Met Office College – Course Notes
China Seas Meteorology - Methods of
Analysis in the Tropics
Contents
1
Methods of analysis in the tropics
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Introduction
Satellite imagery
Isobaric charts
Isallobaric charts
Temperature change charts
Time cross-sections
Streamline and isotach analysis
References and further reading
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1 Methods of analysis in the tropics
1.1
Introduction
The methods of analysis of data in the tropics sometimes
differ from those employed in the middle and high latitudes
because of the following limitations:
 small scale influences such as topographical effects and
diurnal effects usually overshadow changes produced by
synoptic scale systems;
 scarcity of data;
 gradients of surface pressure or contour are usually
slight and cannot be resolved accurately by
observations.
To partly overcome such problems, the following procedures
are usually followed in analysing data over the tropics:
 make maximum use of satellite imagery, whenever
available –especially valuable in regions where surface
data are sparse;
 analyse changes over the past 24 hours;
 climatology and persistence are often taken into
consideration especially over data scarce areas;
 surface isobars are drawn at smaller intervals of 2 hPa;
 streamline analysis of upper air charts is preferred to
contour analysis.
1.2
Satellite imagery
Given the scarcity of data over much of the region,
satellite imagery is an invaluable aid to analysis.
Information from imagery can be used to add detail to chart
analyses and, if a sequence of geostationary satellite
images is available, the movement and development of
systems can be predicted.
The Japanese GMS geostationary satellite series orbits
above the equator at 140°E, and is well placed to provide
frequent, good quality images of the South and East China
Sea areas (see Figure 1).
1.3
Isobaric charts
Isobars are commonly drawn at 2 hPa intervals over the
whole chart area. It should be noted that pressures
reported by mountain stations may sometimes not compare too
well with surrounding stations because of inaccuracies
introduced by reducing the station level pressure to mean
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China Seas Meteorology - Methods Of Analysis In The Tropics
sea level using the station level temperature. In addition
ship pressures can easily be wrong by one or two millibars.
Figure 1. GMS whole-disc visible image for 16 October 1997,
showing weather systems affecting South-east Asia and the
Western Pacific.
1.4
Isallobaric charts
24 hour pressure change charts are drawn in preference to
3-hour change charts, except when the synoptic situation is
changing rapidly.
1.5
Temperature change charts
24-hour temperature changes in winter over inland Asia
often give clues to forecasting minimum temperatures in
coastal regions around the South and East China Seas.
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1.6
Time cross-sections
Time cross-sections are useful in detecting variations of
wind, pressure or contour heights, etc. in the vertical and
in time. Movement of ridges and troughs also shows up
clearly.
1.7
Streamline and isotach analysis
Streamline and isotach analysis is used in preference to
contour analysis in tropical regions, due to the lack of
geostrophic balance at low latitudes. For best results, the
analysis should use the plotted data together with:
 continuity from the previous analysis;
 the climatological mean streamlines for the relevant
month;
 the latest satellite picture.
Features identified on streamline charts include:
 Axes of monsoon/trade wind troughs;
 shear lines;
 sub-tropical ridges and near equatorial buffer zones
(these will usually have an East/west orientation);
 the Pacific tropical upper tropospheric trough (TUTT) on
upper level charts;
 centres of tropical cyclones (use information from TC
advisories and satellite imagery;
 areas of inflow along troughs and outflow along ridge
lines.
Commence the drawing of the streamlines starting with the
outflow from the sub-tropical highs and the large uniform
flow areas of the trade wind belt. Finish with the flow
into the cyclonic vortices and asymptotes of confluence
within the near equatorial portion (i.e. the trade
wind/monsoon trough). Satellite imagery is valuable for
identifying regions of convergence and cyclonic flow.
Resist the temptation to draw streamlines through every
report as this will lead to a cluttered appearance. Also
avoid the tendency to space streamlines more closely in
strong wind regions as this may lead to misinterpretation
when assessing convergence. There should be sufficient
streamlines to enable easy interpolation.
Ensure that streamlines are consistent with all reports
which are considered accurate. An exception to this rule is
permitted for reports of very light wind speed (e.g. less
than 5 kn at the gradient level or 10 kn at upper levels).
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China Seas Meteorology - Methods Of Analysis In The Tropics
In these cases, more leeway in directional analysis is
permitted to achieve a reasonable streamline pattern. An
example of an analysis is shown at Figure 2.
Once the streamline analysis is completed the isotach
analysis may be added. Streamline and isotach analyses show
the wind field at various levels and as such are invaluable
in aviation forecasting.
Figure 2. Example of a streamline analysis for South-east
Asia.
1.8
References and further reading
Chin, P.C. 1970 : Operational procedures for streamline
analysis, Royal Observatory Hong Kong Occasional Papers
(Cyclo.) No.22.
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Atkinson, G.D. 1971 : Forecasters' guide to tropical
meteorology, Air Weath. Service Tech. Report No.240.
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