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The comparison between Moderate Resolution Imaging Spectrometer (MODIS) Total Precipitable Water (TPW) and
Global Positioning System (GPS) TPW showed that the standard deviation for differences between these two data sets
was about 3.3 and 5.2 mm for near-infrared (nIR) and infrared (IR) TPW, respectively. The comparison also showed
that there were biases for both retrieved nIR and IR TPW data. The MODIS nIR values were slightly underestimated in
a dry atmosphere and overestimated in a moist atmosphere, and the overestimation increased as the column water vapor
content increased. This makes it possible to correct the bias associated with these data. The bias correction and trend
removal of MODIS nIR TPW reduced the standard deviation of differences from 3.3 mm to about 2 mm. A similar
trend of differences between MODIS TPW and radiosonde TPW was also obtained, and a dry bias was found in the
radisonde measurements.
Two severe weather simulations, a severe thunderstorm (2004) over land and Hurricane Isidore (2002) over ocean, were
used to assess the impact of assimilating MODIS nIR TPW data on severe weather simulations. The assimilation of
conventional observations alone had a slightly positive impact on both weather simulations. The addition of assimilating
original or bias-corrected MODIS TPW had no impact on simulated rainfall for the thunderstorm over the southern US.
However, for Hurricane Isidore, MODIS nIR TPW with or without bias correction started influencing the simulated
storm intensity positively after a one-day integration. There was almost no impact for the first day of simulation because
almost no MODIS data were available due to cloudiness over the storm region and its vicinity.
While this work is still too preliminary to draw conclusions on the impact of MODIS TPW on forecast improvement, it
shows the type of results that may be expected. When assimilating MODIS TPW, severe weather simulations were
improved over ocean but not over land since the quality of global analysis over land is usually better than over ocean. When over ocean, the assimilation of MODIS data can have a positive impact during the early simulation period if
cloud-free data are available over the region of interest, while the impact can be delayed to a later simulation period if
data are available only away from the region.
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