Lessons from Tropical Cyclone Rusty

By: 24hourprof , 4:54 PM GMT on March 02, 2013

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The 0732Z infrared image of Tropical Cyclone Rusty (below; larger image), which comes courtesy of MTSAT-2 (Japan's geostationary satellite), didn't really reveal Rusty's eye as it approached the northwest coast of Australia on February 27, 2013 (track). There were just too many high clouds that covered the core of the storm from space. The corresponding visible image was a bit more helpful because it at least hinted that there probably was an eye very close to the coast at this time.


The 0732Z infrared satellite image of Tropical Cyclone Rusty from MTSAT-2 on February 27, 2013. Larger image. Courtesy of the Naval Research Laboratory.

When high clouds partially or completely obstruct the "view" of the core structure of a tropical cyclone on standard satellite imagery, weather forecasters can turn to passive microwave imagery as one of the tools to help them better identify the position of the storm's center of circulation (especially when there isn't any aircraft reconnaissance to get a fix on the center). What does "passive" mean? In a nutshell, the radiometer mounted on the satellite passively collects electromagnetic radiation emitted by the earth and the atmosphere at various wavelengths and frequencies. In other words, a passive sensor mounted on a satellite does not actively transmit pulses of electromagnetic energy and then wait for a return signal. At any rate, passive sensors that collect microwave energy emitted by the earth and the atmosphere provide data to create satellite images that help weather forecasters to better locate a tropical cyclone's center of circulation when its core is partially or completely shrouded by high clouds.

Of course, the microwave radiation emitted by natural sources such as the earth's surface and the atmosphere is relatively weak. Indeed, the ocean, for example, doesn't emit enough microwave radiation to heat a TV dinner. Otherwise, we all would be "cooked." Though microwave radiation emitted by the ocean, raindrops, ice crystals, etc., is weak, it can still be detected by sensitive radiometers mounted on some low-flying satellites (these satellites orbit at altitudes much lower than geostationary satellites)...several hundred miles versus 22000+ miles).

There are several passive microwave tools that you can access from the tropical Web site of the Naval Research Laboratory. Microwave satellite images at 37 GHz (37 gigahertz) have fairly reliable utility for estimating the low-level center of circulation of tropical cyclones over remote tropical seas. Let's investigate further.

Check out, below, the 37 gigahertz microwave image of Tropical Cyclone Rusty at 0558Z on February 27, 2013 (37 GHz falls into the band of microwave frequencies; larger image). Note that the swath of microwave data (displayed in color) from the TRMM satellite were superimposed on the 0532Z visible satellite image from MTSAT-2. The various colors represent "brightness temperatures," which essentially is the temperature of the "body" emitting microwave radiation at 37 GHz.


The 0558Z image of 37-GHz brightness temperatures from the TRMM satellite on February 27, 2013. Temperatures are color-coded in Kelvins (along the bottom). The 37-GHz brightness temperatures were superimposed on the 0532Z visible satellite image from MTSAT-2. Note the "unobstructed" eye of Tropical Cyclone Rusty. Larger image. Courtesy of the Naval Research Laboratory.

The first feature you'll notice on the image above is the "unobstructed" eye of Tropical Cyclone Rusty. More importantly, note the color-coded temperature scale along the bottom of the image (expressed in Kelvins). I realize that some readers might not be accustomed to working with Kelvins as a temperature scale, but the only piece of information to keep in mind is that the "rusty" (dark-brownish) colors that indicate relatively high temperatures (consistent with radiating bodies in the lower troposphere).

What's the science that explains how 37-GHz imagery (or imagery based on microwave frequencies close to this value) detect low-level features of tropical cyclones? Keep in mind that conventional infrared imagery displays high-altitude storm clouds (tops of thunderstorms, cirrus clouds, etc.), so this question is a pivotal point for me to establish.

For the record, thunderstorms around the eyes of strong tropical cyclones typically "lean" outward with height. To see my point, check out this idealized schematic of the "leaning" thunderstorms around the eye of a hurricane (courtesy of, and copyrighted by, Penn State's online certificate program in weather forecasting). As it turns out, determining the low-level center of circulation using other frequencies of microwave radiation that detect energy emitted by features at higher altitudes (tops of thunderstorms and cirrus clouds) is fraught with more error. In a future blog, I'll discuss the more serious errors associated with using microwave imagery at 85-91 GHz to locate the low-level center of circulation. For the time being, here's the corresponding 85-GHz image of Tropical Cyclone Rusty. Note how the eye appears larger on this image compared to the eye shown on the 37-GHz image. I'll reveal the details of 85-GHz imagery later this upcoming week, so please stay tuned.


An idealized schematic showing how 37-GHz radiation emitted by raindrops below the melting layer reach the low-flying satellite. Larger image. Courtesy of, and copyrighted by, Penn State's online certificate program in weather forecasting.

