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moving after the last cyclone of the series is called conclusive anticy
clone. On the left-hand side of the figure 25 this large-size anticyclone is
shown. The pressure in its center is 1020 hPa. Such kind of anticyclone
usually forms in the cold air. Arriving to a region, it brings cold weather.
Such anticyclones are fast moving objects. Their trajectories have rather
large component directed southward. Moving southward, they may
merge with subtropical anticyclones strengthening the latter. In ex
tratropical latitudes, after certain development, they may become station
ary anticyclones and create blocking situations.
7. Synoptic situation and synoptic process
Synoptic situation is an aggregate of synoptic objects (air masses,
atmospheric fronts, frontal zones, and pressure systems) over a definite
geographical region at a certain moment of time. To get the complete
idea on the current synoptic situation at a given time, one should analyze
surface weather map. The analysis includes drawing isobars and atmos
pheric front lines, marking centers of cyclones and anticyclones, shading
areas with particular weather phenomena such as fogs, precipitation,
thunderstorms, snowstorms, and glaze.
Upper-level constant pressure charts (absolute topography charts)
serve to determine synoptic situation at upper levels. Geopotential field
expressed by the isobar pattern allows for estimating the degree of cy
clone and anticyclone vertical development and displacement velocity of
the low-level pressure systems. It also allows forecasters to determine
positions of the upper level frontal zones and jet streams. The maximal
wind chart serves for refinement of the jet stream axis position. The
charts of the tropopause heights are composed to analyze the tropopause
position. To refine further the synoptic situation, large-scale vertical mo
tion charts are used. These charts allow for distinguishing area of ascend
ing and distending motion, and, hence, the area of possible of cloudiness
and precipitation development or degradation. Besides, rawinsonde data
and satellite images are accounted for the purpose.
The aim of the synoptic analysis is to get an idea on the character of
the current physical processes going on in the atmosphere and their pos
sible development in the future.
Synoptic process is a temporal sequence of the synoptic situations.
Comparing the results of the synoptic analyses of the previous and cur
31
rent situations, one can determine the tendency of the process develop
ment i.e. to forecast synoptic situation and, at some degree, weather con
ditions.
At present, numerical weather prediction methods allow meteorolo
gists to forecast the pressure, geopotential, large scale vertical motion
fields for the lead time 48-72 hours and more, i.e. to make the charts of
expecting synoptic situations. In turn, these charts can be used to predict
the weather conditions at any region accounting for the local features of
the location the prediction is to be made.
8. Short range weather forecasting
Weather forecast is scientifically based description of expected
weather conditions. Contents and formulation of the weather forecast
depends upon what it is intended for.
The short-range forecasts for general usage by public and for
spreading by mass media (newspapers, radio, TV) include air tempera
ture, relative humidity, pressure, wind speed and direction, cloudiness,
precipitation, and other phenomena such as snowstorm, duststorm, fog,
squall, hail, thunderstorm, glaze, and rime. Except thunderstorm and
rime, these phenomena are divided into weak, moderate and strong ones.
In the forecast text the wind direction is indicated in cardinal points, and
wind speed in ms or knots. The forecast includes night minimal and day
maximal temperatures.
Enumeration of the meteorological parameters and phenomena,
which are to be included into specialized forecast intended for various
category of consumers, is determined by corresponding agreements. For
instance, in the forecasts intended for airport operation the following pa
rameters and phenomena are included.
• Wind speed in m/s and wind direction in degrees.
• Visibility range.
• Cloudiness (cloud amount, form, and height of the cloud lover
boundary).
• Air temperature, and
• Expected atmospheric phenomena (the same mentioned above).
Knowing current and expected synoptic situation, the forecaster can
get an idea on the weather character in the nearest 24-36 hours. How
ever, as rule, it is usually not sufficient. Therefore, the forecaster has to
32
apply various reckoning methods for meteorological parameter and phe
nomenon prediction. These methods are based on functional and statisti
cal relations between current and expected values of the meteorological
parameters. In this case, the parameter to be forecasted is called predic-
tand, and the parameter (parameters) to be used for making the forecast
is (are) called predictor (predictors).
The forecast of winds at the upper levels is usually made from nu
merically predicted geopotential fields at the corresponding levels. To
make this forecast, geostrophic wind velocities are simply calculated
assuming that the actual wind does not significantly deviate from the
geostrophic one.
The situation is quite different at the surface. Here, the friction ef
fect diminishes the geostrophic wind speed value and makes the wind
direction to deviate from isobars to the left, i.e. to the side of the lower
pressure. The angle of deviation depends on the type (roughness) of the
underlying surface and on the wind speed. The stronger the wind, the
smaller the angle. Over land the angle is larger than over the water sur
face. The forecasting procedure, therefore, includes the following steps.
First of all the geostrophic wind speed and direction at the given locality
is determined from the numerically predicted pressure surface field.
