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EVALUATION AND COMPARISON
OF TROPICAL ANALYSES DURING DST-5 ANP DST-6
James C. Sadi el" 4
Department of Meteorology
University of Hawaii
(NASA-CP-170431) EVILOITION AND CCMPABISOM
OF TBOPICAl ANALYSFS DUPING DST-5 AND DST-6
Final Report (Hawaii Univ. ) 31 p
HC A03/MF AC 1 CSCL 04B
National Aeronautics and Space Administration
Goddard Space Flight Centen
Grant No. NAS 5-25376 ^
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Data systems Tests (DST) 5 and 6 were conducted to assess the adequacy of
the global data base for numerical analysis and forecasting and In the pi’oeess
to determine the Impact of meteorological satellite data, A voluminous
literature has accumulated on the evaluation of satellite data, particularly
satellite soundings, and their Impact on analysis and forecasting^ however,
the verdict Is not yet clear. Ghil et a1 ., 1979; Atlas et al . , 1979; and
Atlas, 1979 reported modest improvement in analyses and forecasts using
satellite data with the Goddard Laboratory for Atmospheric Sciences (GLAS)
models and In selected cases the satellite soundings provided critical data
In particular weather situations which led to significant positive impacts
relative to either the GLAS or National Meteorological Center (NMC) models
using no satellite data. Tracton and McPherson, 1977; Miller and Hayden, 1978;
and Tracton et al . , 1980 reported no significant improvement with NMC models
and even a negative impact during DST-5 tests. The results of these exhaustive
tests indicate that the satellite data impact is probably model- and season-
dependent but definitely dependent on the method of data assimilation and the
numerical model used to produce the analyses or initial conditions. None of
the tests have included an evaluation of the satellite data impact in the
tropics for this is a much more difficult task. The pressure height gradients
are small and there is little "ground truth" for evaluating the global sound-
ings. The primary analysis within the tropics must be of the wind field and
the satellite contribution is restricted to essentially two levels and only a
portion of the globe unless there is a dedicated program of bogusing data from
polar-orbiting satellites. The only extensive tropical forecast experiment
was for a 5-day period in 1965 (Miyakoda et al . , 1974). The results were mixed--
good and bad. For the first 24-hour forecast the initial conditions of the
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height and wind fields at all 10 levels were detennined by an experienced
’ • *
analyst. The first 24 hour forecast of the tropical precipitation pattern,
based on this subjective analysis, compared well with observations of deep
convective cloudiness observed by satellite. This severe test indicated that
the model structure and physics were capable of an adequate forecast given
the proper initial conditions. However, subsequent forecasts, based on updated
numerical model analysis and initialization, deteriorated rapidly. This one
test supports the contention of Atlas (1979) that the impact of satellite data
depends primarily on improving the initial conditions and not on differences
in forecast models.
This report discusses a limited exercise to evaluate the adequacy of the
PST data base for analyses in the tropics. A few days of subjective analyses
at two levels are compared with numerical analyses from the models of the
Goddard Laboratory for Atmospheric Science (GLAS), National Meteorological
Center (NMC), and NMC modi fled by the University of Hawaii under a separate
study conducted by Murakami (Sumi, 1980). The comparisons are both subjective
and objective. Subjective comparisons were made of the vorticity and divergence
fields as well as the wind fields and synoptic systems. The energetics of the
upper troposphere over selected regions determined by Sumi (1980) for the
different analyses are also discussed here. In addition, GLAS tropical
analyses and the utility of satellite soundings and derived winds in subjective
tropical analyses are evaluated.