Okay, to understand the underpinning science of 37-GHz imagery, let's start with how radiometers mounted on low-flying satellites can detect radiation emitted at this frequency by features at low levels. Check out (above; larger image) the idealized schematic that tells the story of 37-GHz imagery. For starters, microwave radiation (at frequencies of 36-37 GHz) upwelling from the ocean surface gets largely absorbed and scattered by raindrops and cloud droplets below the melting level. But raindrops (and cloud droplets) also emit microwave radiation at 36-37 GHz, some of it upward. This upwelling microwave radiation from raindrops is not appreciably attenuated by larger precipitation-sized ice particles above the melting level. Moreover, small ice crystals in cirrus clouds higher up also don't attenuate the upwelling microwave radiation. As a result, the 36-37-GHz radiation reaches the satellite from relatively low, warm altitudes. Thus, the brightness temperatures are relatively high. In other words, the temperatures at which raindrops radiate at a frequency of 36-37 GHz are relatively high (hence the "rusty" colors on the 37-GHz image of Tropical Cyclone Rusty).

The bottom line here is that passive microwave sensors detect a relatively large portion of the upwelling 36-37 GHz from the source of the radiation...raindrops below the melting level. In this way, 37-GHz gives forecasters a better sense of the overall low-level structure of strong tropical cyclones, even when their eyes are obscured by high clouds (on conventional satellite imagery).

Here endeth the lesson.

Lee

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5. 24hourprof
5:12 PM GMT on March 05, 2013
Quoting DelWeather:
Thanks for the lesson. I am interested in learning more about how different satellite data show different things in the atmosphere column. It makes sense that microwaves would see "farther down" into a cloud since ice doesn't absorb (or emit) nearly as much microwave energy as water (the rotational motion of water molecules in solid water being constrained), while infrared light would be absorbed and emitted plenty well from the tops of clouds (the vibrational motion of water molecules in solid water not being thusly constrained). Maybe this is too simplistic?

I have gone back and read some of your earlier posts (since I just started following your blog a month or so ago and I now have some vacation time). I like your work on pet peeves. One of mine is that for SI units derived from the proper name of a person, the letter is capitalized when abbreviated, but NOT capitalized when written out in English. Hence, temperature measured in kelvins is abbreviated K. Likewise the newton (N), the watt (W) and the joule (J). Man, I'm getting old, because this pet peeve is really minor.

As a physicist, I have to take issue with your pet peeve regarding why the sky is blue. You are very right to point out that not ONLY short wavelength visible light is scattered by the particles in the atmosphere (and that we never experience single wavelength light except in the lab, maybe). The old saw that molecules only scatter blue light is, indeed, overly simplistic (and I think not even middle schoolers need that level of simplification). But I look at your spectrophotometer data and I see lots more short wavelength radiation than long wavelength radiation, compared to the spectrum of light emitted by the sun (which we perceive as "white"). Take a look:

Link

The spectrum directly from the sun peaks at around 500 nm, and the shorter wavelength end of the spectrum is only about 30% higher than the longer end of the visible spectrum. After scattering (red data in the link above), the shorter wavelength visible light is attenuated more than the longer wavelength visible light. Indeed, this is just what your spectrophotometer data shows: that shorter wavelength light is now a factor of two to three higher than the longer visible wavelengths. Thus, your data agrees nicely with the graph whose link I posted above.

So why do we perceive the sun's spectrum as white and the scattered spectrum as blue? The difference is the relative amounts of different wavelengths of visible radiation. The sun's unscattered spectrum has a more even distribution of wavelengths and peaks around 500 nm. The scattered spectrum has a peak somewhere around 410 nm and much less longer wavelength light. Hence a difference in our perceptions! More short wavelengths means a perception of the color blue. As you point out, nearly monochromatic light in the 400 nm range (laser light is not truly monochromatic due to things like doppler broadening... another one of my pet peeves!) is perceived by us as blue.

A good model for the scattering of light by very small particles is Rayleigh scattering (a more useful model for a wider range of particle sizes is Mie scattering, but it reduces to the Rayleigh scattering model for small enough particle sizes). The mathematical representation of Rayleigh scattering shows a dependence on the inverse of the fourth power of the wavelength of the light being scattered. Thus, shorter wavelengths are scattered more strongly than long ones. Again, this agrees quite nicely with your data. The Rayleigh scattering model also supports your assertion that ALL wavelengths will be scattered. But the model definitely states that there will be a preferential scattering of shorter wavelengths. And this is why the sun and the sky are perceived by us to have different colors.

To summarize, different spectra result in different perceived colors and the preferential scattering of short wavelength light by molecules results in a different scattered spectrum. Or maybe I have misinterpreted the gist of your pet peeve?


Oh I think we're definitely on the same wavelength!!! A little humor...

I agree with everything you said. I probably should have added more about the peak in the shorter wavelength region.

So, in summary, I agree. My main point is that it's a spectrum of wavelengths, not just one.

Thanks!
Member Since: October 24, 2012 Posts: 90 Comments: 798
4. DelWeather
4:43 PM GMT on March 05, 2013
Thanks for the lesson. I am interested in learning more about how different satellite data show different things in the atmosphere column. It makes sense that microwaves would see "farther down" into a cloud since ice doesn't absorb (or emit) nearly as much microwave energy as water (the rotational motion of water molecules in solid water being constrained), while infrared light would be absorbed and emitted plenty well from the tops of clouds (the vibrational motion of water molecules in solid water not being thusly constrained). Maybe this is too simplistic?

