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Файл:Введение в синоптическую метеорологию
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Ministry of Science and Education of the Russian Federation
RUSS IA N S TA TE H U D R O M E T E O R O L O G I C A L UNIV ERSI T Y
V .I. V oro b y e v , G.G . T a r ak an o v
INTRODUCTION TO SYNOPTIC
METEOROLOGY
ВВЕДЕНИЕ В СИНОПТИЧЕСКУЮ
МЕТЕОРОЛОГИЮ
Рекомендовано Учебно-методическим объединением по образованию
в обл ас ти гидрометеорологии в кач естве учебного пособия
для студен тов высших учебных заведений, обучающихся
по направлению «Ги дрометеорология»
RSHCJ
Se. Petersburg
2005
U D K 5 5 1 .5 0 9.3 2 (075 8 )
Vorobyev V. I., Tarakanov G. G. Introduction to synoptic meteorology.
Manuel. СПб. Изд. РГГМУ, 2005 - 40 pp.
IS BN 5-86813-160-9
The book considers the major concepts and terms that students of
hydrometeorology are to know while taking the basic course of synoptic
meteorology. The aim of the manual is the deeper comprehension of the
content of the course.
The manual is intended for students of higher educational institu
tions specializing in hydrometeorology. It can be also useful for geogra
phers and specialists whose work requires account for weather.
IS BN 5-86813-160-9
Российский государственный
гидроммворилогический
би бл и о т е к а
© Vorobyev V.I., Tarakanov G.G., 2005
© Russian State Hydrometeorological University
(RSHU), 2005
университет
INTRODUCTION TO SYNOPTIC METEOROLOGY
Foreword
Variation of weather in a region is closely associated with so called
synoptic objects acting in the region or passing it. Study o f these objects
and the associated weather conditions represents the main content of the
discipline called
related to each other. Therefore, when studying a synoptic object, un
avoidably reference to some other objects should be made. It is why, be
fore one starts systematic studying synoptic meteorology, it is worth be
ing, at least briefly, familiarized with basic notions and definitions re
lated to the processes of origination and evolution of the synoptic ob
jects, and with some special features of the meteorological fields struc
ture within the limit of every object. The matter is that the objects them
selves and the associated meteorological fields create pre-conditions for
developing weather forecasting methods.
Introduction
Synoptic meteorology is the scientific discipline studying macro
scale atmospheric processes with the aim of weather forecasting. Actu
ally this definition gives just a notion of the objects to be studied. It says
nothing about the studying method, and it does not contain appropriate
concept of the weather itself. Therefore, the definition should be added
with some explanations necessary for deeper understanding the disci
pline
“synoptic meteorology”
From the definition of
spectrum of the atmospheric processes is to be studied but only that re
sponsible for weather (weather condition) formation and variation. Vari
ous consumers of the meteorological information understand the term
“weather” or “weather conditions” in a different way. The majority of
general population is interested in air temperature, precipitation, and
wind; sailors are concerned with wind velocity, sea choppiness caused
by the wind, and visibility; pilots-with visibility, amount of clouds and
their form and height. Owing to these facts, weather services are to give
the information on weather to various consumers taking into account
their particular requirements. Therefore, the term “weather’ (weather
conditions) must have a wide interpretation. That is why we understand
the weather as
synoptic meteorology.
and the tasks it’s supposed to solve.
synoptic meteorology
All synoptic objects are closely
it follows that not all
the state o f the atmosphere at a definite point (interval) o f
time in a given region or point described with the combination o f mete
orological parameters and phenomena.
This combination includes: pres
sure, air temperature and humidity, wind velocity, cloudiness, precipita
tion, visibility range, thunderstorms, squalls, fogs, snow storms, dust
storms, and glaze. All these are of interest for a wide range of consumers
and those dealing with weather forecasting.
The following questions arise: What atmospheric processes define
the weather conditions? Which of the atmospheric processes can be
called weather forming ones, and what is their spacial and temporal
variation?
