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SCIE2223WeatherandWaves –ComputerLab Activity1

Weathermapsandforecasts

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Prepared by Dr Danielle Verdon-Kidd, University of Newcastle

During our field activity we discussed how we measure the weather. In this lab activity you will learn how these measurements are used along with models to forecast the weather. You will first build your skills in interpreting synoptic charts to make your own weather predictions. During the lab you will undertake a series of tasks within your group and this will form the basis for your computer lab report (which is individual). Details of the report requirements are included at the end of this document.

Part 1 – Synoptic charts (60-90 mins)

There are a number of key features on a synoptic chart that you need to be able to identify.

1. ISOBARS

Isobars are the lines on weather maps that join areas of equal air pressure. Numbers higher than 1013 hectopascals (hPa) on an isobar indicate high air pressure. Numbers lower than 1013 hPa indicate low air pressure.

Below is a figure of the United States of America with a series of pressure measurements in hPa. Following the rules of equal pressure generate your own isobars. Discuss with your group the weather features that emerge from your isobars. Remember isobars do not cross each other. Isobars are usually drawn for every four millibars (hint start at 1024).

2. LOW AND HIGH PRESSURE CELLS

Low and high pressure cells are often dominant features in a synoptic chart. Low pressure systems (cyclones/depressions) indicate cloudy or rainy regions. High pressure systems (anticyclones) indicate fine cool dry weather. The lowest or highest pressure of a pressure system is contained at the centre of the cell.

3. WINDS

If the isobars are quite close together on the weather map, wind will blow from the higher pressure area to the lower pressure area. The closer the isobars, the stronger wind. The further apart the isobars, the gentler will be the wind.

For the following synoptic charts discuss in your groups where you think the wind would be strongest and which direction are they are blowing.

4. TROUGHS AND RIDGES

Troughs are elongated extensions of areas of low pressure. They bring similar weather to that associated with depressions/lows. They are indicated on a weather map by a blue dashed line.

Ridges are elongated extensions of areas of high pressure. They bring similar weather to that associated with anticyclones. Ridges are represented as a finger-shaped isobars.

In your groups identify the ridges and troughs in the following synoptic chart

5. FRONTS

An air mass is a large section of the atmosphere containing air with a similar temperature and moisture content. A line drawn on a weather map to show where two different air masses meet is called a front. There are two main types of fronts: cold fronts and warm fronts.

Cold Front – is an area where a mass of cold air moves very quickly underneath a mass of warm moist air. The result is a very sudden thunderstorm. It is represented like this:

We can see a series of cold fronts in the following synoptic chart.

Warm Front – A warm front occurs when a mass of warm moist air moves slowly over the top of a mass of cold air. The result is drizzling rain that may last a few days. It is represented like this:

We can see a warm front in the following synoptic chart. These are less common for Australia and tend to occur south of us.

In your group research what the two other types of fronts (occluded and stationary). Table 1: Fronts explained

	Occluded front	Stationary front
Symbol on a weather map
Description
Weather experienced

6. EAST COAST LOWS AND TROPICAL CYCLONES

Tropical cyclones are represented by an intense closed low pressure system (>986hPa). They develop over very warm tropical waters where the sea surface temperature is greater than 26

°C. They have relatively long life cycles, typically about a week. Severe tropical cyclones (Category 3 or greater) can produce significant property damage with wind speeds over 180 km/h near the centre, heavy rainfall and coastal inundation through storm surge.

East Coast Lows are intense low pressure systems that occur off the eastern coast of Australia, in particular southeast Queensland, New South Wales, far eastern Victoria, and sometimes Tasmania. Although they can happen at any time of the year, they’re more common during late autumn and winter, with a maximum frequency in June. East Coast Lows generally have much shorter lifetimes than tropical cyclones and last only a few days. They develop over the Tasman Sea close to the east coast and can intensify rapidly in the overnight period. Unlike tropical cyclones, where the warm seas provide the energy source, East Coast Lows are driven by a dynamic interaction between cold air in the high levels of the atmosphere over the continent, and the surface temperature gradient between the land and the relatively warm Tasman Sea air. They can produce gale force to storm-force winds, very heavy rainfall and in some cases coastal inundation.

For the following synoptic charts identify the TCs and ECLs. What does the arrow symbolise on the TC systems? For each map identify the region(s) most likely to be impacted by the storm systems.

*** End of Part 1 – we will take a quick break here

Part 2 – Making a weather forecast (60-90 mins)

1. HISTORICAL CHARTS

The synoptic charts we have just been looking at can be used to make a general weather forecast for the day (more specific forecasts for days ahead will be discussed in the next section). Based on your knowledge of synoptic charts you will now make some predictions about what the weather was likely to be like at the major cities for a particular date in history.

In your groups go to http://www.bom.gov.au/australia/charts/archive/index.shtml. Choose a synoptic chart that you find interesting (this can be done by trialling different date ranges). For your chosen chart describe the weather at the locations in the table below. You can estimate the wind direction, is it windy or calm? Describe the likely temperatures (‘warmer’, ‘cooler’, ‘about normal for the time of the year’). Is it likely to be raining, is a cold front approaching?

