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NCAS Atmospheric Measurement Facility's (AMF) mobile X-band radar

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The National Centre for Atmospheric Science Atmospheric Measurement Facility's (NCAS AMF) mobile X-band radar operates in the X-band (frequency = 9.375 GHz, wavelength = 3 cm). It has Doppler and dual-polarisation capability. The radar measures the location and intensity of precipitation, radial winds and polarisation parameters. It works by transmitting pulses of electromagnetic radiation and measuring the amount of energy backscattered to the receiver. The time delay between the transmitted and received pulses enables the range of the object to be determined. In the case of precipitation, the magnitude of the backscattered signal is proportional to the size and number of raindrops. The radar also measures the radial velocity through the Doppler effect. In the absence of precipitation, the radar is sensitive to backscattering from insects, known as clear-air echoes, at close range. These signals provide information on the wind field and the organisation of the boundary-layer e.g. features such as convergence lines can be identified. In addition, the radar has dual-polarisation capability; it simultaneously transmits and receives horizontally- and vertically- polarised waves. Through this technique the radar measures a number of additional variables that provide information on the size and shape of precipitation, identification of non-meteorological echoes, attenuation (a decrease in the signal strength due to the absorption and scattering of the radar signal as it propagates through heavy rain), and alternative methods for calculating rainfall rates.

The system has a conventional parabolic antenna of 2.4m diameter and a beamwidth of 1 degree (in the horizontal and vertical), and is operated without a radome. The range resolution is tunable and dependent on the user-selected pulse width (0.5, 1 or 2 micro seconds) and pulse repetition frequency (PRF, 250-2000 Hz). The radar is fully transportable, being mounted on a trailer approved for EU roads that can be towed by a 4x4 vehicle. Signal analysis, data retrievals and data storage are performed by PCs contained within the trailer unit. The radar can be operated via a laptop that connects by wireless, ethernet or 3G to the onboard PCs. The operational software allows the user to set up the radar for deployment and schedule the scanning sequence. The software also provides real-time data visualisation and product generation.

The data frequency is dependant on the requirements for the instrument's deployment, but is typically of the order of 10 360-degree scans at various elevations every 5 minutes.

Parameters available in data files from this instrument include:
DOP - degree of polarization;
KDP - specific differential phase shift;
PhiDP - differential phase shift;
RhoHV - co-polar cross correlation coefficient;
SQI - signal quality index or normalized_coherent_power;
V - radial velocity;
W - spectral width;
ZDR - differential reflectivity;
dBZ - reflectivity or equivalent reflectivity factor.

Abbreviation: ncas-mobile-xband-radar
Keywords: NCAS, AMF, radar, precipitation, rainfall, dual-polarisation, Doppler, clouds, microphysics, clear-air echoes, convection, storms, dynamics, rainfall