Are microwave surface emissivity models good enough to fully
exploit AMSU temperature sounding channels?
Stephen English
Met Office Exeter United Kingdom
Do we need a model of emissivity to use ATOVS over land? If yes, how accurate does it need to be? English (JAM, 1999) calculated
that to extract useful information from ATOVS below 3 km an accuracy of at least 2% is required, and information content increases
rapidly as emissivity errors fall to 0.5%.
Are current models achieving this?
This
This figure
figure shows
shows the
the standard
standard deviation
deviation of
of
the
the dfiference
dfiference between
between the
the NWP
NWP
background
background and
and AMSU
AMSU channel
channel 44 as
as aa
funtion
funtion of
of zenith
zenith angle.
angle.
The
The fit
fit is
is better
better near
near the
the edge
edge of
of the
the scan.
scan.
This
This is
is because
because the
the surface
surface contribution
contribution to
to
the
measured
radiance
is
largest
at
nadir,
the measured radiance is largest at nadir, and
and
errors
errors in
in calculating
calculating surface
surface brightness
brightness
temperature
temperature are
are greater
greater than
than errors
errors in
in
calculating
calculating emission
emission from
from the
the atmosphere.
atmosphere.
This
This figure
figure shows
shows the
the fit
fit of
of AMSU
AMSU channels
channels
3-5
3-5 as
as aa function
function of
of surface
surface to
to space
space
transmittance
transmittance and
and dashed
dashed curves
curves
corresponding
corresponding to
to surface
surface emissivity
emissivity errors
errors of
of
0.5
0.5 (lowest
(lowest curve),
curve), 1.0,
1.0, 1.5,
1.5, 2.0,
2.0, 2.5
2.5 %
% for
for
ocean
ocean points
points only.
only.
The
The mis-fit
mis-fit of
of the
the channel
channel radiances
radiances follow
follow
the
the 1.5%
1.5% curve
curve for
for channels
channels 44 and
and 5,
5, and
and the
the
2.0%
curve
for
channel
3.
For
channel
3
2.0% curve for channel 3. For channel 3
there
there is
is more
more variation
variation with
with scan
scan position.
position.
The
The plot
plot for
for land
land points
points (above)
(above) has
has points
points
following
following closely
closely the
the 2.5%
2.5% emissivity
emissivity curve
curve
(dashed
line)
for
all
channels
when
the
(dashed line) for all channels when the
correct
correct surface
surface to
to space
space transmittance
transmittance for
for the
the
surface
surface elevation
elevation is
is used
used (the
(the set
set of
of points
points to
to
the
left
of
the
line
assume
sea
level
to
space
the left of the line assume sea level to space
transmittance).
transmittance).
For
For land
land points
points the
the points
points for
for channel
channel 55
follow
a
curve
corresponding
to
follow a curve corresponding to aa similar
similar
emissivity
emissivity error
error to
to channels
channels 33 and
and 4.
4.
These simulations use RTTOV-7 (Saunders et al. 2002), with Fastem-2 (Deblonde and English 2000) surface emissivity (ocean) and 0.95 (land).
References: English, S.J., 1999: J. Appl. Meteorol. 38, 1526-1541.; Saunders R.W. et al., 2002: NWP Tech Memo 387; Deblonde and English,
2000: ITSC-XI Proceedings, Budapest, 67-78.
The analysis of the fit of radiances to background
suggests emissivity errors of 1.5% for the ocean
emissivity model.
Land surface emissivity errors using a fixed
emissivity of 0.95 are around 2.5%.
This falls well short of the 0.5% emissivity errors
required to fully exploit ATOVS, and explains why
fit to background is worst near nadir for surface
sensing channels.
However for oceans it is better than the 2%
minimum requirement. For land it is not good
enough.
Given
Given these
these errors
errors in
in surface
surface emissivity
emissivity model
model
which
which channels
channels can
can be
be used?
used?
The
The error
error arising
arising from
from errors
errors in
in surface
surface
emissivity
emissivity can
can be
be compared
compared with
with errors
errors
arising
arising from
from errors
errors in
in the
the background
background
tmperature,
tmperature, expressed
expressed as
as the
the Hessian
Hessian of
of the
the
T
T
cost
cost function,
function, HBH
HBH .. The
The plot
plot above
above shows
shows
emissivity
emissivity errors
errors of
of 0.2%
0.2% (lowest),
(lowest), 0.5%,
0.5%,
1.0%,
1.0%, 1.5%,
1.5%, 2.0%,
2.0%, 5.0%
5.0% as
as dotted
dotted lines
lines and
and
background
background temperature
temperature errors
errors as
as continuous
continuous
lines
lines (for
(for AMSU
AMSU channels
channels 6,
6, 5,
5, 4,
4, 33 from
from left
left
to
to right),
right), as
as aa function
function of
of surface
surface to
to space
space
transmittance.
transmittance.
For
For AMSU
AMSU channel
channel 66 emissivity
emissivity errors
errors in
in
excess
excess of
of 5%
5% are
are tolerable
tolerable for
for most
most of
of the
the
scan.
scan. Therefore
Therefore AMSU
AMSU channel
channel 66 can
can be
be used
used
over
over most
most land
land surfaces
surfaces most
most of
of the
the time,
time,
without
without the
the need
need for
for an
an emissivity
emissivity model.
model.
For
For AMSU
AMSU channel
channel 55 emissivity
emissivity errors
errors of
of 5%
5%
are
tolerable
at
the
edge
of
the
scan,
but
only
are tolerable at the edge of the scan, but only
1%
1% near
near nadir.
nadir. Therefore
Therefore part
part of
of the
the scan
scan
could
be
used
for
AMSU
channel
5
without
could be used for AMSU channel 5 without aa
more
more accurate
accurate model,
model, but
but near
near nadir
nadir the
the use
use
of
of AMSU
AMSU channel
channel 55 is
is sub-optimal,
sub-optimal, even
even over
over
the
oceans.
For
AMSU
channel
4
an
the oceans. For AMSU channel 4 an
emissivity
emissivity error
error of
of 0.2-0.5%
0.2-0.5% is
is required,
required,
depending
depending on
on scan
scan position,
position, which
which is
is beyond
beyond
the
the capability
capability of
of current
current models,
models, even
even over
over
the
the ocean.
ocean. AMSU
AMSU channel
channel 33 has
has negligible
negligible
information
information content
content for
for atmospheric
atmospheric
temperature.
temperature.
Are microwave surface emissivity models good enough to fully exploit AMSU temperature sounding channels? © Crown copyright 2003
Conclusions
• Emissivity models are not
good enough to be able to
exploit the atmospheric
temperature sensitivity of
AMSU channel 4.
• Surface emissivity errors
are around 1.5% for the
ocean and 2.5% for land.
• The outer edges of the
AMSU channel 5 scan
could be used over all
surfaces (if cloud detection
is adequate), whereas near
nadir this channel is
unlikely to be useful even
over oceans.
• Account should be taken
of the variation of RT error
with scan position when
processing AMSU.