The Advanced Microwave Sounding Unit

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While we've already seen how satellite data can help forecasters estimate the intensity of tropical cyclones (via the Dvorak Technique, for example), we're about to see that other remote sensing tools called "microwave sounders" can also provide intensity estimates. But, the functionality of microwave sounders extends far beyond just observing tropical cyclones. These are passive microwave sensors that retrieve atmospheric vertical temperature and moisture profiles throughout the atmosphere, and when it comes to the initialization data that feeds global weather models, the largest fraction actually comes from microwave sounders. So, microwave sounders are an indispensable tool for weather analysis and forecasting.

Besides temperature and moisture profiles, many other applications of data from microwave sounders exist, such as deriving rain rate, sea ice concentration, and snow cover, but our focus here will be on temperature profiles because of their connection to tropical cyclone intensity. Here again, the ability to collect multispectral data is key to making the most out of these instruments, which use multiple channels that are "tuned" to specific atmospheric layers. Having the capability to estimate temperatures in specific layers of the atmosphere is pivotal for getting a handle on the high-altitude warming above the core of a developing tropical cyclone.

For example, check out the cross-section of temperature anomalies in a slice through the center of Hurricane Helene on September 25, 2024, below. The warming in the eye can be correlated to a reasonable estimate for minimum surface pressure (warming decreases mean column density, which results in a decrease in column weight, which, in turn, is closely related to surface pressure). It sounds simple, but deriving these temperatures is actually fairly complicated (more details coming shortly). 

Credit: NASA

The deepest orange and red shadings represent the largest positive temperature anomalies (the warmest air compared to the storm environment), which appear to be in the middle and upper troposphere thanks to compressional warming occurring with sinking air in the eye. This particular cross section captured the maximum warm anomalies aloft in the storm because this slice included Helene's eye. In other parts of the storm, the pattern of temperature anomalies can look quite different. To get an idea of how they can change in different parts of a hurricane, check out the interactive tool below.

In the image on the left, click and drag the white bar to view various cross sections throughout the storm (on the right). Keep in mind that all of these cross sections were created at the same time in an actual hurricane; they simply represent different slices through the storm. As you drag the white bar closer to the eye, the broad warm anomalies (from the release of latent heat in convective clouds) transition to dramatic, focused, warming over the core (in deep red). The magnitude of the compressional warming high above the core and the low central pressure at the ocean surface (and, thus, the powerful surface winds around the periphery of the eye) are connected, and researchers have developed various statistical techniques for estimating minimum central pressure and maximum sustained wind speeds from this connection.

You might have also noticed that in slices closer to the center of the storm, a notable cool anomaly appeared in the lower troposphere on the cross section. This signal actually becomes most prominent as two symmetric cool anomalies on either side of the eye, corresponding to the stormy eye wall. If you look back at the cross-section of Hurricane Helene, you'll notice similar low-level cool anomalies, though they're more subtle. Without mincing words, you should disregard these large cool anomalies because they are phony. Indeed, heavy rain in the eye wall and spiral-band thunderstorms grossly attenuates microwaves from microwave sounders (raindrops scatter and absorb microwaves), causing unrealistically weak upwelling that is accidentally interpreted as a large cool anomaly. So don't believe it! The attenuation of microwaves by heavy rain is one of the limitations of these kinds of remote sensors. I should add that another limitation of microwave sounders is that they're housed aboard polar-orbiting satellites, much like the other passive microwave sensors we've covered. Therefore, they don't offer continuous, universal coverage (i.e. they may miss storms sometimes and hours may go by before a storm is successfully sampled by a satellite pass).

Now that you know about how microwave sounders can give forecasters a look at warm core of a tropical cyclone, let's dig a little deeper.

How does it work?

As I mentioned before, each channel on a microwave sounder is "tuned" to measure brightness temperatures in specific atmospheric layers. Recall that brightness temperature (also known as "equivalent black-body temperature") is the temperature of a hypothetical object that absorbs all radiation that strikes it. Having the capability to estimate brightness temperatures in specific layers of the atmosphere is the key for assessing the high-altitude warming above the core of a tropical cyclone. But, how do these instruments assign brightness temperatures to specific atmospheric layers? We've encountered a similar problem before, when we discussed the complicated methods of assigning altitudes to water vapor targets in order to derive cloud-drift winds. That problem was particularly complex because vertical profiles of water vapor vary in time and space across the globe.

