CLIMATOLOGY OF K DEDUCED FROM THE SATELLITE WIND SPEEDS
J. Boutin and J. Etcheto
February 17th , 1995
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We use the satellite data from the first and second SSM/I (Special Sensor
Microwave Imager) which were launched on June 19 1987 and December 1 1990 on
the U.S. Air Force Defense Meteorological Satellite Program (DMSP) spacecraft
F08 and F10 respectively. A complete description of the SSM/I instrument can
be found in (Hollinger, 1989)
. Its swath is 1400km wide. At latitudes larger than 40degres and at
the equator, the coverage is nearly complete after one day; over the global
ocean, it is nearly complete after 4 days (Minster et al., 1992) .
We use the wind speed derived by Wentz (1992) .The wind speed, originally
retrieved at 19.5m height, U19.5, is converted into a 10m height wind speed,
U, using a neutral atmospheric profile and a drag coefficient equal to 1.5
10-3: U=0.939xU19.5. The resolution of the retrieved wind speed is 25km.
Following the Wentz recommendations (Wentz, 1989) , we discard U19.5 less
than -4m s-1 and we consider the U19.5 values between 0 and -4m s-1 to be 0m
s-1. We also discard U when the liquid water content is greater than 25 mg
cm-2. In order to avoid any pollution by land area,we add a mask around land
areas, 111km away from large land masses and 56km away from islands (diameter
less than 150km) using the method described in (Ozieblo and Etcheto, 1991) .
For SSM/I F08, we use only the "class 0" measurements defined in (Wentz,
1989) and corresponding to water surface measurements far from ice or land
and rain rate less than 1.5mm hr-1. For SSM/I F10, we use the "class 1"
measurements newly defined in (Wentz, 1993) and we add a mask 168km away
from "class 6" measurements which detects sea ice as was suggested by F.Wentz
(personnal communication) and following the method described in (Ozieblo and
Etcheto, 1991) . 1 to 2m s-1 overestimates of the wind speed in regions of
high atmospheric water contents (western equatorial Pacific) as well as 1 to
2m s-1 underestimates of the wind speeds in some high latitudes regions
depending on the wind direction have been evidenced (Boutin and Etcheto,
December 1994)
The climatology is computed from January 1st 1988 to January 1st 1993.
From January 1st 1988 to November 1st 1991, we use the SSM/I F08
measurements. From November 1st 1991 to January 1st 1993, we use the SSM/I
F10 measurements.
The 2.5degres maps are computed as follow:
-U maps:
We compute the monthly average and standard deviation of the satellite
wind speed measurements located in a 2.5 degre square; the square is
kept only if it contains more than 50 measurements per week. The
monthly wind speed climatology is then computed by averaging the
monthly 2.5degres maps.
-K maps (Liss and Merlivat relationship):
We first compute K from the raw satellite wind speed measurements and
using the Liss and Merlivat (1986) relationship at 20degres C and
then take their monthly average and standard deviation in 2.5 degre
squares. We do not take the temperature into account as Etcheto and
Merlivat (1988) show that it is negligible. The monthly K Liss and
Merlivat climatology is then computed by averaging the monthly 2.5
degres K maps.
-K maps (quadratic relationship):
Climatology maps of K computed with a quadratic relationship such as
the one proposed by (Wanninkhof, 1992) can be deduced from a
climatology of the wind speed monthly variance and from a climatology
of the square of the monthly wind speed. Assuming that:
K=a*U*U (1)
where U is the short term wind speed and a is a constant; the 2.5 degre
and monthly average of K, , can be deduced from the monthly average
of U, __, and from the variance of U, V, computed at 2.5 degre and one
month resolution, as follows:
= a* ____ = a*(V + ____*____) (2)
The monthly climatology of K, <>, can then be deduced from the
monthly climatology of V, (= the average of the monthly V values),
and from the monthly climatology of ____*____, <____2> (= the average of
the square of the monthly U values):
<>=a*(+<____2>) (3)
-Nb maps:
We compute the total number of SSMI measurements in 2.5 degre squares
that is the sum of the monthly number of measurements maps. We do not
discard the squares for which there is less than 50 measurements per
week. These maps can be used to roughly determine the ice coverage for
there is no SSMI measurements over the ice and the ratio of the number
of measurements in the squares close to the ice edge to the number of
measurements in the squares 5 to 10 degres away from the ice edge is a
rough indicator of the fractional ice cover in the square close to the
ice edge.
REFERENCES:
Boutin, J., and J. Etcheto, Consistency of Geosat, SSM/I and ERS1
global surface wind speeds; comparison with in-situ data, submitted
to Journal of Atmospheric and Oceanic Technology, December 1994.
Etcheto, J., and L. Merlivat, Satellite determination of the carbon dioxide
exchange coefficient at the ocean-atmosphere interface: a first step,
J. Geophys. Res., 93, 15669-15678, 1988.
Hollinger, J., DMSP Special Sensor Microwave /Imager calibration/validation,
Final report vol 1, Nav. Res. Lab., Washington, D.C., 1989.
Liss, P.S., and L. Merlivat, Air-sea gas exchange rates: Introduction
and synthesis, in The Role of Air-Sea Exchange in Geochemical Cycling,
edited by P. Buat-MŽnart, pp. 113-127, D. Reidel Pu Co, 1986.
Minster, J.F., D. Jourdan, E. Normant, C. Brossier, and M.C. Gennero,
An improved Special Sensor Microwave Imager water vapor correction for
Geosat altimeter data, J. Geophys. Res., 97, 17859-17872, 1992.
Ozieblo, A., and J. Etcheto, A method for masking microwave radiometer
data polluted by the presence of land and ice, Int. J. Remote Sens.,
12, 2379-2388, 1991.
Wanninkhof, R., Relationship between wind speed and gas exchange over
the ocean, J. Geophys. Res., 97, 7373-7382, 1992.
Wentz, F.J., User's manual SSM/I geophysical tapes,Tech. Rep. 060989,
Remote Sensing Systems, Santa Rosa, CA, 1989.
Wentz, F.J., Measurement of oceanic wind vector using satellite
microwave radiometers, IEEE Trans. Geosci. Remote Sensing, 30, 960-972,
1992.
Wentz, F.J., Revision-2 update for SSM/I geophysical tapes user's
manual,Tech. Rep. 040293, Remote Sensing Systems, Santa Rosa, CA,
1993.
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