The Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI) was a passive microwave sensor that provided data related to quantitative rainfall. By carefully measuring the minute amounts of microwave energy emitted by Earth and its atmosphere, TMI quantified atmospheric water vapor, cloud water, and rainfall intensity.
In November 1997, the first microwave radiometer capable of accurately measuring sea surface temperatures (SST) through clouds was launched on the TRMM spacecraft. TMI provided an unprecedented view of the oceans. Its lowest frequency channel (10.7 GHz) penetrates non-raining clouds with little attenuation, giving a clear view of the sea surface under all weather conditions except rain. Furthermore at this frequency, atmospheric aerosols have no effect, making it possible to produce a very reliable SST time series for climate studies.
The one disadvantage of the microwave SST is spatial resolution. The radiation wavelength at 10.7 GHz is about 3 cm, and at these long wavelengths the spatial resolution on the Earth surface for a single TMI observation is about 50 km. Also, the TRMM orbit was selected for continuous monitoring of the tropics. To achieve this, a low inclination angle was chosen, confining the TRMM observations between 40°S and 40°N. Previous microwave radiometers were either too poorly calibrated or operated at too high of a frequency to provide a reliable estimate of SST. The early results for the TMI SST retrievals are quite impressive and are already leading to improved analyses in a number of important scientific areas, including tropical instability waves (TIWs) and tropical storms [Wentz et al., 1999]
TMI is a 5-channel, dual-polarized, passive microwave radiometer. Microwave radiation is emitted by the Earth's surface and by water droplets within clouds. TMI is used to measure several important meteorological parameters over sea surfaces. The TMI, a successor to the SSM/I, measures radiation at frequencies of 10.7, 19.4, 21.3, 37, 85.5 GHz. It orbits at an altitude of 218 miles, much lower than the SSM/I, thus providing better resolution. From the 5 channels of data provided by TMI, scientists calculate several parameters over ocean surfaces. These are:
- sea surface temperature
- 11 GHz 10m wind speed
- 37 GHz 10m wind speed
- columnar water vapor (atmospheric water vapor, integrated water vapor)
- columnar cloud water (cloud liquid water, liquid cloud water)
- 19-37 GHz rain rate (precipitation rate)