Lessons from Tropical Cyclone Rusty
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.