2. DATA AND ANALYSIS
(1 ) Data Base
The primary data base was the level Il-b data compiled for the
DST-5 and DST-6. The file contained (a) conventional data received at NMC
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enhanced by extending the cutoff time to VO hours} (b) VTPR soundings; (c)
cloud motion vectors together with an enhanced tet‘ provided by NESS; (d) a
special set of cloud motion vectors determined at University of Wisconsin;
(3) Nimbus 6 soundings specially processed by 6ISS; (f) aircraft reports from
specially equipped wide-bodied aircraft (AIDS); and (g) TWERLE data processed
While the data set was Impressive, It must be emphasized that no
special effort, beyond extending the cutoff time to 10 hours, was made to
enhance the conventional data base and regular aircraft reports. The greatest
effect of this Is reflected In the tropical regions where the normally scanty
data stem from a combination of factors such as (1) relatively fewer stations;
(2) missed observations; (3) poor communications; and (4) lack of a program
for collecting AIREPs by most tropical weather services (Figs. 1 and 2). In
Fig. 1 our analysis over Africa at 250 mb for 00 GMT, 3 September Is based on
only five rawin observations supplemented by time and space continuity,
climatology, bogused winds estimated from satellite cloud photographs and a
few satellite winds in the Atlantic. Figure 2 shows the aircraft wind
observations available for 12 GMT, 15 August 1974 during the GATE. No special
AIDS data are Included. It Is unlikely that air traffic decreased between
1974 and 1975.
Compared with Africa the data over South America were as sparse;
however, the satellite winds from NESS and Wisconsin permitted a reasonable
analysis at 250 mb. At 850 nib adequate analysis was impossible over either
South America or Africa.
The primary data base was supplemented by satellite photographs
from three sources: (1) the global band tropical mercator projection mosaics
of NOAA-4 from NESS; (2) high resolution DMSP orbital strips borrowed from
the archives of the World Data Center for Glaciology; and (3) |^]es West ‘ 1
pictures front the NESS station In Honolulu. These were used to tbcate and I
estimate the Intensity of synoptic systems such as tropical cyclones, tropical |
Upper tropospheric troughs (TUTT) and embedded upper tropospheric cyclones, ^
troughs, ridges, jet streams, lower tropospheric convergence zones, mid- c
laH^’ide lows and fronts. Through established subjective models, wind fields
were inferred from the cloud patterns (Anderson et al . . 1969; Sadler, 1963,
1976a, 1976b, 1978). As an example, the direction and speed of the easterly
jet entering the east side of Fig. 1 In the equatorial zone was Inferred from
the satellite -observed cirrus clouds. This jet, extending from south of India,
persisted arid on the next day (4 September) penetrated Into equatorial Africa;
55 kt winds were observed at 250 mb at Nairobi. The increase from less than
10 kts on 3 September to greater than 50 kts on 4 September could not have ,
been adequately interpreted without the history of the jet obtained from the
satellite-observed cirrus. This anomalous jet, which deviated considerably 1
from climatology, will be referred to later in a discussion of the energetics. j
(2) Selection of Period and Levels for Analysis
The days for analysis within each DST period were selected mostly
on the basis of the tropical synoptic activity as revealed by the global
satellite pictures. The selected DST-5 period of 30 August - 4 September 1975
contained seven tropical cyclones in various stages of develd^ent and a well-
developed TUTt in the North Atlantic and the North Pacific. In\\the eastern
Pacific, Hurricane Katrina attained an intensity of 115 kts, Jewell became a
minimum strength hurricane and the remnants of lisa passed north of Hawaii.
Hurricane Doris foniied north of 30N In the Central Atlantic and Hurricane
Caroline traversed the Gulf of Mexico and entered Mexico. Typhoon Tess formed
near Guam and tropical storm Susan formed southeast of Japan.
The DST-6 period of 29 February - 3 March 197v contained a typhoon
off northeast Australia and a well-developed, TUTT 'in the southeast Pacific.
In addition a deep trough In the westerlies with a strong frontal system
passed through Hawaii.
The standard levels of 850 mb and 250 mb were chosen for analysis
to maximize the data by utilizing low- and high-level satellite cloud motion
vectors and aircraft reports which are concentrated near the 250 mb level.
