Yuma Downwelling Infrared Flux Comparison
450
y = 0.976x + 4.2104
R2 = 0.6221
400
350
300
250
200
200
250
300
350
400
450
Measured IR Flux(W/m2)
Figure 3.5 Comparison of the measured and model calculated downwelling longwave
flux.
There is considerable spread in the data resulting in a relatively low R2. During the field
program, daytime temperatures were as high as 31 C with relative humidity as low as 3
percent. Out of 1124 observations, a difference of 50 W/m2 or greater between the
measured value and the corresponding calculated occurred only seventeen times. For four
of the seventeen cases the calculated flux was greater than the measured flux. In every
one of these cases, eight tenths or more of thin cirrus was reported. Two cirrus regimes
invaded this area: mid-latitude cirrus and cirrus associated with a subtropical jet stream.
Cirrus associated with the subtropical jet usually consists of smaller ice crystals. In
cirrus (based on LIDAR and aircraft observations taken during the First ISCCP Regional
Experiments [FIRE] program at Coffeeville, KS). Thin cirrus (meteorologically, thin
cirrus is defined as cirrus that is transparent enough that blue sky can be observed) will
have an emissivity that is less than optically thick cirrus. Paltridge and Platt (1981)
reported cirrus emittance as a function of Ice Water Path (IWP). Emittance values range
from 0.1 for IWPs of approximately 10 g/m2 to 0.9 for IWPs of 60 g/m2. The model
implicitly assumes an emissivity of one for all clouds. The calculated base of the clouds
was on the order of seven kilometers. Using an emissivity (0.5 rather than 1.0) and cloud
base altitude (10 km rather than 7 km) that is more characteristic of thin subtropical jet
stream cirrus, the contribution to the flux at the surface is reduced by approximately 36
W/m2. Six of the observations that exceeded the 50 W/m2 criteria occurred when the
relative humidity was 5 percent or less and the temperature was approximately 30 C. It is
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