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Microwave Weather Observation From Space
A conversation with Gregory Porter, Meteorologist/Director of UK Business Development, Weather Stream, Ltd.

 It is difficult to name a country, economy, or activity that is not somehow influenced by weather. Some people spend an inordinate amount of time and effort worrying about what the weather might be and often wishing for a better forecast. Whether considering a personal walk in the park, or a government looking to best plan for a critical weather event, improved and more detailed weather information is always welcome.

Daily weather reports include high-definition visual images from satellites operated by governments and commercial firms. Such images are readily available and document what can be seen as frequently as every thirty seconds. While impressive, most of these images show only the tops of clouds and storms. To look inside and through the clouds and better understand the dynamics of weather, technologies such as passive microwave, synthetic aperture radar (SAR), and infrared must be employed.

This conversation with Gregory Porter, director of UK business development for Weather Stream, Ltd., and a certified meteorologist, explored what is currently being done with satellite-based sensors in the weather landscape to improve weather predictability.

Weather Stream has proprietary access to data from the Orbital Micro Systems’ (OMS) Global Environmental Monitoring System (GEMS) constellation of cubesats that are equipped with passive microwave radiometers. With one satellite on orbit and plans for multiple launches during 2021, OMS will provide their GEMS data to Weather Stream and other select private and public users.

Why are passive microwave sounders crucial to Earth Observation (EO) efforts?

Gregory Porter (GP)
Passive microwave radiometers measure the natural radiation emitted by the earth at specific frequencies. The characteristics of the microwave signals emitted from the earth provide well-defined information at the sensor, enabling the determination of what the radio waves have encountered as they traverse clouds, precipitation, and other atmospheric conditions. As the only viable technology to gather information at multiple altitudes within clouds, passive microwave delivers unique value for multiple use cases.

To achieve better and more accurate weather forecasting and nowcasting, the weather industry should move toward integrating or blending data from multiple sources and observation technologies. To date, we have been observing weather in silos, keeping data from different collectors compartmentalized.

Each technology contributes to the overall picture and can add value and definition, but technologies such as passive microwave are under-sampled. The current government-driven observation model has focused on visual technologies more than microwave. Expanding the global capacity to capture measurements within storms, using passive microwave sounders, can change the value and usability of weather information in commercial and public markets.

What are some critical meteorological challenges facing governments and commercial entities that could be better mitigated through improved frequency of weather data collection and different observation technologies?

The primary challenges break down into categories of resolution and coverage. There are significant observation gaps in temporal resolution — the time between live observations.

Other gaps exist in spatial resolution, relating to areas of the Earth that are observed less frequently or sometimes not at all. With better resolution and coverage in time and space, meteorologists can make better assessments of storm strength, and gain better understanding of the storm track and propensities which could threaten people and commerce.

Take the example of North Atlantic Storm Dennis, which battered parts of Iceland, Ireland, and the UK with catastrophic flooding, damaging winds, and rough seas in mid-February, 2020. The storm was named the second strongest non-tropical storm on record in the North Atlantic.

As it reached peak intensity February 15, the storm was well outside the range of ocean buoys that would typically relay key observations on temperature, wind speed, wave height, and precipitation rates. The only source of information on this storm was from satellites, which themselves only have certain instrumentation capable of capturing observations under specific atmospheric conditions.

The geostationary GOES-16 satellite captured impressive visual images of this enormous storm, and some microwave observations of the storm were also captured by MetOp satellites, but the temporal resolution of these observations was not sufficient to clearly access the intensity of the storm. With more decisive information that could be available through additional microwave observations, civil authorities may have been better prepared for the devastating effects of the storm.

It’s logical that more frequent observations will improve accuracy and enable real-time decisions based on observed information rather than modeled expectations. The OMS GEMS fleet of satellites is planned to observe the entire earth’s surface roughly one time per hour by 2022. This will be a significant bolstering of the current stream of weather observations feeding models.

With existing government-owned satellites, microwave soundings are taken every three to six hours for much of North America and Europe, and revisits in other parts of the world can be days apart, so there is ample opportunity to improve.

The forecast and storm-track images most people see are output from complex models which interpret and interpolate data to estimate what things should look like between sampled datapoints. However, weather doesn’t always comply with the rules of the models. The unpredictability of weather is one reason to eliminate or shorten temporal gaps, thereby improving forecasts.

The low spatial resolution aspects of weather observation are most evident over oceans. Alternative data sources such as land-based radar provide excellent pictures of storm conditions such as precipitation density, but only reach about 50 kilometers beyond the coastline. Over oceans, there are few opportunities to observe precipitation returns, a significant indicator of storm strength.