(3) Data Plotting. Analysis and Grid Point Data Extraction
A large scale (1:15,000,000) map, even though unwieldy (8 ft long)
and time consuming In line drawing, was necessary to accommodate the large
volume of satellite cloud motion vectors procesi-ed from GOES East and West
over the Western Hemisphere. The Il-b data were plotted with varying plotting
models to distinguish the different data types and sources.
The analyst then noted on the charts Information which was obtained
from other sources- -the position and intensity of tropical cyclones from
published sources and the global satellite pictures; the position and intensity
of other satellite-observed significant cloud masses associated with tropical
disturbances, convergence zones, TUTTs, monsoons, fronts, trade winds, etc.;
estimated winds from the polar-orbiting satellites outside the area covered
by the geostationary satellites; the position of other surface features such
as subtropical highs and ridges from the NMC global band surface analysis.
The wind direction and speed were then analyzed using overlying
acetate and various-colored grease pencils. This method facilitates the over-
laying and ajustment of analyses to assure time, space and vertical continuity.
After completion of all analyses, they were traced onto the plotted
charts and grid point data were manually extracted at 2.5° grid spacing for
subsequent use in comparison with other analyses and derived fields.
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The satellite sounding data were plotted on separate charts of the
same scale and analyzed for thickness and temperature patterns. No analyses
were available for the reference level (1000 mb); therefore, height analyses
of the constant pressure surfaces were not accomplished.
3. EVALUATION OF DATA BASE FOR SUBJECTIVE ANALYSES
Prior to evaluating and comparing various tropical analysis schemes,
suitability of the data for subjective analysis should be commented on.
Subjective tropical analysis by an experienced analyst working In a research
mode should be superior to an objective tropical analysis for many obvious
reasons; however, the quality of both Is dependent on data; the greater the
quantity and quality of data the more alike the analyses become and converge
on the true solution. Even though I selected the levels of maximum data
(excluding the surface) for this exercise, on the global scale there remained
large areas with very few data; as discussed In the Introduction, analysis
over these areas by any method Is little better than climatology.
(1 ) Winds
Within most of the Western Hemisphere, due to the GOES satellites
and at least some effort In collecting aircraft observations, the DST data
base approaches that needed for an adequate tropical wind analysis at two
levels after making the reasonable assumption that the satellite and aircraft
winds are representative of the flow at the selected levels of 250 mb and 850
mb. To Illustrate the impact of the satellite winds. Eastern Pacific sections
of the analyses are shown In Figs. 3 and 4. This Is a region of very few
conventional observations and prior to the satellite era its meteorology was
relatively unknown. The 250 mb analysis for 3 March (Fig, 3) obtained by
meshing the few rawinsonde observations (circled), the few aircraft reports
west of 180**' and a good distribution of satellite winds Is probably close to
the true solution of this very complex circulation pattern caused by the
extensive and strong tropical upper tropospheric trough (TUTT) and two
Intense mldlatltude cyclones. The addition of data would not change the
analysis significantly. The 850 mb analysis for 2 Septetrt>er (Fig. 4) with
only satellite data over the ocean depicts a simple trade wind flow In the
South Pacific and a rather complex monsoon type flow In the North Pacific.
The trade wind flow Is well sampled but even In the north the few well placed
satellite winds In combination with the satellite-observed cloudiness and
synoptic modeling of Hurricane Katrina and decayed Hurricane Jewell permits a
Adequate analyses of the pressure height and temperature fields
have never been possible in the tropics due to the sparsity of rawinsonde
stations, small gradients, and the mixture of rawinsonde types whose differences
or biases are comparable to the synoptic gradients.
In such a situation satellite soundings should have maximum impact.
