Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 3:37 PM GMT on March 22, 2013
I am dismayed by the way some weather forecasters continue to misinterpret water vapor imagery. Just this week, I threw my bad-call brick at our television when I heard a forecaster point to a color-enhanced region on a water vapor image and describe it as a "greater amount of water vapor associated with a disturbance emerging from the Rockies." Such areas on color-enhanced water vapor images actually indicate high cloud tops. Yes, ladies and gentlemen, high cloud tops frequently contaminate water vapor images, a fact that often gets lost on most satellite analyses.
Rather than rehash the case that was botched on television earlier this week, I decided to look at a different case more suitable for my blog. Check out this standard infrared satellite image at 1215Z on Wednesday, March 20, 2013, which shows the cold tops of thunderstorms and anvil cirrus clouds over the eastern Gulf of Mexico. Now compare this infrared image with the corresponding standard water vapor image below (larger image). Note that the shape of the cold tops of the thunderstorms and anvil cirrus look pretty much the same, so, yes, high cloud tops contaminate water vapor images! Here's the corresponding color-enhanced water vapor image (green and gold colors indicate high, cold cloud tops).
A portion of the 1215Z water vapor image on March 20, 2013. The yellow circle marks an area of high cloud tops associated with thunderstorms and their cirrus anvils. Larger image. Courtesy of the University of Washington.
Wait just a darn minute, Grenci. Aren't clouds composed of water vapor? No, they're not. For the record, a cloud is a collection of visible cloud droplets (water) and/or ice crystals (here's the entry from the NWS glossary). Despite what you hear on television, we're not looking at water vapor in areas where high, cold cloud tops contaminate water vapor imagery.
Moreover, there's not much water vapor at rarefied altitudes near 400 mb. Check out the 12Z model skew-T from the 12-kilometer NAM (below) in the area of the high cloud tops that we observed off the west coast of Florida on March 20 (Introduction to Skew-Ts). I infer that there were cirrus clouds near 400 mb, where the dew point (more accurately, the frost point) is about minus 27 degrees Celsius (which converts to roughly minus 17 degrees Fahrenheit). Such a low frost point indicates that there was a sparse amount of water vapor at this high altitude (the 400-mb height off the west coast of central Florida was roughly 7500 meters = 24,606 feet at 12Z on March 20, 2013).
The NAM skew-T (12-km resolution) from the 12Z run on March 20, 2013, in the area of high cloud tops off the western coast of Florida. The high relative humidity at 400 mb...the temperature (red) and dew-point (green) soundings are close together...is consistent with cirrus clouds (high cloud tops). Introduction to Skew-Ts. Courtesy of NOAA.
The reason there's not much water is straightforward...it would take dramatic cooling (down to minus 17 degrees Fahrenheit) at constant pressure before net condensation would occur. Hopefully, you can now understand why I threw my "bad-call brick" at the television set when I heard the forecaster state that the color-enhanced area on the water vapor image was "a greater amount of water vapor." Hogwash, I say.
So what's the underlying science associated with water vapor images and how do we interpret them. For starters, check out this graphic that plots the absorptivity of some of the constituent atmospheric gases as a function of the wavelength of electromagnetic radiation. Note that, for wavelengths between six and seven micrometers (alternatively, microns), the absorptivity of water vapor is 1 (100%). In other words, water vapor is the dominant absorbing gas, readily absorbing infrared radiation emitted by the earth's surface at wavelengths between six and seven micrometers. The high absorptivity of water vapor at seven microns means that the radiation emitted by water vapor near the earth's surface has two chances of reaching the radiometers mounted on satellites...slim and none. That because this radiation is absorbed by water vapor at higher altitudes. Thus, the greatest intensity of radiation at wavelengths near seven microns that reaches the satellite originates at relatively high altitudes in the troposphere.
As a general rule, most of the radiation emitted by water vapor at seven microns that reaches the satellite originates in the layer between 600 mb and 300 mb (roughly 11000 to 30000 feet). When the air is very dry, however, radiation emitted by water vapor at pressure altitudes as low as 800 mb can reach the satellite (roughly 6000 feet). Moreover, water-vapor content generally decreases with increasing latitude, so the altitude of the water vapor contributing most to the radiation reaching the satellite lowers toward the poles.
The idealized schematic below (larger image) shows four columns of air with various values of relative humidity and clouds. In all cases, I'm assuming that the concentration of water vapor decreases with increasing altitude (the typical vertical variation of water vapor). The bluish shading indicates the qualitative concentration of water vapor...darker blue indicates greater concentrations and light blue represents lower concentrations. Note how the blue shading decreases from dark to light, indicating that the concentration of water vapor decreases with increasing altitude.
An idealized schematic showing four columns of air with various values of mean relative humidity and clouds. The bluish shading indicates qualitative concentrations of water vapor (dark blue indicates greater concentrations; light blue indicates lower concentrations). The red bars indicate the altitudes where the most significant radiation emitted by water vapor at 6-7 microns reaches the satellite. Courtesy of, and copyright by, Penn State's online certificate program.
The first and third columns have a mean relative humidity of about 25%. Note that the red bar, which indicates the middle of the contributing layer of water whose emitted radiation (at wavelengths of 6-7 microns) reaches the satellite, lies lower in the middle troposphere. In other words, the upper-troposphere is dry. The temperature of the water vapor is relatively "warm" in the middle troposphere, so the intensity of radiation reaching the satellite is relatively high, and the water vapor image is colored black. Note that the low cloud in the third column has no bearing on the final result. That's because the radiation emitted by the low cloud top (essentially tiny water drops) at wavelengths between 6 and 7 microns is absorbed by water vapor higher up. Lesson learned: There are two chances of seeing low clouds on water vapor imagery...slim and none.
I'll add here that you cannot, repeat, cannot, infer low-level moisture on water vapor imagery. I hear it all the time on television that a dark area on a water vapor image automatically means that it's dry near the earth's surface. More hogwash.
The second air column has a higher mean relative humidity, say 80%, for sake of argument. Note that the red bar (the middle of the layer the contributing water vapor) lies at a higher, colder altitude. In this case, the water vapor image is colored gray, indicating that the upper troposphere is moist.
Finally, the last column, which has a higher relative humidity and high clouds. Radiation emitted by water vapor below the high cloud is absorbed by the high cloud. So, the radiation from ice crystals at the top of the very cold high cloud, which also emits at 6 to 7 microns, reaches the satellite. In this case, the water vapor is shaded bright white (or shaded in color on enhanced water vapor images). In other words, high cloud tops contaminate water vapor images.
Suggesting that color-enhanced areas on water vapor images represent areas with "greater amounts of water vapor" is nothing more than bad science.
Here endeth the lesson.
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