Remote sensing specialist and CHARTER project researcher Annett Bartsch of bgeos and others have looked at developing a long-term monitoring system for rain-on-snow (ROS) events across the Arctic using satellite data. They explored the utility of different microwave frequencies from passive and active satellite sensors to detect wet snow and snow structure change during and after ROS events.
The authors compare C-band radar data from ‘scatterometer’ and ‘synthetic aperture radar’ (SAR) with Ku-band radar data, which has been previously used for ROS detection. A scatterometer is a device that uses light or radio waves to measure how something scatters them. It can tell us things like how clear the air is, how fast and where the wind is blowing over the ocean, or how rough a surface is. A scatterometer sends a beam of light or radio waves towards something and then measures how much of it comes back and from what direction. A scatterometer can work in different conditions, like day or night, or cloudy or clear weather C band radar is a kind of radar that uses a part of the radio waves that we can’t see or hear. It can do many things, like sending messages to satellites, connecting wireless devices, making phone calls, and seeing the weather and the ground. It can see the weather and the ground even when it is dark or cloudy, unlike cameras. The Sentinel-1A/B satellites are examples of C band radar that take pictures of the Earth for different uses. Synthetic Aperture Radar and KU band radar use different parts of these KU band uses radio waves that are higher than C band radar and can do some things that C band radar can’t, such as spotting very small things in the air, like birds or drones.
They also used ‘passive microwave data’ from SMOS (Soil Moisture and Ocean Salinity) to complement the active sensors. Passive microwave data is a type of data that comes from sensors that measure the natural microwave radiation that is emitted or reflected by the Earth’s surface or atmosphere. Passive microwave data can tell us things like how much ice, snow, water, or vegetation there is on the ground or in the air.
Bartsch and her colleagues evaluated the results against in situ observations, such as snowpit records, caribou migration data, and other Ku-band products.
They found that C-band radar can capture ROS events and their impacts, but it is affected by temperature dependence of backscatter at VV (V – vertical) polarization. This means that the strength of the radar echo from the ground changes with how warm or cold the ground is when the radar sends and receives signals up and down (VV polarization). This can make it hard to see ice layer formation in the snow, which also changes the radar echo.
Bartsch and her fellow researchers propose a combination of C-band VV and SMOS snow indicators to address this issue and conclude that a multi-sensor approach is needed for long-term monitoring of ROS events across the Arctic. Interestingly, no trends can be observed north of the treeline since 2000, but a number of regional extreme events can be documented which led for example to decline of animal populations and changes in migration routes. Their mapping can be used to identify potential causes and relationships related to ROS formation.
Data capture of Rain on Snow events could have multiple local uses for local land land users and managers. For example, it is interesting to view satellite data captured over time such as this image below. Here is a view of rain on snow frequency events in November over the last 10 years (dark blue low, orange high).
The article upon which this post is based can be read and downloaded here: Towards long-term records of rain-on-snow events across the Arctic from satellite data by Annett Bartsch et al, 2022. You can watch an interview with Annett Bartsch here on our video gallery page.
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