Retrieval of Precipitation from Satellites
271
Figure 11.11: Data coverage by SSM/I during one day.
Wilheit et al. (1991) proposed an interesting procedure to overcome this deficiency by assuming
that the number of occurrances of monthly rain totals within an area should exhibit a log-normal
frequency distribution. Preliminar rain rates are retrieved for 5° x 5° boxes over one month
by an emission algorithm based on a simplified cloud/precipitation model using the 19 GHz
and 22 GHz channels of SSM/I. The height of the freezing level, which is a necessary piece of
information for these types of algorithms, is estimated as a constant for the whole grid-area and
the month from the frequency distributions of the 19 GHz and 22 GHz channel measurements.
The observed rain rate frequency distribution is then used to fit an log-normal distribution
overlapping a normal distribution representative for the non-rain cases. Fitting parameters
are besides a constant the logarithmic mean rainrate, the logarithmic standard deviation of
rainfall, and the distiribution parameters of the normal distribution for the rain-free areas.
Basically, this method predicts the seldomly observed, and thus uncertain, but very important
high rainrate occurrances from the more frequently observed low rainrate cases. The method
is, however, restricted to the tropical oceanic atmosphere due to the emission algorithm used
and the assumptions of constant freezing level over a large area and time.
11.8.3 Three-dimensional geometry
Most algorithms for the microwave region are based on results obtained from one-dimensional
radiative transfer simulations. Microwave radiometers, however, measure at a view zenith angle
around 50 degrees looking mostly at the side walls of the clouds. Especially over the oceans
three-dimensional effects caused by the reflection of radiation off the water surface can amount
to several tens of degrees in radiation temperature (Liu and Simmer, 1994). The effect on the
retrieval algorithms depends on the signal which mainly used by the algorithm. A study by
Petty (1994a) gives an indication that emission based algorithms over the oceans will suffer
more heavily from three-dimensional effects than the scattering-based algorithms.
271
Figure 11.11: Data coverage by SSM/I during one day.
Wilheit et al. (1991) proposed an interesting procedure to overcome this deficiency by assuming
that the number of occurrances of monthly rain totals within an area should exhibit a log-normal
frequency distribution. Preliminar rain rates are retrieved for 5° x 5° boxes over one month
by an emission algorithm based on a simplified cloud/precipitation model using the 19 GHz
and 22 GHz channels of SSM/I. The height of the freezing level, which is a necessary piece of
information for these types of algorithms, is estimated as a constant for the whole grid-area and
the month from the frequency distributions of the 19 GHz and 22 GHz channel measurements.
The observed rain rate frequency distribution is then used to fit an log-normal distribution
overlapping a normal distribution representative for the non-rain cases. Fitting parameters
are besides a constant the logarithmic mean rainrate, the logarithmic standard deviation of
rainfall, and the distiribution parameters of the normal distribution for the rain-free areas.
Basically, this method predicts the seldomly observed, and thus uncertain, but very important
high rainrate occurrances from the more frequently observed low rainrate cases. The method
is, however, restricted to the tropical oceanic atmosphere due to the emission algorithm used
and the assumptions of constant freezing level over a large area and time.
11.8.3 Three-dimensional geometry
Most algorithms for the microwave region are based on results obtained from one-dimensional
radiative transfer simulations. Microwave radiometers, however, measure at a view zenith angle
around 50 degrees looking mostly at the side walls of the clouds. Especially over the oceans
three-dimensional effects caused by the reflection of radiation off the water surface can amount
to several tens of degrees in radiation temperature (Liu and Simmer, 1994). The effect on the
retrieval algorithms depends on the signal which mainly used by the algorithm. A study by
Petty (1994a) gives an indication that emission based algorithms over the oceans will suffer
more heavily from three-dimensional effects than the scattering-based algorithms.