Millimeter cloud radar

MMCR ( millimeter - wave cloud radar ) refers to a user -less cloud radar in the millimeter wavelength range.

Applications

Cloud radars play a major role in the research of the influence of clouds on climate. This influence is expressed on the one hand the role that clouds play in the radiation transfer through the atmosphere and on the other, as they are an important link in the hydrological cycle of the earth.

The American Committee on Earth Sciences (CES ) recognized in 1989 this influence, and in 1994 initiated the U.S. Department of Energy program to measure the atmospheric radiation ( Atmospheric Radiation Measurement program, ARM), which many research projects have been stimulated, including the development of a MMCR for the regular investigation of macro-and microphysical cloud properties such as layer height, number, horizontal extent, particle size, concentration and liquid water content. Previous episodic clouds studies with sounding balloons or aircraft, reported not need monitoring skills.

The development of MMCR goes back to the U.S. military. There (roughly equivalent to a wavelength λ of 8.7 mm ) were used in the late 1960s and early 70s 35 - GHz radars. While the vertically aligned AN/TPQ-11 had neither Doppler nor dual - polarization capability, could cloud structures but still quite good capture. The hardware was very prone to problems, especially the high- magnetron transmitter ( high-power magnetron transmitters ). Despite the decommissioning of the plants in the 1970s several research groups engaged in the 80s on with radars in the millimeter wavelength range (eg Pasqualucci et al. 1983, Hobbs et al., 1985, Lhermitte 1987).

In the 1990s, research has been intensified after the significant role of clouds in climate change was found ( see above). At the same time numerous engineering innovations came on the market, and newly constructed or extensively renovated cloud radars have been used in various current research (eg, Albrecht et al., 1990, Kropfli et al., 1990, Pazmany et al. 1994 Clothinaux et al., 1995).

In the Environmental Technology Laboratory ( ETL) of the American National Oceanic and Atmospheric Administration (NOAA ) was developed in the early 1980s, the so-called NOAA / K radar. This is, by continuous improvements, still in use worldwide, but requires constant care by a scientist and an engineer. His skills, among others, the good recognition of multilayer cloud structures, led the ETL to develop a new, unattended cloud radar for the ARM program. The MMCR is the evolution of NOAA / K, has in addition to the related wavelength λ of 8.7 mm but few similarities. In November 1996, the first MMCR went on the clouds and radiation measurement field (CART) of ARM in the north of Oklahoma ( USA) in operation.

Structure and properties

Externally there is a MMCR from a vertically upward parabolic antenna, which is mounted on an air-conditioned shipping containers. The antenna has a diameter of 3 m (some versions have a diameter of 2 m, resulting in better portability ), the container measures 2.5 × 2.5 × 6 m.

Inside the container there is the electronics and computer technology, which is responsible for the storage and analysis of measured data, the calibration of the system, and the transmission of data via the Internet. About Internet it is also possible to remotely control the system and all measurement options, and to get a real time display of measured values ​​and the condition of the entire system.

The MMCR has by the vertical orientation of the radar beam optimal conditions for the detection of clouds limits (eg upper and lower boundaries of clouds), and by the Doppler capability, the speed of clouds. The narrow beam angle of 0.2 ° ( or 0.3 ° in consideration of the skirt portion of the beam) leads to a beam width of 35-50 m at a height of 10 km (or 105 m in 30 km altitude). Through this small width, a quasi - point observation of the atmosphere is possible directly above the radar. The visualization is carried out in a table in the shape of a treble -day time diagram. The MMCR operating in a wavelength range λ of 3.1 mm ( W band ) to 8.7 mm ( the Ka-band, corresponding to a frequency f from 94 to 35 GHz).

Beam angle / / vertical resolution / / reach (height) / / wavelength range 0.2-0.3 ° MMCR / / 50 m / / 10-30 km / / 3.1 to 8.7 mm

Pros and Cons

Although it was designed and built specifically for cloud watching, there are disadvantages compared to other monitoring systems (see below) that occur due to the special design. The very short wavelength allows for a very high spatial resolution and allows measurements to be almost insensitive to echo radiation. However, the disadvantage of this short wavelength, relatively weak radiation is the high sensitivity to rain effects and the resulting reduced range, even compared with precipitation radar that operates at longer wavelengths. Due to this fact the MMCR provides only under precipitation -free conditions to a maximum of light rain or drizzle reliable results. The influence of liquid water in clouds or water vapor in the wavelength range λ of 3 mm greater than 8 mm, but generally not severe. Especially with ground-based measurements, the Ka-band is therefore preferred. Single crystals or snow cause very little attenuation, so even heavy snow storms cause little attenuation.

Comparison with other radar systems

In addition to the MMCR there are two other types of meteorological radar systems that are able to observe clouds: Wind Profiler and precipitation radar. However, due to their primary use for other purposes, and related to different measurement methods and priorities, they are not equally well suited for the study of clouds. Precipitation radar and wind profiler can detect clouds only to a certain extent.

Storm monitoring precipitation radar

The storm monitoring precipitation radar of the U.S. National Weather Service WSR- 88D ( NEXRAD ) operating at a wavelength of 3 cm ( the so-called X-band ) to 10 cm (S- band). This wavelength is approximately an order of magnitude longer than that of automatic cloud radar ( 3 to 8 mm, W- to Ka-band ). Characterized the bottom echo, which is generated by the radar is substantially influential because it is hardly attenuated in comparison to the signal, and are thus at a large distance ( for example from 15 km) reliable measurements can be made. Other problems are the flat viewing angle, so that the vertical structure of the atmosphere can be recognized only insufficient, and the low spatial resolution and the above temporal inconsistency, ie the non-permanent use of storm monitoring precipitation radars in one place.

Radar Wind Profiler

Although radar wind profiler with much larger wavelengths work (33 cm to 6 m, UHF to VHF ), they also 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 intended use of wind profilers - to measure horizontal winds at heights between 5 and 20 km - there is only a limited sensitivity for small clouds games. Novel developments (see Ecklund et al., 1995) of wind profilers in the S-band (10 cm wavelength), however, make it possible to precipitation measurement also capturing more reflective clouds with medium spatial and temporal resolution.