22
C.L. Gentemann et al.
and Meissner, 2007). Only a few small regions of difference exist that seem to be
related to the 37 GHz observations of the ocean surface and atmosphere.
Combined surface wind data sets have recently become more available and are
very useful in atmospheric and oceanographic research due to the lack of data
gaps. One example, the Cross Correlated Multi-Platform (CCMP) winds (Atlas
et al., 2009), use carefully inter-calibrated PMW wind speeds from radiometers and
wind vectors from scatterometers. Simple interpolation schemes are unable to adequately represent fast-moving storms in mid-latitude regions when making a merged
wind product with no gaps. An advanced 4-dimensional variational analysis method
is used in the CCMP to merge the satellite winds with the European Center for
Medium-range Weather Forecasting (ECMWF) Re-Analysis (ERA)-40 model wind
vectors, providing a gridded wind product consisting of an analyzed wind field every
6 h for 20 years.
The satellite winds used in the CCMP include over 20 years of SSMI winds.
A recent study showed that these carefully inter-calibrated SSM/I winds have no
spurious trends. Wentz et al. (2007) found agreement between ocean buoy trends
and the SSM/I trends for many buoy types and different ocean regions. The overall
difference in wind trend (SSM/I minus buoy) is –0.02 m/s/decade. This gives one
confidence in using the passive microwave winds in climate studies.
2.5.2 Water Vapor
Over 99% of the atmospheric moisture is in the form of water vapor, and this vapor
is the principal source of the atmospheric energy that drives the development of
weather systems on short time scales and influences the climate on longer time
scales. Tropospheric water vapor measurements are an important component to the
hydrological cycle and global warming (Held and Soden, 2006; Trenberth et al.,
2005). The microwave measurement of water vapor can also be used as a proxy
to detect global warming of the lower troposphere with a signal-to-noise ratio that
is five times better than the AMSU method of measuring the temperature change
(Wentz and Schabel, 2000).
Satellite microwave measurements near the 22.2 GHz vapor absorption line
provide the most accurate means to determine the total amount of vapor in the
atmosphere. Quality controlled radiosondes from stations on small islands or ships
are used for validation of the columnar water vapor retrievals. Simulations show
that retrievals are accurate to 0.1 mm total columnar water vapor. Comparisons of
AMSR-E water vapor retrievals with ship based radiosondes show an error of 2.2–
0.5 mm (Szczodrak et al., 2006) which includes errors due to differences between a
radiosonde point measurement and the larger AMSR-E footprint.
2.5.3 Cloud Liquid Water
Cloud water links the hydrological and radiative components of the climate system. Cloud water can be retrieved from passive microwave measurements because
C.L. Gentemann et al.
and Meissner, 2007). Only a few small regions of difference exist that seem to be
related to the 37 GHz observations of the ocean surface and atmosphere.
Combined surface wind data sets have recently become more available and are
very useful in atmospheric and oceanographic research due to the lack of data
gaps. One example, the Cross Correlated Multi-Platform (CCMP) winds (Atlas
et al., 2009), use carefully inter-calibrated PMW wind speeds from radiometers and
wind vectors from scatterometers. Simple interpolation schemes are unable to adequately represent fast-moving storms in mid-latitude regions when making a merged
wind product with no gaps. An advanced 4-dimensional variational analysis method
is used in the CCMP to merge the satellite winds with the European Center for
Medium-range Weather Forecasting (ECMWF) Re-Analysis (ERA)-40 model wind
vectors, providing a gridded wind product consisting of an analyzed wind field every
6 h for 20 years.
The satellite winds used in the CCMP include over 20 years of SSMI winds.
A recent study showed that these carefully inter-calibrated SSM/I winds have no
spurious trends. Wentz et al. (2007) found agreement between ocean buoy trends
and the SSM/I trends for many buoy types and different ocean regions. The overall
difference in wind trend (SSM/I minus buoy) is –0.02 m/s/decade. This gives one
confidence in using the passive microwave winds in climate studies.
2.5.2 Water Vapor
Over 99% of the atmospheric moisture is in the form of water vapor, and this vapor
is the principal source of the atmospheric energy that drives the development of
weather systems on short time scales and influences the climate on longer time
scales. Tropospheric water vapor measurements are an important component to the
hydrological cycle and global warming (Held and Soden, 2006; Trenberth et al.,
2005). The microwave measurement of water vapor can also be used as a proxy
to detect global warming of the lower troposphere with a signal-to-noise ratio that
is five times better than the AMSU method of measuring the temperature change
(Wentz and Schabel, 2000).
Satellite microwave measurements near the 22.2 GHz vapor absorption line
provide the most accurate means to determine the total amount of vapor in the
atmosphere. Quality controlled radiosondes from stations on small islands or ships
are used for validation of the columnar water vapor retrievals. Simulations show
that retrievals are accurate to 0.1 mm total columnar water vapor. Comparisons of
AMSR-E water vapor retrievals with ship based radiosondes show an error of 2.2–
0.5 mm (Szczodrak et al., 2006) which includes errors due to differences between a
radiosonde point measurement and the larger AMSR-E footprint.
2.5.3 Cloud Liquid Water
Cloud water links the hydrological and radiative components of the climate system. Cloud water can be retrieved from passive microwave measurements because