NRL Monterey, Marine Meteorology Division
|The first image on the left shows the TRMM orbit which is inclined 35
degrees with respect to the equator. Thus, the TRMM satellite gives
excellent converage in the tropics, and none north of about 38 degrees
North or South. Coverage is shown by the center image above. The image
on the right above diagrams the TRMM in action. Besides the TRMM
Microwave Imager (TMI) which is what we generally refer to as "TRMM", the
satellite also contains a precipitation radar (PR) and and
visible/infrared instsrument (VIRS). The TMI is inclined 52.8 degrees and
makes a conical scan (see right side of diagram). |
TRMM flies at about half the altitude of the SSM/I and therefore has a narrower swath width (759 km) and smaller footprint sizes. The footprint size for the 37 GHz channels is roughly 13 kilometers. Thus, TRMM images have a "sharper" appearance than SSM/I images of 37 GHz (around 34 km footprint). For a variety of reasons discussed below TRMM 37 GHz images often give superior depiptions of tropical cyclones than the higher-resolution 85 GHz channels.
|Images of 37 GHz are valuable for two main reasons: First, this frequency
is sensitive to low-level rain, not large ice particles in convection
aloft. It is insensitive to most of the precipitation-sized ice particles
that appear on 85 GHz images. Thus, 37 GHz can provide images of the
lowest rainbands within a storm. Secondly, 37 GHz images show the spatial
variations in rain magnitude near the center of the storm. Such
variations often do not appear on images of 85 GHz because this
frequency "washes out" over heavy rain clouds. |
For more information on passive microwave radiometry, please visit the following web site:
|The TRMM 37 GHz channels give spectacular views of precipitation stucture
within tropical cyclones. The spatial resolution is high enough (around
13 km) to resolve small-scale features within the storm structure. (Note,
that the SSM/I 37 GHz channels are much more limited because of a much
coarser resolution.) Images of 37 GHz can show low-level rain with little
obscuration by precipitation-sized ice particles aloft. Therefore,
obscuration is not a problem as it is at 85 GHz. |
TRMM 37 GHz images often show storm eyes before they are fully developed on visible and infrared images. In fact, 37 GHz images often depict eyes before they appear on TRMM 85 GHz images. Caution should be exercised, however, because false eye signatures can appear.
A final advantage of the TRMM 37 GHz images is that both the V and H polarizations can be imaged without the need for further processing. There are no difficulties distinguishing between the sea surface and precipitation as with 85 GHz.
|Limitations to TRMM 37 GHz images:|
1. They do not show embedded convection shown by 85 GHz. Also, they do not show other kinds of convective debris aloft. So effects such as shearing are hard to observe. To observe shearing, use 85 GHz images, especially the three- color composite image.
2. They are at a lower spatial resolution than 85 GHz, so that the images are somewhat less sharp.
3. The advantages of 37 GHz images are reduced for SSM/I because of that satellite's much coarser spatial resolution.
4. Precipitation and clouds that appear over oceans often disappear over land on images of 37 GHz. For precipitation over land, examine images of 85 GHz.
|1A 85 GHz H Image of Steve, Western Australia||1B Color composite (85 GHz) Steve, Center is diffuse||1C 37 GHz H, Center well-defined|
|These are three TRMM views of a southern hemisphere storm, Steve, all imaged at the same time. The left two are derived from 85 GHz information. Notice that outer convection is prominent in both Fig. 1A (green/yellow/red) and in Fig. 1B (red). However, the exposed inner core on these two images shows little detail. This is because the cloud physics at 85 GHz produce a "flat" appearance over rain clouds. However, Fig. 1C gives an improved view of the inner rain bands (red). An inner eye wall can be discerned which was absent on Figs. 1A-1B.|
|2A Dennis: 85 GHz Image shows convection aloft well but washes out low-level rain (dark blue) detail||2B 85 GHz Color Composite shows convection aloft as red, little detail near the surface||2C 37 GHz H Image shows improved rainband structure and smaller eye|
|Figs. 2A-B show Hurricane Dennis based on 85 GHz information. Convection containing large ice particles aloft is seen in yellow\green\red in Fig. 2A and in red in Fig. 2B. These images show a large convection-free eye. Additional important information appears in Fig. 2C, an image of 37 GHz H. It shows rain bands that could not be observed in Figs. 2A-b because of obscuration by large ice particles. It also shows a smaller eye than the previous two images. This indicates a low-level eye in the rain clouds that is smaller than the eye in the convection aloft.|
|3A 85 GHz Image shows storm center in "washed out" blue (low-level rain clouds)||3B 85 GHz Color Composite Image shows convection aloft as red||3C 37 GHz H sees "eye" embedded in low-level rain clouds|
|These three images are coincident images of a WESTPAC storm that would soon (five hours later) be named VIRGIL. Figs. 3A-3B show images based on 85 GHz information. Both images show convection away from the center of the storm and no sign of an eye. 37 GHz is mainly sensitive to liquid rain (and not large ice particles aloft), and its image (Fig. 3C) shows the rain band structure near the surface. It sees the eye that can not be sensed at 85 GHz.|
Author: Tom Lee
Last Updated: Mon Dec 9 16:53:49 2002
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