Through a series of studies spanning two decades, Dr Thurai and team have used a 2DVD (Joanneum Research Digital) to probe the physical properties of rainfall in meticulous detail. The 2DVD has been used to study raindrops at many different locations around the world, at altitudes ranging from sea level to as high as 1.4km. When combined, these techniques make the disdrometer accurate enough to analyse the characteristics of individual raindrops in real time. More recently, the 2D video disdrometer (2DVD) incorporates both the observations of two cameras facing in perpendicular directions across the sensing surface, and measurements of the drop silhouttes imparted on different parts of the detector by individual droplets. To measure the velocities, shapes and size distributions of raindrops, meteorologists typically use a device named a ‘disdrometer,’ (or drop size meter) which measures the properties of the droplets as they fall onto a sensing surface. This remarkable consistency has been widely studied in the past, including by Dr Thurai and her colleagues. In most circumstances, a vast majority of raindrops will retain their rotational symmetry as they oscillate, until they hit the ground. Kaprisove/Īs they form, these shapes will influence the ‘terminal velocity’ of the droplets – a speed limit for falling objects which is reached when their weights equally match their drag due to air resistance. Such shapes are defined through the principle of ‘rotational symmetry,’ which allows an object to be spun around on one particular axis without any apparent change to its shape. As they interact with the environment and collide with other slower falling ones, they come to steadily oscillate over time. Mathematically, falling raindrops can be described as spheres for tiny drops but for larger drops the forces of gravity, surface tension and aerodynamic drag cause a flattening of the base and a convex shape on the top which have been squashed and stretched to varying degrees along its vertical axis, but not as much horizontally. Dr Thurai uses novel techniques to measure the intriguing attributes of raindrops, gaining new insights into how they fall and change shape. There is now active research to use the data from such links to infer raindrop characteristics. At the same time, they have important implications for communications systems which use millimetre wavelength electromagnetic waves along earth-satellite links. Through the various environmental factors and collisions (with other drops) that they encounter on their journeys, they adopt different shapes and sizes, oscillate in characteristic patterns, and fall at different speeds.įor meteorologists, it is critically important to consider these factors, which have significant influences on the radar techniques used to observe and predict the weather. Her team’s work offers important new insights into the physical properties of rainfall, and could lead to improvements in weather forecasting and radio communications in rainy conditions.Īlthough we barely notice them until they hit the ground, raindrops can display some fascinating behaviours as they descend from the sky. Through her research, Dr Merhala Thurai at Colorado State University uses innovative techniques to learn more about what happens as they fall, particularly during stormy weather. Raindrops can adopt remarkably similar shapes, sizes and movements as they fall to the ground, but so far, our understanding of their behaviour has remained far from complete.
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