6 Precipitation
125
observations established, over water, an association of areas of relatively warm
brightness temperature with areas of rainfall. Curves of 19.35 Ghz brightness temperatures and rainfall rate were published by Wilheit et al. (1977) using a model
based upon the concept of many optically thin layers bounded on top by a variable
freezing level. Improvements to the technique, including the use of both vertical and
horizontal polarisation for measurements over land, have been proposed by Wilheit
(1975).
Over land the passive MW algorithms can detect rain mainly by the ice scattering
mechanism (b) above. This indirect rain estimation method is less accurate. Moreover, rainfall over land from clouds which do not contain significant amounts of ice
aloft goes mostly undetected. A passive radiation method that is able to detect such
rain is the 'effective radius' method of Rosenfeld and Gutman (1994).
Spencer (1986) proposed a technique based upon analysis of radiometric data
gathered from the NIMBUS-7 SMMR (Scanning Multichannel Microwave Radiometer) at a frequency of37 Ghz (0.81 em wavelength). This radiometer system provides
dual-polarized data at five different wavelengths (0.81, 1.42, 1.66,2.80 and 4.54 cm)
and can be used to detect a wide range of snow and ice features as well as rainfall.
Spencer (1986) suggests that 37 Ghz can provide the best compromise for measuring
rainfall over the ocean between an easily detectable precipitation signal (such as at
19.35 Ghz) and cloud penetration ability. However, it was noted that the scattering
signal at 37 Ghz is not as strong as the emission signal of light precipitation at
19 Ghz. Hence the 37 Ghz technique may be better for observing heavy convective
precipitation, and observations at 19 Ghz may relate better to measurements oflight
precipitation. Recently SMMR data have been used to derive rainfall fields over the
Indian Ocean (Martin et aI., 1993).
Much work is now being carried out to investigate the ability to measure precipitation of the Special Sensor Microwave Imager (SSM/I) 88.5 Ghz channels of the US
Defense Meterological Satellite Programme (DMSP) satellite at present in polar orbit.
The scattering-based technique is applicable at this frequency, and early results
reported by Spencer et al. (1989) are very encouraging albeit for convective rainfall.
It remains to be seen to what extent mid latitude frontal rainfall can be measured
unambiguously. Recently, Ferraro et al. (1994) described how the identification of
surfaces having signatures similar to that of rain can improve SSMII estimation
techniques. Figure 6.4 summarizes the relationship between rainfall rate and various
passive microwave frequencies.
Measurements of precipitation have also been made using passive microwave
imagery near 118 Ghz (Spencer et al. 1994, Schwartz et aI., 1996). At these frequencies scattering from large, dense graupel in the tops of convective cells produces large
decreases in radiances. However, liquid hydrometeors produce much smaller changes
and overlying cloud may obscure precipitation at low levels. Finally, Kummerowand
Giglio (1995) have described a technique for combining microwave and infrared
observations, and Alder et al. (1991) have simulated microwave satellite observations
using a three-dimensional cloud model.
125
observations established, over water, an association of areas of relatively warm
brightness temperature with areas of rainfall. Curves of 19.35 Ghz brightness temperatures and rainfall rate were published by Wilheit et al. (1977) using a model
based upon the concept of many optically thin layers bounded on top by a variable
freezing level. Improvements to the technique, including the use of both vertical and
horizontal polarisation for measurements over land, have been proposed by Wilheit
(1975).
Over land the passive MW algorithms can detect rain mainly by the ice scattering
mechanism (b) above. This indirect rain estimation method is less accurate. Moreover, rainfall over land from clouds which do not contain significant amounts of ice
aloft goes mostly undetected. A passive radiation method that is able to detect such
rain is the 'effective radius' method of Rosenfeld and Gutman (1994).
Spencer (1986) proposed a technique based upon analysis of radiometric data
gathered from the NIMBUS-7 SMMR (Scanning Multichannel Microwave Radiometer) at a frequency of37 Ghz (0.81 em wavelength). This radiometer system provides
dual-polarized data at five different wavelengths (0.81, 1.42, 1.66,2.80 and 4.54 cm)
and can be used to detect a wide range of snow and ice features as well as rainfall.
Spencer (1986) suggests that 37 Ghz can provide the best compromise for measuring
rainfall over the ocean between an easily detectable precipitation signal (such as at
19.35 Ghz) and cloud penetration ability. However, it was noted that the scattering
signal at 37 Ghz is not as strong as the emission signal of light precipitation at
19 Ghz. Hence the 37 Ghz technique may be better for observing heavy convective
precipitation, and observations at 19 Ghz may relate better to measurements oflight
precipitation. Recently SMMR data have been used to derive rainfall fields over the
Indian Ocean (Martin et aI., 1993).
Much work is now being carried out to investigate the ability to measure precipitation of the Special Sensor Microwave Imager (SSM/I) 88.5 Ghz channels of the US
Defense Meterological Satellite Programme (DMSP) satellite at present in polar orbit.
The scattering-based technique is applicable at this frequency, and early results
reported by Spencer et al. (1989) are very encouraging albeit for convective rainfall.
It remains to be seen to what extent mid latitude frontal rainfall can be measured
unambiguously. Recently, Ferraro et al. (1994) described how the identification of
surfaces having signatures similar to that of rain can improve SSMII estimation
techniques. Figure 6.4 summarizes the relationship between rainfall rate and various
passive microwave frequencies.
Measurements of precipitation have also been made using passive microwave
imagery near 118 Ghz (Spencer et al. 1994, Schwartz et aI., 1996). At these frequencies scattering from large, dense graupel in the tops of convective cells produces large
decreases in radiances. However, liquid hydrometeors produce much smaller changes
and overlying cloud may obscure precipitation at low levels. Finally, Kummerowand
Giglio (1995) have described a technique for combining microwave and infrared
observations, and Alder et al. (1991) have simulated microwave satellite observations
using a three-dimensional cloud model.