The spatial resolution of the data is good and from the same instrument the
gradients should be accurately determined even though the absolute values may
be biased. The problem of specifying the height of the reference level remains
but even here, due to small pressure gradients, the error should be less than
In higher latitudes (except in the vicinity of tropical cyclones). Tropical
cyclones must be man-"bogused" into the data base whether it be wind or height
analysis. Assuming the soundings are useful in the tropics, the added
advantage is their availability for all standard levels.
Our subjective analyses of the thickness field indicate that the
satellite soundings can reasonably depict the height field at 250 mb over the
tropical oceans. Figure 5 shows an analysis of the 1000-250 mb thickness
field from the Nimbus 6 soundings In the Central and Western Pacific on
4 September. Added to the chart are the ridge and trough positions taken
from the wind analysis. The positions of the N.H. subtropical ridge (STR)
extending from China across Japan and northeastward, the N.H. subequatorlal
ridge (SER) oriented east-west near ION, the S.H. ridge from northern Australia
to the equator near 175W; the N.H. tUTT system extending across the West
Pacific and separating the STR from the SER In eastern China and the omega
pattern north of Hawaii are all reasonably well located by the satellite
soundings; certainly better than could be obtained by determining the height
field only from the few radiosonde stations. How much the pattern would be
altered by the addition of the 1000 mb height field is unknown.
It was noted during analyses of the thickness fields that certain
areas have repetitive patterns which appear to be caused by strong horizontal
near-surface thermal gradients anchored by topography. The best examples are
over the mountains of eastern Africa, particularly in Ethiopia. The thickness
pattern from 1000 to 850 mb (Fig. 6) is essentially the same as from 1000 to
250 mb (Fig. 7) and the addition of the 1000 mb height would not appreciably
alter this relationship over this area. It is highly unlikely that such a
strong cold low exists through the troposphere and experience with upper
tropospheric winds during GATE (see Fig. 2) indicate that these features are
not real but must be due to some flaw in the method or assumptions in produc-
ing the soundings.
4. COMPARISONS OF ANALYSES
The plan to use subjective analyses to evaluate data and serve as a
comnion base for comparing objective analyses in the tropics included both
tropical systems were not adequately depicted and no time continuity could
be established. However* In the couse of attempting the comparison some
general features of the GLAS analyses In the tropics were noted and will be
In conjunction with a parallel project under the direction of Prof.
Murakami. University of Hawaii, comparisons were made between parameters
derived from the Level III grid point data of the NMC and University of Hawaii
objective analyses and our subjective analyses. The University of Hawaii
analysis modified the NMC analysis over data sparse areas by applying a
divergence term proportional to the satellite-observed outgoing longwave
radiation (Sumi , 1980).
(1) ReUtIve Vorticitv
Figure 8 shows the 250 mb subjective analysis at 00 GMT on 1 Sep-
tembar 1974 for the section from India eastward to 70W illustrating the
typical data distribution and the contrast between the simple flow pattern of
the'winter hemisphere and the complex pattern of the summer hemisphere. The
wild data supplemented by satellite photographs and time continuity were
sulficlent on which to base reasonable analyses for this section. The lack
of satellite winds in the eastern South Pacific on 1 September was not typical
of most days. The grid point winds between the equator and 20N on Fig. 8a
ar‘:3 from the GLAS 300 mb analysis and will be discussed in a later section.
Figure 9 shows the vorticity field derived from the analysis of
Fig. 8 and for comparison Fig, 10 shows the vorticity derived from the Level
III NMC 200 mb analysis. In general the vorticity patterns are alike. The
main exceptions are in the southern Bay of Bengal, the western North Pacific
and the South Pacific in the region centered near 12S and 173E. In general
the subjective analysis vorticity is greater than the NMC vorticity. Notable
examples are ovef the southern United States, along the TUTT across the
North Pacific and from Japan southwestward to Indie.
