The techniques used for ingesting SEASAT-A SASS wind retrievals into the existing operational software are described. The intent is to assess the impact of SEASAT data in he marine wind fields produced by the global marine wind/sea level pressure analysis. This analysis is performed on a 21/2 deg latitude/longitude global grid which executes at three hourly time increments. Wind fields with and without SASS winds are being compared. The problems of data volume reduction and aliased wind retrieval ambiquity are treated

Langland, R. A.

Pihos, G. G.

Stephens, P. L.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19830002203 2020-03-21T06:56:36+00:00Z
OCEAN DATA SYSTEMS, INC.
..	 2400 GARDEN ROAD, MONTEREY. CALIFORNIA 93940 • 408/649-1133
i
Prepared for: JPL NO. 9950 — 7 3^
JET PROPULSION LABORATOP.Y	 I
Pasadena, California
This work was performed for the Jet Propulsion Laboratory,
California Institute of Technology, sponsored by the National
Aeronautics and Space Administration.
OC
SEASAT-A SASS WIND
PROCESSING
March 1980
(NASA-CR-169433) SEASAT-A SASS iilhD
PRuCESSING (Ocean Data Systeas, inc.) 9 p
HC AU21CIF A01	 CSCL UdC
J Prepared under:
Contract No. 955370
Prepared by:
Rodger A. Langland
Pamela L. Stephens
Gregory G. Pihos
OCEAN DATA SYSTEMS, INC.
Monterey, California 93940
N83-10473
Unclas
G3/4 3 3 5'. 05
A
I. Introduction
The following pages briefly describe the techniques used in work which
Ocean Data Systems, Inc. has been doing for Fleet Numerical Ocestnography
Center for ingesting SEASAT-A SASS wind retrievals into the existing opera-
tional software.
The intent is to assess the impact of SEASAT data in the marine wind
fields produced by the global marine wind/sea level pressure analysis. This
analysis is performed on a 2140 latitude/longitude global grid which executes at
three hourly time incremem-. Comparisons of wind fields with and without SASS 	 1
winds are being made.
The Jet Propulsion Laboratory is continuing this study by funding experi-
ments of driving the Spectral Ocean Wave Model with the resulting wind fields.
Comparisons of the output will be made with Altimeter data, also received from
SEASAT-A.
The problems of data volume reduction and allased wind retrieval ambi-
guity are treated as described in the following sections.
II. Production of SUPEROB Input
The JPLSASS program converts SASS reports from the JPL format to a
format compatible with FNOC requirements, to be incorporated into SUPEROB
generation. This program also performs some alteration on the SASS reports to
alleviate the aliasing problem caused by multiple wind solutions for the same
observation. The concept is to reduce the number of possible solutions from a
maximum of four to a maximum of two solutions which are approximately in
opposite directions from each other. It should then be easier to later discard the
one solution which departs the most from the guess field.
If there are more than two solutions for a report, the techniques for
reducing the number of solutions to two are as follows:
For three solutions, if any two solutions have directions within a specified
angle of each other, then the directions and wind speeds of these twc solutions
I', •	 * .
are averaged with equal weight. The modified report then consists of the
averaged direction and speed plus the remaining solution which was not included
in the average. If the angular criterion is not met by any of the solutions the
report is discarded.
For four solutions, if any vvo solutions have directions within a specified
angle of each other, then the directions and wind speeds of these two solutions
are averaged with equal weight. The directions and wind speeds of the remaining
two solutions are also averaged regardless of their angle with equal weight. The
modified report then consists of these two averaged solutions. If the angular
criterion is not met by any solutions, the report is discarded.
For a special data set provided by JPL, it was estimated that for a
specified angle of 400, only about 10% of the SASS reports with more than two
solutions would be accepted. For wider angles, the acceptance increased
significantly. Computer runs show that 56% of the reports were accepted for
S0°, 68% for 60°, and 84% for 70°. Preliminary tests are being made using 600
as the acceptance angle.
In the following description of the production of SUPEROBS, one should
remember that references of up to four solutions is reduced to a maximum of
two solutions as a result of the JPLSASS processing.
III. Production of the SUPEROBS
A SUPEROB is a pseudo-report which is generated at a gridpoint and
represents a. weighted average of one or more SASS reports. The contribution of
a SASS observation to t hib composite is specified by its weight and is proportion-
al to (1) the time difference between the observation and the analysis (range
+90 minutes) and (2) the distance between the locations of the SASS report and
the SUPEROB. A SASS report is only incorporated into the SUPEROB at the
closest gridpoint.
Since each SASS report consists of between one and four solutions for both
the wind speed and direction, there are several ways to utilize this information.
Five options have been included in the subroutine SUPEROB and the particular
method to be used is specified via sense switches which are set prior to program
execution.
2
 _.,-
In Option 1 each of the (up to four) SASS wind speed solutions is multiplied
by the report's weight (see above) and added to the cumulative total for that
SUPEROB location. Thus, each of the possible four solutions has the same
contribution. The wind speed is a weighted average of all SASS report solutions
at the gridpoint location.
NSASS(
Wk f SPDSASSRI
(SUPEROB Speed) 	 k=1 	 1_1
	
//
NSASS CSOL l
M	 E wk/k	 =1
I,J	 =	 gridpoint locations
where NSASS	 =	 number of SASS reports close to gridpoint 1,3
NSOL
	
