MMCR

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An overview of an MMCR

MMCR (millimeter-wave cloud radar) denotes an Unattended cloud radar in the millimeter wave range.

Applications

Cloud radars play a major role in researching the influence of clouds on the climate. This influence is expressed on the one hand in the role that clouds play in the transfer of radiation through the earth's atmosphere and on the other hand because they are an important link in the earth's water cycle.

background

The American Committee on Earth Sciences (CES) recognized this influence in 1989, and in 1994 the American Department of Energy initiated a program to measure atmospheric radiation (ARM), which stimulated numerous research projects, including the development of an MMCR for the regular examination of macro- and microphysical cloud properties such as layer height, number, horizontal extent, particle size, particle concentration and liquid water content. Previous, episodic cloud surveys with balloon probes or airplanes did not show the necessary monitoring capabilities.

development

The development of the MMCR goes back to the US military . As early as the late 1960s and early 1970s, 35 GHz radars (corresponds to a wavelength λ of 8.7 mm) were used there. The vertically aligned AN / TPQ-11 had neither Doppler nor dual polarization capabilities, but could still capture cloud structures quite well. The hardware was very problem-prone, especially the high-power magnetron transmitter. Despite the decommissioning of the systems in the 1970s, some research groups continued to work on radars in the millimeter wavelength range in the 1980s (e.g. Pasqualucci et al. 1983, Hobbs et al. 1985, Lhermitte 1987).

Research intensified in the 1990s after the important role of clouds in climate change was established (see above). At the same time, numerous engineering innovations came onto the market, and newly constructed or extensively renewed cloud radars were used in various current researches (e.g. Albrecht et al. 1990, Kropfli et al. 1990, Pazmany et al. 1994, Clothinaux et al . 1995).

The so-called NOAA / K radar was developed at the Environmental Technology Laboratory (ETL) of the American Weather and Oceanography Authority (NOAA) in the early 1980s. Thanks to continuous improvements, it is still in use worldwide today, but requires constant support from a scientist and engineer. Its capabilities, including the good detection of multi-layered cloud structures, prompted the ETL to develop a new, unattended cloud radar for the ARM program. The MMCR is the further development of the NOAA / K, but apart from the related wavelength λ of 8.7 mm, it has little in common. In November 1996, the first MMCR went into operation on the ARM cloud and radiation measuring field (CART) in northern Oklahoma (USA).

Structure and properties

Externally, the MMCR consists of a parabolic antenna pointing vertically upwards, which is mounted on an air-conditioned shipping container. The antenna has a diameter of 3 m (some versions have a diameter of 2 m, making it easier to transport), the container measures 2.5 × 2.5 × 6 m.

Inside the container is the electronics and computer technology that is responsible for storing and evaluating the measurement data, calibrating the system and forwarding the data via the Internet. It is also possible to remotely control the system and all measurement options via the Internet and to get a real-time display of the measured values ​​and the status of the entire system.

Due to the vertical alignment of the radar beam, the MMCR has optimal conditions for the detection of cloud boundaries (e.g. upper and lower boundaries of clouds), as well as the speed of clouds due to the Doppler capability. The narrow beam angle of 0.2 ° (or 0.3 ° taking into account the fringe area of ​​the beam) leads to a beam width of 35 to 50 m at a height of 10 km (or 105 m at a height of 30 km). This narrow width enables quasi-point viewing of the atmosphere directly above the radar. The visualization takes place in a table in the form of an altitude-time of day diagram. The MMCR works in a wavelength range λ from 3.1 mm (W band) to 8.7 mm (Ka band, corresponds to a frequency f of 94 to 35 GHz). 

Ausstrahlwinkel//vertikale Auflösung//Reichweite(Höhe)//Wellenlängenbereich

MMCR 0.2-0.3 ° // approx. 50 m // 10-30 km // 3.1-8.7 mm

Advantages and disadvantages

Although it was specially developed and constructed for cloud observation, there are disadvantages compared to other observation systems (see below) that arise due to the special construction. The very short wavelength enables a very high spatial resolution and makes the measurements almost insensitive to echo radiation. The disadvantage of this short-wave, relatively weak radiation, however, is the high sensitivity to rain effects and the resulting reduced range, also compared to precipitation radar, which works in longer wavelength ranges. Due to this fact, the MMCR only delivers reliable results under conditions free of precipitation up to a maximum of light rain or drizzle. The influence of liquid water in clouds or water vapor is greater at a wavelength range λ of 3 mm than at 8 mm, but generally not severe. The Ka band is therefore preferred, especially for ground-based measurements. Ice crystals or snow only cause very slight weakening, which is why even strong snow storms hardly cause any weakening.

Comparison with other radar systems

In addition to the MMCR, there are two other types of meteorological radar systems that are capable of observing clouds: wind profilers and precipitation radar. However, due to their primary use for other purposes and the associated different measurement methods and priorities, they are not equally suitable for the investigation of clouds. Precipitation radar and wind profilers can only record clouds to a certain extent.

Wavelengths resolution Range disadvantage
Wind profiler 6 m-33 cm (VHF-UHF) 60-500 m 5-20 km bad resolution
Precipitation radar 10-3 cm (S- to X-band) 150-1000 m 100-450 km flat viewing angle
MMCR 9–3 mm (Ka to W band) 30-90 m 10-30 km strong rain effect

Storm monitoring precipitation radar

The American National Weather Service's storm monitoring precipitation radar, WSR-88D (NEXRAD) works with a wavelength of 3 cm (the so-called X-band) to 10 cm (S-band). This wavelength is about a power of ten longer than that of the automatic cloud radar (3 to 8 mm, W to Ka band). As a result, the ground echo generated by the radar is much more influential, as it is hardly attenuated in comparison to the signal, and therefore reliable measurements can only be made at a great distance (from around 15 km). Further problems are the flat viewing angle, which means that the vertical structure of the atmosphere cannot be adequately captured, as well as the low spatial resolution and the above-mentioned temporal inconsistency , i.e. the non-permanent use of storm monitoring precipitation radars in one place.

Radar wind profiler

Although radar wind profilers work with much larger wavelengths (33 cm to 6 m, UHF to VHF), they too have the ability to observe both clouds and precipitation (see White et al. 1996, Orr and Martner 1996, Ralph et al. 1995). However, due to the actual purpose of wind profilers - to measure horizontal winds at heights of 5 to 20 km - there is only a limited sensitivity for small parts of the cloud. However, new developments (see Ecklund et al. 1995) of wind profilers in the S-band (10 cm wavelength) enable not only the recording of precipitation but also the recording of more reflective clouds with medium spatial and temporal resolution.

literature