.1

MPI ECHAM 5

Bias-corrected precipitation, CMIP5 models

GFDL CM3 and MIROC5

MPI ESM-LR and MPI-ESM-MR

BCSD on 1/8 degree grid, CMIP3 models

CCCma CGCM 3.1 and MIROC 3.2 medres

GFDL CM2.0 and CM2.1

ECHAM 5

BCSD on 1/8 degree grid, CMIP5 models

GFDL CM3 and MIROC5

MPI ESM-LR and MPI-ESM-MR

BCCA on 1/8 degree grid, CMIP3 models

CCCma CGCM 3.1 and MIROC 3.2 medres

GFDL CM2.0 and CM2.1

MPI ECHAM 5

BCCA on 1/8 degree grid, CMIP5 models

GFDL CM3 and MIROC5

MPI ESM-LR and MPI-ESM-MR

Ratio of BCCA to BCSD, CMIP3 models

CCCma CGCM 3.1 and MIROC 3.2 medres

GFDL CM2.0 and CM2.1

MPI ECHAM 5

Ratio of BCCA to BCSD, CMIP5 models

GFDL CM3 and MIROC5

MPI ESM-LR and MPI-ESM-MR