Microwave sounders, however, remotely sense microwave radiation emitted by molecular oxygen. That's a big deal because unlike water vapor, the decrease in the concentrations of molecular oxygen with increasing altitude is roughly the same at any place and at any time. Moreover, the presence of clouds does not meaningfully interfere with microwave emissions from molecular oxygen reaching the satellite. The bottom line here is that we know how oxygen is distributed in the atmosphere. And, this knowledge is the basis for how we can assign specific altitudes to brightness temperatures measured at microwave frequencies with these instruments.

Between roughly 50 GHz and 60 GHz (the microwave band for the channels that create temperature profiles), molecular oxygen absorbs strongly at some frequencies, but not as strongly at other frequencies. For example, let's look at the channels corresponding to 50.3 GHz and 54.9 GHz. Molecular oxygen weakly absorbs microwave radiation at a frequency of 50.3 GHz, so it virtually passes through the atmosphere without much absorption at this frequency (see graph on the left below). As a result, the greatest contribution to upwelling microwave radiation at 50.3 GHz that reaches the satellite comes from the earth's surface (see graph on the right below).

Credit: David Babb @ Penn State is licensed under CC BY-NC 4.0 (opens in a new window)

Meanwhile, at a frequency of 54.9 GHz, molecular oxygen much more strongly absorbs microwave radiation. This means that microwave emissions from the ground at 54.9 GHz do not reach the satellite because this radiation is absorbed by molecular oxygen higher up. Nor do microwave emissions (at 54.9 GHz) from oxygen in the low-to-middle troposphere ever reach the satellite. In the final analysis, microwave emissions from molecular oxygen at approximately 200 mb (about 12 kilometers) provide the greatest contribution to upwelling radiation that reaches the satellite at this frequency.

The primary microwave sounders used for tropical cyclone monitoring are the Advanced Microwave Sounding Unit (AMSU) and the Advanced Technology Microwave Sounder (ATMS), which both have a dozen or more channels dedicated to temperature profiling (and other channels dedicated to water and ice detection), so it's not difficult to imagine that they can generate a temperature profile through virtually the entire atmosphere. If you're interested in knowing the specific level of maximum contribution to upwelling microwave radiation for each ATMS channel (including other channels not used for temperature profiles), check out this graph of weighting functions (opens in a new window). In simplest terms, you can think of a weighting function as the level of maximum contribution to upwelling microwave radiation that reaches the satellite at the given channel's frequency.

In addition to viewing cross sections of tropical cyclones, we can also view data from individual channels to identify temperature anomalies near single pressure altitudes. Historically, the maximum warming over the eye of a hurricane was thought to occur near 200 mb, and it does appear there often on microwave sounder images; however, research suggests that the maximum warm anomaly can meander between the middle and upper troposphere at various times during the storm's life cycle. Therefore, forecasters commonly monitor four channels that allow them to evaluate temperature in the upper half of the troposphere and lower stratosphere, though the specific channel numbers are slightly different on each instrument, as outlined in the table below.

So, these four channels (AMSU channels 5-8 and ATMS channels 6-9), give forecasters a "top down" or plan view of the temperature anomalies at various levels within a tropical cyclone. For example, the image below shows the warm anomalies in a hurricane from AMSU Channels 5 - 8. The warm core really stands out, especially on channels 6 and 7 (350 mb and 200 mb, respectively), marked by yellows, oranges, and reds.

Credit: CIMSS

Ultimately, data from these microwave sounders gets incorporated (along with other satellite data) into statistical models for estimating tropical cyclone intensity. These approaches are imperfect, but on average, they have smaller errors than automated Dvorak-based methods alone. If you're looking for where you can access these satellite-based intensity estimates, as well as AMSU/ATMS data, check out the Explore Further section below after you've tested your basic knowledge from this section in the Quiz Yourself below. Up next, we have one more stop on our tour of remote sensing from satellites -- remote sensing of surface winds from space with satellite-based radars.