Anticyclones are high pressure systems with closed isobars. Wide spread concept of fine weather accompanied anticyclones is not really correct. Actually the weather related to anticyclones can be of various kinds. It depends on the air mass acting over concrete location and the part of the anticyclone. Anticyclones
There are two opinions concerning the vorticity nature of anticyclones. Some meteorologists consider them as huge negative vortexes. The others believe that they just pressure systems.
An impression on clockwise circulation appeared due to wind systems caused by cyclonic vortexes surrounding the anticyclones. Inside the anticyclonic body the winds are usually weak and have chaotic character. However, on periphery of any anticyclone one can observe well developed clockwise circulation. It is quite possible that this kind of circulation is created by numerous cyclonic vortexes arpond the high pressure system. L
L L
H
L
L Conclusive anticyclone Intermediate anticyclone P+5 P+10
P+15 P-10 P L P-5 H H P+5 L P P-5 P
L H
P+5
Intermediate anticyclone Anticyclone formation
As well as depressions, anticyclones (highs) can be of frontal origin and local (thermal). First of all we’ll consider formation of frontal ones. No anticyclone can appear without cyclogenesis processes. The first a low (a few lows) must start forming.
Formation of a low Excess of the mass High pressure (a few lows) of air availability system formation
Developing of the low (lows) supporting high pressure system and even cause its further development. As soon as the low stars filling in, the high pressure system begins decaying. P=const P=const dm dm dm < 0 > 0 < 0 dt dt dt Low High Low dP dP dP < 0 > 0 < 0 dt dt dt Evidences of anticyclone formation
Evidences Criterion Type of anticyclone
Vorticity advection ( 2HH <∇ 0, ) Frontal
Divergence r Frontal VDiv p > 0 Low temperature seat ∂ 2T >∇ 0 Frontal and local formation ∂t m Evidences indicated in the table above must be regarded as just some signs of the anticyclone formation, but not the reasons. The reasons should be looked for in the equation of tendency. ∞∞ ∂P0 r ⎛ ∂ ∂ρρ⎞ −= ∫∫ρ − ⎜ugdzVdivg + v ⎟dz ∂t 00⎝ ∂x ∂y ⎠ The first term is about 100 times larger than the second at least in a frontal zone. Outside the frontal zone it still larger, although at some cases it can be of the same order of magnitude. Therefore, this term can be neglected. The major role belong to the thermal factor. Together with divergence it can be regarded as one of the reason for the anticyclone formation. However, for the frontal anticyclone development the most important role belongs to divergence, and for the local ones to the thermal factors. The life of an anticyclone conditionally can be divided into three stages: developing stage, mature stage, and decaying stage. Distribution of divergence at these stages can be schematically presented in the following way.
z z z
Div V<0 Div V>0 Div V<0 Div V>0 Div V<0 Div V>0 Rear part Central part Leading part
Developing Mature Decaying stage stage stage Unlike lows, anticyclones do not allow for the fronts to be situated at or near the central part of the high. Outflow in all direction from the centre (positive divergence) excludes the interaction of different air masses their. A front can be situated on the periphery of the anticyclone, or the front may cross the ridge of high pressure approximately perpendicular to the ridge line. Inversions in anticyclones
On of important features of anticyclones is the possibility for inversions to form within them. Z km 12 TROPOPAUSE
9
6 Warm air 3 Cold air Cold air
H L H Surface inversion Subsiding inversion
Dynamic inversion Surface inversion formation
In the central parts of anticyclones, at clear sky condition, this kind of inversions arises at nights. In winter, when days are short, they can arise any time.
* = 0 −δBBB A
B0 z
δBA
T
0 >> δBB A B >> 0*
∂T ∂Ta s < 0 < 0 ∂t ∂t Subsiding inversions
∂T w( −−= γγ) ∂t a Within anticyclones downward vertical motion prevail. r r r ∂T w = −z ⋅ DivV DivV > 0 w = −z ⋅ VDiv < 0 > 0 ∂t
z2 w2 Inversion > ww 12 z1 w1 T Dynamic inversions
This types of inversions arises over periphery of anticyclones. They are associated with jet streams. ∂T > 0 ∂t z2 z1 ∂T < 0 ∂t
z2 Inversion z1
T Weather in anticyclones
It was commonly agreed to divide anticyclone into 5 parts according to weather condition distribution.
Weather in the central part 1. Central part 2. Eastern part State of the Clear 5 sky 3. Southern Precipitation No part 1 2 Phenomena A fog is 4. Western part 4 H possible at 5. Northern night part 3 Wind Light Temperature Normal diurnal variation variation Eastern part
Weather Winter Summer Cu hum: State of the Clear Cu cong. sky Precipitation No No
H Phenomena Blizzard due to Rarely dust strong surface storm wind
Wind Cold gusty wind Cold gusty wind
Temperature Grow down due Grow down due variation to cold to cold advection advection Southern part
Weather Winter Summer
State of the Clear Clear. sky Precipitation No No
Phenomena No No
The wind can be Wind Cold NE wind Cold NE wind stronger if a low is found to the South of the anticyclone Temperature Grow down due Grow down due variation to cold to cold advection advection Western part
Weather Winter Summer St, Sc, As Ci, As, Ns. State of the Sometimes Ns Cb sky
Precipitation Widespread Widespread rain H snow, Pouring rain Drizzle Phenomena Snowstorm, Rarely Thaw, Fog thunderstorm Glaze
Wind S, SE moderate S, SE moderate wind wind Indicated here weather is subject to vary Temperature Grow up due to Grow up due to depending on the lows variation warm advection warm advection existing to the West of and insolation the high. Northern part
Weather Winter Summer St, Sc Ci, Cc, Ac State of the Cb sky Precipitation Drizzle, Shower type rain Sleet In case of unstable air mass H
Phenomena Snowstorm, Thunderstorm Thaw, Fog In case of Glaze unstable air Indicated here weather is mass subject to vary Wind W, SW moderate W, SW moderate depending on the lows wind wind existing to the North of Temperature Stable Grow up due to the high. variation insolation