GPM – Backscatter Modeling of Winter Precipitation
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Project consortium: |
Finnish Meteorological Institute |
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Finnish Environment Institute |
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University of Helsinki |
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Aalto University |
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Vaisala Oy |
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Project funding: |
Academy of Finland |
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Project duration: |
1 January 2009 – 31 December 2012 |
Introduction
The GPM is a NASA managed project of spaceborne measurements of the global precipitation and its influence on Earth’s hydrological cycle. Mission includes an international network of satellites, where the “core” satellite is carrying a Ku/Ka-band Dual-frequency Precipitation Radar (DPR) and a multi-channel GPM Microwave Imager (GMI) ranging in frequencies from 10 GHz to 183 GHz, and where the planned eight supporting satellites are carrying duplicates of the GMI. (http://pmm.nasa.gov/GPM/)
In Finland the objective of the Academy funded project is to validate existing algorithms and to develop new ones for monitoring winter precipitation, like water, slush, snow and ice. In Aalto the work is concentrated on modeling the melting snow in atmospheric layer, in, so called, melting layer, and researching the polarimetric properties of melting hydrometeors (Figure 1). One of the aims is to clarify the changes in the propagation attenuation at different frequencies.
Figure 1. Image of radar reflectivity on 10 December, 2011. Melting layer is located on the ground and its top at 0.35 km is marked.
Research
In modeling of the melting layer two models are driven side by side, microphysical and electromagnetic model. The microphysical model utilizes the temperature and humidity profile, and possible also wind speed fields to define the rate of melting, the changes in size distribution and fall speed of hydrometeors. It specifies the volume fractions of ice, air and water of an assembly of particles at specific height. There exits both empirical and theoretical models. The theoretical models differ from each other in respect of the components taken into account in heat balance equation, or whether they consider particle formation or breakup inside the melting process. The electromagnetic model calculates the scattering properties of hydrometeors as function of their size, proportion of melting and radar wavelength. A common practice in the past was that the scattering particles were assumed to be spherical with equivalent radius, since the widely known analytical solution of vector Helmholtz equation derived by Mie could solve the scattering problem with a high numerical accuracy. In recent research the emphasis is on more realistic modeling of the particles and thus deriving the scattering characteristics induced from the complexity of the particle structure.
In Aalto the scattering properties of the melting hydrometeors are calculated with discrete dipole approximation (DDA), and compared to Mie solution (Figure 2). The DDA is a flexible and a general technique for calculating the electrical properties of particles in arbitrary shapes, where the dielectric scatterer is described as a collection of small subunits. These subunits behave as dielectric dipoles in terms of their response to an applied electronic field and in addition the dipoles interact with each other. The scattered electric field of the particle is then calculated as linear combination of excited fields of each dipole as a function of incident electric field.
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Figure 2. A melting spherical hydrometeor described for the scattering calculations with DDA-method.