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1. Modtran Tutorial

The MODTRAN model simulates the emission and absorption ofinfrared radiation in the atmosphere. The smooth curves aretheoretical emission spectra of blackbodies at differenttemperatures. The jagged lines are spectra of infrared light at thetop of the atmosphere looking down.

The total energy flux from all IRlight is labeled as Upward IR Heat Flux, in units of Watts /meter 2. The model demonstrates the effect ofwavelength-selective greenhouse gases on Earth's outgoing IR energyflux.MODTRAN results (red) compared with data (solid black) from theNimbus 3 IRIS instrumentfrom Hanel et al., 1972). The spectrum plot shows the intensity of infrared light as afunction of the frequency of the light. The smooth curves areblackbody spectra for different temperatures as indicated in thelegend. You can think of these curves as a scale on a thermometer.The jagged blue curve is the actual model output. Where the air isoptically thick, looking down from space, you see the cold upperatmosphere.

Modtran Tutorial

Where it is optically thin, the IR light comes up fromthe ground, and the light is as intense as that from a warmblackbody.You can modify the greenhouse gases and weather, and seehow it effects the outgoing infrared energy spectrum. The totalinfrared energy flux leaving the planet is labeled Upward IR HeatFlux. The model does not compute global warming, that is, itdoesn't change the Earth temperature in response to changes in theatmosphere. However, you can alter the temperature of the ground andatmosphere yourself to compensate for a change in IR energy fluxThe greenhouse gases CO 2, methane,and tropospheric ozone can be set by entering concentrations aslabeled. Stratospheric ozone andwater vapor are scaled by the number in the box (where a valueof 1 would take the default value with no change).Ground Temperature Offset changes the temperature of the groundand the overlying atmosphere by the amount you specify. The changeimposed in the atmosphere is not very realistic but gives thefirst-order behavior, by simply decreasing as much as the groundtemperature up to an altitude of 10 km.

You can see a atmosphericprofiles of temperature, water vapor, and ozoneby selecting the options underneath the profile plot.If you alter the atmospheric temperature you raise the question of whatto do with the water vapor. The options appear when T offset is not equalto 0 of Holding Fixed Vapor Pressure (absolute number of vapor molecules)or Holding Fixed Relative Humidity (scaling the vapor pressure by thechange in equilibrium vapor pressure as you changed the temperature).There are several standard atmospheric settings, and standardcloud settings. The Altitude (km) refers to the virtual sensor; if youset this to zero you will see the ground with no atmospheric interference,and if you set it Looking Up you will see the sky by IR.You can compare two model runs by running the first, thenselecting Save This Run to Background. The run will be saved on theplot, and you can superimpose a second by changing model options.

Get ridof the backround plot by Deleting it.The default way to plot the spectra are by wavenumber, but you can alsoview it by wavelength on either linear or log axis. The intensity plottedon the y-axis is an energy flow integrated between two values on the x axis.The different ways of tabulating the light frequencies are nonlinear, soit's as if the plot were being squished around horizontally, squeezingtogether in some regions and stretching apart in others as you change theplot type. The intensity, representing the area in that integral, getshigher where the plot is geting squeezed, and lower where it is stretched. Demonstrate the by making a plot of I out as a function of a greenhouse gas concentration, from say 0 to 1000 ppm, with lots of points at low concentrations.Calculate the average temperature that the Earth radiates to space by setting the model I out to epsilon sigma T 4 and solving for T. What altitude is this (assuming the light is coming from the troposphere, the lowermost region in the temperature profile plot)?

How does this altitude change as you change the greenhouse gas concentration?What is the effect of clouds on the upwelling IR to space, and to the downwelling IR seen from the ground?Compare CO 2 vs. Methane as greenhouse gases. Which would be the stronger if they had the same concentrations? Which is stronger given their current concentrations?Simulate the temperature response to greenhouse gas IR forcing by adjusting the ground temperature to maintain a constant I out as you increase the gas concentration.

Demonstrate the water vapor feedback effect by doing this using both options of constant vapor pressure and constant relative humidity.Find the model's climate sensitivity.