Global and Synoptic Circulations Bureau of Meteorology Fire Weather Training Course Module 2: Global and Synoptic Circulations, “The big picture” Fire Weather Course Global and Synoptic Circulations “The Big Picture”

sun’s rays strike equatorial regions from directly above Fire Weather Training Module 2: Global and Synoptic Circulations Sun’s rays sun’s rays strike equatorial regions from directly above energy concentrated over a smaller area at the poles, energy dispersed over a wider area little if any arrives in winter Main point here is the differential heating on different parts of the earth creates pressure differences and the wind flow tries to balance this pressure difference, ie from high pressure areas to low pressure areas.

Fire Weather Training Module 2: Global and Synoptic Circulations Vernal equinox 21 March - sun overhead equator 149.5M km Winter solstice 21 June - sun overhead 23.5ºN 152M km 21 Dec - sun overhead 23.5ºS Summer solstice 147M km 22 Sept - sun overhead equator Autumnal equinox Seasons are again the result of differential heating of the earth by the sun. During the year the sun slowly travels from overhead of the Tropic of Capricorn to overhead of the Tropic of Cancer and back again. The summer season will follow the movement of the Sun.

Fire Weather Training Module 2: Global and Synoptic Circulations Circulation on a Non-rotating Earth Now looking horizontally across the earth’s surface but keeping the vertical structure in mind. In the subtropical high pressure belt the winds spiral anti-clockwise outwards (trying to get rid of the air) and the wind spirals clockwise into the lows (trying to replace the air).

Fire Weather Training Module 2: Global and Synoptic Circulations Looking vertically through the atmosphere. Maximum heating near the equator means maximum rising of the air leads to lower pressure, which is called the equatorial trough. This air has to fall down when it cools and when it does this area is called the subtropical high pressure belt. The poles are tremendously cold and thus air falls over the poles creating another high pressure area. Between the poles and the subtropical high pressure belt is another generally low pressure area.

Features of the Weather Map

The Weather Map Isobars – lines of equal pressure Form patterns Determine wind direction/speed and weather.

Pressure Pressure - the weight of a vertical column of air above a point decreases with height Stations at high elevation record lower station level pressures We convert all station pressures to MSL to remove this effect and reveal the “true” pressure pattern

Fire Weather Training Module 2: Introduction to the Weather Map PRESSURE CHANGES Pressure rises and falls a few hPa on a regular daily cycle we subtract this diurnal variation to look for “real” changes rapidly changing pressure is a sign that weather systems are developing or moving, is better indicator than absolute values

Fire Weather Training Module 2: Introduction to the Weather Map The Mean Sea Level Pressure analysis (MSLP chart) is a standard way that meteorological data is displayed. TV stations and newspapers use a surface chart to show weather systems. A chart should always have the validity time on it to tell you when it is valid for. Remember that the lines or isobars join places of equal mean sea level pressure. The H symbol indicates a centre of a high pressure area, the L symbol indicates the centre of a low pressure area. Cold fronts are indicated by a line with barbs on it and low pressure troughs are indicated by a dashed line. Warm fronts are indicated by a line with half circles on it.

Access Model 3 hourly

Fire Weather Training Module 2: The weather map. High pressure system Closed pressure pattern with high pressure at the centre characterised by descending air normally associated with “settled” weather This doesn't mean we don't have rain falling when the pressure is high cloud trapped beneath inversions can produce fine drizzle often when the wind is onshore to the coast/ranges rain falling from upper level systems

Fire Weather Training Module 2: The weather map. High Pressure System

IR Image at the same time

Fire Weather Training Module 2: The weather map. Lows Closed pressure pattern with low pressure at the centre rising air may result in cloud and rain if the air is sufficiently moist to condense as it rises and cools characterised by ascending air which is fed from below air flows clockwise around low and tending inwards towards lower pressure generally associated with unsettled weather and rainfall

Fire Weather Training Module 2: The weather map. Low With Associated Front

Fire Weather Training Module 2: The weather map. Troughs Regions of low pressure - not closed circulation like a low possess similar characteristics to lows air rises within trough can be the focus of thunderstorms and rain troughs can occur in a westerly stream or easterly stream

Fire Weather Training Module 2: The weather map. Troughs Westerly generally associated with cold fronts where the wind will change abruptly from a NW to SW may precede a cold front giving a “staggered” wind change, Ash Wednesday and January 97 are examples

moist NE winds east of the trough shifting SE Fire Weather Training Module 2: The weather map. Trough in easterly moist NE winds east of the trough shifting SE May have dry W’ly wind to west of trough on summer afternoons (due downward mixing) result in many warm season thunderstorms

Fire Weather Training Module 2: The weather map. Pre-Frontal Trough (Trough in westerlies)

Typical summer chart Typical summer pattern, with Heat low to North, Sub-Tropical Ridge south of the state and dry easterly flow across the lower West Coast. This pattern must break down for frontal rain to reach inland regions. Otherwise the only precipitation that can be expected is thundery activity or rain from an upper level disturbance.

Local Summer Conditions Sub-Tropical Ridge axis approx 35°-40° S Dry easterly air flow across state Sea Breeze Thunderstorms

Typical winter chart Picture showing typical positions of SubTropical Ridge and frontal systems. With the ridge in this position, well north of Perth, the fronts can penetrate to inland regions.

Fire Weather Training Module 2: The weather map. Cold Fronts Boundary zones between airmasses with different characteristics such as temperature, moisture or wind. move from west to east with average speeds of around 30-60 km/h Develop when pre-existing temperature gradients are strengthened

Fire Weather Training Module 2: The weather map.

Fire Weather Training Module 2: The weather map.

Cold Fronts and Pre-frontal Troughs Fire Weather Training Module 2: The weather map. . Cold Fronts and Pre-frontal Troughs very common for a prefrontal trough to develop ahead of a cold front during summer due to the land/sea temperature gradient computer models are picking up the development of these PFTs just a slight change in the near surface processes can significantly alter the speed of the PFT.

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