Then the expected actual wind speed is calculated from the simple for
mula
Va = k V g
Here
к
is transfer coefficient. Its average value over land is equal to
(8.1)
0,55, and over water surfaces is equal to 0,70. Deviation angle is deter
mined for every place based on the local underlying surface features.
The basis for the air temperature forecasting is the equation of en
ergy (heat influx equation).
ЭГ r
dt
where
u, v,
and w are the wind velocity components along axis
respectively;
дТ 8T
---
1- v—
и
у дх ду j
I ^ д dT sr
Щ Га-Г) + — к — +
dz dz cp
+
£Ph ro ^
----— (8.2),
x, y,
y a
is the dry adiabatic lapse rate; у is actual lapse rate; к is
and z
eddy coefficient; er and £ph are radiation and water phase transfer heat
influxes; cp is the air heat capacity at a constant pressure.
This equation represents all known reasons for air temperature
variation at a given point. The
first term
in the right hand part of the
33
equation represents the temperature variation due to advection; the
ond term
third term
the temperature variation due to radiation and water phase transfer heat
influxes. The first term in the equation is the largest one, the second one
does not play any role at the ground surface since the vertical motion
speed here strives to zero. However it is not the case for the free atmos
phere, here this term is rather large, and it should be accounted for when
forecasting temperature at upper levels. The third term plays an impor
tant role at the surface and boundary layers. It responsible for heat ex
change between the surface and the atmosphere and, hence, for the diur
nal temperature variation. In the free atmosphere it is very small and can
be neglected. The fourth term, although influences somehow the tem
perature variation, but in majority if cases it is much smaller than other
terms, and, usually, it is neglected.
tion.
describes the temperature variation due to vertical motion, the
represents eddy heat transfer, and the
The basis for air humidity forecasting is the moisture transfer equa
d q _
dt
да da
u — + v —
d x d y
v
fourth term describes
’ d z d z
dt
sec
Here, q is specific humidity; p is air density;
DivV = — + —
is air-
d x d y
flow divergence; m is the mass of evaporated (condensed) water; Az is
thickness of the layer we deal with. The meanings of the rest notations
are the same as in the equation (8.2).
The
first term
air specific humidity advection. The
humidity divergence. The
water vapor. The
evaporation process takes place in the atmosphere or from the underlying
surface, the amount of water vapor in the atmosphere increases, i.e.
— >
0 and — < 0 ; in case of condensation the situation is just oppo
se
site i.e. — < 0 and — > 0 .
dq .d m .
in the right hand part of the equation represents the
last term
second term
third term
is to be explained in more detail. As the
represents the eddy exchange of the
describes the specific
dt
dt dt
In the frontal zones all terms of the equation (8.3) are of the same
order of magnitude, in a homogeneous air mass the first and the second
34
terms strive to zero. The third term is responsible for the water vapor
exchange between the surface layer and the atmosphere, its value de
pends upon the state of the underlying and the properties of the air mass
acting in the region of interest. The fourth term is difficult for account
ing. Its impact on air humidity variation depends on many circum
stances. Therefore, it is taken into account qualitatively.
When forecasting the air tem perature near the ground surface,
the following steps are to be done.
1. Determination of advective temperature variation from the initial
moment of time to the time the prediction is to be made for. To do this,
one should determine the place the air parcel will come from during the
lead-time and “bring” its temperature value T, for this purpose the air
parcel displacement trajectory is to be constructed.
2. Estimation of a possible temperature transformation in the mov
ing air parcel due to interaction with the underlying surface (the third
term).
3. Accounting for the diurnal variation (the third and the fourth
terms).
Similar steps are to be done when forecasting air humidity. An ad
ditional step must be made, namely, the forecaster must account for di
vergence in case of an atmospheric front approaches the region of inter
est.
When forecasting the air temperature in the free atmosphere, the
forecaster is to determine advection temperature variation in the same
manner as it is done for the surface layer. As to the transformation, the
reason for it in the free atmosphere is the vertical motions (the second
term in the equation (8.2)). Thus, accounting for the transformation, pre
dicted vertical motion charts must be used. The diurnal variation in the
free atmosphere is negligibly small.
Similar steps are to be done when forecasting air humidity in the
free atmosphere.
Methods for short-range prediction of various phenomena (fog,
thunderstorm, hail, etc) are based on the relations between the phenome
non formation and values of initial (actual) and expected atmospheric
parameters. For instance, forecasting shower type precipitation, thunder
storms, and squalls is based on calculated convection param eters such
as heights of condensation and convection levels, thickness of the con-
vectively unstable layer (CUL), convective available potential energy,
35
convective motion speed, etc. These parameters are calculated from ob
served and predicted values on the air temperature and humidity at all
standard levels. Temperature of the convectively ascending air is deter
mined with the aid of aerological diagram, where stratification and state
curves are constructed (figure 26). If the convection parameters reach
critical values, then corresponding phenomenon is forecasted. The criti
cal values of the convection parameters were determined as result of
processing a large number of cases with various convective phenomenon
formations.
