NRL Monterey, Marine Meteorology Division
|Example of TRMM Precipitation Image||Track Map of Dan|
|On the left is an example of a TRMM precipitation product for Typhoon Dan
approaching China. On the right is the track map for Dan. |
The Tropical Rainfall Measuring Mission (TRMM) is a satellite that orbits the tropical areas of the earth with a mission to derive rainfall estimates for climate studies. However, it has also proved invaluable as a means to observe tropical cyclones in near realtime. Aboard the TRMM is the TRMM Microwave Imager (TMI) that was based on the SSM/I instrument. The key differences on TRMM are the addition of a pair of 10.7 GHz channels with horizontal and vertical polarizations. 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 widely used 85 GHz channels is roughly 6 kilometers. Thus, TRMM images have a "sharper" appearance than SSM/I images.
|The parameter we show is the TRMM "surface rain" paramteter. This based
on a sophisticated Baysian Methodology to construct a vertical profile of
precipitation. The addition of the 10.7 GHz channels make the TRMM
estimates signifantly better than those available using the SSM/I.
Validation is assisted by an active precipitation radar aboard TRMM. It
shows an instaneous rain rate at the surface in mm/hr. |
The estimates are best over water where more the channels of the TRMM TMI can be used to estimate precipitation. Over land, the estimates are of less value, because of the reliance on just the 85 GHz channels.
For more information on passive microwave radiometry, please visit the following web site:
|The TRMM Precipitation estimates give spectacular views of precipitation stucture within tropical cyclones. Quantitative estimates of precipitation are available for rainbands and the eye wall. The accuracy of the values is greater than the comparable retrievals from the SSM/I because the TRMM has a smaller footprint and additional channels at 10.7 GHz. The satellite orbits only in the tropics, such that coverage of tropical cyclones is relatively good, despite the infrequent coverage. The retrievals are continuous over coastlines, so that observation of precipitation patterns over landfalling storms are possible.|
|There are several limitations to the TRMM precipitation retrievals: |
1. The TRMM swath is so narrow that the product is available only infrequently. Often, several days pass without a good pass over a given tropical cyclone.
2. The land precipitation algorithm is sensitive to convective rain only and misses low-level "warm rain". Thus, in landfalling storms important areas of precipitation may be missed over land.
3. The TRMM satellite does not operate out of the tropics, so that storms that move into the midlatitudes, e.g., near Japan or the east coast of the United States will be missed.
4. Drizzle or very light rain rates might be missed entirely.
5. Due to the nature of "precipitation contamination", wind speed retrievals are not possible in regions of rain, so that rain and wind retrievals can not be compared directly.
6. The rain rate is instantaneous--it can not give accumulations at a given spot over an extended period of time.
|1A TRMM rainmap image for Dan in the development stage||1B Corresponding TRMM wind speed image|
|The heaviest rains in green (Fig. 1A) appear over the center of this storm. Embedded areas of red show rates of about 1 mm/hr. Bands containing generally lighter amounts appear around the edges. Infrared data (black and white) are substituted where the rain rate is zero and in regions outside of the TRMM pass (yellow lines).|
|2A TRMM Rainfall Image shows heavy convective bands||2B TRMM Windspeed image is only valid where it's not raining!|
|Two days later than Fig. 1A, Fig 2A shows concentric rain bands in Dan, which is now a typhoon. Note that it provides a "poor man's radar in space" observing precipitation structure that is not observable from infrared images. Note that the image indicates precipitation over northern Taiwan. The rain-free eye can be seen near the edge of the TRMM pass near the exact center of the image. Notice that where it is raining, wind speed is not retrieved. In fact, even in regions of heavy clouds surrounding rain areas, wind speed retrieval is impossible,so infrared imagery is substituted.|
|3A Rain reaches southern coast of Taiwan||3B Wind speed image can not see speeds over most of Dan|
|Fig. 3A shows heavy rain near the center Dan with lighter rains around the edges. A heavy cell is in southern Taiwan.|
|4A Rain reaches Chinese coast||4B Corresponding Wind Speeds|
|Seventeen hours later than the examples shown in Figs. 3A-B, Dan has reached the Chinese coast with precipitation (Fig. 4A). Notice the heavier amounts in the convective storm core and lighter amounts in the periphery. No rain is indicated over the Chinese mainland; however, it is probably raining there based on the heavy cloudiness seen on the infrared data over the coastal areas. However, stratiform ("warm") precipitation which is shown surrounding Dan over the oceans is not detectable over land.|
|5A Scattered Rain over China as Dan dissipates||5B Wind speed sugests light winds over Chinese offshore waters|
|In Fig 5A Dan is dissipating over China. There might be considerable low- level light rain over the coastal areas, based on the visible image. However, the TRMM rain rate product can not detect this warm, non- convective precipitation. Thus, only limited convective precipitation appears over the mainland north-northwest of Taiwan. The wind speed image (Fig 5B) shows mostly light winds offshore, suggesting that damaging winds have mostly diminished over the storm domain.|
Author: Tom Lee
Last Updated: Mon Dec 9 16:54:45 2002
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