The Sensitivity of Microwave Remote Sensing Observations of Precipitation to Ice Particle Size Distributions

Ralf Bennartz Institut für Weltraumwissenschaften, Freie Universität Berlin, Berlin, Germany

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Grant W. Petty Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana

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Abstract

This study investigates the effect of variable size distribution and density of precipitation ice particles on microwave brightness temperatures. For this purpose, a set of self-consistent relationships among rain rate, size parameters of an exponential drop size distribution, and the radar reflectivity–rain rate relations for frozen and liquid precipitation was derived. Further, a scaling factor was introduced that is the ratio between the average melted diameter of the frozen and liquid precipitation and allows the specification of different sizes of the frozen particles. For given radar observations, this method allows size distributions of frozen and liquid precipitation to be derived, which are then used as input for a radiative transfer model.

These relationships were used to perform Mie calculations for different precipitation rates and different types of hydrometeors (snow, graupel, and hail) and to investigate the dependence of their respective optical properties on rain rate as well as on the radar reflectivity. It is shown that, for a given rain rate, variations of particle density as well as of particle size may result in variations of the extinction coefficient by an order of magnitude. However, a comparison of volume extinction coefficients with radar reflectivities found that, for a broad range of particle sizes, the particle density has little effect in comparison with the liquid-equivalent size of the ice particles.

The proposed method was applied to cases of coincident Special Sensor Microwave Imager (SSM/I) and radar volume scans, the latter being provided by the Swedish Gotland radar. Microwave optical fields for all four SSM/I frequencies were derived from the radar data, and the observed brightness temperature was simulated using a three-dimensional Monte Carlo radiative transfer model. A comparison of scattering indices at 85 GHz derived from the SSM/I overpass with those derived from the model data found that the size of the precipitation-sized ice particles governs the variability of the scattering index. For the particular cases investigated here, the ice particle size varied considerably depending on the type of the precipitation event. For the case of an intensive thunderstorm, ice particles are roughly four times larger than liquid precipitation at the same rain rate, but the ice particles of a frontal system are inferred to be only 20% larger than liquid ice particles. A further evaluation of the relation between scattering index and radar-derived precipitation intensity above the freezing level found high correlations (0.8–0.9) for all precipitation events. However, the sensitivity of scattering index to precipitation intensity varies within a broad range for different types of precipitation events. A convective event had a sensitivity of 9 K (mm h−1)−1, but frontal and small-scale convective events showed sensitivities in the range between 20 and 50 K (mm h−1)−1.

Corresponding author address: Ralf Bennartz, Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045.

bennartz@ukans.edu

* Current affiliation: Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas.

Current affiliation: Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin.

Abstract

This study investigates the effect of variable size distribution and density of precipitation ice particles on microwave brightness temperatures. For this purpose, a set of self-consistent relationships among rain rate, size parameters of an exponential drop size distribution, and the radar reflectivity–rain rate relations for frozen and liquid precipitation was derived. Further, a scaling factor was introduced that is the ratio between the average melted diameter of the frozen and liquid precipitation and allows the specification of different sizes of the frozen particles. For given radar observations, this method allows size distributions of frozen and liquid precipitation to be derived, which are then used as input for a radiative transfer model.

These relationships were used to perform Mie calculations for different precipitation rates and different types of hydrometeors (snow, graupel, and hail) and to investigate the dependence of their respective optical properties on rain rate as well as on the radar reflectivity. It is shown that, for a given rain rate, variations of particle density as well as of particle size may result in variations of the extinction coefficient by an order of magnitude. However, a comparison of volume extinction coefficients with radar reflectivities found that, for a broad range of particle sizes, the particle density has little effect in comparison with the liquid-equivalent size of the ice particles.

The proposed method was applied to cases of coincident Special Sensor Microwave Imager (SSM/I) and radar volume scans, the latter being provided by the Swedish Gotland radar. Microwave optical fields for all four SSM/I frequencies were derived from the radar data, and the observed brightness temperature was simulated using a three-dimensional Monte Carlo radiative transfer model. A comparison of scattering indices at 85 GHz derived from the SSM/I overpass with those derived from the model data found that the size of the precipitation-sized ice particles governs the variability of the scattering index. For the particular cases investigated here, the ice particle size varied considerably depending on the type of the precipitation event. For the case of an intensive thunderstorm, ice particles are roughly four times larger than liquid precipitation at the same rain rate, but the ice particles of a frontal system are inferred to be only 20% larger than liquid ice particles. A further evaluation of the relation between scattering index and radar-derived precipitation intensity above the freezing level found high correlations (0.8–0.9) for all precipitation events. However, the sensitivity of scattering index to precipitation intensity varies within a broad range for different types of precipitation events. A convective event had a sensitivity of 9 K (mm h−1)−1, but frontal and small-scale convective events showed sensitivities in the range between 20 and 50 K (mm h−1)−1.

Corresponding author address: Ralf Bennartz, Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045.

bennartz@ukans.edu

* Current affiliation: Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas.

Current affiliation: Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin.

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