Weather and Climate Prediction Exercise 7
Due Friday Feb 25

In this exercise you will learn about climate feedbacks compare with different types of forcings. One type of forcing is the familiar doubling CO2. The other is a volcanic aerosol forcing or you can think of it as geoengineering via "Solar Radiation Management" or (SRM).


I. Run the EBM and compute climate feedbacks. In the EBM SRM is accomplished by simply lowering the solar constant.

II. Analyze equilibrium GCM runs already done for you, compute their feedbacks, and discuss the results. In the GCM (which is CCSM3 here, so not the same as last week) SRM is accomplished by adding volcanic aerosol to the stratosphere.

I. Run the EBM

cd /home/disk/p/atms380/$LOGNAME/camruns/climsens
matlab &

Run a default case and compute the "radiative forcing" that would come from reducing the solar constant by 1% by

deltaQ99 = -0.01*mean(S.*(1-alb))
Tx1=T;  Finx1=(1-alb).*S; Foutx1=A+B*T; divFx1=divF;  % rename the data for later use

Run a 2XCO2 case by lower A by 2.1

Tx2=T;  Finx2=(1-alb).*S; Foutx2=A+B*T; divFx2=divF;  % rename the data for later use

Return A to 203.3 and run a case with Q/Qo = 0.99

T99=T;  Fin99=(1-alb).*S; Fout99=A+B*T; divF99=divF;  % rename the data for later use
save hw7_ebmresults.mat  % this will make it easy to retrieve the data later if you wish

Compute the feedback parameters for both radiative forcings. We did the 2XCO2 compared to 1XCO2 case in class, but I'll take you through it again.


lambda_LW = (deltaQx2 + deltaFout)/DeltaT
lambda_SW = (-deltaFin)/DeltaT
lambda_total = deltaQx2/DeltaT

Now repeat for the reduced solar constant case. Notice that the deltaQ moves from lambda_LW to lambda_SW because the radiative forcing affects SW rather than LW when we lower the sun


lambda_LW = (deltaFout)/DeltaT
lambda_SW = (deltaQ99-deltaFin)/DeltaT
lambda_total = deltaQ99/DeltaT

Make a table like so for the EBM (to turn in):

                                     Delta T    lambda_LW    lambda_SW   lambda_total

 99% sun   
doubling CO2                          

Turn in: Discuss the extent to which the lambda's are the same. Try to understand the changes in terms of what is happening with the albedo.

Remember all these lambda's have the opposite sign of their actual feedback. The signed feedback, or capital lambda, is just the opposite sign of each variable. You may wish to use make a second table with the capital lambda's to assist in your discussion.

II. Analyze the GCM

Double CO2 Alone
Volcanic Aerosol Forcing Alone
Both Forcings Together. This is only FYI and is not needed for the assignment

First spend some time just looking at whatever interests you in the myriad of figures provided. The sets I find most useful are 1 Tables, 3 Line plots, 4 Vertical contour plots, and 5 Horizontal Contour plots.
The figures and web sites are made automatically using a diagnostics package provided by NCAR. Some of the figures are turned off for various reasons --- just hit back if you find a missing figure.

In the links above, the first item in the list you are offered is "Tables". Select it and then again select "global" "ANN", which brings you to a large table of data. The variables in CAM that correspond to the names that we use in class are as follows:

Fin = FSNT             which stands for Flux Shortwave Net absorbed Top of atmosphere
Fout = FLNT           which stands for Flux Longwave Net outgoing Top of atmosphere
T = TS                    which stands for Surface Temperature

FLNT and FSNT are "all sky" quantities, which means they are averages of cloudy and clear sky areas. FLNTC and FSNTC are just these quantities for the clear part of the grid cell. The difference gives you the cloudy sky part of the grid cell. Note that for doubling CO2, Delta FSNT is 1.079 W/m2 and Delta FSNTC is 2.342 W/m2. This tells you that the clear sky increase in absorbed shortwave radiation is much higher than when clouds are overhead. The clouds also are changing (fewer low clouds/more high clouds). Hence the cloud presence obscures the change in surface ice loss somewhat and moderate the positive ice albedo feedback. The cloud changes themselves also change the albedo from the top of atmosphere and hence muddy FSNT.

Note that deltaQ = 3.7 W/m2 in CAM for doubling CO2 and -3.7 W/m2 for adding volcanic aerosols. Just as for the EBM, the radiative forcing from volcanic aerosol figures into the lambda_SW while the radiative forcing from doubling CO2 figures into lambda_LW. Use the instructions in part I above as a guide. If it helps, verify that lambda_LW+lambda_SW=lambda_total and lambda_total and lambda_LW > 0 and lambda_SW < 0.

Make a table like so for the GCM (to turn in):

                                                Delta T    lambda_LW    lambda_SW   lambda_total

 Volcanic aerosol      
  doubling CO2                            

To turn in:

A) Discuss
the extent to which the lambda's are the same. Can you make sense of why they differ?

B) Set 3 "Line Plots" (zonal mean annual mean plots) are useful for determining where the action is in latitude. Just look at the change from doubling CO2 on the annual mean. Discuss the distribution of Delta TS and how it relates to Delta FLNTC.

C) Where is there an increase in low and medium to high clouds?
Discuss how the change in medium to high clouds corresponds to changes in FLNT by looking at the Line Plots for FLNT and FLNTC.

D) Discuss how the change in clouds corresponds to changes in FSNT by looking at the set 3 Line Plots for FSNT and FSNTC. You don't need to repeat the discussion about clouds obscuring surface ice changes. Instead discuss how the cloud changes themselves affect FSNT.

E) For something different... What is the percent change in global mean annual mean total precipitation (PRECT in the table under set 1) for (i) doubling CO2 and (ii) adding volcanic aerosol forcing? Also include the percent change normalized by the global mean temperature change in (i) and (ii).

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