From the Lee Side

Using Water Vapor Images to Quantify Atmospheric Water Vapor Is Bad Science

By: 24hourprof, 6:27 PM GMT on January 07, 2017

Last evening, while watching a national weather show on cable TV, a meteorologist began his presentation about the threat of heavy precipitation in California with an enhanced water vapor image (see below; Larger image). As I have preached and preached for years, using water vapor images to quantify atmospheric water vapor is just plain old bad science. And, yet, that's exactly what the meteorologist did.

The enhanced water-vapor image from GOES-15 at 02 UTC on January 7, 2017. The two areas circled in white indicate high, cold cloud tops, not water vapor. Larger image. Courtesy of Penn State.

I'll begin today's sermon with the simple observation that a water-vapor image is really nothing more than a glorified infrared satellite image (the wavelengths for water-vapor imagery cluster near 7 micrometers, which falls squarely in the infrared portion of the electromagnetic spectrum). For more tangible proof, focus your attention of the standard infrared satellite image (below) at 02 UTC on January 7, 2017 (Larger image). Note that I circled two areas that encompass high, cold cloud tops. Now compare the two areas on this IR image with the two areas I annotated on the water-vapor image. They're identical!!!! I...dentical! Would anyone here argue that they're looking at two areas of water vapor (instead of cold cloud tops) on this standard infrared satellite image? Man, I hope not.

I think we all agree that we're looking at high, cold cloud tops inside the two black circles on the standard IR image. On the enhanced water-vapor image, the patterns of reds and yellows you see inside the corresponding two circles are obviously identical to the two patterns of bright white on the standard infrared image.

The standard infrared image from GOES-15 at 02 UTC on January 7, 2017. The two areas circled in black clearly indicate the presence high, cold cloud tops. Larger image. Courtesy of Penn State.

Inescapable Conclusion: The patterns of reds and yellows within the two white circles on the enhanced infrared satellite image are high, cold clouds tops and NOT water vapor.

By the way, the temperature of the highest cloud tops (dark red on the color-enhanced water-vapor image) was roughly 205 Kelvins, which converts to approximately minus 68 degrees Celsius. Dew points equal to minus 68 degrees Celsius indicate that very, very, very little of water vapor was present at the rarefied altitudes where the high cloud tops were observed. So pointing to color-enhanced cloud tops on water vapor imagery and pontificating that these features represent a lot of water vapor is completely absurd. And yet, the practice continues by some TV broadcasters.

On Friday evening, the broadcast meteorologist on the national cable show claimed that the plume of reds and yellows arcing northeastward from low latitudes (Area #1) was water vapor. He further stated that these reds and yellows indicated a LOT of water vapor. On the second count, the meteorologist was right but for the wrong reason. Take a look at the reds and yellows inside Area #1 (again these are high, cold cloud tops). Now look at the corresponding precipitable water over southern British Columbia. Good grief!!!! Looks like bad science to me.

The GFS model analysis of precipitable water (PWAT) at 00 UTC on January 7, 2017. Courtesy of Penn State.

In my view, the only way to assess the amount of water vapor in the troposphere is to look at precipitable water. The 00 UTC model analysis of PWAT (above) shows that the plume of reds and yellows did indeed correspond to a lot of water vapor.

Lessons Learned: 1) A water-vapor image is a special kind of infrared satellite image; 2) High, cold cloud tops contaminate water-vapor images; and 3) High, cold cloud tops on water-vapor images do NOT guarantee that there is a lot of water vapor present in the troposphere.

Here endeth the lesson.


Commentary Satellite Meteorology

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

From the Lee Side

About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.