February 2022 Issue
Research Highlights
Atmospheric science
Night cloud watching from space
Polar mesospheric clouds (PMCs), also called noctilucent clouds, occur in the upper layer of the Earth’s atmosphere, the so-called mesosphere, at altitudes between 76 km and 85 km. They consist of ice water particles and can typically be observed at high latitudes at night when the lower part of the atmosphere is already in the Earth’s shadow but the upper part is still in the sunlight. Since 2007, PMCs are being studied from space by means of the NASA-operated AIM (Aeronomy of Ice in the Mesosphere) satellite, a low-Earth-orbit satellite launched with the aim to better understand why PMCs form, and what the origin of their variations is. Now, Takuo Tsuda from the University of Electro-Communications and colleagues have looked into the possibility of using data from a geostationary-Earth-orbit (GEO) satellite to detect PMCs. They developed a two-step processing scheme for data recorded by the GEO Himawari-8 satellite operated by the Japan Meteorological agency, and found that results generated in this way can be used for scientific PMC research.
GEO satellites are in an orbit with the same rotational speed as the Earth, so that they are always located above the same spot on the Earth’s surface. The Himawari-8 satellite is located at 140.7° East so that it covers the Asia-Pacific region, and has instruments on board that can detect visible and infrared light. Its spectral images have a spatial resolution of about 1 km, and a relatively high time resolution of 10 minutes — the latter is a particular advantage of Himawari-8 in the context of studying PMCs.
Tsuda and colleagues analysed spectral signals for various frequency bands, and first calculated average intensity values for a 3D point grid of the part of the atmosphere captured by the Himawari-8 satellite; a point has coordinates latitude, longitude and altitude. To decide whether, at a given time, a point is ‘occupied’ by a PMC or not, they compared the intensity to the value for a dark (i.e. cloudless) spot. If the intensity does not lie above a certain threshold, it is assumed that there is no PMC there. In a second, refining step, the scientists corrected the selection threshold for Rayleigh scattering effects (stemming from electromagnetic radiation being scattered by particles that are much smaller in size than the radiation’s wavelength), to ascertain the inclusion of weaker PMC signals.
The researchers studied PMC signal maps obtained in this way with data generated since 2015. They were not only able to recover the typical day-to-day and year-to-year variations in PMC formation, but they also noted many shorter time scale structures. These could be detected thanks to the 10-minute time resolution of the Himawari-8 satellite, and may be related to atmospheric tidal waves. Tsuda and colleagues also performed a thorough validation of their data processing scheme by comparing their results to the data recorded by the AIM satellite, and found that there is excellent consistency between the two different PMC datasets. The scientists concluded that “the current data product would be of benefit for research on various PMC science in the longer time scales” and expect that in the future, further data analysis will enable to “produce newer PMC science in the shorter time scale (with the 10-min) resolution”.
Exemplary plot of the day-to-day variation in detected PMCs by analysing data from the Himawari-8 satellite. Dark blue indicates PMCs detected in the first processing step, light blue indicates PMCs detected in the second, refining processing step taking Rayleigh scattering effects into account.
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