Validation of Satellitederived Clearsky Atmospheric Temperature Inversions in the Arctic

1
Validation of Satellite-derived Clear-sky
Atmospheric Temperature Inversions in the Arctic
Yinghui Liu1, Jeffrey R. Key2, Axel Schweiger3,
Jennifer Francis4 1Cooperative Institute for
Meteorological Satellite Studies, University of
Wisconsin 2Office of Research and
Applications, NOAA/NESDIS 3Polar Science Center,
University of Washington, Seattle, Washington
4Rutgers University,
NewBrunswick, New Jersey

• Results
• Figure 4 gives the spatial distribution of
  monthly mean 2-channel inversion strength in
  Arctic under clear-sky condition in November,
  December, January, February, March, and winter
  (DJF) averaged over the period 1980-1996. The
  spatial distributions of 2-channel inversion
  strength have similar pattern in Arctic from
  November to March, but with different magnitudes.
  The monthly trend of the 2-channel inversion
  strength over Arctic region under clear
  conditions in November, December, January,
  February, March, and winter (DJF) from 1980-1996
  is shown in Figure 5 based on the monthly mean
  data, and the trend with confidence level larger
  than 90 based on F test is labeled with symbol
  in the figure.

Abstract A 17-year time series of clear-sky
temperature inversion strength in the Arctic is
derived from TOVS data using 2-channel
statistical method, and using TOVS retrieved
temperature profile product respectively from
1980 through 1996. The inter-satellite
calibration problem is alleviated by using one
retrieval equation for each of the 17 years. The
satellite-derived products are compared to the
results based on weather station observations.
The spatial distribution and temporal changes of
monthly mean inversion strength and trends are
also shown.
Fig. 1. Weighting function for bands at 6.7 µm,
7.3 µm, 11 µm, 13.3 µm, and 13.6 µm using the
Subarctic Winter standard atmosphere profile.

• Data
• The radiosonde data in this study is from the
  Historical Arctic Rawinsonde Archive (HARA),
  which comprises over 1.5 million vertical
  soundings of temperature, pressure, humidity and
  wind, representing all available rawinsonde
  ascents from Arctic land stations north of 65 oN
  from the beginning of record through mid 1996.
  The inversion strength, defined as the difference
  between the surface temperature and the maximum
  inversion temperature, is derived from the
  sounding data. The TIROS Operational Vertical
  Sounder (TOVS) onboard the NOAA series of
  satellites provides Brightness Temperature (BT)
  at 7.3 µm, 11 µm and 13.3 µm channels from 1980
  to 1996. In this study, NOAA-6 (1979-1982),
  NOAA-7 (1983-1984), NOAA-9 (1985-1986), NOAA-10
  (1987-1991), and NOAA-12 (1992-1996) data are
  used. The spatial resolution of the TOVS data is
  around 17 km at nadir. The cloud detection tests,
  which are described by Chedin et al. (1985) and
  Francis (1994), are applied to the TOVS data to
  determine clear and cloudy. Rawinsonde soundings
  are matched with TOVS overpasses, based on time
  and spatial distance. The operational TOVS Polar
  Pathfinder (Path-P) dataset provides the
  temperature profiles retrievals at surface and 13
  pressure levels (10, 30, 50, 70, 100, 300, 400,
  500, 600, 700, 850, 900, 1000 hPa) with100 km x
  100 km spatial resolution, based on which
  inversion strength can also be derived.
• 2. Theoretic Basis and Method
• The peaks of the weighting functions for the 7.3,
  11, 13.3 µm channels are approximately 650 hPa,
  the surface, and 850 hPa, respectively, as Figure
  1 shows. The brightness temperature of the window
  channel at 11 µm, BT11, will be most sensitive to
  the temperature of the surface. The 7.3 µm water
  vapor channel brightness temperature, BT7.3, is
  most sensitive to temperatures near 650 hPa. The
  magnitude of the brightness temperature
  difference (BTD) between the 7.3 µm and 11 µm
  channels, BT7.3-BT11, will therefore be
  proportional to the strength of temperature
  difference between the 650 hPa layer and the
  surface, which is related to the inversion
  strength. This should also be true for the
  13.3 µm carbon dioxide channel. The inversion
  strength can be estimated by the linear
  combination of BT7.2, BT11, and the product is
  called 2-channel inversion strength. Good
  inter-satellite calibration is essential for
  climate change studies. In this paper, in situ
  sounding data are used to alleviate uncertainties
  in inter-satellite calibration of the different
  TOVS sensors by deriving a different set of
  retrieval coefficients for each year. The initial
  TOVS clear-sky BTs, determined by TOVS cloud
  detection tests, are converted to inversion
  strength using the retrieval equations, then the
  inversion strength is mapped to the 100 km
  Equal-Area Scalable Earth Grid (EASE-Grid) based
  on longitude and latitude of the original data on
  the daily base. Then monthly mean value is
  calculated based on the daily mean. The monthly
  trend is also calculated based on the 17-year
  monthly mean. Inversion strength can also be
  derived by using the Path-P temperature profiles
  instead of radiosonde observations. This
  inversion strength product is called profile
  inversion strength. Monthly mean and trend are
  also derived based on this product.

