Dynamical Meteorology 2

8/10/2019 Dynamical Meteorology 2

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Teaching Week 2 Dynamical Meteorology 2 of 5

Teaching Week 2 - Dynamical Meteorology 2

Just as the wind flow has been described for part of the atmosphere below the friction or boundarylayer, now the wind flow for the region above the boundary layer is outlined. There are thee cases andthey are for straight line, cyclonic and anticyclonic wind flows.

2.1 Straight Line Wind Flow

When the pressure gradient force (PGF) and the Coriolis Force (CF) are in balance→ straight line flowoccurs and this means wind doesn't f low toward low or high pressure.

PGF = CF→ No flow towards HIGH or LOW pressure.

When you look at a weather map you will find these highand low pressures systems and more often than not theyhave curved isobars around them. Generally thepressure gradient force is greater than the Coriolis forcearound low pressure systems whereas for high pressure

systems the Coriolis force is greater than the pressuregradient force. The combination of these forces leads tocyclonic (clockwise) and anticyclonic (anticlockwise)flow. These are described below.

2.2 Curved, Cyclonic Flow

When the PGF > CF, the wind flow follows a curved patharound LOW pressure.

2.3 Curved, Anticyclonic Flow

When the CF > PGF the curved flow is around HIGH

pressure.

The diagrams (left) refer to straight-line flow, cyclonicand anticyclonic flow respectively.

(Source: Manual of Aviation Meteorology,Commonwealth Bureau of Meteorology, 2003, Wind pg51)

Because the Coriolis force is negligible in equatorialregions of the Earth these relationships break down. TheCF is zero at the Equator and negligible from there toabout 15 degrees south. (Manual of Aviation Meteorology,

Commonwealth Bureau of Meteorology, 2003).Streamline charts showing wind direction are depicted intropical parts of the globe rather than pressure patterncharts and this is due to the fact the relationship betweenpressure gradient and wind speed in equatorial regionsas well (and isobaric charts are not used to depict thecurrent weather pattern).

2.4 Variations in Wind

2.4.1 Backing and Veering

Changes in the wind direction can be described using theterms backing and veering. The diagram below

describes these terms. When the wind direction changes in an anticlockwise direction it is backing,




for example from NW to SW. A veering wind is one in which the wind direction changes in a clockwisefashion, namely winds which go from SW to NW for example. When it is said the prevailing wind isSW for example, this means the wind is blowing from this direction.

2.4.2 Squalls

These are strong winds that rise suddenly, lasting for some time and then rapidly decrease. Bydefinition, from Manual of Aviation Meteorology, Commonwealth of Meteorology, 2003, Glossary ofWeather Terms, pg 194, a squall is "a sudden onset of strong winds with speeds increasing to atleast 16 knots and sustained at 22 knots or more knots for at least one minute." The intensity andduration is longer than that of a gust. The aviation abbreviation is SQ.

2.4.3 Gusts

These are variations from the mean wind speed of lesser duration than a squall. Gustiness isindicative of instability and turbulence in the boundary or friction layer. (Manual of Meteorology Part 2,Aviation Meteorology, Bureau of Meteorology, 1992, pg 29)

The diagram above also shows what is meant by the "mean" wind speed as well as "lulls" in the windspeed. It also shows the general variability of wind direction and speed over time.




2.5 Thermal Wind

High level pressure distribution is usually represented by pressure height contours, rather thanisobars. Pressure height contours are drawn for a constant pressure surface, in the case below 500hectopascals (hPa)or at a height of around 18 500 feet, so that the lines on this map show how theheight of the 500 hPa surface varies. Just like the isobaric charts, the orientation and spacing of thecontours provides wind speed and directional information. In many ways it is similar to the weather

map you see in the newspaper.

Upper winds are related to the contour gradient just as low level winds are related to the pressure

gradient. The variation of wind with height (above a given location) is caused by the horizontaldistribution of temperature. The vector difference between the winds at two levels is known as thevertical wind shear (see Teaching Week 12) and is also known as the thermal wind. The thermal windflow in the chart above would follow the white contour lines (which are temperature related) flowingwest to east with cold air to the right (coloured blue and green in high latitude regions) and warm air tothe left (coloured orange and red in low latitude regions). Strong thermal winds are a feature of frontalregions and jet streams (see Teaching Week 4) where temperature gradients are particularly great.The speed of the thermal wind is proportional to the thermal gradient, the closer the contour spacingthe stronger the thermal wind - just as the closer the isobars on the pressure chart the stronger is thewind and the pressure gradient.

The thermal wind is parallel to the isotherms on an upper air constant pressure chart with colder air tothe right in the southern hemisphere (and to the left in the northern hemisphere). Please do not thinkthe thermal wind is an actual wind - it is simply a name given to the vertical wind shear of the actualwind (prevailing between two levels). (Manual of Meteorology Part 2, Aviation Meteorology, Bureauof Meteorology, 1992, pg 42)




2.6 Temperature Advection

There are two types of temperature advection, cold and warm. CAA (Cold Air Advection) occurswhen cold air flows towards warmer air. WAA (Warm Air Advection) occurs when warm air is movedtowards colder air. In the southern hemisphere, CAA usually occurs on the eastern flank of a highpressure system and on the left flank of a low pressure system while WAA usually occurs on thewestern flank of a high pressure system and on the eastern flank of a low pressure system.

Since we have established the two main pressure systems, high pressure and low pressure and thedirection of wind around each, this mean sea level pressure analysis clearly demonstrates cold andwarm air advection.

2.7 A baroclinic or a barotropic atmosphere

The diagram below depicts a map of isobars (the solid lines) and associated low pressure system.

The dotted lines represent isotherms or lines of equal temperature. When isotherms are parallel tothe isobars - no temperature advection occurs and this is known as a barotropic atmosphere. Whenthe isotherms are at an angle to the isobars - either CAA or WAA can occur and this is described as anatmosphere which is baroclinic.




A baroclinic atmosphere is the type of atmosphere that will lead to the formation of cloud andprecipitation.

2.8 Useful URLs

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/adv/cadv.rxml

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/grad.rxml

http://www.auf.asn.au/meteorology/section1b.html then click on Module content 1.15 Thermalgradients and the thermal wind

Dynamical Meteorology 2

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