(2) Five-day Averaged Wind Fields and Energetics
Under the Murakami project grid point data from the analyzed wind
• fields were used to compute divergence, vorticity, streamfunctlon, velocity
potential and energetics over the area from 40E eastward to lOOM. The results
were reported In Sumi (1980) and selected figures from that report are used
herein for additional discussion and to note differences In Interpretation of
the results. The averaged wind fields are shown In Fig. 11, There Is little
perceptible difference between the UHM and NMC objective analyses. The
subjective anflysis, except for being somewhat smoother, is in general agree-
ment with the objective analyses since data are plentiful over most of the
area, Major differences are found in the equatorial Indian Ocean and the
equatorial Pacific east of 170E. Sumi (1980) attributed the differences in
the Indian Ocean to the use of climatology by the subjective analyst; however,
this is not correct for the significantly greater wind speeds of the subjective
analyses differ from climatology and are due to the use of "bogused” winds
obtained from extensive and persistent cirrus streamers observed by satellite
(discussed earlier). In the Pacific the subjective analyses maintained the
buffer system ju^lt north of the equator with resulting west winds along the
equator in contrast to east winds in the objective analyses.
These differences in the wind field produced considerable differences
in the energy conversions and the eddy kinetic energy fluxes over these regions
as reported by Sumi, 1980 (see his Table 1 and Figs. 8 and 9), In the Indian
Ocean between thequator and 15S the barotropic energy conversion was negative
(barotropically stable) for the subjective analysis and strong positive
(barotropically unstable) for the objective analyses. Between the equator and
1SN the kinetic energy flux at the eastern boundary of 110E was six times
greater In the subjective analysis. In the Pacific between the equator and
15N the kinetic energy flux at the western boundary of 140E was ten times
greater In the subjective analysis.
The outgoing longwave radiation (OLR) obtained from satellite data
Is a gross measure of the area and depth of convective cloud systems In the
tropics and Is therefore related to the divergence of the upper winds and can
be used as an Independent check on the quality of wind analyses. The
divergent components of the wind fields from Fig. 11 are shown in Fig. 12
together with the five-day averaged OLR. The minimum OLR values, located In
the Bay of Bengal and over Borneo, agree best with the divergent flows of the
subjective analyses even though OLR was used as a correction factor In the UHM
5. COMMENTS ON GLAS ANALYSES IN THE TROPICS
The GLAS objective analyses and our subjective analyses could not be
adequately compared for reasons stated earlier; however, for those few
tropical features whi»-n could be compared the GLAS analyses proved unsatis-
factory. Some specific examples are;
( ^ ) Tropical Cyclones
None of the six tropical cyclones during the August- September period
were adequately depicted on either the surface or 850 mb analyses. On most
days there was no evidence of the cyclones at all. Since these systems are
seldofu reflected in the conventional data network, they must be bogused.
( 2 ) Surface Pressu re Analysis
In general the surface pressure analysis was very poor over the
tropics, apparently due to unsatisfactory data checking. There were daily
u ’ \ P ^ \ '
- •■ • H"
examples of Isolated "bull's-eye" type circular systems* each obviously
caused by a singular poor observation. Extreme examples Mere a 1020 Isolated
high on the equator In the Atlantic and a 1016 mb high near a 1004 mb low on
the equator In the eastern Pacific, The systems pop In and out of the analyses
with, of course, no continuity In time or space.
A good surface pressure analysis Is a prerequisite for determining
a good reference level for the satellite soundings and It Is unfortunate if
their ultimate utility In the tropics Is limited by the quality of the surface
(3) 300 mb Wind Analyses
A most discouraging feature was the 300 mb wind analysis In the
tropical belt since it is presumably based on many observations from satellite
wind vectors, aircraft reports and rawins. The GLAS grid point wind direction
vectors and interpolated speeds from the 00 GMT 1 September 300 mb analyses
are plotted on Fig. 8a. The analysis Is very "nof^^y" with dirsiCtions "flip
flopping" between grid points and speeds varying erratically. One rawin check
is afforded by Johnston Island which is very near a grid point. The analyzed
and observed directions differ by some 50 degrees. The analysis for this day
is typical of the other days and it is difficult to define the problem without
more knowledge of the analysis scheme.
