TRMM: Tropical Rainfall Measuring Mission
Fig. 1. The TRMM satellite
Image courtesy of Goddard Space Flight Center, NASA
The Tropical Rainfall Measuring Mission (TRMM) satellite is a joint project between the United States (under the leadership of NASA's Goddard Space Flight Center) and Japan (under the leadership of the National Space Development Agency, or NASDA). The first spacecraft designed to monitor rain over the tropics, was successfully launched from Tanegashima, Japan, on November 27, 1997, at 13:27pm Los Angeles (California) time. TRMM travels between ± 35 degrees latitude in a low earth and low inclination orbit.
TRMM is the first mission to measure precipitation quantitatively from space. It includes the first precipitation radar (PR) to be flown in space, along with a 9-channel SSM/I-like passive microwave imager (TMI), an AVHRR-like visible-infrared radiometer (VIRS), a lightning sensor and a cloud sensor. The PR, TMI , and the VIRS are designed to obtain rainfall and other relevant information (e.g. rain type, height of the bright band, cloud type, cloud top height) individually.
TRMM is giving scientists a better understanding of what parts of a hurricane produce rainfall and why, as well as possibly resolve the question of how much latent heat or "fuel" hurricanes of differing strengths release into the atmosphere and whether they affect overall weather circulation.
Precipitation Radar
The Instrument. The PR actively emits radar pulses toward the ground at frequencies of 13.796 and 13.802 GHz, with horizontal polarization, and measures the strength of the backscatter ("echo" or return signal). Precipitation Radar has a horizontal resolution at the ground of about 2.5 miles (four kilometers) and a swath width of 137 miles (220 kilometers). One of its most important features will be its ability to provide vertical profiles of the rain and snow from the surface up to a height of about 12 miles (20 kilometers). The Precipitation Radar will be able to detect fairly light rain rates down to about .027 inches (0.7 millimeters) per hour. At intense rain rates, where the attenuation effects can be strong, new methods of data processing have been developed that help correct for this effect. The Precipitation Radar is able to separate out rain echoes for vertical sample sizes of about 820 feet (250 meters) when looking straight down. It will carry out all these measurements while using only 224 watts of electric powerÑthe power of just a few household light bulbs. The Precipitation Radar was built by the National Space Development Agency (NASDA) of Japan.
The PR measures the echo backscattered from rain: because the strength of the echo is roughly proportional to the square of the volume of falling water, the PR produces very accurate estimates of rain profiles. The PR will determine the vertical distribution of precipitation by measuring the "radar reflectivity" of the cloud systems and the weakening of a signal as it passes through the precipitation . Thus, it will measure the 3-D rainfall distribution over both land and ocean. More specifically, this instrument will define the layer depth of the precipitation and provide information about the rainfall reaching the surface, the key to determining the latent heat input to the atmosphere.
Summary of Features of the PR sensor:
1) uses radar frequencies of 13.796 and 13.802 GHz, with horizontal polarization
2) horizontal resolution = 4.3 km at nadir
3) obtains data in 220 km swaths
4) can perceive rain through clouds
5) makes quantitative measurements of rain (mm/h) over land and ocean with a sensitivity better than 0.5 mm/h
6) measures rain from the ground to an altitude of 15 km, with a vertical ("range") resolution of 250 m. (Range resolution is the ability of the radar equipment to separate two reflecting objects on a similar bearing, but at different ranges from the antenna. The ability is determined primarily by the pulse length in use)
7) provides 3-dimensional rainfall distribution
Fig. 2. Example of horizontal and vertical cross sections of rainfall rate
measured
by TRMM PR for Typhoon #8 on 2 August 2000.
Image courtesy of NASDA
Data Products Available for the PR. Algorithms have been developed for PR data to provide estimates of rainfall rate and vertical rainfall profiles. Below is a brief description of the level 2 data product produced by these algorithms.
TRMM 2A25, precipitation radar (PR) profile, produces an estimate of the vertical rainfall rate profile for each radar beam. The rainfall rate estimate is given at each resolution cell of the PR radar. To compare with ground-based radar data, the attenuation corrected Z factor profile is also given. The average rainfall rate between the two pre-defined altitudes is calculated for each beam position. Other output data include parameters of Z-R relationships, integrated rain rate of each beam, range bin numbers of rain layer boundaries, and many intermediate parameters.
The objective of 2A25 is to correct for the rain attenuation in measured radar reflectivity and to estimate the instantaneous three-dimensional distribution of rain from the TRMM Precipitation Radar (PR) data. The estimates of attenuation-corrected radar reflectivity factor and rainfall rate are given at each resolution cell of the PR. The estimated near-surface rainfall rate and average rainfall rate between the two predefined altitudes (2 and 4 km) are also calculated for each beam position.
Fig. 3. Horizontal cross section, along track (A-B) and cross track (C-D)
vertical cross sections of rainfall rate in Cyclone Pam from TRMM PR.
