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Atmospheric data assimilation is a process that
incorporates observational data into numerical atmospheric models with
consideration of both observation and model errors. Conventional global
assimilated data sets currently contain significant errors in primary
hydrological fields, such as precipitation and evaporation, especially in the
tropics. Part of these errors is related to relatively coarse precipitation
observations. The TRMM-derived rainfall and total precipitable water (TPW)
estimates may be used to constrain these fields in assimilation systems, and
make improvements on assimilated data sets.
Atmospheric scientists at Goddard Space Flight Center NASA have successfully
developed analysis techniques to bring TRMM rainfall observations into their
global numerical model, called the Terra Goddard Earth Observation System (GEOS)
data assimilation system (DAS). By assimilating the 6-hour averaged
TMI surface rain and other TPW data into the Terra GEOS-DAS, they found that
not only the primary hydrological fields, but also key climate parameters, such
as clouds and radiation, have been improved significantly.
The following figures illustrate the good job that Terra GEOS-DAS did for
super-Typhoon Paka (December 10, 1997) assimilation. The top one is the observed Paka
imagery, where red color denotes the thick clouds around the typhoon center.
The bottom panel shows the assimilated surface wind and pressure field
resulting from assimilating TMI and TPW into the GEOS-DAS system. Compared
with the middle panel, which was produced by the same GEOS-DAS system but
without using TMI and TPW data, the bottom one clearly indicates
an intensive low surface pressure and strong wind convergence at
Paka's position.
![[rainfall
assimilation]](/precipitation/TDST_SCI/Paka_assim.gif)
Courtesy TRMM Data Assimilation, NASA
TMI is a
multi-channel dual polarized, conically scanning passive microwave radiometer.
The frequencies for dual polarization are at 10.65, 19.35, 37, and 85.5 GHZ,
and at 21 GHZ for the vertical polarization. TMI is designed to provide
quantitative rainfall information over a wide swath. By carefully measuring
the minute amounts of microwave energy emitted by the Earth and its atmosphere,
TMI is able to quantify the water vapor, the cloud water, and the rainfall
intensity in the atmosphere.
TMI measurements also have been used to derive sea surface temperature
(SST). Scientists have emphasized the great role of TMI SST data in the
monitoring and the forecast of tropical weather and climate, because the
microwave radiation penetrates clouds with little loss of signal, and thereby
provides an uninterrupted view of the ocean surface. Currently GDAAC does not
archive the TMI sea surface temperature data, but users can find TMI SST data
and information through Remote Sensing System
.
For the current released data there are five TRMM standard products derived
from TMI, namely 1B-11, 2A12, 2B31, 3A11, and 3B31. The beginning number denotes
the data processing level: level one data are radiances, level two is
instantaneous geophysical parameters, and level three is integrated
grid-averaged data. 2A-12 data provides surface rain and hydrometeors
profiles, and 2B31 combine TMI profiles with that of precipitation radar.
There two data sets have been described in the "Cloud and Precipitation
Formation" topic above. 3A-11 data are the TMI monthly mean product, consisting
of surface rain rate, rain frequency, and freezing height. 3B31 algorithm
uses the combined rainfall structure (2B31) to calibrate TMI rainfall structure
(2A12) on a monthly basis. The outputs consist of monthly mean surface rainfall and hydrometeors profiles at 14 layers, see "Cloud and Precipitation
Formation" topic above.
Monthly mean surface rainfall for 1999
. Data: TRMM 3B31, TMI and PR combined. Data are at a 5o
longitude x 5o latitude resolution. These figures are plotted by
using GrADS, and the contour levels
are the same as that in the figures of TRMM 3B43 for ENSO study topic above.
The precipitation patterns are comparable, but the differences in the center values are perceptible. Note that TRMM 3B43 has a high 1o x 1o spatial resolution.
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Atmos Composition