Any other phenomena (fog, snowstorm, glaze, etc) are also related
to some appropriate parameters. Their critical values are determined in
the same way as it was done for the convection phenomena, i.e. by statis
tical processing of a big amount of the observed and calculated parame
ter values, as well as observed the phenomena themselves.
36
Figure 26. Determination of the convection
parameters.
CUL means convectively unstable layer
Concluding remarks
Studying this “Introduction to Synoptic meteorology” will allow
students to manipulate without difficulties with basic notions and terms
they will use while learning the main course of synoptic meteorology
and other meteorological disciplines such as long-term weather forecast
ing, nowcasting, tropical meteorology, etc.
Having been well acquainted with the bases of synoptic meteorol
ogy, students will easily understand regularities of the atmospheric proc
ess development and weather variations caused by the synoptic proc
esses. On this ground and using recent statistical and hydrodynamic
achievements, they will be able to work out some new methods and
techniques for short-range weather prediction.
Knowledge of Synoptic Meteorology will facilitates students to get
ideas on basic directions and methods of research activity in the field of
weather forming processes, and, hence, to be prepared for further sophis
tication of existing weather forecasting methods.
Acknowledgement
Authors express their sincere gratitude to Professor A. V. Belot-
serkovsky for his valuable work as language editor of this book.
37
Questions
1. What do we understand by the terms “synoptic object” and “ synoptic scale”?
2. What methods are used to study synoptic objects?
3. What data are plotted on the synoptic maps?
4. What is the difference between constant pressure level and thickness charts?
5. Write the formula to calculate geostrophic wind speed from the pressure field charts.
6. How does the confluence (diffluence) of the airflows in the boundary layer at the central
parts of lows (highs), and at the trough (ridge) lines affect the weather variations?
7. Why does a low become a cold pressure system in its course of development, and an anti
cyclone become a warm pressure system?
8. What is the air mass transformation, and how does the process of the transformation go
on?
9. How can one classify the air masses on the basis of the weather conditions observed in the
air masses? When answering this question, take into account a season (summer, winter)
and the type of underlying surface (water, land, plane, and mountains) the air mass has
formed over.
10. What principles were put into the base for air mass classification?
11. How are the atmospheric fronts classified by their movement direction and by geographi
cal types of the air masses they divide?
12. What are the particularities of the pressure, temperature, wind velocity, cloudiness, and
precipitation fields at the warm front?
13. What are the particularities of the pressure, temperature, wind velocity, cloudiness, and
precipitation fields at the cold fronts of the first and the second kind?
14. Describe the occlusion front formation process'.
15. Give the definition ofthe upper-level frontal zone (ULFZ). What are the particularities of
the geopotential field at this zone?
16. What is the jet stream? How does the WMO define them?
17. What are the common stages the cyclones and anticyclones go through in course of their
development?
18. Describe the probable weather conditions at every stage a frontal low go through.
19. What are the sizes of the extratropical cyclones and anticyclones? What is the pressure
range in their centers? How long is the “life” time of these cyclones and anticyclones?
20. What are the sizes of the tropical cyclones? What is the pressure range in their centers?
How long is the “life” time of these objects?
21. What do we understand by the terms ‘synoptic situation” and “synoptic process’?
22. Why is it necessary to predict synoptic situation prior to weather condition forecasting?
23. Define the terms
24. What are the “convection parameters”, and why should we know their values?
predictand
and
predictor.
38
CONTENTS
Foreword..................................................................................................... 3
Introduction....................................................................................................... 3
1. Basic means used for the synoptic-sale process analysis and short-range
weather forecasting....................................................................................... 7
2. Some weather-forming features of meteorological magnitude fields
3. Air masses................................................................................................... 13
4. Atmospheric fronts...................................................................................... 16
5. Upper-level frontal zones and jet streams...................................................... 23
6. Cyclones and anticyclones
7. Synoptic situation and synoptic process........................................................ 31
8. Short-range weather forecasting................................................................... 32
Questions........................................................................................................... 38
...................................................... .................... 25
................
10
39
^ O - .p v
Teaching publication
Valery Igorevich Vorobyev, Gennady Grigoryevich Tarakanov
INTRODUCTION TO SYNOPTIC
METEOROLOGY
A manual
Victor V. Bankevich
Editor:
ЛР № 020309 от 30.12.96.
Подписано в печать 10.03.05. Формат 60x90 1/16. Гарнитура Times New Roman.
Бумага офсетная. Печать офсетная. Усл.-печ.л. 2,5. Уч.-нзд.л. 2,5. Тираж 300 экз. Заказ № 10
Р1ТМУ, 195196, Санкт-Петербург, Малоохтинскии пр., 98.
ЗАО «Лека», 195112, Санкт-Петербург, Малоохтинскии пр., 68.
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