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VERTICAL STRUCTURE OF HIGH PRESSURE systems
TERTIARY CIRCULATION

Aerographers Mate, Module 05-Basic Meteorology
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time the tropopause is reached. The subtropical highs are  good  examples  of  this  type  of  high.  Therefore, anticyclones found in tropical air are always warm core. Examples   of   warm   core   highs   are   the   Azores   or Bermuda High and the Pacific High. VERTICAL STRUCTURE OF LOW-PRESSURE SYSTEMS Low-pressure systems, like high-pressure systems, are  generally  a  reflection  of  systems  aloft.  They,  too, experience  shifts  in  location  and  changes  in  intensity and shape with height. At times, a surface system may not be evident aloft and a well-developed system aloft may not reflect on a surface analysis. Cold Core Lows The  cold  core  low  contains  the  coldest  air  at  its center   throughout   the   troposphere;   that   is,   going outward in any direction at any level in the troposphere, warmer  air  is  encountered.  The  cold  core  low  (figure 3-20) increases intensity with height. Relative minimums  in  thickness  values,  called  cold  pools,  are found in such cyclones. The temperature distribution is almost  symmetrical,  and  the  axis  of  the  low  is  nearly vertical.  When  they  do  slope  vertically,  they  slope toward the coldest temperatures aloft. In the cold low, the   lowest   temperatures   coincide   with   the   lowest pressures. The  cold  low  has  a  more  intense  circulation  aloft from  850  to  400  millibars  than  at  the  surface.  Some cold  lows  show  only  slight  evidence  in  the  surface pressure  field  that  an  intense  circulation  exists  aloft. The cyclonic circulation aloft is usually reflected on the surface  in  an  abnormally  low  daily  mean  temperature often accompanied by instability and showery precipitation. A cold core low is accompanied by a low warm tropopause. Since the pressure surfaces are close together,  the  tropopause  is  reached  at  low  altitudes where   the   temperature   is   relatively   warm.   Good examples   of   cold   core   lows   are   the   Aleutian   and Icelandic  lows.  Occluded  cyclones  are  generally  cold core in their later stages, because polar or arctic air has closed in on them. At high latitudes the cold pools and their associated upper air lows show some tendency for location in the northern Pacific and Atlantic Oceans where, statistically,  they  contribute  to  the  formation  of  the Aleutian and Icelandic lows. Warm Core Lows A warm core low (figure 3-21) decreases intensity with  height  and  the  temperature  increases  toward  the center   on   a   horizontal   plane.   The   warm   low   is frequently  stationary,  such  as  the  heat  low  over  the southwestern  United  States  in  the  summer;  this  is  a result  of  strong  heating  in  a  region  usually  insulated from intrusions of cold air that tend to fill it or cause it to move. The warm low is also found in its moving form as a stable wave moving along a frontal surface. There is  no  warm  low  aloft  in  the  troposphere.  The  tropical cyclone, however, is believed to be a warm low because its   intensity   diminishes   with   height.   Because   most warm lows are shallow, they have little slope. However, intense warm lows like the heat low over the southwest United States and hurricanes do slope toward warm air aloft. In general, the temperature field is quite asymmetrical around a warm core cyclone. Usually the southward  moving  air  in  the  rear  of  the  depression  is 3-19 H H 800MB 800MB 600MB   700MB 900MB  1000MB 1000MB  900MB 700MB  600MB AG5f0319 Figure 3-19.—Warm core high. L L 600MB 600MB 700MB 700MB 800MB 800MB 900MB 900MB 1000MB 1000MB AG5f0320 Figure 3-20.—Cold core low. H L 600MB 600MB 700MB 700MB 800MB 800MB 900MB 900MB 1000MB 1000MB AG5f0321 Figure 3-21.—Warm core low.







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