First of all it is worth to note that the state of the atmosphere known
as weather is referred to- the comparably thin layer
-troposohere,
where
the larger part of the atmosphere mass is found. It is obvious that the tro
posphere is the layer, where the main weather forming processes de
velop. At the same time the processes within thinner layer, nearest to the
ground surface, such as airflows streamlining a building, do not signifi
cantly affect weather conditions. Hence the vertical scale of the atmos
pheric weather forming processes, which are considered in synoptic me
teorology, is of order of magnitude 10 lO'km. As to the horizontal
scale, it must be adopted according to the sizes of the troposhperic for
mations, which possess homogeneity of weather conditions, or, just op
posite, sharp space variation of the meteorological parameters. Accord
ing to observations, these sizes vary from 101 up to 103 km. This range
corresponds to the horizontal scale of the processes studied in synoptic
meteorology. The time scale is determined by the “life’-span of the
above formations i. e. fromlO1 to 102 hours. However, it should be noted
that some atmospheric processes and disturbances with the life time less
than 12 hours and horizontal sizes having the order of magnitude from
10° to 102 km are also studied in the course called “meso-meteorology”
intended for very short range weather prediction.
The atmospheric processes of the synoptic scale, first of all, are re
lated to development and displacement of the synoptic objects such as
depressions (cyclones), anticyclones, frontal zones, atmospheric fronts,
jet streams, and air masses. To study these objects is the main concern of
the synoptic meteorology. Also synoptic meteorology includes modem
methods for every components of weather prediction with the lead-time
from 12 up to 48 hours. These methods are based on the relation between
synoptic objects and weather conditions. The forecasting techniques for
4
larger and smaller ranges are studied in some special courses namely
range weather forecasting
weather prediction.
The matter is that they are based on the processes of
and
meso-meteorology and very short-range
log
the scales other than those in synoptic meteorology.
Now let’s turn our attention to the discussion of the methods used in
synoptic meteorology for studying synoptic objects, their origin, devel
oping, evolution and displacement, and the technique for weather pre
diction.
One of the special features of the methods used in synoptic meteor
ology is that the atmospheric processes are studied over a large area tak
ing into account geographic particularity of the area. The necessity to
examine the processes over a large territory arises from the fact that the
atmosphere is always in motion. For 24 hours synoptic objects being
thousand kilometers or so in their horizontal sizes can travel a long dis
tance displacing from one region to another. Coming to the region the
weather to be forecasted for, they define weather conditions at the re
gion. Besides, process development and weather variation in any region
the weather to be forecasted for results from the process interaction over
vast areas.-
The atmospheric process character greatly depends upon geographi
cal latitude defining radiation conditions, type and state of the underlying
surface, the region orographic features, etc. For instance, extratropical
depressions may occupy the area of 106 km2, while a tropical cyclone
only 104km2, although it is precisely these cyclones produce the most
thick clouds, the heaviest precipitation, and the strongest wind; the ex
tratropical depressions move mainly eastward, while the tropical cy
clones move mainly west ward.
Underlying surface significantly influences atmospheric process de
velopment and weather character. As we'll below, the atmospheric proc
esses are quite different over oceans and lands, over planes and moun
tains etc. It is why, that when analyzing weather conditions over vast
territory, we use geographical maps. On these maps with meteorological
observation data plotted are called synoptic (weather) maps. They al
low for a broad view of the weather over large geographical regions. The
word synoptic came to us from Greeks. It means “ affording a general
view o f a whole”, we understand it as
tions as they exist simultaneously over a broad area.
a display of atmospheric condi
For the same reason
we call this discipline synoptic meteorology.
5
The second feature of the synoptic meteorology method is the statis
tical approach the physical atmospheric process analysis. The same ap
proach is used for development Of the weather forecasting techniques.
What is the essence of this kind of analysis, and what is the difference
from other kinds of analysis?
As we know, in the process of learning one can distinguish three
stages:
- accumulation and primary processing o f the data
plation
");
- analysis and interpretation of the processed data
(“live contem
( ‘‘abstract think
ing’J,
correction, verification of those theories, models, and hypothesis
-
which were created as result of the analysis of the empirical materials,
and obtaining some practical recommendations.
