Zhenzhan Wang

WCOM: THE MISSION CONCEPT AND PAYLOADS OF A GLOBAL WATER CYCLE OBSERVATION MISSION

WCOM, the Water Cycle Observation Mission, is proposed to improve the capability of synergetic observation of key water cycle variables. By developing innovative active-passive and multi-frequency combined sensors and retrieval models and techniques, the scientific objectives of this mission is to deepen the understanding on global water distribution, transportation and phase conversion by synergistic observations; and based on the improved model and data, to rebuilt long-term data series for revealing of the responses and feedbacks of water cycle to global changes.

Design of the second generation microwave humidity sounder (MWHS-II) for Chinese meteorological satellite FY-3

Design and Technical specification parameters of MWHS-II (MicroWaves Humidity Sounder) of FY-3 satellite are presented, which include much improvements compared to MWHS onboard FY-3A/B satellite launched in 2008 and 2010. Especially MWHS-II adds 8 channels at 118.75GHz to sound temperature information for the first time internationally. The simulation results show that MWHS-II will play an important role in the meteorological sounding system for measuring global atmospheric temperature and water vapour profiles.

Prelaunch Calibration of Microwave Humidity Sounder on China's FY-3A Meteorological Satellite

Chinas Feng Yun-3A (FY-3A) meteorological satellite is a second-generation polar-orbiting meteorological satellite launched in May 2008. The Microwave Humidity Sounder (MWHS) is the main payload, designed for atmospheric humidity sounding. Before the launch of MWHS, a series of experiments was conducted in a thermal/vacuum (T/V) chamber. This letter describes the MWHS T/V calibration, in which the flight model currently operating on FY-3A was tested. The calibration procedure and data-processing methods are presented. Calibration results, such as radiometric resolution, receiver nonlinearity, and calibration bias, were obtained. The results meet the specifications on bias and sensitivity of MWHS. Since the instrument temperatures will be approximately 10°C-20°C on orbit, the calibration errors of MWHS over the range of 100-300 K would be less than 0.3 K using the nonlinearity coefficients derived in the T/V test.

Calibration Analyses for Sounding Channels of MWHS Onboard FY-3A

The Microwave Humidity Sounder (MWHS) is a five-channel microwave radiometer in the range of 150-191 GHz onboard FengYun 3 (FY-3). Before the launch of FY-3A, the intensive thermal vacuum (TVAC) tests for MWHS had been carried out in a 2-m TVAC chamber, and the basic parameters such as receiver nonlinearity were obtained. Four-year operation shows that the performance of FY-3A MWHS remains stabile. The variations among space views and warm target views in the interval of scan lines were generally within 20 counts. In addition, the temperature fluctuation of the warm target in a single pass is within 0.5 K. Primarily, due to the nonlinearity correction obtained from the TVAC test, radiance measurements of MWHS agree well with NOAA-17 AMSU-B and NOAA-18 Microwave Humidity Sounder (MHS) data. Averaged brightness temperature differences between FY-3 MWHS and NOAA-17 AMSU-B, at simultaneous cross-overpass points, are less than 1.08 K for the channels at 183 ± 1 GHz. In the observation-minus-background comparison, the observed and simulated brightness temperature biases of FY-3 MWHS and NOAA-18 MHS exhibit similar performance in the 183.31 ± 1 GHz channel; but in the 183.31 ± 7 GHz channel, MWHS agrees better with background. It is anticipated that FY-3 MWHS data will contribute to numerical weather prediction.

Hurricane Wind Speed Estimation Using WindSat 6 and 10 GHz Brightness Temperatures

The realistic and accurate estimation of hurricane intensity is highly desired in many scientific and operational applications. With the advance of passive microwave polarimetry, an alternative opportunity for retrieving wind speed in hurricanes has become available. A wind speed retrieval algorithm for wind speeds above 20 m/s in hurricanes has been developed by using the 6.8 and 10.7 GHz vertically and horizontally polarized brightness temperatures of WindSat. The WindSat measurements for 15 category 4 and category 5 hurricanes from 2003 to 2010 and the corresponding H*wind analysis data are used to develop and validate the retrieval model. In addition, the retrieved wind speeds are also compared to the Remote Sensing Systems (RSS) global all-weather product and stepped-frequency microwave radiometer (SFMR) measurements. The statistical results show that the mean bias and the overall root-mean-square (RMS) difference of the retrieved wind speeds with respect to the H*wind analysis data are 0.04 and 2.75 m/s, respectively, which provides an encouraging result for retrieving hurricane wind speeds over the ocean surface. The retrieved wind speeds show good agreement with the SFMR measurements. Two case studies demonstrate that the mean bias and RMS difference are 0.79 m/s and 1.79 m/s for hurricane Rita-1 and 0.63 m/s and 2.38 m/s for hurricane Rita-2, respectively. In general, the wind speed retrieval accuracy of the new model in hurricanes ranges from 2.0 m/s in light rain to 3.9 m/s in heavy rain.