3. Validation To validate the monthly mean and
trends of 2-channel and profile inversion
strength, the monthly mean and trends of
inversion strength based on the radiosonde data
from weather stations are used. The twice-daily
temperature profiles from 1980 to 1996 are used
to derive the monthly mean and trends of the
inversion strength for all sky conditions for
some weather stations. Each of these stations has
at least 50 of all possible 0000 and 1200 UTC
soundings in each month. The monthly mean is the
average of all the inversion strength values in
that month, and the monthly trend is derived
using linear least squares regression based on
the monthly mean from 1980 to 1996.   The
inversion strength monthly mean and trends from
weather stations, and both 2-chanel and profile
monthly mean inversion strength and trend from
TOVS data from 1980 through 1996 are compared in
Figure 2 and Figure 3, in which the 2-chanel
retrieval values are shown as stars, and TOVS
retrieved temperature profile based values are
shown as diamonds. Considering the different sky
conditions, the monthly means of 2-channel
inversion strength agree with those from station
data very well in the cold season, with mean bias
around 0.3 K. The monthly means of profile
inversion strength are lower than those from
station data. The mean bias is around 1.5 K, and
the biases are as high as 5.5 for some
individual stations. The trends from 2-channel
and profile methods agree with the trends from
station data reasonably well.
Fig. 4. Monthly mean clear-sky inversion strength
(K) in November, December, January, February,
March, and winter (DJF) over 1980-1996 using
2-channel statistical method.
Fig. 5. Monthly trend of clear-sky inversion
strength (K/Year) in November, December, January,
February, March, and winter (DJF) over 1980-1996
using 2-channel statistical method. The trend
with confidence level larger than 90 based on F
test is labeled with .
Acknowledgments This research was supported by
NOAA and NSF grants OPP-0240827 and OPP-0230317.
The views, opinions, and findings contained in
this report are those of the authors and should
not be construed as an official National Oceanic
and Atmospheric Administration or U.S. Government
position, policy, or decision.
Fig. 3. Comparison of Inversion Strength (INVST)
trends from weather stations and from TOVS data
in November, and December, January, February, and
March over 1980-1996. The star symbols represent
the results from the 2-channel statistical
method. The diamond symbols represent the results
from the TOVS retrieved temperature profiles.
Fig. 2. Comparison of monthly mean Inversion
Strength (INVST) from weather stations, and from
TOVS data in November, and December, January,
February, and March over 1980-1996. The star
symbols represent the results from the 2-channel
statistical method. The diamond symbols represent
the results from the TOVS retrieved temperature
profiles.