5: Adjust threshold for deep convection over land

The purpose of this exercise is to introduce a simple CAM code modification.

You will modify the deep convection code so that convection is harder to trigger over land. Specifically, you will increase the minimum required convective available potential energy, or CAPE, needed to initiate deep convection over land.

Scientific Motivation

Deep convection affects precipitation, clouds, radiation, and atmospheric circulation. By increasing the CAPE threshold over land, we delay the onset of deep convection in land grid cells. This can change the timing and intensity of precipitation and may allow more instability to build before convection is triggered.

In this short tutorial run, the goal is not to produce a fully equilibrated climate response. Instead, the goal is to verify that the code change compiles, runs, and produces output that can be compared with the control case.

Exercise: Effects of delaying the initiation of convection

Create a case similar to control case but add code to delay the initiation of convection over land by increasing the minimum required convective available potential energy (CAPE) to initiate convection.

Figure: CAPE.

Create, configure, build, and run a case called fhist.cape using:

• Compset: FHISTC_LTso

• Resolution: ne16pg3_ne16pg3_mg17

Use the same CAM history output settings as in the control case.

Modify the deep convection code so that over land, the minimum CAPE threshold required to initiate deep convection is increased by a factor of 10.

Run for 5 days.

Questions to Think About

Before looking at the solution, consider:

• Which CAM source file controls this part of the deep convection calculation?

• Why do we copy the source file into SourceMods/src.cam instead of editing the original source tree?

• How can you confirm that your modified code was compiled?

• Which output variables are most likely to respond to a change in deep convection?

• Why might a 5-day simulation show only a limited response?

Click here for hints

How do I output 3 hourly instantaneous variables?

• Use namelist variables: nhtfrq, mfilt, fincl.

• For more information, look at the chapter:
  NAMELIST MODIFICATIONS -> Customize CAM output

Where do I change threshold for CAPE?

• Look for the file zm_convr.F90 into the cesm code

• For this exercise, copy that file into: SourceMods/src.cam and modify the copy.

Click here for the solution

# Set environment variables

Set environment variables with the commands:

Tcsh user

set CASENAME=fhist.cape
set CASEDIR=/glade/u/home/$USER/cases/$CASENAME
set RUNDIR=/glade/derecho/scratch/$USER/$CASENAME/run
set COMPSET=FHISTC_LTso
set RESOLUTION=ne16pg3_ne16pg3_mg17

bash user

export CASENAME=fhist.cape
export CASEDIR=/glade/u/home/$USER/cases/$CASENAME
export RUNDIR=/glade/derecho/scratch/$USER/$CASENAME/run
export COMPSET=FHISTC_LTso
export RESOLUTION=ne16pg3_ne16pg3_mg17

# Create a new case

Create a new case with the command create_newcase:

cd /glade/u/home/$USER/code/my_cesm_code/cime/scripts/
./create_newcase --case $CASEDIR --res $RESOLUTION --compset $COMPSET --run-unsupported

# Change the job queue and account number

If needed, change job queue and account number.
For instance, to run in the queue tutorial and the project number UESM0014. You should use the project number given for this tutorial.

cd $CASEDIR
./xmlchange JOB_QUEUE=tutorial,PROJECT=UESM0014 --force

This step can be redone at anytime in the process.

# Setup

Invoke case.setup with the command:

cd $CASEDIR
./case.setup    

# Modify code

• copy zm_convr.F90 into the SourceMods/src.cam directory and modify it

cp /glade/u/home/$USER/code/my_cesm_code/components/cam/src/atmos_phys/schemes/zhang_mcfarlane/zm_convr.F90 SourceMods/src.cam

• In the subroutine zm_convr

Replace the code:

if (cape(i) > capelmt) then 
 lengath = lengath + 1 
 ideep(lengath) = i 
 end if

by

if (landfrac(i) > 0.5_kind_phys) then
    capelmt_mask = 10._kind_phys*capelmt
else
    capelmt_mask = capelmt
end if

if (cape(i) > capelmt_mask) then 
    lengath = lengath + 1 
    ideep(lengath) = i 
end if