When data collection from satellites occurs hours, or even days apart, the data is insufficient for useful weather predictions. Over oceans we are basically blind between satellites, leaving commercial traffic and weather tracking systems to operate in free-run with little to no information.

Figure 1. Storm Dennis captured by GOES-16

How does passive microwave differ from other satellite-based weather data collection technologies?

While passive microwave is one of multiple sensing technologies that provide critical information to comprehensive weather models, it has some significant performance advantages.

Microwave is considered an All-Sky technology as it works in the same way and provides consistent information in daylight or night and with cloudy or clear skies. This enables the ability to see into clouds.

One of the biggest limitations of satellite instruments employing other technologies is that most cannot see through the clouds, and nothing other than microwave can see what is happening and where the activity is taking place inside the clouds. Without this penetrating view, predicting weather conditions is somewhat akin to diagnosing a car engine problem without lifting the hood.

When you can only observe what is happening on the upper surface, it can be very difficult to accurately diagnose or model a problem.

Of the multiple observation technologies, microwave provides the most versatile utility, but all contribute to making model outputs complete and useable. Figure 2 on the previous page is an excellent illustration of different observation technologies published by the European Centre for Medium-Range Weather Forecasts (ECMWF), one of the world’s leading sources for weather forecast information.

It is important to note that visual and infrared observations can produce high-resolution images of the first thing they encounter – which may be the tops of clouds. Microwave observations can measure the conditions at multiple altitudes from surface to the troposphere regardless of obstructions.

Figure 2 “Recent progress in allsky radiance assimilation,” Geer, Bormann, et al., ECMWF Newsletter, No. 161 Autumn 2019.  Click to enlarge. 

How many microwave sounders are in use today?

There are approximately 12 microwave radiometers providing weather observation data from orbit. With such a small number of instruments taking measurements, the addition of the first GEMS satellite in 2019 increased the size of the observation pool by six to eight percent.

Because it is a cubesat, the cost to put it in place was a small fraction of the larger systems in space. With each addition to the GEMS constellation, the total amount of observed data available for input to models will increase substantially. In addition to cost, operating in Low Earth Orbit (LEO) enables higher definition due to a smaller observation spot than instruments in higher orbits, and revisits are more frequent than the larger systems. Each GEMS satellite covers the earth every 10 to 11 hours.

Currently, microwave data is underutilized due to the resolution gaps discussed earlier. However, with more data of a single type which can provide a clearer look at cloud dynamics or cloud physics, the weather world has a tremendous opportunity to improve general knowledge and modeling.

What is the potential for “Weather Radar” type information away from population centers and land masses?

The idea of weather radar over oceans is of interest to anyone who crosses or uses that space. The simple fact is there is a direct correlation between better information and safety and efficiency.

More frequent and more detailed observed data inputs to models will decrease errors and improve accuracy. When available data over oceans can be reliably measured at revisit rates of less than one hour, we can begin to consider the model outputs as a type of weather radar away from land.

Are there commercial markets for improved density and frequency of microwave weather observations?

Weather Stream is beginning to open new markets for dense and frequent microwave observation data. In the past, microwave data was not inherently valued because it was not available with the regularity deemed necessary for effective forecasting.

With revisit intervals below the one-hour mark, the details of internal cloud dynamics will be of more use and value to weather analysts. At the projected resolution, GEMS data could be the foundation for integrated nowcasting models that have only been dreamed of previously.

Any industry looking for a more dynamic observation of weather will benefit from more microwave data. Commercial markets such as transportation and insurance have unmet needs for frequent weather data to empower business decisions and enable expansion to new products and geographies.

Analysts and users will soon be presented with data in volumes and detail they have never experienced, and we anticipate that in addition to the applications we’ve identified, a vast array of opportunities, markets, and products will evolve from the data analytics ecosystem.

The marketplace will likely determine the optimum temporal frequency that makes business sense, but until that saturation point is reached, Weather stream will leverage the growing GEMS constellation to deliver unmatched data that can better inform business decisions and help assure safety across the globe.

Gregory Porter is the Director of UK Business Development and Lead Meteorologist for Orbital Micro Systems and is responsible for expanding the company’s engagement with partners and customers throughout the EMEA region. As a certified meteorologist, he is a key contributor to shaping the company’s market-ready weather data offerings to address critical needs of commercial and public-sector customers globally. Porter is posted in Edinburgh, Scotland, the home of OMS’ primary data center, the International Center for Earth Data (ICED).

Porter has extensive expertise in aviation and weather science garnered from working for the U.S. Federal Aviation Administration on air traffic planning and management, authoring weather related materials for NOAA (National Oceanic and Atmospheric Administration), and as a meteorologist and weather forecaster for the U.S. National Weather Service and The Washington Post newspaper.