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Tuesday, September 07, 2010

Today's Weather

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

Sources of error

RADAR BEAM FILLING
In many cases, and especially at large ranges from the radar, the pulse volume is not completely filled with homogeneous precipitation. Precipitation intensities often vary widely on small scales; at large distances from the radar, the pulse volume increases in size. At the same time, the effects of the Earth curvature become important. In general, measurements may be quantitatively useful for ranges less than 100 km. This effect is important for cloud-top height measurements and estimation of reflectivity.

NON-UNIFORMTY OF THE VERTICAL DISTRIBUTION OF PRECIPITATION
The first parameter of interest when making radar measurements is usually the precipitation at ground level.Because of the effects of beam width, beam tilting, and Earth curvature, radar measurements of precipitation are higher than average over a considerable depth. These measurements are dependent on the details of the vertical distribution of precipiation and can contribute to large errors for estimates of precipitation on the ground.

VARIATIONS IN THE Z-R RELATIONSHIP
A variety of Z-R relationships have been found for different precipitation type. However, from the radar alone, these variations in types and size distribution of hydrometeors cannot be estimated. In operational applications, this variation can be an important source of error.

ATTENUATION BY INTERVENING PRECIPITATION
Attenuation by rain may be important, especially at the shorter radar wavelengths (5 and 3 cm). Attenuation by snow, although less than for rain, may be important over long path lengths.

BEAM BLOCKING
Depending on the radar installation, the radar beam may be partly or completely occulted by topography or obstacles, located between the radar and the target. This results in underestimations of reflectivity and, hence, of rainfall rate.

ATTENUATION DUE TO A WET RADOMIE
Most radar antennas are protected from wind and rain by a radome, usually made of fiberglass. The radome is engineered to cause little loss in the radiated energy. For instance, the two-way loss due to this device can be easily kept to less than IdB at the C band, under normal conditions. However, under intense rainfall, the surface of the radome can become coated with a thin film of water or ice, resulting in a strong azimuth dependent attenuation.

ELECTROMAGNETIC INTERFERENCE
Electromagnetic interference from other radars or devices, such as microwave links, may be an important factor of error in some cases. This type of problem is easily recognized by observation. It may be solved by negotiation, by changing frequency, by using filters in me radar receiver, and sometimes by software.

GROUNID CLUTTER
Contamination of rain echoes by ground clutter may bring very large errors in precipitation and wind estimation. The ground clutter should first be minimized by good antenna engineering and a good choice of the radar location. This effect may be greatly reduced by a combination of hardware clutter suppression devices (Aoyagi, 1983) and through signal and data processing. Ground clutter is greatly increased in situations of anomalous propagation.

ANOMAILOUS PROPAGATION
Anomalous propagation distorts the radar beam path and has the effect of increasing ground clutter by refracting the beam towards the ground. It may also cause the radar to detect storms located far beyond the usual range, making errors in their range determination because of range aliasing. Anomalous propagation is frequent in some regions, when the atmosphere is subject to strong decreases in humidity and/or increases in temperature with height. Clutter returns due to anomalous propagation may be very misleading to untrained human observers and are more difficult to eliminate fully by processing them as normal ground clutter.

ANTENNA ACCURACY
The antenna position may be known within 0.2° with a weh-engineered system. Errors may also be produced by the excessive width of the radar beam or by the presence of sidelobes, in the presence of clutter or of strong precipitation echoes.

ELECTRONICS STABILITY
Modem electronic systems are subject to small varia tions with time. This may be controlled by using a well-engineered monitoring system, which will keep the variations of the electronics within less than I dB, or activate an alarm when a fault is detected.

PROCESSING ACCURACY
The signal processing must be designed to take the best advantage of the sampling capacities of the system. The variances in the estimation of reflectivity, Doppler velocity, and spectrum width must be kept to a minimum. Range and velocity aliasing may be important sources of error-

RADAR RANGE EQUATION
There are many assumptions in interpreting radarreceived power measurements in terms of the meteorological parameter Z by the radar range equation. Non-conformity with the assumptions can cause error.

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