question about CLM5 output and runoff

[xgao304]

Dear Sir:

I am using cesm2.1.3 and have several questions regarding CLM5 output.

1. I tried both startup and branch runs and found out that the netcdf file size of first monthly output is always different (larger) than
other months (see below). This causes some issues if GrADs is used to read the output continuously. I am not sure what causes this problem,
or simply we should always discard the first monthly output?

startup run (1951-2014) on a regional domain:

-rw-r--r-- 1 xgao users 20106204 Feb 15 13:10 Clm5Sp2000_USA.clm2.h0.1951-01.nc
-rw-r--r-- 1 xgao users 14188868 Feb 15 13:10 Clm5Sp2000_USA.clm2.h0.1951-02.nc
-rw-r--r-- 1 xgao users 14188868 Feb 15 13:11 Clm5Sp2000_USA.clm2.h0.1951-03.nc
-rw-r--r-- 1 xgao users 14188868 Feb 15 13:12 Clm5Sp2000_USA.clm2.h0.1951-04.nc
-rw-r--r-- 1 xgao users 14188868 Feb 15 13:12 Clm5Sp2000_USA.clm2.h0.1951-05.nc

branch run (2021) on a regional domain:
-rw-r--r-- 1 xgao users 20106204 Mar 10 10:34 PFmedAWI2021_USA.clm2.h0.2015-01.nc
-rw-r--r-- 1 xgao users 14188872 Mar 10 10:35 PFmedAWI2021_USA.clm2.h0.2015-02.nc
-rw-r--r-- 1 xgao users 14188872 Mar 10 10:36 PFmedAWI2021_USA.clm2.h0.2015-03.nc
-rw-r--r-- 1 xgao users 14188872 Mar 10 10:37 PFmedAWI2021_USA.clm2.h0.2015-04.nc

startup run (1951-2014) on a globe domain
-rw-r--r-- 1 xgao users 592076372 Oct 6 2018 CRU5016clm5.clm2.h0.1951-01.nc
-rw-r--r-- 1 xgao users 415819032 Oct 6 2018 CRU5016clm5.clm2.h0.1951-02.nc
-rw-r--r-- 1 xgao users 415819032 Oct 6 2018 CRU5016clm5.clm2.h0.1951-03.nc
-rw-r--r-- 1 xgao users 415819032 Oct 6 2018 CRU5016clm5.clm2.h0.1951-04.nc

2. Is there any way to derive the runoff due to snowmelt?

3. for QRUNOFF and QRUNOFF_TO_COUPLER, I assume they are not equal when land use change is active.
I know QRUNOFF is derived as follows:

Sub-surface drainage (QDRAI) + sfc runoff (QOVER) + runoff from glaciers, wetlands, & lakes (QRGWL) + surface water runoff (QH2OSFC) + drainage from perched water table (QDRAI_PERCH) - irrigation (QIRRIG).

I am just wondering what other terms are used to derive the QRUNOFF_TO_COUPLER?

Thanks,

 

[oleson]

1. The output for the first month is valid. The first monthly file contains some useful time-invariant variables. These are not on every file since they are time-invariant.
2. I don't think there are any variables that capture the part of snowmelt that runs off.
3. Yes, they will be different when land use change is active. QRUNOFF_TO_COUPLER includes the liquid runoff due to a change in the area of a landunit. See Chapter 27 in the technical note for details:

2.27. Transient Land Use and Land Cover Change — ctsm CTSM master documentation

"Water and energy are conserved by summing up the total water and energy content of each gridcell before and after a change in area. Differences in liquid and ice water content are balanced by liquid and ice runoff terms, which can be either positive or negative. (Negative runoff is effectively a withdrawal of water from the ocean.) Differences in energy content are balanced by a sensible heat flux term, which again can be either positive or negative. These balancing fluxes are spread evenly throughout the following year."