Peering at Precipitation (Extras)

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Key Data Resources

Perhaps the best resource on the Web for accessing products from active and passive microwave sensors aboard satellites is the Naval Research Laboratory's (NRL) Tropical Cyclone page. In addition to a whole host of conventional satellite imagery focused on tropical cyclones around the world, you'll find a number of products for qualitatively and quantitatively assessing the precipitation structure of tropical cyclones.

In the discussion of 85-91-GHz imagery, I mentioned that a handful of "twists" on standard 85-91-GHz imagery exist, and you can find them on the NRL site. The standard 85-91-GHz image that you learned about is listed as "85 GHz H" on the NRL page. But, one of the drawbacks of such images is that the brightness temperatures in ocean areas with few clouds (away from hurricanes) are relatively low (the ocean doesn't emit much microwave radiation). For example, focus on the green swath of relatively low brightness temperatures to the west of Hurricane Jimena in the 91-GHz image of Hurricane Jimena that I showed you previously. Note that brightness temperatures in this green swath are similar to those in some areas near the core of the storm, which could get confusing (since the precipitation in each area is likely much different).

To correct this issue, the NRL page has a product that uses something called "polarization-corrected temperatures" (listed as 85 GHz PCT) which effectively eliminates the possible confusion with the ocean or low cloud areas and focuses on precipitation in the layer between roughly five and nine kilometers. For example, check out the 1453Z 91-GHz PCT image of Hurricane Jimena on September 1, 2009 (below). It's superimposed on the 1430Z visible image from GOES-11. Remember that the 91-GHz data on this PCT image are the same as those displayed on the 91-GHz H image (just a different color scheme). The structure of the deep convection within Jimena really stands out with this product.

When tropical cyclones are weaker (tropical depressions or tropical storms), I recommend checking out the "85 GHz Weak" product. In a nutshell, NRL uses a different color scheme to spotlight higher brightness temperatures, which are more consistent with the "relatively modest" convection in tropical storms and tropical depressions (less attenuation by sparser concentrations of precipitation-sized ice particles). As a result, the microwave footprints of tropical storms and tropical depressions are easier to observe on this special imagery.

The NRL site also has images that show quantitative precipitation estimates, but you may also be interested in some of the products available on the GPM site. They include real-time 30-minute, 24-hour, and 7-day rainfall estimates. Many of these products make use of a GPM-based, Multi-satellite Precipitation Analysis (MPA). This technique combines all available passive microwave rain data from GPM and other polar orbiting satellites In a nutshell, MPA is basically a merger of all available space-based estimates adjusted per GPM calibration. In this way, meteorologists try to minimize the weaknesses and capitalize on the strengths of the various IR and microwave estimates that are currently available from space.

Locating the center: 85-91-GHz Imagery vs. 36-37-GHz

One of the important uses of 85-91-GHz and 36-37-GHz imagery is that these microwave images can help forecasters see the core structure of a tropical cyclone even when its masked by high clouds on conventional satellite imagery. Being able to see the "hidden eyes" of tropical cyclones also help forecasters pinpoint the center of a tropical cyclone when it's outside the range of aircraft reconnaissance. But, as I mentioned before, 36-37-GHz imagery is a better choice than 85-91-GHz imagery for locating a tropical cyclone's center. Let's explore the reason more in-depth.

For starters, you'll learn later that that eye-wall thunderstorms tend to lean outward with increasing altitude. To see what I mean, check out this schematic displaying a vertical cross section through a hurricane. In light of this "stadium effect" and the fact that passive microwave sensors sample high altitudes within eye-wall thunderstorms (and outer rain-band storms), it stands to reason that the diameter of the eye on 85-91-GHz images tends to be larger than the diameter at low altitudes. For example, the 89-GHz image of Hurricane Wilma at 1845Z on October 20, 2005 (below), shows the apparently inflated diameter of the eye. At this time, maximum sustained winds were 125 knots, and the central barometric pressure was 915 mb.

If we simply estimate the center of the "circle" that roughly coincides with Wilma's eye, we've located the center of the storm, right? Not so fast. An inherent error associated with the viewing geometry of the satellite exists. Allow me to explain. First, keep in mind that, in the context of 85-91-GHz imagery, the passive microwave sensor samples relatively high altitudes within eye-wall thunderstorms. Now check out this schematic (not drawn to scale), which illustrates the problem that arises from the viewing geometry of the satellite. Focus your attention on a point above the freezing level in an eye-wall thunderstorm. This point lies directly above Point X (on the earth's surface). The passive microwave sensor onboard the satellite detects 89-GHz radiation upwelling from this point. But, given the angled view of the satellite, the source of this radiation, relative to the earth's surface, appears to be located at Point Y. Satellite meteorologists refer to this displacement (the satellite-perceived offset from Point X to Point Y) as parallax error.

Because of the relatively large parallax error, professional meteorologists don't usually look at the eye of a hurricane on 85-91-GHz imagery to estimate the center of circulation. Instead, they utilize 36-37-GHz imagery like the 36-GHz image of Hurricane Wilma below, from the same time as the 89-GHz image above.

Note that the diameter of Wilma's eye on 36-GHz imagery is noticeably smaller than the diameter indicated on the cousin 89-GHz image. That's because 36-GHz radiation detected by the satellite originates at much lower altitudes where the diameter of the eye is typically smaller. Clearly, the smaller, circular eye on 36-GHz imagery reduces the potential error while trying to locate the center of circulation (compared to 89-GHz imagery). More importantly, the parallax error is smaller because the source of the 36-GHz radiation comes from lower altitudes. Thus, 36-37 GHz imagery gives forecasters a more accurate way to determine the center of circulation of a tropical cyclone over remote seas.