=	 number of solutions for (SASS report)k
w k	=	 weight of (SASS report)k
SPDSASSj	=	 a speed solution for (SASS report)
When all of the SASS reports have been processed, a weighted average for the
wind speed is obtained at each gridpoint. This averaged speed is coupled with
the wind direction which has been taken from the guess wind field, to create a
SUPEROB report.
NOTE: When the SASS instrument is looking straight down at the nadir point,
the wind direction is considered to be unresolvable. In such rases, the SASS
report will have only one solution, with the wind direction set to zero. The wind
speed is still valid, however. Since option 1 is the only option which does not
utilize the SASS wind direction, these nadir reports are included in the average
only for this option, and rejected for all others.
For Option 2, the procedure is the same as in Option 1 except that the
weighted average procedure is used for both the wind direction and the wind
speed. The direction solutions are averaged as above, so that:
NSASS
E
(SUPEROB Direction)43 = k=1
NSASS
k =1
where DIRSASSR = a direction solu
/ NSOL
(i
 E	 wk * DIRSASSR
£_1
C NSOL
wk
f.^l
tion for (SASS report)
In this cas`, then, each of the SASS wind direction solutions contributed to the
final SUPEROB direction. Due to the method of revieval of SASS obs, this
option should not be used unless retrievals have bet.3 reduced to one solution only
(i.e. if the aliasing problem is solved externally).
In Option 3 each of the wind direction solutions is compared to the guess
field wind direction. The solution which is closest to the guess wind field
direction is assumed to be the correct solution. Only this direction and its
corresponding wind speed solution are incorporated into the average of the
SUPEROB location. For one particular SASS report, the first step in the
processing is to determine which solution is the closest in direction to the guess
field, i.e.:
DIRSASSk = SASS so!ution such that
DIRGFD - DSASSn ! is the minimum
ford<n number of solutions
in (SASS report)k
DIRGFD = direction of guess field
The SUPEROB is a weighted average of the SASS reports which are closest to a
particular gridpoint. Thus:
NSASS
E	 (wk * DIRSASSk
(SUPEROB Direction)	 = k=1
I ' 3	 NSASS
E	 wk
k=1
NSASS	 = number of SASS reports for which 1,3 location
represents the nearest gridpoint
wk
	= weight for (SASS report) , a function of distance
from gridpoint and time la^.
DIRSASSk 	= is defined above.
Similarly, the speed is a weighted average of the "best" solution, Le:
NSASS
L	 Cwk * SPDSASSk
k=I
NSASS
E	 wk
k=1
(SUPEROB Speed)l'i _
where SPDSASS k = SASS speed solution co.-responding to the DIRSASS k solution.
MOTE: In the actual calculation of a wind direction a runni;ig average is done
rather than dividing two sums. This method is utilized due to the problem of
averaging wind directions which are close in direction, but degree values are
quite different (e.^. 3550 and 100). If the difference between the two angles is
greater than 1800, then 360 is added to the smaller angle before the averaging is
done. (In the example, the 10 0 solution is changed to 3700.) Once the new
average is found, it will have 360 subtracted if it ex,,eeds 3600
The running average is an average which is determined with each new addition to
the solution at a particular gridpoint.
S
Thus:
k-1
E W DIRECTIONk = W	 * DIRECTION k-1
k
E w^
Tr 1
+ k 
wk	 * DIRSASSk
E wk1G=1
where
DIRECTION  = new weighted average at a gridpoint location
DIRSASSk	 = new SA SS direction solution
wk
	= weight for the new solution
w P
	= weights for other solutions
Because there may be several SASS reports to be included in a particular
SUPEROB, the SASS contributions are weighted as described above.
In Options 4 and S the wind speed and direction of a solution are
transformed into the zonal and meridional wind components USASS and VSASS.
These components are then averaged according to the same methods that are
used for the speeds in options 2 and 3. The resulting averaged SUPEROB U and V
are then transformed back to SUPEROB speed and direction. Thus:
USASS = - SPDSASSR * COS (DIRSASS )
VSASS -SPDSASS * SIN (DIRSASS )
NSASS
E
For option 4, (SUPEROB U)i,, = k+1
NSASS
E
k=1
NSOL
E	 wk
 * USASS I~
k+l
NSOL
E wk
R=1
6
ORi
of PcO
NSASS
E
and, (SUPEROB V)1'3 =	 k=1
NSASS
E
k=1
NSOL
E	 wk * VSASSI
1=1
NSOL
E wk
1=1
For Option S, only the solution whose direction is closest to the guess wind field
direction is used, so that:
NSASS
(SUPEROB U) = k 1 (wk * USASSk)
1'^ NSASS
E	 wk
k+1
NSASS
and (SUPEROB V) 1^^ = k = 1 (wk 
* VSASSk)
NSASS
Ek=1 wk
Then, for both options 4 and S,
(SUPEROB V)113
(SUPEROB direction) 1' ^ = 270° - ARCTAN (SUPEROB U)1,1
and (SUPEROB speed )41 _ ((SUPEROB U) 1,J ) 2 + ((SUPEROB V)1J)2
FNOC SUPEROB EXECUTION(3 hourly cycle - SASS . 1.5 tours)
Preliminary SLP Analysis(60 sec)
Gradient wind enerotion(2 sec
Superob generation(average FF/gradient wind DD-Option 1)( I sec)
SLP/Marine Wind Analysis - NVA Balancing(III sec)
Superob regeneration(Closest SASS ob to marine wind analysis-Option 5)( I sec)
Cycle as
advantogeous
SLP/Marine Wind Analysis - NVA Balancing(I I I sec)
NOTE: (X) represents CDC Cyber 175 CPU time
8


SEASAT-A SASS wind processing

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