• Near the top of subroutine zm_convr:

Add the line

real(r8) :: capelmt_mask

right after:

real(r8) pblt(pcols) 	

# Customize namelists

Edit the file user_nl_cam and add the lines:

nhtfrq(2) = -3
mfilt(2) = 240
fincl2 = 'TS:I','PS:I', 'U850:I','T850:I','PRECT:I','LHFLX:I','SHFLX:I','FLNT:I','FLNS:I'
interpolate_output(2) = .true.
interpolate_nlat(2)   =  91
interpolate_nlon(2)   = 180

This creates a second CAM history stream, h1, with 3-hourly instantaneous output.

You can edit the file with a text editor. Alternatively, you can add the lines using echo:

echo "nhtfrq(2) = -3">> user_nl_cam    
echo "mfilt(2) = 240">> user_nl_cam
echo "fincl2 = 'TS:I','PS:I', 'U850:I','T850:I','PRECT:I','LHFLX:I','SHFLX:I','FLNT:I','FLNS:I'">> user_nl_cam
echo "interpolate_output(2) = .true.">> user_nl_cam    
echo "interpolate_nlat(2)   = 91">> user_nl_cam    
echo "interpolate_nlon(2)   = 180">> user_nl_cam    
echo "">> user_nl_cam    

You build the namelists with the command:

./preview_namelists

This step is optional as the script preview_namelists is automatically called by case.build and case.submit. But it is nice to check that your changes made their way into:

$CASEDIR/CaseDocs/atm_in

# Set run length

If needed, change the run length. If you want to run 5 days, you don’t have to do this, as 5 days is the default.

./xmlchange STOP_N=5,STOP_OPTION=ndays

# Build and submit:

qcmd -- ./case.build
./case.submit

---

# Check your solution

When the run is completed, look at the history files into the archive directory.

(1) Check that your archive directory on derecho (The path will be different on other machines):

cd /glade/derecho/scratch/$USER/archive/$CASENAME/atm/hist
ls 

As your run is only 5-day, there should be no monthly file (h0)

(2) Look at the contents of the h1 files using ncdump.

• The file should contain the instantaneous output in the file h1 for the variables:

        float FLNS(time, lat, lon) ;
                FLNS:Sampling_Sequence = "rad_lwsw" ;
                FLNS:units = "W/m2" ;
                FLNS:long_name = "Net longwave flux at surface" ;
        float FLNT(time, lat, lon) ;
                FLNT:Sampling_Sequence = "rad_lwsw" ;
                FLNT:units = "W/m2" ;
                FLNT:long_name = "Net longwave flux at top of model" ;
        float LHFLX(time, lat, lon) ;
                LHFLX:units = "W/m2" ;
                LHFLX:long_name = "Surface latent heat flux" ;
        float PRECT(time, lat, lon) ;
                PRECT:units = "m/s" ;
                PRECT:long_name = "Total (convective and large-scale) precipitation rate (liq + ice)" ;
        float PS(time, lat, lon) ;
                PS:units = "Pa" ;
                PS:long_name = "Surface pressure" ;
        float SHFLX(time, lat, lon) ;
                SHFLX:units = "W/m2" ;
                SHFLX:long_name = "Surface sensible heat flux" ;
        float T850(time, lat, lon) ;
                T850:units = "K" ;
                T850:long_name = "Temperature at 850 mbar pressure surface" ;
        float TS(time, lat, lon) ;
                TS:units = "K" ;
                TS:long_name = "Surface temperature (radiative)" ;
        float U850(time, lat, lon) ;
                U850:units = "m/s" ;
                U850:long_name = "Zonal wind at 850 mbar pressure surface" ;

Note that these variables have no cell_methods attribute becasue the output is instantaneous.