Weather by Malcolm RileyBureau of Meteorology’s Doppler radar system

Doppler radar - Gust Front and ThunderstormThe amount of electronic aids appearing on even small vessels is growing. Even on the humble tinny, the skipper may have a smart phone in their pocket that can access a whole range of information through the internet.

Many know about the Bureau of Meteorology radar system, and use the system to monitor the movement of storms to help with their boating activities.

A quick view of the radar pages can give you an idea of where the rain or storms are in your area and which way they are moving. The colours that are shown on the radar represent different levels of reflectivity caused by the size of the rainfall droplets.

For instance drizzle that is made up of very small droplets are usually shown as white or pale blue and indicate a rainfall intensity of around 1 millimetre per hour. The red colours (usually in a thunderstorm) are reflecting off large rain drops and the rainfall intensity can be in excess of 100 millimetres per hour.

Often these large storms are moving quickly so that the high rate of rainfall (red) may only be over a given point for 10 or 20 minutes.

From most thunderstorms a downdraft of cold air moves down to the land surface and then outwards from the storm, this is a gust front from the storm. A gust front comes with a rapid and large increase in wind speed and often direction. The air is often much cooler (as it has come from higher levels) and a marked temperature decrease occurs.

To the mariner on the water this gust front would be experienced as a squall and may be some distance from the actual storm.

The radar image of a thunderstorm near Sydney (above) shows the gust front some 20 kilometres from the actual storm, these gust fronts do not always “show up” on the radar.

Above: A Doppler radar image of a cold front that has just passed the radar and the surrounding wind field indicated by the white arrows that have been added.The Bureau of Meteorology has a program to update many of the radars around the country and part of this upgrade is to add Doppler to the radar’s capability. Radars with this capability are already in operation, and for mariners the radars in Adelaide, Melbourne, Sydney, Wollongong and Brisbane are of interest.

Most have experienced the Doppler effect, probably without knowing it. When you are standing on the street and a vehicle drives past you with a siren going the noise of the siren changes as it moves towards and past you; that is the Doppler effect.

Doppler technology is also use in radar guns by the police and to measure the speed of balls at the cricket or tennis.

Actual WindDoppler radars use a sophisticated technology to measure the movement of particles in the atmosphere and whether they are moving towards or away from the radar. This gives a wind field around the radar. This wind field allows the user to pick features such as gust fronts that often do not show up on the regular radar image.

It then gets a little more complicated, the wind field shown by a Doppler radar is a radial wind direction and speed. That is motion either toward or away from the central point of the radar. The convention is that blue colouring is moving towards the radar and orange away from the radar.

Doppler radars are not picking up the surface wind, and the height of the measurement increases the further you get from the radar. For instance at a distance of 50km from the radar the level being sampled is approximately one thousand metres in height.

The radar products are complex, to gain a better understanding visit these sites.

Radar Images and loops
http://reg.bom.gov.au/australia/radar/about/using_radar_images.shtml

Doppler images and loops
http://reg.bom.gov.au/australia/radar/about/doppler_wind_images_intro.shtml 

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.