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The superpower of millimeter-wave radar: How does it capture the secrets of small water droplets and heavy rain?
Millimeter wave radar, also known as cloud radar, is designed specifically for monitoring clouds and operates in the frequency range of 24 to 110 GHz. Such a special frequency makes the wavelength of millimeter-wave radar about 1 mm to 1.11 cm, which is about ten times shorter than traditional S-band radars such as NEXRAD. The core purpose of this technology is to study the nature of clouds and how they evolve.
These radar systems typically operate in the Ka-band at 35 GHz and the W-band at 94 GHz, which have the highest efficiency in atmospheric transmission.
Millimeter wave radar has very high time and distance resolution. The time resolution is usually adjustable, ranging from 1 to 10 seconds, while the range resolution depends on the design and purpose of the radar. Generally speaking, the maximum detection range of cloud radar can reach 14 to 20 kilometers, and its Doppler velocity resolution is a few centimeters per second.
Cloud radars are mostly polarimetric systems, which allows them to measure particle irregularities via the linear depolarization ratio (LDR). Radars are usually pointed straight up at the zenith, but as technology has improved, many radars have added scanning units that allow the radar to scan at different angles at higher speeds, thereby obtaining additional information such as vertical wind profiles and air volume information. .
Long-wavelength radars have less attenuation for small raindrops and rainfall, while short-wavelength radars are more sensitive to smaller particles, which means that choosing the right radar is particularly important in different weather conditions.
Currently, millimeter-wave radar is widely used in many fields, including detecting cloud boundaries (such as cloud base and cloud top) and estimating cloud microphysical characteristics (such as particle size and mass content). These data help understand How clouds reflect, absorb, and transform radiant energy passing through the atmosphere. Radar is also widely used in fog studies and has been used for over 40 years in entomological research, especially to detect targets that are almost exclusively insects on clear warm days. In addition, it has recently been discovered that millimeter-wave radar can be used to study giant aerosols.
The operating environment of cloud radar is not limited to the ground, it can also be in the air or space. Examples of airborne systems include radars mounted on the HALO (High Altitude Long Range Research Aircraft) and the KingAir research aircraft in Wyoming. A cloud profiling radar in space has been operating since 2006 on the CloudSAT satellite. The Earth Clouds, Aerosols and Radiation Explorer (EarthCARE) mission, scheduled to launch in March 2023, will carry the first space-based cloud profiling radar with Doppler capabilities.
Measurement with radar: from IQ to spectrum
Pulsed radar systems are considered active measuring instruments because they transmit electromagnetic waves into the atmosphere and receive the signals that reflect back. Radar is made up of different hardware components, each of which contains different elements. The electromagnetic waves generated by the oscillator in the transmitter unit are transferred to the antenna via a waveguide, which radiates them into the atmosphere.
After each transmitted pulse is scattered by the volume of air containing water vapor, the returning signal is collected by the radar antenna and digitized after filtering, enhancement and down-conversion.
Although the transmission of each return signal changes with time, the electric field reflected in the signal is obtained from the mixing of a large amount of water vapor. Therefore, the received signal is composed of echoes from many water vapor particles, and these echoes cannot be analyzed individually. Therefore, by sampling the signal, we can determine the distance of the wave at a specific time delay to focus on the diversity of the echoes.
In addition, when performing Doppler processing of the radar, a spectrum obtained from the return signal is automatically generated through calculation of the I/Q signal, making it possible to measure the Doppler frequency of the echo. This helps scientists assess the range of velocities of different particles within the sample volume.
Characteristics of Doppler spectroscopy
In the radar's sample volume, there are usually multiple scattering targets. Each target has its own specific frequency shift, which enables us to analyze the Doppler spectrum by measuring the returned power. The reflectivity can be calculated from the spectrum. By integrating the spectrum, we can obtain relevant meteorological data and deduce weather changes.
The first moment of the spectrum represents the average Doppler velocity, reflecting the radial velocity in the entire sample volume, while the second moment indicates the Doppler width, providing the degree of variability in the detected velocity range.
What should we pay attention to among the many parameters?
Doppler width, skewness and peakedness are all key parameters to describe Doppler spectrum. Studying these parameters helps reveal the microphysical and dynamic changes in cloud structure, which is crucial for predicting weather changes. In addition, the radar's polarimetric measurements provide a deeper insight into how precipitation works and the impacts of climate change.
With the advancement of technology, the application scope of millimeter-wave radar is becoming more and more extensive, but in this endless exploration, can we fully grasp and understand the physical principles behind these technologies?
