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Tuesday, September 07, 2010

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General Weather: Normal summer weather , but dust haze at first.

Wind: Mainly n'ly 05 to 10kt reaching 10 to 15kt at times.

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Radar


	Education Centre

General Meteorology :	Air Masses - Cold Front - Fog - High Pressure - Humidity - Introduction to Weather - Large Thunderstorms - lightning - Low-Pressure - Meteorology - Occlusion Fronts - Rain - Sea Breaze & Land Breaze - Temperature - The Water Cycle - Tornadoes - Warm Front - Wind

Clouds :	Cirrus - Clouds - Cumulus - Cumulonimbus -

Radar :	General - Radar Technology - Attenuation in the atmosphere - Velocity measurements - Sources of error - Optimizing radar characteristics - Radar installation - Precipitation measurements

Radar Technology

Principles of radar measurement
Basic weather radar
Doppler radar
Ground clutter rejection

Principles of radar measurement
The principles of radar and the observation of weather phenomena were established in the 1940s. Since that time, great strides have been made in improving equipment, signal, and data processing and its interpretation.
Most meteorological radars are pulsed radars. Electromagnetic waves at fixed preferred frequencies are transmitted from a directional antenna into the atmosphere in a rapid succession of short pulses.
A parabolic reflector in the antenna system concentrates the electromagnetic energy in a conical shaped beam which is highly directional. The width of the beam increases with range, for example, a nominal I° beam spreads to 0.9, 1.7 and 3.5 km at ranges of 50, 100, and 200 km, respectively.
The short bursts of electromagnetic energy are absorbed and scattered by any meteorological targets encountered. Some of the scattered energy is reflected back to the radar antenna and receiver. Since the electromagnetic wave travels with the speed of light (that is, 2.99 x 108 m s-1), by measuring the time between transmission of the pulse and its return, the range of the target is determined. Between successive pulses, the receiver listens for any return of the wave.
The return signal from the target is commonly referred to as the radar echo.

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Basic weather radar
The basic weather radar consists of:
(a) A transmitter to produce power at microwave frequency;
(b) An antenna to focus the transmitted microwaves into a narrow beam and receive any returning power;
(c) A receiver to detect, amplify, and convert the microwave signal into a low frequency signal;
(d) A processor to extract the desired information from the received signal; and
(e) A system to display the information in an intelligible form.
Other components that maximize the radar capability are:
(a) A processor to produce supplementary displays; and
(b) A recording system to archive the data for training, study and record.

A basic weather radar may be non-coherent, that is the phase of successive pulses is random and unknown. Almost exclusively today's systems use computers for radar control, digital signal processing, recording, product displays, and archiving.
The useful range of weather radar, except for long range detection only of thunderstorms, is of the order of 200 km. The beam at an elevation, of say 0.5°, is at a height of 4 km above the Earth's surface. Also, the beamwidth is of the order of 1.5 kin or greater. For good quantitative precipitation measurements, the range is less than 200 km. At long ranges, the beam is too high for ground estimates. Also, beam spreading reduces resolution and the measurement can be affected by underfilling with target. Technically, there is a maximum unambiguous range determined by the pulse duration and the pulse repetition frequency (equation 9.5) since the range must be measured during the listening period between pulses. At usual PRFs this is not a problem. For example, with a PRF of 250 pulses per second, the maximum range is 600 km. At higher PRFS, typically I 000 pulses per second, required for Doppler systems, the range will be greatly reduced to about 100 km. New developments may ameliorate this situation (Joe, et al., 1995).

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Doppler radar
The development and introduction of Doppler weather radars to weather surveillance provides a new dimension to the observations. Doppler radar provides a measure of the targets' velocity along a radial from the radar in a direction either towards or away from the radar. A further advantage of the Doppler technique is the greater effective sensitivity to low reflectivity targets near the radar noise level when the velocity field can be distinguished in a noisy Z field.
At the normal speeds of meteorological targets, the frequency shift is relatively small compared to the radar frequency and is very difficult to measure. An easier task is to retain the phase of the transmitted pulse, compare it with the phase of the received pulse and then determine the change in phase between successive pulses. 'Me time rate of change of the phase is then directly related to the frequency shift, which in turn is directly related to the target velocity - the Doppler effect. If the phase changes by more than ±180', then the velocity estimate is ambiguous. The highest unambiguous velocity that can be measured by a Doppler radar is the velocity at which the target moves, between successive pulses, more than a quarter of the wavelength. At higher speeds, an additional processing step is required to retrieve the correct velocity.

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Ground clutter rejection
Echoes due to non-precipitation targets are known as clutter, and should be eliminated. An exception is the echoes due to clear air or insects that can be used to map out wind fields. Clutter can be the result of a variety of targets including buildings, hills, mountains, airplanes, other radars and chaff, to name just a few. Good radar siting is the first line of defense against ground clutter effects. However, clutter is always present to some extent. The intensity of ground clutter is inversely proportional to wavelength (Skolnik, 1970), whereas backscatter from rain is inversely proportional to the fourth power of wavelength. Therefore, shorter wavelength radars are less affected by ground clutter.
Clutter can be reduced by careful site selection. Radars used for long-range surveillance, such as for tropical cyclones or in a widely scattered network, are usually placed on hills to extend the useful range, and are therefore likely to see many clutter echoes. A simple suppression technique is to scan automatically at several elevations, and to discard the data at the shorter ranges from the lower elevations, where most of the clutter exists.

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