(4) The system centers and trough and ridge lines seldom coincide in the
pressure and wind analyses. Again, knowledge of the analysis scheme is needed
to determine the cause since we do not know if the analyses are independent or
if some type of dynamic balance procedure is used.
6. ADDITIONAL COMMENTS
Satellite Input to the tropical data bate has been impressive and has
continued to increase since the DST-5 and DST-6 with the addition of improved
soundings, better specification of the SST and more geosynchronous satellites.
However, purely objective analysis by any current scheme, even for the few
levels of maximum data, does not take full advantage of satellite data and can
be considerably improved by experienced human intervention. The impact of
satellite soundings, although promising, has yet to be tested.
Beyond the analysis problem is the more formidable one of numerical
tropical forecasting which has scarcely been posed much less tested. If our
main interest lies in higher latitudes, perhaps we should continue to "wire
around" the tropics while posing the question: "What imijact would a better
tropical analysis have on higher latitude forecasting?" Most meteorologists
arbitrarily assume it would have a large effect because the tropical region
is the major energy source and the large international programs of GATE and
FGGE have been conducted to deal mainly with the tropical problems. However,
success in linking the tropical convection scale to the large scale circulation
has not been reported from the GATE program. Perhaps a detailed tropical
analysis, beyond the proper specification at the interface between the tropics
and extratropics, is unnecessary and would have little impact on the quality
of midlatitude forecasts. The data collected during the special observing
periods of FGGE may be sufficient to address the question and is worthy of
Mr. Hiroshi Nishimoto aided in the analysis, Mr. Louis Oda conducted the
data management and plotting and Mrs. S. Arita typed the manuscript.
Appreciation Is expressed to Or. Hurakami and Mi*. Sumi for their cooperation
In Integrating the research efforts and for ^he’us’e of their results.
Anderson, R. K. • J. P. Ashman, F. Bittner, 6, R.* Farr, E. W. Ferguson, V. J.
Oliver, A. H. Smith, F. C. Parmenter, 0. Slebers, R. W. Skidmore,
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Influence of satellite temperature sounding data and Increased model
resolution on numerical weather forecasting. Preprint, Proc. of the
Fourth Conference on Numerical Weather Prediction, 319-328.
, 1979: A comparison of GLAS SAT’and NMC high resolution NOSAT
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Gh11, M. , M. Halem, and R. Atlas, 1979: Time-continuous assimilation of reirote-
soundlng data and its effect on weather forecasting. Mon. Wea. Rev . .
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Miller, A. J., and C. M. Hayden, 1978; The impact of satellite derived
temperature profiles on the energetics of the NMC analyses and forecasts
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, 1976a: Tropical cyclone initiation by thu tropical upper
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Sadler* J» C. . 1976b: A role of the tropical upper tropospheric trough In
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, 1978: Mid'season typhoon development and Intensity changes and
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over the monsoon region. Dept, of Heteor. , Univ. of Hawaii unnumbered
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, A. J. Desmarais, R. J. van Haaren, and R. D. McPherson, 1980:
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five rawin observations are circled.
2. Aircraft data over Africa near 250 mb for 12 GMT, 15 August 1974 illustrating the typical distribution of the
approximately 200 available pilot reports.
analysis over the tropical eastern Pacific Ocean at 00 GMT, 2 Septefnber 1975.
onQINM. PftflE ®
at 00 QfT, 1 September 1975 from 180 to 70E.
9a. 250 mb relative vorticity at 00 GMT, 1 September derived from analysis of Fig. 3a. Positive values (N.H.)
are thin solid lines.
relative vorticity at 00 GMT. 1 September derived
vorticity at 00 GMT, 1 September derived from NMC analysis
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relative vorticity at 00 GMT, 1 September derived from NMC analysis