Federico C

The wind watchman

 

Dr Federico Cossu combines data from sources such as EUMETSAT’s Advanced Scatterometer (ASCAT) to make better wind observations for weather forecasts

Federico C
Federico C

Meet EUMETSAT’s Research Fellows: Dr Federico Cossu explains how he is bringing together data from different sources to provide better observations of winds, one of the biggest challenges for improving forecasting

Last Updated

04 April 2022

Published on

07 March 2022

You cannot see it or hold it, but the force of wind can propel ships across the ocean, power cities with renewable energy, and rip trees and houses from the ground. And despite its transparent nature, wind has dimension, shape, and ultimately plays a central role in weather and climate. 

“Wind transfers heat, moisture and dust around the world, and it plays a key role in how weather evolves at many different scales," says Dr Federico Cossu, an EUMETSAT fellow and postdoctoral researcher at the Institut de Ciències del Mar (ICM-CSIC), Barcelona, Spain.

"But because it is difficult to observe over the entire globe with a high spatial and temporal resolution, achieving more accurate wind observations remains one of the biggest challenges for improving weather forecasts."

Gaps in observations

Using super computers and specialised software, meteorologists can predict weather fairly accurately up to a week into the future. Nevertheless, gaps in wind observations – especially over the open ocean and in remote parts of the world – can affect the reliability of forecasts.

"Filling in these gaps is crucial because wind can have a driving influence on adverse weather such as major storms, heavy rain, and other destructive meteorological events," Cossu says. “This is especially the case at smaller scales, where fast-evolving weather is largely driven by wind rather than by temperature and pressure gradients."

By revealing the effects of winds on clouds, the ocean surface, and air particles, satellite data provide an essential part of the arsenal of instruments needed to build multidimensional wind profiles. These can then be used for more accurate forecasts.

During his EUMETSAT fellowship, Cossu wants to help fill in gaps in wind observations, working together with meteorologists at the Spanish State Meteorological Agency (AEMET), the Royal Dutch Meteorological Institute (KNMI), and the Portuguese Institute of the Sea and the Atmosphere (IPMA). 

“My project aims to build consistency between independent wind datasets taken using different methods – spanning satellites, commercial planes, the outputs from weather models, and in situ instruments such as ocean buoys,” Cossu explains.

“Our ultimate goal is to develop four dimensional datasets – horizontally, vertically, and over time – that provide accurate and consistent observations of winds and their evolution. This can provide essential information for nowcasting and numerical weather prediction models.”

Key reflections

To do this, Cossu makes use of data provided by satellite instruments such as EUMETSAT’s Advanced Scatterometer (ASCAT) on Metop polar-orbiting satellites. ASCAT transmits pulses of microwave energy, as a radar, towards the sea surface, recording the resulting echos.

The instrument collects these echoes, which are ‘backscattered’ from small waves on the ocean surface – with a wavelength of a few centimetres – and translates it into a wind measurement through advanced software developed by EUMETSAT.

Despite being at an altitude of 817 km, ASCAT can accurately measure the unique characteristics of the backscattered radar signal caused by the wind-roughened ocean surface. As the sea surface roughness correlates closely with near-surface wind speed and direction, ASCAT can provide a clear picture about wind conditions over a broad area. 

“Each method we use has distinct advantages and disadvantages for observing wind,” Cossu says. “Buoys, for instance, have the advantage of taking direct wind measurements in situ, where the winds are happening. But in the open ocean they are often in short supply. On the other hand, scatterometers can provide wind information across the global ocean.

ASCAT-550 km-wide swathes
ASCAT's two sets of antennae provide two 550 km-wide swathes, ensuring an excellent coverage of the Earth’s surface.

“But as satellite instruments make these measurements much further away from the actual source, they do not provide as much detail as in situ instruments. Additionally, orbiting satellites revisit the same area with a repeat cycle of several days, therefore creating time gaps in the observations that buoys do not have.”

Just as weather systems can be completely altered by small changes in initial conditions, so too can the predictions made by weather models.

“Any weather forecast needs to account for the reality that our atmosphere is hugely complex and chaotic,” Cossu says. “Meteorologists have developed ways to quantify uncertainties in forecasting and to provide probabilistic forecasts using numerical weather predictions.

“To do this, they need to harmonise different measurements that would otherwise introduce 'noise' into forecast models, and make weather predictions inaccurate. Moreover, the scales resolved by the observations need to match those resolved by the weather models in order to avoid the generation of more unwanted noise into the forecast.

“Therefore, it’s not possible to simply feed the data collected directly into these weather models: we need to first account for potential discrepancies and to characterise the observational datasets in terms of their spatial scales and measurement errors.” 

So Cossu turns to a mathematical technique called triple colocation analysis, which fits together the collocated observations of three independent sources. “For example, in the ocean you might have a buoy taking direct measurements of wind, while at the same time a satellite passes overhead remotely sensing the wind’s effects on the sea surface,” he explains.

“These measurements are combined with a third source – usually the output of models that predict the dynamics of the atmosphere – by selecting the wind information from the nearest geographical grid point. At the end of an iterative process, the triple collocation analysis provides not only the inter-calibrated datasets but also an estimate of their measurement errors.

“Forecasters need to account for potential errors: co-locating datasets not only mutually reinforces observations of wind at a particular place and time, but also supports the calibration and validation of the different datasets. For the national weather centres that use these models, this means the data is more robust. That means more accurate forecasts.” 

Vital network

Cossu's project is also benefiting from newly available wind products, such as those derived from the recently launched Atmospheric Dynamics Mission-Aeolus (ADM-Aeolus). ADM-Aeolus uses a space-based doppler wind lidar, beaming an ultraviolet laser through the atmosphere, which scatters off air molecules, water droplets, and dust particles, enabling meteorologists to take 'direct' wind measurements from satellites. 

“I feel privileged because I am one of the first users to have access to such a broad range of data,” Cossu says. “The work that goes into the delivery of these data is the collective effort of hundreds of people over many years.

“Being an EUMETSAT fellow has the advantage of having direct access to an amazing network of researchers to collaborate with, learn from, and explore ideas with. I have been provided with excellent tools, supervision, and funding. When I see how our research is appreciated by the community, it is a huge motivation.” 

EUMETSAT Research Fellowships

In close partnership with its Member States, EUMETSAT funds a limited number of research fellowships to support young scientists in developing new uses and applications of satellite data. Through this series of articles, we showcase some of the fascinating projects Research Fellows work on, which include assimilation and analysis of satellite data to support weather prediction models; the use of satellite data for climate applications; and the development of new products and applications in areas spanning oceanography, hydrology, and atmospheric composition. EUMETSAT Research Fellows are hosted by institutions within EUMETSAT Member States, who also coordinate applications to the programme in response to an annual call for proposals.

Main image courtesy of Federico Cossu.

Author: Adam Gristwood