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C.G. Collier
particles absorb the 1.6J.lD1 and 3.7J.lD1 radiation much stronger than small cloud
droplets. This effect makes it possible to calculate the effective radius (reff = integral
volume divided by integral surface area) of particles (Arking and Childs, 1985).
Rosenfeld and Gutman (1994) have shown that reff = 14J.lD1 can serve to delineate
precipitating clouds regardless of the temperature of their tops.
A useful skill in estimating rain areas has been identified, although experience
indicates that the probability of rain table should be changed seasonally. The radar
verification shows that the main skill lies in identifying three classes, namely clear,
cloudy with low probability of rain, and cloudy with a significant probability of rain.
The accuracy of the results diminishes with increasing distance from the training
radar. However, it was pointed out that use of a coarse resolution might allow the
introduction of texture matrices. Rainfall over periods from Y2 to 2 hours may be
estimated with an accuracy of around 49% over areas of 10 5 km 2 . However, more
extensive assessments for different rainfall types are required to confirm this performance.
6.3.6 Passive microwave estimates of rainfall from space
Microwaves provide the measurements that are physically best related to the actual
precipitation, especially in the longest wavebands. The interactions of passive MW
with precipitation clouds and the surface are discussed by Rosenfeld and Collier
(1998) using two wavebands, shorter (85 Ghz) and longer (19 Ghz).
(a)
Absorption-based measurements
Water drops have relatively large absorption/emission coefficient, increasing for
the higher frequencies. The emission is proportional to the vertically integrated
cloud and rain water in the low frequencies, but due to the increased emissivity for
the higher frequencies the emission saturates for light rain intensities.
(b)
Scattering-based measurements
Ice particles have relatively small absorption/emission, but they are good
scatterers of the MW radiation, especially at the higher frequencies. Therefore, at high frequencies (85 Ghz) the large scattering from the ice in the upper portions of the clouds makes the ice an effective insulator, because it reflects back down most of the radiation emitted from the surface and from the
rain. The remaining radiation that reaches the MW sensor is interpreted as
a colder brightness temperature. A major source of uncertainty for the scattering-based retrievals is the lack of a consistent relationship between the
frozen hydrometers aloft and the rainfall reaching the surface.
Simple comparisons of the ESMR-5 (Electrically Scanning Microwave Radiometer)
imagery operating at 19.33 Ghz (1.55 cm wavelength) with imagery from visible and
infrared wavelength radiometers, ground-based radars and conventional meterological
C.G. Collier
particles absorb the 1.6J.lD1 and 3.7J.lD1 radiation much stronger than small cloud
droplets. This effect makes it possible to calculate the effective radius (reff = integral
volume divided by integral surface area) of particles (Arking and Childs, 1985).
Rosenfeld and Gutman (1994) have shown that reff = 14J.lD1 can serve to delineate
precipitating clouds regardless of the temperature of their tops.
A useful skill in estimating rain areas has been identified, although experience
indicates that the probability of rain table should be changed seasonally. The radar
verification shows that the main skill lies in identifying three classes, namely clear,
cloudy with low probability of rain, and cloudy with a significant probability of rain.
The accuracy of the results diminishes with increasing distance from the training
radar. However, it was pointed out that use of a coarse resolution might allow the
introduction of texture matrices. Rainfall over periods from Y2 to 2 hours may be
estimated with an accuracy of around 49% over areas of 10 5 km 2 . However, more
extensive assessments for different rainfall types are required to confirm this performance.
6.3.6 Passive microwave estimates of rainfall from space
Microwaves provide the measurements that are physically best related to the actual
precipitation, especially in the longest wavebands. The interactions of passive MW
with precipitation clouds and the surface are discussed by Rosenfeld and Collier
(1998) using two wavebands, shorter (85 Ghz) and longer (19 Ghz).
(a)
Absorption-based measurements
Water drops have relatively large absorption/emission coefficient, increasing for
the higher frequencies. The emission is proportional to the vertically integrated
cloud and rain water in the low frequencies, but due to the increased emissivity for
the higher frequencies the emission saturates for light rain intensities.
(b)
Scattering-based measurements
Ice particles have relatively small absorption/emission, but they are good
scatterers of the MW radiation, especially at the higher frequencies. Therefore, at high frequencies (85 Ghz) the large scattering from the ice in the upper portions of the clouds makes the ice an effective insulator, because it reflects back down most of the radiation emitted from the surface and from the
rain. The remaining radiation that reaches the MW sensor is interpreted as
a colder brightness temperature. A major source of uncertainty for the scattering-based retrievals is the lack of a consistent relationship between the
frozen hydrometers aloft and the rainfall reaching the surface.
Simple comparisons of the ESMR-5 (Electrically Scanning Microwave Radiometer)
imagery operating at 19.33 Ghz (1.55 cm wavelength) with imagery from visible and
infrared wavelength radiometers, ground-based radars and conventional meterological