Cross-validation of spaceborne radar and ground polarimetric radar observations

Abstract

There is great potential for spaceborne weather radar to make significant observations of the precipitating medium on global scales. Unlike other instruments, such as light detecting and ranging (lidar) systems that operate in the optical region, and thermatic mapping systems that use short wavelengths in the infrared, etc., radar has the ability to penetrate the cloud and rain layer to make observations of the vertical profile of the rain field. Though the scientific reasons for making weather observations using spaceborne radar are varied, the benefits of using space-based weather radar have been largely unrealized. The technological advances in spaceborne weather radar systems that were required to reduce the size, weight and cost of deployment, and to meet the efficiency, reliability and sensitivity needs of the scientific community were not available until the 1990's. In 1997, the Tropical Rainfall Measuring Mission (TRMM) satellite was launched. TRMM was the first dedicated mission to study rainfall in the tropics from space. The satellite operates in a nearly circular orbit at 350 km nominal altitude and 35 degree inclination. It carries several meteorological observing instruments, including the 13.8 GHz Precipitation Radar (PR). This instrument is designed to yield information about the vertical storm structure so as to gain insight into the intensity and distribution of rainfall. Attenuation effects on PR measurements, however, can be significant, which can be as high as 10-15 dB. This can seriously impair the accuracy of rain rate retrieval algorithms derived from PR signal returns, which makes it necessary to compensate for attenuation effects. Independent evaluation and verification of attenuation correction, and subsequent rain rate estimation, derived from spaceborne algorithms is an important task in order to accurately determine the rain rate from space. Direct inter-comparison of meteorological measurements between space radars with polarimetric ground radar observations can be used to evaluate and validate spaceborne processing algorithms. Though conceptually straightforward, this can be a challenging task. Differences in viewing aspects between space and earth point observations, propagation frequencies, resolution volume size and time synchronization mismatch between measurements can contribute to direct point-by-point intercomparison errors. The problem is further complicated by spatial geometric distortions induced into the space-based observations caused by the movements and attitude perturbations of the spacecraft itself. A method is developed to align space and ground radar observations so that a point-by-point inter-comparison of measurements can be made. The method uses variable resolution volume matching to match observations between the two systems and a polynomial alignment technique to minimize the effects of potential geometric distortion in space radar observations relative to ground measurements. Examples of the alignment method are presented using TRMM PR reflectivity measurements and ground radar measurements from the National Center for Atmospheric Research (NCAR) S-band polarimetric (SPOL) radar. After appropriate resolution volume matching and alignment, in situ data collected from the TRMM TExas and FLorida UNderflights (TEFLUN-B) experiment, and the Large-scale Biosphere Atmosphere (LBA) field campaign was used to calibrate PR measurements, and to evaluate the PR attenuation correction algorithm relative to ground-based measurements. Ground-based polarimetric observations are used to quantitatively estimate the attenuation on PR signal returns along individual PR beams, and a technique is formulated to determine the true PR return via theoretical modeling of specific attenuation (k) at PR wavelength with ground-based S-band radar observations. The reflectivity factor (Zh) at horizontal polarization state and specific differential phase (Kdp) are determined along the PR beam from GR measurements, and a theoretical relationship is used to determine the expected specific attenuation along the space-earth path at Ku-band frequency from these measurements. A theoretical k-Kdp relationship is determined for moderate rain rate (when Kdp ≥ 0.5 deg/km); and, a power- L law relationship, k = a Zbh, is used for light rain and for other types of hydrometers encountered along the path. The two-way path-integrated attenuation (PIA) is calculated along the PR propagation path by integrating the specific attenuation along the path. The true PR reflectivity is then determined from PIA estimates and the measured PR reflectivity. Ground-based polarimetric observations are also used to quantitatively estimate the parameters of a three-parameter gamma raindrop size distribution (RSD) model along individual PR beams in the presence of rain. The mean shape of the raindrops is used along with polarimetric measurements to retrieve the model parameters Nw, Do and μ. The current algorithm used by the PR for attenuation correction is evaluated using GR polarimetric estimates of the functional behavior of the RSD along PR beams. The statistical behavior of the gamma model parameters, and PR attenuation correction algorithm, are presented in histogram form along the vertical profile through the rain layer, which is used to evaluate the PR attenuation correction algorithm. Additionally, PR rain rate estimates near the earth surface are compared to polarimetric ground-based radar observations. This analysis leads to direct evaluation of the PR-derived RSD model parameter values. The analysis, and summary, of this work is presented in the dissertation.