As applied to meteorological problems, the first stage means accu
mulation and processing of the data on the object to be studied. The sec
ond stage means construction on the model of the process (phenomenon)
to be studied on the basis o f the first stage results. The set of equations
describing the process (that is more typical for dynamic meteorology), or
a physical system behaving similarly to the process to be studied may
define the model. For instance, a set of equation can describe depression
displacement (dynamic meteorology approach) or the same displacement
can be presented by a physical model according to which depression is
regarded as a solid spinning body transferring by air currents (synoptic
meteorology approach). Every of these approaches has both advantages
and disadvantages and, correspondingly, its field of application.
Once the process physical model has been constructed, the numeri
cal values of its parameters are determined with statistical processing of
the observational data related to the process. In our example these are the
values of the parameters relating the rate of depression displacement and
the velocity of non-disturbed airflows. That is the approach typical for
the synoptic meteorology. When making a forecast on the basis of the
process physical model, first of all, the initial state of the atmosphere is
determined, namely those parameters of the state which defined the val
ues to be forecasted. Then the links between initial and forecasting pa
rameters are obtained using statistical technique.
The third stage is the verification of the model practical application
effectiveness that should be made using independent observational data.
6
Accounting for above reasoning, definition of the synoptic meteor
ology can be formulated in the following way: synoptic meteorology is
the scientific discipline studying weather forming processes with the
aim o f weather prediction on the geographical basis with the aid o f
statistical techniques.
1. Basic means used for the synoptic scale process analysis
and short-range weather forecasting
The basic means used for the synoptic scale process analysis and
short-range weather forecasting are synoptic maps. They are plotted with
the information on weather conditions at the Earth surface, namely, at
mospheric pressure reduce to the sea level, air temperature, dew point
temperature, wind speed and direction, meteorological visibility range,
cloud amount and forms at all levels, weather phenomena at the time and
between the times of observation, the value and the sign of pressure ten
dency. The plotting is done with digits and special symbols.
At the surface stations the time interval between observations is 3
hours. The main times are 00, 06, 12, and 18 GMT. Using data of these
observations, basic synoptic maps are constructed. The scale of these
maps is 1: 1,5* 107. It means 1 cm on the map represents 150 km. To
make more detailed analysis of the synoptic process development and
weather conditions assessment in the region of interest, somewhat larger
scale maps are used, namely 1:5* 106 or even 1: 2,5 * 106 i.e. 50.^nd 25
km in 1 cm respectively. In this case some denser network of weather
stations must be used, and forecasters can be enabled to scrutinize synop
tic process development and make more accurate forecasts.
At the first sight the distribution of the various meteorological pa
rameters, as they are seen on the synoptic map, seems to be o f a chaotic
character. Actually it is not so. That is just the matter of presentation. To
present the weather parameters in more systematic way, various isolines
are drawn. The main isolines on the synoptic maps are
isobars
and
isotendencies (isallobars).
Isobar has two definitions: the first one is simpler. Isobar
connecting points with the equal values of the atmospheric pressure.
second involves the notion of
pressure (isobaric surface) i.e. the surface
at every point o f which the pressure is of the same value.
line o f intersection o f the pressure surface with some other surface,
is a line
Isobar
The
is the
in
7
our case with the sea level surface.
P — P(x , y, z
Accordingly,
) , while
isobar
isallobar is the line connecting the points with equal
Isobaric surface
as
P = P(x, y ) .
can be described as
value o f pressure variation with time.
The atmospheric processes are of three dimensions. Therefore, to
predict the weather, the upper atmosphere state should be analyzed too.
To do this, the upper air charts are used. These charts are constructed for
so called standard pressure surfaces. They plotted with the geopoten
tial heights of the pressure surfaces, temperature and dew point tempera
ture at the surfaces and wind velocities. The data to plot the charts are
taken from radio soundings of the atmosphere. The standard pressure
surfaces are adopted to be 1000, 925, 850, 700, 500, 400, 300, 200, 150,
100, 50, and 30 hPa. The average heights of these surfaces are cited in
the table 1.