Image courtesy of Goddard Space Flight Center, NASA.
TMI
The Instrument. The TRMM Microwave Imager (TMI) is a passive multi-channel radiometer whose signals in combination can measure rainfall quite accurately over oceans and somewhat less accurately over the land. TMI is not a new instrument. It is based on the design of the highly successful Special Sensor Microwave/Imager (SSM/I) which has been flying continuously on Defense Meteorological Satellites since 1987. The TMI measures the intensity of radiation at five separate frequencies: 10.7, 19.4, 21.3, 37, 85.5 GHz. These frequencies are similar to those of the SSM/I, except that TMI has the additional 10.7 GHz channel designed to provide a more-linear response for the high rainfall rates common in tropical rainfall. The other main improvement that is expected from TMI is due to the improved ground resolution which will result from the lower altitude of TRMM 218 miles (350 kilometers) compared to 537 miles (860 kilometers) of SSM/I). TMI has a 487 mile (780-kilometer) wide swath on the surface. The higher resolution of TMI on TRMM, as well as the additional 10.7 GHz frequency, will make TMI a better instrument than its predecessors.
Summary of Features of the TMI instrument:
1) uses a scan angle of 65 degrees
2) collects data over a swath width of 760 km.
3) can perceive rain through clouds
4) makes quantitative measurement of rain intensity (mm/h) as integrated column precipitation content and areal distribution
| Bands |
Frequency (GHz)
|
polarization |
| 1 | 10.65 | dual |
| 2 | 19.35 | dual |
| 3 | 22.235 | single |
| 4 |
37.0
|
dual |
| 5 | 85.5 | dual |
How the TMI measures Rainfall with Microwaves
The TMI measures the microwave radiation emitted by Earth's surface and by cloud and rain drops. Calculating rainfall rates from TMI requires some fairly complicated calculations. The basis of these calculations is in Planck's radiation law, which describes how much energy a body radiates given its temperature. Water surfaces such as oceans and lakes have an additional property which is very important. The surfaces emit only about one half the microwave energy specified by PlanckÕs law and therefore appear to have only about half the real temperature of the surface. Water surfaces therefore look very "cold" to a passive microwave radiometer.
Raindrops on the other hand, appear to have a temperature that equal their real temperature. They appear warm to a passive microwave radiometer and therefore offer a contrast against "cold" water surfaces. The more raindrops, the warmer the whole scene appears, and research over the last three decades now make it possible to obtain fairly accurate rainfall rates based on the temperature of the microwave scene.
Land is very different from oceans in terms of the emitted microwave radiation, appearing to have about 90 percent of its real temperature. In this case, there is little contrast to observe the "warm" raindrops. Certain properties of rainfall, however, still can be inferred. The high frequency microwaves (85.5 GHz) measured by TMI are strongly scattered by ice present in many raining clouds. This reduces the microwave signal at the satellite and offers a contrast against the warm land background. Because large ice particles (often present in upper cloud regions) tend to scatter this emitted radiation, the TMI uses its various channels along with cloud models to discriminate between these processes and quantify the rain and ice responsible for the observed microwave signatures.
Data Products Available for the TMI.
The TRMM 2A12 hydrometeor profile provides the level 2 data product for the TMI. The profiling techniques of the algorithm use the Goddard Cumulus Ensemble Model, and generate vertical hydrometeror profiles on a pixel by pixel basis. For each pixel, cloud liquid water, precipitation water, cloud ice water, precipitation ice, and latent heating are given at 14 vertical layers based upon the nine channels of the TRMM microwave imager (TMI). The top of each layer is given at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 8.0, 10.0, 14.0, and 18.0 km above the surface. The surface rainfall and the associated confidence indicator is also calculated. Each data granule is one orbit plus 50 scan lines of pre-orbit overlap and 50 scan lines of post-orbit overlap. Each data granule consists of two parts: metadata and swath data.
Fig. 4. Rainfall Rate obtained from TMI (TRMM 2A12) from Typhoon Paka, 16-December-1997.
Visible Infrared Scanner (VIRS)
The VIRS measures radiance in five bandwidths from the visible through the infrared spectral regions. T he VIRS is a 5 channel cross-track scanning radiometer operating at 0.63, 1.6, 3.75, 10.80, and 12.0 microns. It is similar to AVHRR.
The VIRS is intended to provide very high resolution information on cloud coverage, type, and cloud top temperatures and also serve as the link between these data and the long and virtually continuous coverage by the geosynchronous meteorological satellites. Scientists will use the infrared (IR) data to make rough estimates of tropical precipitation. The instrument, with a swath width of 720 km, will eventually provide cloud distributions by type and height and rain estimates from brightness temperatures at a horizontal resolution of 2.1 km (nadir).
At this point in time, there are no level 2 data products available for the VIRS. A level 1 data product, 1B01: Visible and Infrared Radiance is available which produces calibrated radiances for each of the five wavelengths measured by the instrument.