Radiation Objective:

1. Work with ERBE data to better understand Earth's radiation budget.
2. Calculate zonal averages of radiation quantities.
3. Calculate shortwave and longwave cloud radiative forcing.

1. Become familiar with Matlab's Mapping Toolbox and the following new commands:  axesm, meshm, plotm.
2. Understand the use of missing values that have been designated as NaN; not-a-number.
3. Exchange rows and columns of a matrix by taking its transpose; A' is the transpose of A.

 

A)  Download the following MAT file to your local directory:  erbe.mat   This can be loaded into Matlab using the 'load' command.  erbe.mat contains 8 variables that can be mapped.

sw -  SHORTWAVE RADIATION
lw -  LONGWAVE RADIATION
net -  NET RADIATION
albedo -  ALBEDO
cssw -  CLEAR-SKY SHORTWAVE RADIATION
cslw -  CLEAR-SKY LONGWAVE RADIATION
csnet -  CLEAR-SKY NET RADIATION
csalbedo -  CLEAR-SKY ALBEDO

(The 'all-sky' variables are monthly averages of both clear and cloudy observations.  The 'clear-sky' variables are averages of the clear-sky observations only.)

Each of these variables is a 70 x 144 matrix, containing 70 latitudes from 87.5 N to 87.5 S and 144 longitudes every 2.5°.  In other words, each variable has a value for each 2.5° x 2.5° box covering the earth (except the poles).  Missing values have been set to NaN, which stands for Not-a-number.  erbe.mat also contains the latitude (mlat) and longitude (mlon) of the center of each grid box.

B)  Download a sample script to map some ERBE data:  map_erbe.m.  Execute this script to create maps of the sw, lw, and net variables.  NOTE that the mouse cursor must be within the frame of the figure for the colors to be correct.  Explain the main features on each of these maps.  For instance, why is the shortwave flux at the top of the atmosphere so large over Africa?

Part II.

Calculate zonal averages of the sw and lw data.  Zonal averages are means of all the longitudes at a given latitude.  Plot the zonal averages of sw and lw as a function of latitude (mlat), both on the same graph.  Turn in this graph.  Explain the shape of these two curves.

[HINT:  sw' is called the transpose of sw, and simply has its rows and columns exchanged.  Therefore, sw' is a 144 x 70 matrix with each column being a given latitude.  Now check out the mean function by typing 'help mean'.]

Part III.

A)  Calculate the shortwave and longwave cloud radiative forcing (CRF) at the top of the atmosphere.  Cloud radiative forcing is defined as "all-sky conditions - clear-sky conditions".

B)  Generate maps of the CRF variables by adapting the map_erbe.m script.  Make sure you overlay the outline of the continents onto your map.  Also, add a color bar to explain the values of the different colors on your maps.  You do NOT have to hand in copies of these maps, since they take a long time to print out.  However, you must hand in a script that will create the CRF variables and generate maps for each variable.

C)  Explain the main features of the shortwave and longwave CRF.  Why do the sub-tropics differ from the tropics in both the shortwave and longwave?  Why is the shortwave CRF over Greenland so different from that over the  North Atlantic Ocean?  How does the shortwave and longwave CRF differ over Africa and India for this month?  Why?

[HINT:  It may be useful for you to plot the clear-sky shortwave and clear-sky longwave, as well as the all-sky conditions to sort out the CRF plots.]