Where the air is clear

The MTP 5 Meteorological Temperature Profiler is an instrument for the remote measurement of the air temperature profile from ground level to 600m, typically used in studies of air pollution, atmospheric stability and climatology.

The unique technology employed in the MTP 5 is a completely passive microwave radiometer, which determines the air temperature profile using a technique called microwave remote sensing, based on the measurement of thermal radiation in the atmosphere at the centre of the molecular oxygen absorption band – around 5mm wavelength.

Temperature profile

The brightness temperature measured at different zenith angles (from 0º up to 90º) enables retrieval of PABL temperature profiles by means of calculation. After signal processing, the graphs of the temperature profile (as a function of altitude and time) are immediately displayed on a PC screen.

A unique aspect of the MTP 5 is its calibration. As the central frequency of the receiver is at the centre of a strong absorption band of molecular oxygen (14dB/km at 5mm wavelength), the brightness temperature of the atmosphere, when looking in a horizontal direction, is practically equal to the ambient air temperature. The ambient air therefore can be used as a high quality microwave target. The natural fluctuations of the ambient air temperature during a day are used to check the calibration of the receiver. Calibration is continuously carried out according to a pre-programmed protocol, making an important contribution to the quality assurance of the data.

As complex as the theory may be, using the MTP 5 is not difficult – the intelligence is built in. After installation, the device automatically performs its measurement sequence, usually doing a measurement and a subsequent self-test every 10 minutes.

The MTP 5 scans in one direction across 11 angles from horizon to zenith. The location must offer a free field of view inside the plane of measurement for about 500m. This implies that the height of installation be 15m or higher above ground level.

There are several possible methods for measurement of the PABL temperature profiles. However, none of the conventional or direct measurements are capable of providing continuous monitoring of a temperature profile above an arbitrary site.

Examples of profiling techniques are: radiosondes (a rocket, probe or balloon); sensors mounted on a meteorological tower; and remote sensing measurements with a Radar Acoustic Sounding System, or RASS. A radiosonde can measure a temperature profile at about 50m height resolution from about 50m to 30km. This requires the launching of sondes and does not give the possibility of continuous measurement. Tethered balloons and meteorological towers can provide fast response turbulence measurements in the lower 1 km (balloon) or 10-300m (tower) altitude range, however, there are strong location restrictions. A Radio Acoustic Sounding System can provide continuous profiling of the temperature from 100m to about 1km, however the technique uses a powerful sound source, which creates a noise nuisance. Also, the lower levels are not measurable, and the system will give false readings in conditions with water droplets (rain, mist etc), and with strong wind. Meteorological satellites can provide temperature profile measurements on a global scale, but cannot measure the lower 3km with sufficient accuracy.

A ground-based passive meteorological temperature profiler, like MTP 5, offers many features that are not available with the other techniques, for example a wide operational range (including conditions of fog, snow and rain), very high sensitivity, zero emission of radiation, a high level of quality assurance, automatic calibration and low operational costs, all whilst operating continuously. Further, the added features of portability and low weight are important for

conducting experiments.

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