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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

Attenuation in the atmosphere

Microwaves are subject to attenuation due to atmospheric gases, clouds, and precipitation by absorption and scattering.

ATTENUATION BY HYDROMETEORS

Attenuation by hydrometeors can result from both absorption and scattering. It is the most significant source of attenuation. It is dependent on the shape, size, number and composition of the particles. This dependence has made it very difficult to overcome, in any quantitative way using radar observations alone. It has not been satisfactorily overcome for automated operational measurement systems yet. However, the phenomenon must be recognized and the effects reduced by some subjective intervention using general knowledge.

Attenuation is dependent on wavelength. At 10 cm wavelengths, the attenuation is rather small while at 3 cm it is quite significant. At 5 cm, the attenuation may be acceptable for many climates particularly in the high mid-latitudes. Wavelengths below 5 cm are not recommended for good precipitation measurement except for short-range applications.

One-way attenuation relationships

Wavelength (cm)	Relation (dB km^-1)
10	0.000 343 R^0-97
5	0.00 18 R^1-05
3.2	0.01 R^I.21

After Burrows and Attwood (1949). One way specific attenuations at 18°C. R is in units of mm hrl.

For precipitation estimates by radar, some general statements can be made with regard to the magnitude of attenuation. The attenuation is dependent on water mass of the target, thus heavier rains attenuate more; clouds with much smaller mass attenuate less. Ice particles attenuate much less than liquid particles. Clouds and ice clouds cause little attenuation and can usually be ignored. Snow or ice particles (or a hailstone) can grow to a size much larger than a raindrop. They become wet as they begin to melt and result in a large increase in reflectivity and, therefore, in attenuation properties. This can distort precipitation estimates.

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