US space agency NASA is developing new technologies to dissect the Earth’s planetary boundary layer (PBL), helping meteorologists to better understand the interaction between the Earth’s surface and weather, and its evolution in a changing climate.
According to the agency, the PBL is the hardest part of the atmosphere to measure from space due to the volume of gases above it. Accurate, real-time measurement of PBL temperature and water vapor from space would help facilitate more accurate predictions of rapidly changing extreme weather events.
Dr Antonia Gambacorta, research physical scientist in the climate and radiation laboratory at NASA’s Goddard Space Flight Center, said, “The PBL is where we live and where we experience weather. It’s been studied in great detail with lots of ground-based measurements, but there are large gaps, like over oceans and polar regions where we don’t have as many ground-based instruments. Having the capability from space to probe and measure the boundary layer is important to study the connections between this layer and the rest of our atmosphere on a global basis.
“From space, you’re trying to measure a signal that gets weakened as it gets absorbed and re-emitted along the way. You also have interfering noise from the surface and clouds.”
The ‘signal’ scientists use to study Earth’s surface and atmosphere include infrared and microwave frequencies of the electromagnetic spectrum. Infrared wavelengths of lights are absorbed by liquid droplets and ice particles that make up clouds. However, those same clouds are partially transparent to microwave light. In addition, different parts of the microwave spectrum provide information about different properties of the PBL that can affect weather, such as water vapor, clouds and temperature.
Existing spaceborne microwave sensors and even planned instruments, however, detect only a couple dozen channels within the microwave spectrum. This limits the vertical resolution and accuracy of the data, especially from the boundary layer.
Hyperspectral microwave measurements break the microwave region of the spectrum down into hundreds or even thousands of individual frequencies. Capturing and interpreting these frequencies would provide as clear a view as possible of this crucial layer from a global perspective, Gambacorta said.
According to NASA researcher Dr Joseph Santanello, a leading contributor to the NASA Planetary Boundary Layer Incubation Study Team report and co-investigator of this project, hyperspectral microwave measurements have been long advocated for by meteorological and space agencies worldwide to improve data collection from orbit to improve climate and weather prediction. The PBL Incubation Study report lists hyperspectral microwave sensors as an essential component of the future global PBL observing system.
Technical challenges
To provide meaningful information and visualizations for scientists and meteorologists as well as big-data users such as NOAA, Gambacorta’s team has to solve two major technical hurdles.
They must first dramatically reduce the size, weight and power requirements of existing radio frequency signal processors by converting microwave frequencies to optical signals. These signals will be processed by multiple photonic integrated circuits (PICs).
Dr Mark Stephen, an engineer with Goddard’s Research Engineering Partnership, said, “We have computer models and a preliminary breadboard system assembled to show that optical processing on a PIC should work. If we can convert radio frequency into an optical signal and back into radio, we can show that we can reduce the size weight, power and cost of the instrument without degrading the information content in the signal.”
These savings will allow them to integrate many of these PICs, each processing a different set of bandwidths. The result will be the first of its kind hyperspectral microwave photonic instrument (HyMPI) capable of measuring Earth’s full microwave radiation spectrum. This instrument has been matured through three years of funding from Goddard’s Internal Research and Development (IRAD) program, and is now being backed by further development resources from the Earth Science Technology Office.
The second hurdle is processing the massive amounts of data these sensors will capture into something scientists and meteorologists can understand. Gambacorta is working to advance the data processing software effort with help from another IRAD grant.
“We have already concluded how hyperspectral microwave sounding performs under simulated clear sky conditions,” Gambacorta said. “This year, we’re modeling how clouds affect hyperspectral microwave measurements. This remote sounding capability will help achieve a global perspective on the make-up and behavior of the boundary layer.”