I have gone back and read some of your earlier posts (since I just started following your blog a month or so ago and I now have some vacation time). I like your work on pet peeves. One of mine is that for SI units derived from the proper name of a person, the letter is capitalized when abbreviated, but NOT capitalized when written out in English. Hence, temperature measured in kelvins is abbreviated K. Likewise the newton (N), the watt (W) and the joule (J). Man, I'm getting old, because this pet peeve is really minor.

As a physicist, I have to take issue with your pet peeve regarding why the sky is blue. You are very right to point out that not ONLY short wavelength visible light is scattered by the particles in the atmosphere (and that we never experience single wavelength light except in the lab, maybe). The old saw that molecules only scatter blue light is, indeed, overly simplistic (and I think not even middle schoolers need that level of simplification). But I look at your spectrophotometer data and I see lots more short wavelength radiation than long wavelength radiation, compared to the spectrum of light emitted by the sun (which we perceive as "white"). Take a look:

Link

The spectrum directly from the sun peaks at around 500 nm, and the shorter wavelength end of the spectrum is only about 30% higher than the longer end of the visible spectrum. After scattering (red data in the link above), the shorter wavelength visible light is attenuated more than the longer wavelength visible light. Indeed, this is just what your spectrophotometer data shows: that shorter wavelength light is now a factor of two to three higher than the longer visible wavelengths. Thus, your data agrees nicely with the graph whose link I posted above.

So why do we perceive the sun's spectrum as white and the scattered spectrum as blue? The difference is the relative amounts of different wavelengths of visible radiation. The sun's unscattered spectrum has a more even distribution of wavelengths and peaks around 500 nm. The scattered spectrum has a peak somewhere around 410 nm and much less longer wavelength light. Hence a difference in our perceptions! More short wavelengths means a perception of the color blue. As you point out, nearly monochromatic light in the 400 nm range (laser light is not truly monochromatic due to things like doppler broadening... another one of my pet peeves!) is perceived by us as blue.

A good model for the scattering of light by very small particles is Rayleigh scattering (a more useful model for a wider range of particle sizes is Mie scattering, but it reduces to the Rayleigh scattering model for small enough particle sizes). The mathematical representation of Rayleigh scattering shows a dependence on the inverse of the fourth power of the wavelength of the light being scattered. Thus, shorter wavelengths are scattered more strongly than long ones. Again, this agrees quite nicely with your data. The Rayleigh scattering model also supports your assertion that ALL wavelengths will be scattered. But the model definitely states that there will be a preferential scattering of shorter wavelengths. And this is why the sun and the sky are perceived by us to have different colors.

To summarize, different spectra result in different perceived colors and the preferential scattering of short wavelength light by molecules results in a different scattered spectrum. Or maybe I have misinterpreted the gist of your pet peeve?
Member Since: October 9, 2012 Posts: 0 Comments: 50
3. WunderAlertBot (Admin)
4:04 PM GMT on March 05, 2013
24hourprof has created a new entry.
2. 24hourprof
1:41 PM GMT on March 03, 2013
Quoting vis0:
The lesson has just beginneth/beguneth.
As one day science will learn how to DIRECTLY influence weather (as what i call the ml-d does) using microwaves and later a more not yet understood science i call Galacsics.
Sticking to the known only, when microwaves (passive too) are pointed at storms microwaves are influencing it at sub atom levels, but since its not being recorded as to "compare"  microwaves ascending/descending; angles,strength,rotation as to every storm observed by microwaves, i figure it'll be discovered by accident after a few years when some man or woman reads records and notices a pattern. And we do need microwaving to read weather, so in time science will learn how to use sounds(x3) to ground microwaves, then science will begin to "read" twinning energies and that will lead to Galacsics.
Thus the opening sentence, keep teaching Mr. Lee Grenci we need both book smarts and experience smarts so future generations can continue to open new horizons of the many sciences.


Many thanks.
Member Since: October 24, 2012 Posts: 90 Comments: 798
1. vis0
2:21 AM GMT on March 03, 2013
The lesson has just beginneth/beguneth.
As one day science will learn how to DIRECTLY influence weather (as what i call the ml-d does) using microwaves and later a more not yet understood science i call Galacsics.
Sticking to the known only, when microwaves (passive too) are pointed at storms microwaves are influencing it at sub atom levels, but since its not being recorded as to "compare"  microwaves ascending/descending; angles,strength,rotation as to every storm observed by microwaves, i figure it'll be discovered by accident after a few years when some man or woman reads records and notices a pattern. And we do need microwaving to read weather, so in time science will learn how to use sounds(x3) to ground microwaves, then science will begin to "read" twinning energies and that will lead to Galacsics.
Thus the opening sentence, keep teaching Mr. Lee Grenci we need both book smarts and experience smarts so future generations can continue to open new horizons of the many sciences.
Member Since: December 15, 2006 Posts: 243 Comments: 398

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

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