Emissivity Measurements of Foam-Covered Water Surface at L-Band for Low Water Temperatures

For a foam-covered sea surface, it is difficult to retrieve sea surface salinity (SSS) with L-band brightness temperature (1.4 GHz) because of the effect of a foam layer with wind speeds stronger than 7 m/s, especially at low sea surface temperature (SST). With foam-controlled experiments, emissivities of a foam-covered water surface at low SST (-1.4 degrees C to 1.7 degrees C) are measured for varying SSS, foam thickness, incidence angle, and polarization. Furthermore, a theoretical model of emissivity is introduced by combining wave approach theory with the effective medium approximation method. Good agreement is obtained upon comparing theoretical emissivities with those of experiments. The results indicate that foam parameters have a strong influence on increasing emissivity of a foam-covered water surface. Increments of experimental emissivities caused by foam thickness of 1 cm increase from about 0.014 to 0.131 for horizontal polarization and 0.022 to 0.150 for vertical polarization with SSS increase and SST decrease. Contributions of the interface between the foam layer and water surface to the foam layer emissivity increments are discussed for frequencies between 1 and 37 GHz.

Emissivity measurements and theoretical model of foam-covered sea surface at C-band

To investigate the effects of sea foam (or whitecaps) on the emissivity of sea surface, microwave radiometric experiments of foam-covered sea surface at C-band (6.8 GHz) were conducted by artificial foam during September 2012 at Tangdao bay of Qingdao. The sea surface temperatures (SST) were from 22°C to 26.4°C, and the thickness and the air bubble size of the foam layer were controlled by a compressed air pump and monitored with a video recorder. The emissivities were obtained with the measured brightness temperatures. Furthermore, based on the experimental data, a theoretical model of sea foam emissivity is proposed by the wave approach method of two-layer medium, where the foam complex permittivity is estimated by an effective medium approximation. Experimental results show that the averaged emissivity increments of both horizontal and vertical polarizations are approximately from 0.25 to 0.35 for 1 cm of foam thickness with different SST and air volume fractions (AVF). For a given thickness of the foam layer, although both AVF and SST play important roles in increasing the sea surface emissivity, the influence of AVF on the emissivity is much stronger than that of SST. The brightness temperature increases by about 113K compared to that of the foam-free surface. Moreover, the theoretical model implies that the emissivity is increased during the initial aggregation process of foam, and then fluctuates on increasing the foam thickness up to a larger saturation value. The effect of the meniscus at the foam–water boundary on foam emissivity is also discussed.

A digital correlation full-polarimetric microwave radiometer design and calibration

Since the first polarimetric radiometer satellite WindSat was launched on January 6, 2003 [1] and has been working normally until now, microwave polarimetry has demonstrated its ability on retrieving global wind vector [2].The first digital correlation microwave polarimeter was designed in 2001 [3], and digital technology has made a great progress during the last decade. We describe design of a digital-correlation full-polarimetric microwave radiometer system (DPMR), which operates at the same frequencies as WindSat, i.e. 6.8, 10.7, 18.7, 23.8 and 37.0 GHz. A direct digital cross-cerrelation technique is used in the system to produce four Stokes parameters at each frequency simultaneously. The main specifications are list in TABLE 1.

Chaos particle swarm optimization combined with circular median filtering for geophysical parameters retrieval from Windsat

This paper established a geophysical retrieval algorithm for sea surface wind vector, sea surface temperature, columnar atmospheric water vapor, and columnar cloud liquid water from WindSat, using the measured brightness temperatures and a matchup database. To retrieve the wind vector, a chaotic particle swarm approach was used to determine a set of possible wind vector solutions which minimize the difference between the forward model and the WindSat observations. An adjusted circular median filtering function was adopted to remove wind direction ambiguity. The validation of the wind speed, wind direction, sea surface temperature, columnar atmospheric water vapor, and columnar liquid cloud water indicates that this algorithm is feasible and reasonable and can be used to retrieve these atmospheric and oceanic parameters. Compared with moored buoy data, the RMS errors for wind speed and sea surface temperature were 0.92 m s−1 and 0.88°C, respectively. The RMS errors for columnar atmospheric water vapor and columnar liquid cloud water were 0.62 mm and 0.01 mm, respectively, compared with F17 SSMIS results. In addition, monthly average results indicated that these parameters are in good agreement with AMSR-E results. Wind direction retrieval was studied under various wind speed conditions and validated by comparing to the QuikSCAT measurements, and the RMS error was 13.3°. This paper offers a new approach to the study of ocean wind vector retrieval using a polarimetric microwave radiometer.

In-orbit calibration scanning microwave radiometer on HY-2 satellite of China

In this paper, we give a method of calibrating Tb on HY-2 satellite by cross comparing them with those from WindSat satellite. The results show that the re-calibrated Tb were obviously closer to the simulated Tb by using ECWMF data. We will further investigate the accuracies with newly launched microwave radiometer AMSR2.