When you've finished this section, you should be able to interpret the positive and negative temperature anomalies on cross-sections created by the Advanced Microwave Sounding Unit (AMSU), as well as images created by a single channel. You should also know what pressure levels correspond to Channels 5 - 8.
Our tour of remote sensing instruments aboard satellites will now focus on the Advanced Microwave Sounding Unit (AMSU), which is a sophisticated instrument carried on some satellites in NOAA's fleet that can be used to remotely estimate the strength of tropical cyclones. These estimates can be particularly helpful when storms swirl outside of the range of reconnaissance aircraft, as Hurricane Isabel did at 11 A.M. on September 8, 2003 (Isabel was located over the eastern Atlantic at the time). Even without aircraft reconnaissance, however, forecasters were able to use data from the AMSU (along with some of the other remote sensing techniques we've discussed) to help gauge Isabel's intensity, as the 11 A.M. discussion indicates:
ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM HURRICANE ISABEL DISCUSSION NUMBER 10 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 11 AM EDT MON SEP 08 2003 ISABEL HAS CONTINUED TO RAPIDLY INTENSIFY. THE INITIAL INTENSITY IS INCREASED TO 100 KT BASED ON SATELLITE INTENSITY ESTIMATES OF 115 KT FROM TAFB AND AFWA...102 KT FROM SAB...AND 102 KT/T5.5 3-HOUR OBJECTIVE DVORAK INTENSITY ESTIMATES. THE 100 KT INITIAL INTENSITY IS ALSO CONSISTENT WITH THE LATEST AMSU INTENSITY ESTIMATES OF 100 KT AND 960 MB.
Each AMSU unit consists of passive radiometers that sense microwave radiation emitted from the earth and atmosphere, and has two components -- AMSU-A and AMSU-B. The AMSU-B unit is primarily dedicated to detecting humidity profiles and liquid-water and ice profiles within atmospheric columns. I won't get into any more details about AMSU-B here, but you can feel free to study this overview of AMSU-B if you wish. Instead, our focus here is going to be the AMSU-A unit, which is primarily devoted to determining vertical profiles of temperature in the atmosphere.
The AMSU-A unit consists of two independent instruments (AMSU-A1 and AMSU-A2). As a whole, the AMSU-A unit detects microwave emissions at 15 different microwave wavelengths (frequencies). The AMSU-A1 module uses two antenna-radiometer systems to provide 12 channels in the 50- to 60-GHz band (0.50 cm to 0.60 cm in wavelength) for retrieving the atmospheric temperature profile from the Earth's surface to about 42 kilometers (or 2 mb, which lies near the "top" of the atmosphere). The other AMSU-A1 channel and the two AMSU-A2 channels provide forecasters with rain rate, sea ice concentration, and snow cover, but our focus here is on temperature profiles. By and large, each of the 12 AMSU-A1 channels 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, which is the primary use of AMSU-A data.
For example, check out the cross section of AMSU-derived temperature anomalies (below) through Hurricane Floyd at 2332Z on September 11, 1999. The anomalies were calculated by comparing AMSU-derived temperatures inside the storm with those outside the storm (the "storm environment"). 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).
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. Meanwhile, note the large cool anomalies that appear in the lower troposphere on the cross section through Hurricane Floyd. The two symmetric anomalies on either side of Floyd's eye correspond to the stormy eye wall and the other anomaly (to the "left" of the eye) likely coincides with a thunderstorm in a spiral band coiling inward toward the eye. 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 the AMSU instrument (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.
We can see how the vertical structure of temperature anomalies changes within a storm by investigating these interactive cross sections of Hurricane Erin at 1739Z on September 10, 2001. In the image on the left, click and drag the blue line 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; they simply represent different slices through the storm. As you drag the blue line closer to Erin's eye, note the dramatic warming over Erin's core (in deep red). Clearly, there is a connection between the magnitude of the compressional warming high above the core of Erin and the low central pressure at the ocean surface (and, thus, the powerful surface winds around the periphery of the eye).
In addition to viewing cross sections of tropical cyclones, we can also view data from individual AMSU-A1 channels to identify temperature anomalies near single pressure altitudes. Forecasters commonly monitor four specific channels that allow them to evaluate temperature in the upper half of the troposphere and lower stratosphere
- Channel 8 (55.5 GHz) approximately 100mb (about 15 kilometers)
- Channel 7 (54.9 GHz) approximately 200mb (about 12 kilometers)
- Channel 6 (54.5 GHz) approximately 350mb (about 10 kilometers)
- Channel 5 (53.6 GHz) approximately 550mb (about 5 kilometers)
For example, the image below shows the Channel 5 - 8 images from Hurricane Fabian from 02Z on September 5, 2003. The warm core of Fabian really stands out, especially on channels 6 and 7 (350 mb and 200 mb, respectively), marked by yellows, oranges, and reds.
Historically, the maximum warming over the eye of a hurricane was thought to occur near 200 mb, and it does appear there often on AMSU-A1 images. Therefore, channel 7 is closely monitored. However, more recent research suggests that instruments like the AMSU have insufficient vertical resolution to truly pinpoint the exact altitude of the maximum warm anomaly. In fact, the maximum warm anomaly may meander between the middle and upper troposphere at various times during the storm's life cycle and be located more frequently toward the middle troposphere. So, monitoring channels 5-8 is prudent to keep an eye on the entire upper half of the troposphere (and lower stratosphere).
Now that you've seen what kinds of data we can get from the AMSU-A1 unit, and how to interpret it, we'll get into how it works a bit more. By the way, if you're interested in finding out where you can access AMSU images like the ones shown on this page for current and past storms, check out the AMSU page at the Cooperative Institute for Meteorological Satellite Studies (CIMSS).
How does it work?
As I mentioned before, each of the 12 AMSU-A1 channels are "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 developing tropical cyclone. But, how does the AMSU-A1 unit 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.
The AMSU-A1 unit, however, remotely senses 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 the AMSU-A1 unit.
Between 50 GHz and 60 GHz (the microwave band that the key AMSU-A1 channels cover), molecular oxygen absorbs strongly at some frequencies, but not as strongly at other frequencies. For example, let's look at channels 3 and 7. Molecular oxygen weakly absorbs microwave radiation at a frequency of 50.3 GHz (channel 3), and virtually passes through the atmosphere without much absorption (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).
Meanwhile, at a frequency of 54.9 GHz (channel 7), 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.
Given the 12 channels on the AMSU-A1 unit, it's not difficult to imagine that it 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 AMSU-A1 channel, check out this graph of weighting functions. 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.
Now that we've covered the AMSU and its ability to detect vertical temperature profiles, we have one more stop on our tour of remote sensing from satellites. Up next, we'll be looking at the remote sensing of surface winds from space using scatterometry. Read on.