Recent studies have shown that polarimetric radars are capable of providing distributions of rain intensity with high
accuracy. Variables obtained by the polarimetric radars include radar reflectivity factor (Zhh), differential propagation
phase (Φdp) and differential reflectivity (Zdr). A number of methods to estimate rain intensity from these variables have
been proposed. In this study, the rain intensity estimated from the differential reflectivity and radar reflectivity factor
measured with a C-band polarimetric radar is used to analyze a local heavy rainfall event as a case study because the
differential reflectivity measured with C-band radar is more sensitive to large raindrops associated with heavy rainfalls
than is radars operating at other frequencies. Results show that the estimated rainfall intensity agrees well with surface
observations made during the event. Moreover, the so-called high Zdr column, a large differential reflectivity region was
clearly analyzed aloft about 10 minutes prior to the local heavy rainfall on the ground, suggesting that the differential
reflectivity observed with C-band polarimetric radar can be a good index to detect heavy precipitation events in advance.
A radar is a powerful tool for measurement of the 3-D structure of precipitation. Recently, polarimetric radar is widely
used because it can measure the size of raindrops to some degree and therefore can measures more accurate rainfall rate
than the conventional weather radar. A space-borne radar is also widely used in precipitation studies. The Tropical
Rainfall Measuring Mission (TRMM) satellite has been continuously monitoring precipitation on a global scale since the
launch in November, 1977. Following the TRMM, the Global Precipitation Mission (GPM) is scheduled to launch in
2013. The polarimetric parameters observed with the polarimetric radar depend on various precipitation properties in a
complex way. Multiple scattering contributions cannot be neglected for a radar operated at higher frequency of 35 GHz
higher onboard the GPM. To develop a robust algorithm for more accurate measurements of precipitation from those
radars, we should evaluate how micro-physical properties of precipitation link to the received signals. We have
developed a generalized radar simulator for polarimetric and space-borne radar (GPASS). This is a physically-based
simulator in which the scattering properties of cloud and raindrops are calculated by using radio wave scattering theory.
Thus we can make detailed study how the radar signals vary with micro-physical properties of precipitation by using the
simulator. We will present the simulator in detail and the limit of the Rayleigh approximation for polarimetric radar.
We measured clouds and precipitation by a combined use of a radar and a passive solar/infrared radiometer onboard the
Tropical Rainfall Measuring Mission (TRMM) satellite and examined how precipitation characteristics are linked to
cloud properties. The radar is used to measure precipitation. Optical thickness (τ) and effective radius (re) of convective clouds whose top is composed of ice particles were estimated from radiances at two wavelengths measured with the
radiometer. Particularly, the sensitivities of retrieved cloud parameters to the assumption of ice crystal shape models
(spherical or nonspherical) were examined. Assumption of spherical model results in larger τ comparing with that from
nonspherical model. The re retrieved from liquid water spherical model is significantly larger than that with hexagonal
columns and ice sphere models. The influence of ice particle shape (hexagonal columns and spheres) on the retrieved re
is noted by several microns. The use of nonspherical model is essential for retrieving τ and re of convective clouds whose
top is composed of ice particles by using visible and near-infrared channels of satellite measurements.
On the basis of the numerical results by using finite-difference time-domain (FDTD) methods, we discuss the light
scattering and its dependence on the particles' physical properties, such as particle shape, surface roughness, and porosity.
The FDTD calculations were conducted for single hexagonal columns and aggregates with size parameter up to 50 taking
into account the particles' random orientation. The discussion includes the depolarization ratio as well as basic radiative
transfer parameters: phase function, extinction efficiency and asymmetry factor. Results of our light-scattering
calculations show that the simple deformation of the original shape alters the optical properties. Comparing with the
shape effects, surface micro-roughness and porosity cause minor changes in some scattering parameters However, the
micro-roughness makes the depolarization ratios increase as well as the irregular shaped aggregate particles.
Cloud feedbacks are known to be the largest sources of uncertainty in the climate system. The cloud radiative
forcing is strongly affected by microphysical properties of clouds which are closely related to precipitation as well as
aerosol concentrations. We measured clouds and precipitation by a combined use of a radar and a passive solar/infrared
radiometer onboard the Tropical Rainfall Measuring Mission (TRMM) satellite and examined how precipitation
characteristics are linked to cloud properties. The radar is used to measure precipitation. Optical thickness and effective
radius of clouds are estimated from radiances at two wavelengths measured with the radiometer. It is found that the
sizes of cloud drops are closely related to precipitation: Cloud drop sizes are smaller for non-precipitating clouds than
those for precipitating clouds in which clouds and precipitation coexist. Clouds with large drop size are almost
precipitating. The processes that precipitation modifies cloud microphysics, therefore, the radiative properties are
critical to evaluate the cloud feedbacks in the global warming prediction.
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