Sea Surface Wind Vector Measured By Polarimetric Microwave Radiometer

Wind speeds and directions play important roles in studying global ocean-air interactions, which serve as inputs for meteorological forecasting models, whether short-term weather forecasts or long-term climate forecasts. Although conventional microwave radiometer, such as Special Sensor Microwave/Imager (SSM/I) of the Defense Meteorological Satellite Program (DMSP), can provide wind speeds for the purpose, it can be retrieved no accurate direction information yet. Since the 90's of last the century, there has been increasing interests, whether in polarimetric microwave remote sensing theories or in measurements using air-borne or ground-based polarimeter, in polarimetric microwave sensing of nature targets, such as ocean, tilled land. The paper published have showed that microwave polarimeter was able to measure not only wind speed, but also wind direction, which finally opened a new field of wind vector measurements. Microwave polarimeter is a sensor of measuring the third and/or the forth Stokes parameters, as well as the vertical-and horizontal-polarization. In the dissertation, we will expand on three main topic related remote sensing ocean wind vector by polarimetric microwave radiometer: (1) The mechanism on microwave polarimetry of wind vector; (2) Principle, design, and calibration methods of microwave polarimeter; (3) Pre-processing algorithms and retrieving methods of wind speed and direction from measurements of polarimeter. 1. The mechanism on microwave polarimetry of wind vector The topic is divided into two sections, which will be dissertated in chapter 2 and chapter 3. In chapter 2, full polarization radiative transfer equations and their models of ocean-air system for passive microwave remote sensing were developed, which are of basis of wind vector polarimetry. The interactions between air and ocean contribute to responses received by microwave radiometer on-aboard satellite. The four Stokes of ocean emissions suffer attenuation through the atmosphere before they arriving the antenna of polarimeter. Atmosphere scattering may be neglected in no rain condition. So only its absorption leads to reduction of emission from the surface, meanwhile it radiate energy as well. Therefore accurate absorption model plays a key role in full Stokes simulations and algorithms development for wind retrievals. We compared in detail some absorption model widely used for the purposes with the model embedded in RTTOV, named by fast radiative transfer model used operationally at the European Center for Medium-Range Weather Forecasts (ECMWF). After combining with polarimetric ocean models, we derived a composite absorption models, named N2-MPM93+O2-MPM93+H2O-PWR98, which can best simulate ocean polarimetric signals at the top of the atmosphere. We listed simulated results by applying 3962 sampled databases of 60-level atmospheric profiles above global ocean from the ECMWF analyses. The emission and scattering of ocean depend on surface roughness, as well as temperatures and dielectric constant of seawater. we dedicate to modeling and simulating emissivity and reflectivity models at various wind vectors. There are a few polarimetric emissivity models derived from solving ocean surface electromagnetic boundary value problem theoretically. In chapter 3, the full Stokes parameters of ocean surface emission simulated by three emissivity models at various ocean state are reviewed, inter-comparisons of the results from numerical simulation are carried out in detail, the effects of wave spectrum on the ocean surface emissivity are evaluated. The simulated four Stokes parameters of emission are compared with those by experimental formulations derived from airborne or spaceborne microwave radiometers. 2. Principles, design, and calibration methods of microwave polarimeter Microwave polarimeter is basic for testing and realizing the mechanism of wind vector polarimetry. In chapter 4, we begin with introducing principles, then describing system design program put forward by NMRSL and test procedures, as well as error analyses. Firstly we give a genaral description of principles of different types of polarimeter, compare their sensitivity with that of idea total-power radiometer. Then we list the key questions related to design a space-borne polarimeter, including realization of the sensor and application of the measurements. The core of the chapter is the design of digital correlation radiometer. The scheme we selected here is used for test the mechanism of direct correlation radiometer, which employs fast multi-bit digital correlators. In order to apply the measurements from polarimeter, we have to calibrate the sensor well in advance. Conventional calibration methods can only produce non-polarization signals and they cannot be used for calibrating a polarimeter. In Microwave polarimetry, it is necessary to make sure sensitivity of one channel to all the other channels in calibration, so we have to utilize other methods to calibrate a polarimeter, which demands more than two signals inter-independent as inputs. In chapter 5, we first introduce concepts and classification on calibration, and then derive three-and four-polarization calibration equations for Passive Polarization Calibration Standards (PPCS). We expound the principle and realization of PPCS. Furthermore, we also describe some other methods of calibrating a radiometer, internally and externally, which include active load calibration, atmospheric tipping curve calibration, and cold-end vicarious calibration. The effects of different methods on calibration accuracies are analyzed and intercompared with each other. Finally, calibration...