Model Input and Output

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using MODTRAN (MODerate resolution TRANsmission) and tied to the visibility and

the cause of the restricted visibility, parameters reported on standard meteorological

observations. MODTRAN is the Department of Defense's standard atmospheric

IR/VIS/UV radiance and transmission band model for lower altitudes.

3.4 Upwelling Longwave Radiation

The upwelling radiance can be specified in terms of the surface temperature,

downwelling radiance, and the surface emissivity ( ε g ). If the surface temperature is not

available it may be necessary to assume the surface temperature is approximately equal to

the ambient temperature. In addition, the surface emissivity is parameterized in terms of

the land cover. The values of the land cover emissivities can be obtained from the

literature. Assuming land cover (or land use) category is available from GIS information.

The upwelling radiance is given as

Iir↑ = ε gσ Tg4 + (1 - ε g ) Iir↓ .

(3.15)

3.5 Model Input and Output

The model inputs are

1. The latitude of the location.

2. The Day-Of-the-Year (DOY) and hour of the day for the model calculations.

3. The land surface category. The land surface categories currently modeled are grass,

all USCS soil types, and snow. It is assumed that the land category is input by the

user.

4. Ambient (air) temperature. The model will not run unless this parameter is provided.

5. Relative Humidity. If this is missing the model assumes a value of 60%.

6. Surface Temperature. If the surface temperature is missing, it is set equal to the air

temperature. For daily average conditions this assumption is not that bad since at

times during the day the surface temperature is greater than the air temperature while

at other times it is less than the air temperature.

7. Low, middle, and high fractional cloud amount. If all three cloud amounts are

missing, the low cloud amount is set equal to the global average cloud amount of 0.5.

8. Low, middle, and high cloud type. If the cloud type information is missing and the

cloud amount is not zero, a cloud type is assigned based on climatological averages.

9. Low, middle, and high cloud height. If the cloud base altitude is missing and the

cloud amount is not zero, a cloud-base altitude is calculated based on the cloud type,

location (latitude), DOY, and climatological information.

The model outputs are the downwelling, upwelling, and net longwave radiation (W/m2) at

the surface for the input conditions. If the input conditions are representative of some

time period, then the number of Joules/m2 for that period is given as the flux (W/m2)

times the length of the period in seconds.