Table 2: Interpretation of synoptic chart Date of chart:

Location	Weather expected
Melbourne
Sydney
Brisbane
Darwin
Perth
Adelaide
Hobart

Now check the rainfall and temperature map for the day of your forecast to see how correct you were. To obtain the rainfall and temperature map for the day in question go to here: http://www.bom.gov.au/climate/maps/

2. WEATHER MODELS

You can view forecasts out to 7 days for any region in Australia using the BoM MetEye. We can learn more about this service here:

When viewing MetEye the BoM uses a range of symbols to indicate particular weather categories. See table below:

The forecasts provided by MetEye are derived from three sources: weather observations, computer weather models and meteorological knowledge and experience. Observations are the readings of the weather that we take —not only quantities like air pressure, temperature and rainfall at the surface, but measurements in the upper atmosphere from weather balloons and aircraft, and also data from weather radars and satellites.

Weathermodelsor technically, ‘numerical weather prediction’ models, are the main tools the BoM use to forecast the weather. These models are developed using mathematical equations that explain the physics of the atmosphere and require enormous computing power. The models divide the atmosphere into a large number of grid boxes. Then at each grid point, mathematical equations based on physics that characterise how the air moves, and how heat and moisture are exchanged in the atmosphere, are applied and stepped forward in time, predicting what the weather will be.

The BOM global numerical prediction model is the Australian Climate Community Earth Systems Simulator (ACCESS). BoM meteorologists receive updated forecast data from ACCESS four times per day and compare this information from several other weather models from scientific organisations around the world. Meteorologists also draw on their own knowledge of how the atmosphere works, gained through on-the-job experience to produce a forecast.

Within your groups go to here: http://www.bom.gov.au/australia/charts/viewer/index.shtml to see the Australia wide MSLP forecast for the next 7 days. Choose two locations that are likely to experience very different weather for the coming week and complete the table below for the 7-day forecast. To obtain the forecast navigate to MetEye on the BoM website and select your locations

Table 3: Prediction of weather of the next week

Location Name
Forecast for Friday
Forecast for Sunday
Forecast for Tuesday
Forecast for Thusday

3. HOW ACCURATE ARE WEATHER FORECASTS?

Since scientists began using numerical weather prediction models about 60 years ago, their accuracy and reach have steadily improved. The BoM provides seven-day forecasts, but this becomes increasingly error-prone towards the end of the week-long outlook. That’s because of how forecast models work. They are forced initially with the observational data (note Australia’s network is more sparse than some other countries – how do you think this influences our accuracy?). Mathematical equations then predict what will happen next. As the timestep lengthens, any small initial errors propagate to a point at which the forecast is not reliable – just like the butterfly effect.

Getting the forecast wrong can have devastating effects, particularly with extreme events – Cyclone Tracey in 1974 is a really good example. Thankfully cyclone predictions have improved since then! However, they still aren’t perfect.

In your groups read and discuss the article analysing the prediction of TC Debbie (2017) here. https://knowledge.aidr.org.au/resources/ajem-april-2019-comparing-sources-of-weather- prediction-information-in-the-aftermath-of-cyclone-debbie/

What are the key takeaway messages?

*** End of Part 2 – we will take a quick break here

***Any time remaining in the lab can be used to start on your report

Part 3 – Instructions for Report on Weather maps and forecasts

Due date: 5pm Tuesday 26^th April Submitted through: Canvas Weighting: 10%

You will now produce a report based on what you have learnt in today’s session (and your online modules). While you have worked in groups until this point, your report must be individual. Reports are to be submitted through Turnitin via the BB link provided in your assessments tab. When writing your report please use the headings provided and stick to the word limits (i.e. answer each question under the heading specified).

1. FRONTS (4 images + max 250 words) – 20 marks

Using the synoptic chart archive identify one example of each type of weather front (Cold, Warm, Occluded and Stationary). Discuss which regions would be most affected by each of your front examples.

2. INTERPRETING A SYNOPTIC CHART (max 200 words) – 15 marks

Prepare a national weather forecast suitable for an online newspaper using the historical synoptic chart you chose in Part 2 and your analysis of each city in Table 2. The forecast will need to include the appropriate weather terminology.

3. DEVELOPING A SYNOPTIC CHART (1 chart + max 250 words) – 30 marks Choose one of the following scenarios and develop a figure showing the ideal synoptic chart to match that situation (in your opinion). Explain why the synoptic chart would be ideal. This can be hand drawn and scanned, or you can use computer-based tools.

Scenarios:

1. You are hosting a large-outdoor party (e.g. a wedding or big birthday party) in Brisbane
2. You are a farmer located in Wagga Wagga and you have just planted a crop of Wheat
3. You manage Warragamba Dam and the storage is at 20% capacity
4. You own a ski resort in Southern Victoria and there have been no snow falls yet and you have guests arriving in two weeks’ time.

4. ASSESSING THE ACCURACY OF 7-DAY WEATHER FORECASTS (1 Table + max 250 words) – 20 marks

In part 3 of the prac you recorded the 7-day weather forecast at two locations in Australia. Now you will assess how accurate the forecasts were. Compare your forecast to the actual weather that occurred on the day (this can be done in a table). Discuss how accurate the forecast was and if this changed as the lead time increased. Comment on any particular synoptic conditions that may be have made the forecast more/less accurate. * Note this task will need to be completed after the 7 days has elapsed.

5. FORECASTING EXTREME EVENTS (max 250 words) – 15 marks

We discussed the issues of forecasting TC Debbie in our groups. Choose another significant weather-related event and discuss the challenges associated with its forecast. You can find details on previous events here: http://www.bom.gov.au/announcements/sevwx/

** END OF PRAC..