For data collected prior to 1 Nov 2001, the infrared total water vapor
product (SDS Water_Vapor_Infrared) should be considered invalid for any
retrieval location where the total precipitable water vapor exceeds 7.0
centimeters (especially obvious at times over the African Sahara). In
addition, the infrared total precipitable water vapor has a wet bias on
the order of 10-20%.
The infrared water vapor algorithms have not been corrected in order to
maintain consistency in this first year of MODIS Collection 3 products.
Spatial and temporal gradients are believed to be representative and useful.
Corrections will be in place for data collected after 1 Nov 2001.
The radiometrical calibrations for some of the MODIS channels,
in particular the channel centered at 1.24 micrometer, are problematic.
Errors in radiometeric calibrations can introduce errors
in water vapor values retrieved from MODIS data causing some of
the water vapor values to be higher than 20 cm, out of range of
typical water vapor values.
Water Vapor results compared well between MODIS near-IR vapor and the DOE ARM microwave
radiometer water vapor over an extended time period (about 1 year).
We also got excellent agreement between MODIS vapor and microwave
radiometer data collected in a recent China field experiment.
Typical errors are 10% or less for water vapor values over land.
Additional validation needs to be done for retrievals over snow and ice-covered surfaces.
DOE ARM has a microwave radiometer in Barrow, Alaska.
We didn't get good agreement between microwave radiometer vapor and MODIS vapor over Barrow.
This microwave radiometer is known to have calibration problems.
As a result, we cannot reach a conclusion based on comparison between
MODIS vapor and microwave radiometer vapor over snow and ice covered surfaces.
By looking our L3 near-IR vapor products, all seasonal variations seem to be
realistic. We cannot see any obvious errors with the near-IR vapor, but the IR vapor from
U. of Wisconsin showed obvious unrealistic large amounts of water vapor over desert areas.
Information provided by Dr. Bo-Cai Gao, Naval Research Laboratory.