272
C. Simmer
11.8.4 Aspherical hydrometeors
The shape of large raindrops differs from spheres and the particles exhibit predominant orientations due to the air flow (e.g. Pruppacher and Klett, 1978), and hail particles are also
inhomogeneous inside. Usually these particles are treated as homogeneous spheres in the simulations, causing errors which have not been quantified so far. Some authors take into account
aspherical particles but only in an empirical approach to correct for polarization differences observed by SSM/I above optically thick rain clouds (e.g. Kummerowand Giglio, 1994). Theories
to take into accout non-spherical particles are known (e.g. Wauben and Hovenier, 1992; Jin,
1992; Wauben et aI., 1993; Mishchenko and Travis, 1994) but adequate information about the
distribution of these particles in precipitating clouds is still not at hand.
11.8.5 Variability of the state of the atmosphere
The main reason for the observed discrepancies in quantitative rain retrieval by different algorithms is that the precipitation process has many more degrees of freedom than can be fixed
by only a few number of radiation measurements. Any algorithm inherently assumes its own
cloud/precipitation model and most of the differences can be retraced to differences between
those. To reduce the number of degrees of freedom, measurements from different spectral ranges must be combined and sound assumptions must be made about the vertical and horizontal
structure of the observed raining system. One possibility is the assimilation of the satellite
measurements into dynamic cloud models which has been pursued recently by several authors
(e.g. Mugnai et aI., 1992; Smith et aI., 1992; Kummerowand Giglio, 1994).
11.9 Outlook
From the vast amount of published simple I-line either statistically-based or indexing-based
rainrate retrieval algorithms, and from the large scatter when the results are compared with
each other or with independent estimates it is clear, that the determination of rainrate at the
surface from satellite measurements is still an unsolved problem. But we must also admit,
that rain retrieval from satellites is at the present time also the most accurate way to estimate
rainfall at least from the global to the regional scale.
It can be observed from still on-going studies within PIP-2 and PIP-3, that the estimates
from the more complicated microwave-based algorithms, which take into account explicitly
the different footprint sizes and antenna patterns of the radiometers and head at consistency
between observed and modelled radiances, do converge on similar results. This makes clear
that rainfall can only be accurately determined when a complete description of the physical
parameters of atmosphere and surface, both vertically and horizontally, is part of the retrieval.
If this is accepted it is obvious, that apt consideration of other spectral channels like the visible
and infrared window channels and sounding channels in both infrared and microwave will further
enhance the quality of rainrate retrieval and thus our knowledge of a most important variable
in the climate system.
11.10 References
Adler RF, Negri AJ (1988) A satellite infrared technique to estimate tropical convective and
stratiform rainfall. Journal of Applied Meteorology 27: 30-51
C. Simmer
11.8.4 Aspherical hydrometeors
The shape of large raindrops differs from spheres and the particles exhibit predominant orientations due to the air flow (e.g. Pruppacher and Klett, 1978), and hail particles are also
inhomogeneous inside. Usually these particles are treated as homogeneous spheres in the simulations, causing errors which have not been quantified so far. Some authors take into account
aspherical particles but only in an empirical approach to correct for polarization differences observed by SSM/I above optically thick rain clouds (e.g. Kummerowand Giglio, 1994). Theories
to take into accout non-spherical particles are known (e.g. Wauben and Hovenier, 1992; Jin,
1992; Wauben et aI., 1993; Mishchenko and Travis, 1994) but adequate information about the
distribution of these particles in precipitating clouds is still not at hand.
11.8.5 Variability of the state of the atmosphere
The main reason for the observed discrepancies in quantitative rain retrieval by different algorithms is that the precipitation process has many more degrees of freedom than can be fixed
by only a few number of radiation measurements. Any algorithm inherently assumes its own
cloud/precipitation model and most of the differences can be retraced to differences between
those. To reduce the number of degrees of freedom, measurements from different spectral ranges must be combined and sound assumptions must be made about the vertical and horizontal
structure of the observed raining system. One possibility is the assimilation of the satellite
measurements into dynamic cloud models which has been pursued recently by several authors
(e.g. Mugnai et aI., 1992; Smith et aI., 1992; Kummerowand Giglio, 1994).
11.9 Outlook
From the vast amount of published simple I-line either statistically-based or indexing-based
rainrate retrieval algorithms, and from the large scatter when the results are compared with
each other or with independent estimates it is clear, that the determination of rainrate at the
surface from satellite measurements is still an unsolved problem. But we must also admit,
that rain retrieval from satellites is at the present time also the most accurate way to estimate
rainfall at least from the global to the regional scale.
It can be observed from still on-going studies within PIP-2 and PIP-3, that the estimates
from the more complicated microwave-based algorithms, which take into account explicitly
the different footprint sizes and antenna patterns of the radiometers and head at consistency
between observed and modelled radiances, do converge on similar results. This makes clear
that rainfall can only be accurately determined when a complete description of the physical
parameters of atmosphere and surface, both vertically and horizontally, is part of the retrieval.
If this is accepted it is obvious, that apt consideration of other spectral channels like the visible
and infrared window channels and sounding channels in both infrared and microwave will further
enhance the quality of rainrate retrieval and thus our knowledge of a most important variable
in the climate system.
11.10 References
Adler RF, Negri AJ (1988) A satellite infrared technique to estimate tropical convective and
stratiform rainfall. Journal of Applied Meteorology 27: 30-51