Table 1. Average height o f the standard pre ssure surfaces
PhPa 1000 925 850
Z km 0 0,7
700 500 400 300 200 150 100 50 30
5,5 7,0 9,0 12,0 13,5 16,0 20,0
3,0
1,5
24,0
For the weather forecasting purposes the following charts are usu
ally worked out: 850 hPa chart, 700 hPa chart, and so on up to 200 hPa.
In some special cases the charts are drawn up for some other surfaces. It
is easy to notice that, at the fixed pressure surface, the higher altitude of
the surface corresponds to the higher pressure at the sea level and vice
versa (see figure 1). Hence we may state that the chart of the pressure
surface heights gives the view of the pressure field structure in the free
atmosphere which is similar to what the surface synoptic map does.
The main isolines drawn on the upper level charts are contour lines
(isohypses).
Contour line (isohypse) is the line o f equal values of the
geopotential heights of the pressure surface.
The heights in the pressure constant charts are plotted in geopoten
tial meters (or decameters) which have dimension of energy but not
usual meters, although numerically they are approximately equal to each
other’s.
Figure 1. An example of the upper level chart of the pressure surface P=Const
Unit of geopotential is the potential energy equal to the work done
as a particle of a unit mass has been ascended by 1 m in the Earth gravity
field with the acceleration g = 9,8
unit Ф = 9,8
m2/s2.
This unit is not convenient for the practical applica
mis1.
It means that the geopotential
tion. To avoid this inconvenient, the Ф value is deviled by non
dimensioned figure 9,8. Doing so, we receive geopotential meters, al
though their dimension will be m2/s2. Thus, the geopotential height can
be calculated from the formula
In addition so-called thickness charts are assembled. These charts
are plotted with the vertical distance (geopotential decameters) between
two pressure surfaces. As we know from dynamic meteorology, the
thickness of the layer between two isobaric surfaces is prop.ortional to
the average temperature of the layer and depends on this temperature
only. It can be calculated from the formula
H Pl =6 ,lA T \g^-d am
P i
For the practical purposes the thickness 500 hPa over 1000 hPa
chart is prepared. This chart represents the average temperature of the
lower half of the troposphere.
Time variation of the meteorological parameters in the free atmos
phere is much smaller than that at the surface. This allows for making
soundings only four times a day: 00, 06, 12, and 18 GMT, 00 and 12
GMT being the main ones. The results o f the soundings allow us to con
9
struct upper level charts, to determine the static stability of the atmos
phere, and, hence, to predict convection phenomena such as thunder
storms, squalls, hail storms, etc. When forecasting, radar data, satellite
images, and pilot reports are also used.
2. Some weather forming features of the meteorological magni
tude fields
An important feature of the pressure field is its continuity and
smoothness. Consequence of the continuity is that there are no disrup
tions in the pressure field. Therefore, isobars can be disrupted at the
margins of the synoptic map only. Consequence of the smoothness is the
possibility to identify synoptic scale systems in the pressure field (see
figure 2)
.....................ifiS*
V /
/
( t r
....
...
...
..........!ои>1а* г й — с
- / m b ; ) )
■
a) depression (low ); b ) anticyclo ne (high); c) trough: d) ridge
.....................
Figure 2. Pressure systems
( f m
An area of lower pressure well defined with closed isobars is called
low
or
depression
closed isobars is called
(a). An area of higher pressure well defined with
high
or
anticyclone
(b). Since pressure systems
determine associated wind systems the low is also regarded as cyclonic
vortex of the synoptic scale or cyclone, and the high-anticyclonic vortex
or anticyclone.*
An area of low pressure defined with non-closed isobars is called
trough
(c), and that of high pressure is called
ridge
(d).
The close relation between of pressure and wind fields results in the
fact that the pressure systems. In the free atmosphere, where the friction
' A ccording to recent findings h igh pressure system can not be regarded as a large-scale
vortex. It seems to be existing as lon g as cyclonic vortexes surround it. As soon as the
surrounding cyclones disappear, the anticyclones disappear too. However, this poin t o f
view needs to have more evidences.
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